Syntheses of per-15N labeled etioporphyrins I-IV and a related tetrahydrobenzoporphyrin for applications in organic geochemistry and vibrational spectroscopy

Article abstract of DOI:10.1016/j.tet.2005.09.105

Nitrogen-15 labeled pyrroles have been prepared from commercially available ¹⁵N glycine or sodium nitrite using the Barton-Zard, Knorr, and Kleinspehn approaches. These pyrroles were used as intermediates in the synthesis of per-¹⁵N labeled porphyrins needed for the analysis and assignment of vibrational spectra for sedimentary porphyrins. Etioporphyrin-I was prepared via pyrromethene intermediates, while etioporphyrins II-V and a related tetrahydrobenzoporphyrin were synthesized via stepwise routes involving the copper(II) mediated cyclization of a,c-biladienes as the key step. Detailed analyses of both the proton and carbon-13 NMR spectra provide nitrogen-15 coupling constants for these important structures.

Full text of DOI:10.1016/j.tet.2005.09.105

Tetrahedron 61 (2005) 11577–11600
Syntheses of per-15N labeled etioporphyrins I–IV and a related
tetrahydrobenzoporphyrin for applications in organic
geochemistry and vibrational spectroscopy*
*
Timothy D. Lash and Shaohua Chen
Department of Chemistry, Illinois State University, Normal, IL 61790-4160, USA
Received 4 July 2005; revised 18 September 2005; accepted 21 September 2005
Available online 14 October 2005
Abstract—Nitrogen-15 labeled pyrroles have been prepared from commercially available 15N glycine or sodium nitrite using the Barton-
Zard, Knorr, and Kleinspehn approaches. These pyrroles were used as intermediates in the synthesis of per-15N labeled porphyrins needed for
the analysis and assignment of vibrational spectra for sedimentary porphyrins. Etioporphyrin-I was prepared via pyrromethene intermediates,
while etioporphyrins II–V and a related tetrahydrobenzoporphyrin were synthesized via stepwise routes involving the copper(II) mediated
cyclization of a,c-biladienes as the key step. Detailed analyses of both the proton and carbon-13 NMR spectra provide nitrogen-15 coupling
constants for these important structures.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
of vibration for various structural types have been analyzed.
These analyses can be aided by selective isotopic
substitutions, such as the incorporation of deuterium,
carbon-13, and nitrogen-15, but in many cases the
isotopomers can only be obtained by total synthesis.
Metalloporphyrins are commonly found in organic-rich
sediments such as oil shales and petroleum.1,2 These
materials are believed to be degradation products, or
molecular fossils, of biological pigments such as the
chlorophylls.1,2 The distribution of porphyrin structural
types can have diagnostic value in relation to determining
thermal maturity and the origins of fossil fuels,2 and may
have applications in chemical prospecting.1–3 Furthermore,
analyses of petroporphyrins can also have value in
environmental investigations.4 The porphyrin distributions
in oil shales and petroleum differ considerably from one
source to another and hence can provide a characteristic
fingerprint for specific sedimentary materials. As the
metalloporphyrins found in petroleum are fairly resistant
to weathering and biodegradation, chromatographic finger-
prints for these compounds can be used in determining the
origins of crude oils and tar balls resulting from oil spills.
Resonance Raman spectroscopy has shown considerable
promise in the analysis of petroporphyrins,5–11 and for this
reason a data base of vibrational spectra for metallo-
porphyrins has been developed and the characteristic modes
Several families of porphyrins are found in oil shales and
petroleum, generally in the form of nickel(II) or vanadyl
chelates, including cycloalkanoporphyrins (CAPs; 1) with
five-, six- or seven-membered exocyclic rings.1,2 The best
known of these geoporphyrins is deoxophylloerythroetio-
porphyrin (DPEP; 2), a structure that is closely related to the
chlorophylls (Chart 1).1,2 Indeed, all of the known
sedimentary porphyrins are believed to be derived from
the chlorophylls and to a lesser extent the hemes, although
the origins of some petroporphyrins are poorly under-
stood.1,2,12 In addition to cycloalkanoporphyrins, etiopor-
phyrins 3 and 4 have been isolated and characterized by 1H
NMR spectroscopy and mass spectrometry.13 There are four
etio ‘type isomers’ where each pyrrole subunit has one ethyl
and one methyl substituent: etioporphyrins-III (3), I (5), II
(6), and IV (7).14 However, virtually all naturally occurring
porphyrins are structurally related to etioporphyrin-III, and
while 3 and its 13-desethyl analogue 4 are present in
sedimentary materials there is no evidence for the
occurrence of etioporphyrins-I, II or IV (5–7).2 The absence
of the latter type isomers provides a degree of confirmation
for the biological origins of petroporphyrins.2 On the other
hand, the four etioporphyrin isomers provide an important
group of structures for spectroscopic analysis.5–11
* Part 18 in the series porphyrins with exocyclic rings. For part 17, see:
Manley, J. M.; Roper, T. J.; Lash, T. D. J. Org. Chem. 2005, 70, 874.
Keywords: Pyrroles; Nitrogen-15; NMR spectroscopy; Petroporphyrins;
Organic geochemistry.
Corresponding author. Tel.: C309 438 8554; fax: C309 438 5538;
e-mail: tdlash@ilstu.edu
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.105

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T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Chart 1.
In addition to the CAPs and etioporphyrins, other families of
sedimentary porphyrins include tetrahydrobenzo-
porphyrins13,15 (e.g., 8) and the related benzoporphyrins.13,16
A series offour isomeric nickel(II) chelates of 8, together with
the symmetrical tetrabutanoporphyrin 9, have been studied in
detail using resonance Raman spectroscopy7 and labeled
versions of 9 with 4 13C meso-carbons and/or 4 15N’s were
synthesized as spectroscopic models.17 per-15N labeled
samples of etioporphyrins I–IV and a related tetrahydroben-
zoporphyrin were also required as spectroscopic standards in
order to more fully assign the vibrational spectra. These
compounds represent a considerably greater synthetic
challenge due to their relative lack of symmetry, particularly
in the case of etioporphyrin-III and butanoporphyrin 8. In this
paper, syntheses of per-15N labeled porphyrins 3 and 5–8 are
reported.18,19 In addition, we took the opportunity to
investigate 15N coupling in both the proton and carbon-13
NMR spectra for these important parent structures.20
2. Results and discussion
2.1. Etioporphyrin-I
The four etioporphyrin type isomers have varying degrees
of symmetry and this led us to consider a number of
methods for their synthesis.21 Etioporphyrin-I represented
the most symmetrical of these systems and hence was the
easiest to synthesize. In this case, we adapted the classical
Fischer pyrromethene methodology using conditions
Scheme 1.

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11579
reported earlier by Smith (Scheme 1).22,23 Initially, a
labeled pyrrole 10 was required and this was prepared by
the Johnson modified Knorr pyrrole condensation.24
Treatment of tert-butyl acetoacetate with 15N labeled
sodium nitrite and acetic acid afforded the labeled oxime
11. This was taken on in crude form and reduced with zinc
dust and acetic acid in the presence of 3-ethyl-2,4-
pentanedione. Reduction affords an unstable aminoketone
Figure 1. 400 MHz proton NMR spectra of per-15N etioporphyrin-I. (A). Free base in CDCl3. The NH protons appear as a quintet at K3.8 ppm from coupling
(1JNHZK24 Hz) from all four nitrogen-15’s due to rapid intramolecular exchange within the macrocyclic cavity. The meso-protons produce a triplet at
10.6 ppm due to coupling from 2 equiv nitrogen-15 nuclei. (B). Dication in TFA–CDCl3. Only the upfield and downfield regions are shown. The meso-
protons again appear as a triplet due to 15N coupling but the internal protons are now fixed and only couple to the directly attached nitrogen to give a doublet
(JZK94.4 Hz).

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T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
15
25
1
and this further condenses with the b-diketone to give the
required pyrrole 10 in an overall 38% yield from sodium
nitrite (Scheme 1). The proton NMR spectrum for the 15N
(for N, IZ ⁄2 ) for the pyrrole proton of K97 Hz. In
some spectra, broadened or distorted peaks were observed
for the NH resonance and this phenomenon was also seen
for other pyrrolic intermediates in our studies. This was due
1
labeled pyrrole showed a characteristic JNH coupling
Figure 2. 100 MHz carbon-13 NMR spectrum of per-15N etioporphyrin-I in TFA–CDCl3. Due to the high degree of symmetry in etioporphyrin-I, there should
only be three alkyl resonances, one meso-resonance (observed at 98.5 ppm) and four pyrrole resonances (137–145 ppm). The a-pyrrole carbons give doublets
(1JNCZ13 Hz) at 141.4 and 142.3 ppm, while the b-carbons show broadened resonances at 137.9 and 144.6 ppm.

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11581
to slight temperature changes after the NMR tube was
introduced into the probe and was caused by minor shifts
occurring during the data acquisition. This effect is not
observed for unlabeled pyrroles due to quadrupole broad-
ening and in any case the problem is easily dealt with by
allowing the temperature of the sample to equilibrate for a
few minutes prior to running the spectrum. Coupling was
also observed to the 5-methyl unit, which appeared as a
doublet (3JNHZ2.8 Hz). In the carbon-13 NMR spectrum,
1
the a carbons gave JNC coupling of 14–15 Hz, while the
Z
b-carbons afforded more weakly coupled doublets (2JCN
4 Hz). Weak couplings to two of the b-aliphatic resonances
were also noted.
Treatment of 10 with bromine in acetic acid afforded the
pyrromethenes 12 and these were treated with refluxing
formic acid to give etioporphyrin-I (5) in an overall 21%
yield (Scheme 1). The 400 MHz proton NMR spectrum in
CDCl3 (Fig. 1A) showed a triplet for the 4 equiv meso-
protons at 10.1 ppm due to coupling from 2 equiv nitrogen-
15 nuclei with a coupling constant of 4.4 Hz. The four
methyl units are equivalent and give rise to a singlet at
3.67 ppm, the downfield shift being due to the expected
strong diamagnetic ring current for the porphyrin nucleus,
while the 4 equiv ethyl moieties afford a 12H triplet and an
8H quartet at 1.87 and 4.10 ppm, respectively. Finally, the
internal NH protons give a quintet with a coupling constant
JZK24 Hz.25 This arises from rapid intramolecular NH
exchange such that both protons are coupled equally to all
four nitrogen-15 nuclei. The data also demonstrates that
intermolecular exchange is not occurring on the NMR
timescale. This phenomenon has been reported previously
for other porphyrins and is, of course, temperature
dependent.26 Addition of TFA affords the corresponding
Scheme 2.
the pyrrole ethyl ester 15. This key pyrrolic intermediate
could be synthesized by the methodology used in preparing
the related tert-butyl ester 10, but in this case we selected
the use of Kleinspehn’s pyrrole synthesis32 using the
modified conditions reported by Paine and Dolphin.27,33
Treatment of a slight excess of diethyl malonate with
aqueous sodium nitrite in acetic acid afforded the oxime 16
and this was further reduced with hydrogen over 10%
palladium–charcoal to give diethyl aminomalonate (17)
(Scheme 2). In this work, a slight excess of diethyl malonate
was used over literature procedures. While this resulted in
residual diethyl malonate contaminating both 16 and 17, the
presence of this material did not interfere with the
subsequent chemistry. Reaction of crude 17 with 3-ethyl-
2,4-pentanedione (18) in refluxing acetic acid afforded the
pivotal pyrrole 15 in 33–38% overall yield. The 400 MHz
proton NMR spectrum for 15N labeled 15 in CDCl3 (Fig. 3)
dication 5H22C. The meso-protons still give a triplet (3JNH
Z
5 Hz) but the four NH protons now appear as a strongly
coupled doublet (1JNHZK94.4 Hz) at K3.8 ppm (Fig. 1B).
In the diprotonated species, the individual NH’s are fixed
and do not exchange, so only one bond coupling to a single
nitrogen-15 is observed. The carbon-13 NMR spectrum for
per-15N etioporphyrin-I in TFA–CDCl3 showed the
presence of three alkyl carbons between 12 and 21 ppm, a
single meso-carbon at 99 ppm and four sets of resonances
for the 4 equiv pyrrole units between 137 and 145 ppm
showed the NH as a strongly coupled doublet (1JNH
Z
K96 Hz) at 8.8 ppm. In addition, the 5-methyl substituent
gave a doublet (3JNHZ2.8 Hz) at 2.20 ppm. In the carbon-
13 NMR spectrum (Fig. 4), all four pyrrole carbons
appeared as doublets and some long range coupling to two
of the alkyl resonances was also noted. Treatment of 15 with
sodium hydroxide in ethylene glycol at 180–190 8C cleaved
(Fig. 2). The a-carbons afforded two doublets (1JNC
Z
13 Hz) near 142 ppm, while the b-carbons gave broadened
resonances at 137.9 and 144.6 ppm (the 2 bond NC coupling
did not resolve in this case, although this can be observed for
some of the examples noted below).
2.2. Etioporphyrin-II
Synthesis of the remaining etioporphyrins requires the use
of a more stepwise approach for generating the porphyrin
nucleus. A number of routes have been developed for
synthesizing structures of this type,21 but it is critically
important that the method chosen would not give rise to any
isomeric impurities. Due to our previous experience in this
area,27,28 we selected the well established a,c-biladiene
methodology29–31 for these studies. The synthesis of
etioporphyrin-II required the pyrrole aldehyde 13
(Scheme 2) and the symmetrical dipyrrylmethane 14
(Scheme 3). Both of these compounds were prepared from
Scheme 3.

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T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 3. 400 MHz proton NMR spectrum of 15N labeled pyrrole ethyl ester 15. The doublet at 8.8 ppm corresponds to the pyrrole NH (1JNHZK96 Hz).
Insert: the 15N coupled doublet at 2.2 ppm corresponds to the 5-methyl substituent (3JNHZ2.8 Hz).
the ester moiety to give the a-unsubstitued pyrrole 19 and
subsequent formylation using Clezy’s modified Vilsmeier–
Haack conditions34 afforded the required pyrrole alde-
hyde 13. The proton NMR spectrum for 15N labeled 19
gave a doublet of doublets at 7.47 ppm for the NH
NMR spectrum to be assessed with ease. Of particular
note, the terminal methyl produced a doublet (3JNH
Z
2.8 Hz) at 2.7 ppm, the bridging CH2 afforded a broadened
partially resolved triplet at 5.2 ppm and the 2 equiv
bridging methines gave a triplet (3JNHZ5.2 Hz) at
7.1 ppm. Naturally, the NH protons appeared as two
overlapping strongly coupled doublets (1JNHZK95 Hz)
near 13.2 ppm. Cyclization of a,c-biladiene 22 with
copper(II) chloride in DMF at room temperature30 afforded
the copper(II) complex of 6 and subsequent demetallation
with 15% sulfuric acid–TFA gave etioporphyrin-II
(Scheme 4) in 31–48% yield. The 400 MHz proton NMR
spectrum of per-15N-labeled 6 in CDCl3 (Fig. 6A)
again showed a quintet for the internal NH protons
(1JNHZK23 Hz). Two different types of meso-protons
are present in this type isomer and these produce two
overlapping triplets at 10.1 ppm. In the presence of TFA
(Fig. 6B), the dication 6H22C again gave two overlapping
triplets for the meso-protons, albeit shifted downfield to
10.6 ppm, and the 4 equiv NH protons afforded a doublet
at K3.4 ppm (1JNHZK94 Hz). The carbon-13 NMR
spectrum for 6H22C in TFA–CDCl3 (Fig. 7) gave two
singlets between 98 and 99 for the meso-carbons and four
doublets for the four types of pyrrolic carbons in this
structure. The a-carbons appear as clear doublets near
142 ppm (1JNCZ13 Hz), while the b-carbons resonated at
137.4 and 144.0 ppm (2JCNZ2 Hz).
3
(1JNHZK94.5 Hz, JHHZ2.7 Hz) due to coupling from
the 15N and the adjacent CH. The pyrrole CH gave a
broadened multiplet from these interactions. In the case of
the aldehyde 13, the proton NMR showed the formyl
proton as a doublet (3JNHZ2.4 Hz) and the 5-methyl
group also demonstrated 15N coupling. The NH showed
up as the usual strongly coupled doublet at 9.9 ppm
(1JNHZK97 Hz).
Reaction of pyrrole 15 with lead tetraacetate in acetic acid
afforded the related acetoxymethyl derivative 20 and
subsequent self-condensation in refluxing HCl–methanol
gave the symmetrical dipyrrylmethane 21 (Scheme 3). The
400 MHz proton NMR spectrum of labeled 21 showed the
usual NH doublet and a triplet for the bridging methylene
(3JNHZ2.4 Hz) at 3.86 ppm due to coupling from the
2 equiv nitrogen-15 nuclei (Fig. 5). Treatment of 21 with
sodium hydroxide in refluxing ethylene glycol afforded the
required diunsubstituted dipyrrylmethane 14. Condensation
of 14 with 2 equiv of pyrrole aldehyde 13 in the presence
of HBr afforded the corresponding a,c-biladiene 22 in
67–76% yield (Scheme 4). The plane of symmetry in this
structure allows the coupling interactions in the proton

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11583
Figure 4. (A). 100 MHz carbon-13 NMR spectrum of 15N labeled pyrrole ethyl ester 15. Inserts: long range coupling to the resonances at 11.5 and 17.4 ppm is
evident. (B). The aromatic region shows four doublets for the pyrrolic carbons.
2.3. Etioporphyrin-III
26 and this condensed with methyl formate in the presence
of triethylamine to give the formamide 27. Dehydration
with POCl3–Et3N then afforded the isocyanoacetate 28.
Condensation of 28 with 1-nitrocyclohexene in the presence
of DBU and THF gave 25, while the nitroacetate 29 reacted
under these conditions to give the required pyrrole n-butyl
ester 23 (Scheme 5). In the latter case, an extra equivalent of
DBU was used. This aids in eliminating the elements of
acetic acid from 2935 to generate the reactive nitroalkene 30
in situ and this further reacts to afford pyrrole 23 in an
overall 36% yield from glycine. The proton NMR spectrum
of 23 in CDCl3 shows a doublet of doublets for the NH
(1JNHZK98 Hz, 3JHHZ3 Hz) and a triplet for the a-proton
at 6.65 ppm (Fig. 8). The triplet results from coupling due to
The synthesis of per-15N etioporphyrin-III presented more
of a challenge due to the complete asymmetry of this type
isomer. This required the availability of an a-unsubstituted
pyrrole 23, (Scheme 5) which was needed for the synthesis
of an unsymmetrical dipyrrylmethane 24 (Scheme 6).
Previously, we reported17 the synthesis of a nitrogen-15
labeled a-unsubstituted tetrahydroisoindole 25 from com-
mercially available 15N-glycine using the Barton-Zard
pyrrole condensation35 (Scheme 5). Due to solubility
considerations, the pyrroles were synthesized as n-butyl
rather than ethyl esters.17 Treatment of glycine with n-butyl
alcohol and 1 equiv of p-toluenesulfonic acid gave the ester

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T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 5. 400 MHz proton NMR spectrum of 15N labeled dipyrrylmethane 21. The doublet at 8.7 ppm corresponds to the pyrrole NH’s (1JNHZK96.8 Hz).
Insert: the 15N coupled triplet at 3.86 ppm corresponds to the bridging methylene substituent (3JNHZ2.4 Hz).
the 15N (2 bond) and the NH (3 bond), as both of these
interactions have coupling constants of approximately 3 Hz.
NaOH in refluxing ethylene glycol afforded the deprotected
dipyrrylmethane 31 and this condensed with 2 equiv of
formylpyrrole 13 to give the a,c-biladiene 32. The per-15N
labeled a,c-biladiene showed up to four overlapping
doublets below 13 ppm for the four nonequivalent NH
protons. Cyclization with CuCl2–DMF and demetallation
with 15% H2SO4–TFA gave etioporphyrin-III in good
overall yields. The proton NMR spectra (Fig. 10) for
per-15N etioporphyrin-III showed many of the expected
features, although the decreased symmetry made the
resolution of coupled interactions more difficult. The
400 MHz proton NMR spectrum of 3 in CDCl3 showed a
quintet for the NH’s at K3.75, but the four nonequivalent
meso-protons afforded a multiplet near 10.1 ppm (Fig. 10A).
The dication 3H22C in TFA–CDCl3 showed strong 15N
coupling to the internal NH’s (two overlapping doublets)
and the meso-protons were better resolved, although they
still produced a complex multiplet from 10.63–10.68 ppm
(Fig. 10B). The carbon-13 NMR spectrum in TFA–CDCl3
was poorly resolved due to the number of very similar
carbons in this structure, but the meso-carbons showed four
well resolved resonances between 98 and 99 ppm (Fig. 11).
Condensation of acetoxymethylpyrrole 20 with 23 in the
presence of Montmorillonite clay36 in dichloromethane
gave the asymmetrical dipyrrylmethane 24 (Scheme 6).
Purification by flash chromatography and recrystallization
from methanol gave 24 as a white powder in 81% yield. The
400 MHz proton NMR spectrum for labeled 24 in CDCl3
showed two overlapping doublets for the NH protons and
the bridging methylene gave a triplet (3JNHZ2.6 Hz) at
3.86 ppm (Fig. 9). Cleavage of the ester moieties with
2.4. Etioporphyrin-IV
The fourth etioporphyrin type isomer has a higher degree of
symmetry and can be prepared from the same pyrrolic
intermediates used in the previous studies. Condensation of
pyrrole 23 with paraformaldehyde in the presence of
p-toluenesulfonic acid gave the symmetrical dipyrryl-
methane 33 in good yields (Scheme 7). Cleavage of the
ester protective groups with sodium hydroxide in refluxing
ethylene glycol gave the a-unsubstituted dipyrrylmethane
Scheme 4.

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11585
Figure 6. 400 MHz proton NMR spectra of per-15N etioporphyrin-II. (A). Free base in CDCl3. The NH protons appear as a quintet at K3.75 ppm from
coupling (1JNHZK23.2 Hz) from all four nitrogen-15’s due to rapid intramolecular exchange within the macrocyclic cavity. The two types of meso-protons
produce two overlapping triplets at 10.1 ppm due to coupling from 2 equiv nitrogen-15 nuclei. (B). Dication in TFA–CDCl3. The meso-protons again appear as
a two overlapping triplets (10.6 ppm), while the internal protons now produce a doublet at K3.4 ppm (JZK94 Hz).
34 and this condensed with 2 equiv of pyrrole aldehyde 13
to give the a,c-biladiene 35. The unlabeled a,c-biladiene
showed two broad 2H singlets for the chemically distinct
NH protons at 13.1–13.3 ppm, the methine bridge protons
appeared as a singlet at 7.06 ppm and the central methylene
bridge gave a singlet at 5.2 ppm (Fig. 12A). In the labeled
a,c-biladiene, the NH protons produced two overlapping
doublets (1JNHZK95 Hz), the methine bridges were
coupled to two nitrogen-15’s each to give a triplet
(3JNHZ5 Hz) and the methylene bridge afforded a
broadened partially resolved triplet near 5.2 ppm
(Fig. 12B). In addition, the terminal methyls afforded

11586
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 7. Downfield region for the 100 MHz carbon-13 NMR spectrum of per-15N etioporphyrin-II in TFA–CDCl3. Due to the symmetry in etioporphyrin-II,
there are only two meso-resonances (observed at 98–99 ppm) and four pyrrole resonances (137–144 ppm). The a-pyrrole carbons give doublets (1JNCZ13 Hz)
at 141.5 and 142.3 ppm, while the b-carbons show more weakly coupled doublets at 137.4 and 144.0 ppm.
a 6H doublet (3JNHZ2.4 Hz) at 2.7 ppm. Cyclization of 35
with copper(II) chloride in DMF, following by demetalla-
tion with 15% H2SO4–TFA gave etioporphyrin-IV (7) in
32–40% yield. Etioporphyrin has three types of meso-
protons, although these are poorly resolved for either the
free base or dication proton NMR spectra (Fig. 13). The NH
protons give the usual quintet at K3.8 ppm for the free base
(CDCl3) and two overlapping doublets for the four internal
protons in the dication (TFA–CDCl3). The carbon-13 NMR
spectrum for per-15N 7 in TFA–CDCl3 (Fig. 14) was
reasonably well resolved. The four types of a-carbons gave
rise to four doublets near 142 ppm, while the four different
b-pyrrolic carbons gave two unresolved multiplets at 137.7–
137.9 and 144.3–144.4 ppm. In addition, the three types of
meso-protons afforded three resonances between 98 and
99 ppm that superficially resemble a triplet (ratio 1:2:1).
2.5. Tetrahydrobenzoporphyrin 8
The synthesis of tetrahydrobenzoporphyrin 8 was accom-
plished (Scheme 8) in a similar fashion to labeled
etioporphyrin-III. Condensation of acetoxymethylpyrrole
20 with tetrahydroisoindole 25 in the presence of
Montmorillonite clay gave, following careful flash
chromatography to remove dipyrrylmethane biproducts,
the required dipyrrole 36 in good yields. This was treated
with NaOH in refluxing ethylene glycol to afford 37 and
subsequent condensation with 2 equiv of 13 in the
Scheme 5.
Scheme 6.

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11587
Figure 8. Downfield region for the 300 MHz proton NMR spectrum of 15N labeled pyrrole butyl ester 23. The pyrrole NH produces a doublet of doublets
centered on 8.7 ppm (1JNHZK96.8 Hz; 3JHHZ3 Hz). The triplet at 6.65 ppm corresponds to the pyrrole CH, which is coupled to both the nitrogen-15 and the
NH; the coupling constants for these two interactions are coincidentally both 3 Hz).
presence of HBr gave the a,c-biladiene 38. Of note, the
400 MHz proton NMR spectrum showed four overlapping
doublets for the NH protons between 12.9 and 13.5 ppm,
while the two methine bridges gave two triplets near
7 ppm (Fig. 15). Cyclization with CuCl2–DMF and
demetallation as before gave the targeted tetrahydroben-
zoporphyrin 8 (Scheme 8). This asymmetrical system
gave well resolved proton NMR data (Fig. 16). The free
base of per-15N 8 in CDCl3 gave a quintet at K3.8 ppm
while the meso-protons produced four overlapping triplets
near 10 ppm (Fig. 16A). The dication 8H22C in
TFA–CDCl3 gave three overlapping doublets for the
inner NH protons, while the meso-protons again gave rise
to four overlapping triplets (Fig. 16B). The carbon-13
NMR data for the per-15N dication 8H22C in TFA–CDCl3
was too complex to allow for complete analysis, although
the meso-carbons gave four resonances in the 98–99 ppm
range (Fig. 17).
3. Conclusions
By applying several methods, a series of nitrogen-15 labeled
pyrroles have been prepared from commercially available 15
N
sodium nitrite and 15N-labeled glycine. These intermediates
have been used to synthesize all four per-15N labeled
etioporphyrin type isomers, as well as a butano- or
tetrahydrobenzoporphyrin. Etioporphyrin-I was prepared
from a single pyrrole precursor via pyrromethene intermedi-
ates, but the remaining porphyrins were synthesized in a
rational stepwise fashion via a,c-biladiene intermediates. The
nitrogen-15 labeled intermediates and porphyrin products
were carefully analyzed by proton and carbon-13 NMR
spectroscopy and 15N coupling constants for these important
systems are reported. The labeled porphyrins will provide
important information for the assignment of vibrational
spectra and these results will have value in the analysis of
fossil fuels and in environmental applications.5–11,37
Figure 9. 400 MHz proton NMR spectrum of 15N labeled dipyrrylmethane 24. The two nonequivalent NH’s produce two overlapping doublets at 8.8–9.0 ppm
(left insert), while the 15N coupled triplet at 3.86 ppm (right insert) corresponds to the bridging methylene substituent (3JNHZ2.6 Hz).

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T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 10. 400 MHz proton NMR spectra of per-15N etioporphyrin-III. (A). Free base in CDCl3. The NH protons appear as a quintet at K3.77 ppm from
coupling (1JNHZK24 Hz) from all four nitrogen-15’s due to rapid intramolecular exchange within the macrocyclic cavity. The meso-protons are all
nonequivalent for this type isomer and produce a multiplet at 10.1 ppm. (B). Dication in TFA–CDCl3. The meso-protons again appear as a complex multiplet,
although more detail can be discerned. The internal NH protons are all nonequivalent and produce two overlapping nitrogen-15 coupled doublets K3.8 ppm.
4. Experimental
at 30–40 psi. Chromatography was performed using Grade 3
neutral alumina or 70–230 mesh silica gel. Melting points
were determined on a Thomas Hoover capillary melting
point apparatus and are uncorrected; unless otherwise noted,
the nitrogen-15 labeled compounds afforded the same mp
values as the unlabeled materials. IR spectra were recorded
on a Perkin-Elmer 1600 Series FT-IR Spectrometer and
only selected absorptions (in reciprocal centimeters) are
listed, while UV–vis spectra were obtained on a Beckmann
4.1. General
Nitrogen-15 labeled glycine (99%) and sodium nitrite
(98%) were purchased from Cambridge Isotope Labora-
tories, Inc. All other reagents were purchased from Aldrich
Chemical Co. and were used without further purification.
Hydrogenations were carried out using a Parr hydrogenator

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11589
Figure 11. Downfield region for the 100 MHz carbon-13 NMR spectrum of per-15N etioporphyrin-III in TFA–CDCl3. Due to the lack of symmetry in
etioporphyrin-III, there are four meso-resonances (observed at 98–99 ppm). In principle, there are 16 different pyrrolic carbons all of which may be coupled to
the nitrogen-15 nuclei. These resonances fall into four regions but there is insufficient resolution to allow for further analysis.
DU-40 spectrophotometer. Proton and carbon-13 NMR
spectra were recorded on Varian Gemini 300 or 400 MHz
NMR spectrometers using in the solvent CDCl3 as a
standard (7.26 ppm of residual CHCl3 in proton NMR and
77.23 ppm for the middle line of CDCl3 in carbon-13 NMR
spectroscopy). The deuteriochloroform used to record the
spectra of free base porphyrins was purified by running it
through basic alumina. Mass spectral determinations were
made at the Mass Spectral Laboratory, School of Chemical
Sciences, University of Illinois at Urbana-Champaign,
supported in part by a grant from the National Institute of
General Medical Sciences (GM 27029). Elemental analyses
were obtained from the School of Chemical Sciences
Microanalysis Laboratory at the University of Illinois.
4.2. Synthetic procedures
4.2.1. Synthesis of the pyrrolic precursors.
4.2.1.1. Ethyl 4-ethyl-3,5-dimethylpyrrole-2-car-
boxylate (15). Sodium hydroxide (0.35 g) was added to
stirred glacial acetic acid (2.5 mL) in a 25 mL round bottom
flask. Once the sodium hydroxide was dissolved, 3.0 mL of
diethyl malonate were added followed by the dropwise
addition of a solution of sodium nitrite (1.340 g; 0.020 mol)
in water (2.2 mL) over a period of 1 h. The resulting mixture
was stirred at room temperature overnight to allow the
complete evolution of nitrogen dioxide. A solution of
sodium hydroxide (0.633 g) in water (2.0 mL) was added to
the mixture and an oily layer separated. Cold ether was
added to the solution and gently swirled to allow the organic
layers to mix together. The aqueous phase was removed and
the organic layer washed with a solution of sodium
bicarbonate (0.40 g) in water (2.5 mL), followed by water
(2!3.5 mL). The organic layer was dried with magnesium
sulfate and evaporated under reduced pressure to give the
oxime 16 as an oil. The oil was dissolved in ethanol (25 mL)
and placed in a hydrogenation vessel and washed in with
additional ethanol (25 mL). After purging the flask with
nitrogen gas, 10% palladium–charcoal (100 mg) was added,
and the mixture was shaken under an atmosphere of
hydrogen (45 psi) at room temperature overnight. The
catalyst was filtered off and the solvent evaporated under
reduced pressure to give diethyl aminomalonate (17) as a
pale yellow oil. A solution of the crude aminomalonate in
acetic acid (3 mL) was added to 3-ethyl-2,4-pentanedione
(18) (1.90 g; 0.015 mol) in a gently boiling solution of
glacial acetic acid (10 mL). After the evolution of carbon
dioxide had subsided, the mixture was gently boiled for an
additional 2 h. The light brown solution was diluted with
ice/water and allowed to stand overnight. The resulting
precipitate was filtered and recrystallized with ethanol/
water to yield the title pyrrole as white crystals (1.260 g;
6.46 mmol; 33% from NaNO2), mp 87.5–88 8C (lit. mp32
88–89 8C). IR (nujol mull): n 3302 (st, sh, NH), 1665 cmK1
(st, sh, C]O); 1H NMR (CDCl3): d 1.05 (3H, t), 1.35 (3H,
t), 2.21 (3H, s), 2.28 (3H, s), 2.38 (2H, q), 4.29 (2H, q), 8.55
(1H, br s). Nitrogen-15 labeled 15. Using the same
Scheme 7.

11590
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 12. 400 MHz proton NMR spectra of a,c-biladiene 35. (A). Unlabeled a,c-biladiene in CDCl3. The plane of symmetry in 35 leads to two environments
for the NH protons and these appear as two broadened singlets near 13.2 ppm. (B). per-15N labled 35. The NH protons now show up as two overlapping
doublets between 13.0 and 13.4 ppm (1JNHZK95 Hz). The 2 equiv methine bridges give a triplet at 7.06 ppm (3JNHZ5 Hz) while the bridging methylene
produces an unresolved triplet at 5.2 ppm. The terminal methyl units are also coupled to the nearby nitrogen-15 nuclei and give a doublet at 2.7 ppm (3JNH
Z
2.4 ppm).
3
procedure, 3.80 g of sodium nitrite (5.43 mmol; O98%
nitrogen-15 labeled) gave the required pyrrole 15 (4.138 g;
21.1 mmol; 38%) as white crystals. IR (nujol mull): n 3292
(st, sh, NH), 1665 cmK1 (st, sh, C]O). 1H NMR (CDCl3): d
3JHHZ7.2 Hz), 4.29 (2H, q, JHHZ7.2 Hz), 8.81 (1H, d,
1JNHZK96 Hz); 13C NMR (CDCl3): d 10.6, 11.5 (d,
2JNCZ1.9 Hz), 14.8, 15.5, 17.4 (d, JNCZ1.2 Hz), 59.7,
3
1
2
116.8 (d, JNCZ14.8 Hz), 124.0 (d, JNCZ3.8 Hz), 127.0
3
3
1
1.05 (3H, t, JHHZ7.2 Hz), 1.35 (3H, t, JHHZ7.2 Hz),
(d, 2JNCZ4 Hz), 129.3 (d, JNCZ14.0 Hz), 162.0; MS (EI,
3
2.20 (3H, d, JNHZ2.8 Hz), 2.28 (3H, s), 2.38 (2H, q,
70 eV): m/z (%): 196 (61) [MC], 181 (51) ([MKCH3]C),

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11591
Figure 13. 400 MHz proton NMR spectra of per-15N etioporphyrin-IV. (A). Free base in CDCl3. The NH protons appear as a broadened quintet at K3.77 ppm
while the two types of nonequivalent meso-protons produce a broadened triplet at 10.1 ppm. (B). Dication in TFA–CDCl3. The meso-protons again appear as
two overlapping triplets at 10.6–10.7 ppm. There are two types of internal NH protons and these produce two overlapping nitrogen-15 coupled doublets at
K3.8 ppm (1JNHZK94 Hz).
167 (6) ([MKC2H5]C), 151 (20) ([MKOC2H5]C), 135
(100) ([MKCH3 and EtOH]C).
into ice/water and the resulting precipitate was collected by
filtration and washed with water until the washings
remained neutral. The product was dried under vacuum
and recrystallized from chloroform/petroleum ether to give
the product as white crystals (0.504 g; 1.99 mmol; 78%), mp
128–129.5 8C; (lit. mp38 128 8C). 1H NMR (CDCl3): d 1.09
(3H, t), 1.36 (3H, t), 2.08 (3H, s), 2.29 (3H, s), 2.46 (2H, q),
4.31 (2H, q), 5.03 (2H, s), 8.95 (1H, br s). 15N labeled 20.1H
4.2.1.2. Ethyl 5-acetoxymethyl-4-ethyl-3-methyl-
pyrrole-2-carboxylate (20). Ethyl 3,5-dimethyl-4-ethyl-
pyrrole-2-carboxylate (15) (0.500 g; 2.56 mmol) were
dissolved in a mixture of glacial acetic acid (8 mL) and
acetic anhydride (4 mL). Lead tetraacetate (95%; 1.20 g;
2.56 mmol) was added and the resulting mixture was stirred
overnight at room temperature. The red solution was poured
3
NMR (CDCl3): d 1.08 (3H, t, JHHZ7.6 Hz), 1.37 (3H, t,
3JHHZ7 Hz), 2.07 (3H, s), 2.28 (3H, s), 2.46 (2H, q,

11592
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 14. Downfield region for the 100 MHz carbon-13 NMR spectrum of per-15N etioporphyrin-IV in TFA–CDCl3. Due to the plane of symmetry in
etioporphyrin-IV, there are three meso-resonances observed between 98 and 99 ppm. There are potentially a total of eight pyrrole resonances. The four a-
pyrrole carbons give four doublets (1JNCZ13 Hz) near 142 ppm, while the b-carbons show two multiplets near 138 and 144 ppm.
3
3JHHZ7.6 Hz), 4.31 (2H, q, JHHZ7.2 Hz), 5.05 (2H, d,
4-ethyl-3,5-dimethylpyrrole-2-carboxylate (15) (0.500 g;
2.56 mmol) was heated with sodium hydroxide (1.00 g)
and ethylene glycol (10 mL) under reflux for 1 h. The
mixture was diluted with water and extracted with hexane.
The hexane solution was dried over sodium sulfate and the
solvent evaporated under reduced pressure to give a yellow
oil (315 mg; quantitative). 1H NMR (CDCl3): d 1.08 (3H, t),
2.02 (3H, s), 2.17 (3H, s), 2.39 (2H, q), 6.34 (1H, br), 7.5
(1H, br s). 15N labeled 19. 1H NMR (CDCl3): d 1.08 (3H, t,
1
3JNHZ2.8 Hz), 8.97 (1H, d, JNHZK98 Hz); 13C NMR
3
(CDCl3): d 10.4, 14.7, 16.2, 17.3 (d, JNCZ1.5 Hz), 21.1,
57.1 (d, JNCZ1.9 Hz), 60.1, 88.68, 119.4 (d, JNC
14.8 Hz), 126.2 (d, JNCZ4.5 Hz), 126.5 (d, JNC
14.4 Hz), 127.1 (d, JNCZ3.8 Hz), 161.6, 171.8.
2
1
Z
Z
2
1
2
4.2.1.3. 4-Ethyl-3,5-dimethylpyrrole (19). Ethyl
4
3JHHZ7.5 Hz), 2.02 (3H, d, JNHZ1.2 Hz), 2.17 (3H, d,
3
3JNHZ2.1 Hz), 2.39 (2H, q, JHHZ7.5 Hz), 6.34 (1H, m),
1
3
7.47 (dd, JNHZ94.5 Hz, JHHZ2.7 Hz).
4.2.1.4. 4-Ethyl-3,5-dimethylpyrrole-2-carboxalde-
hyde (13). The foregoing oil (315 mg; 2.56 mmol) was
dissolved in DMF (3 mL) and cooled to 0 8C on a salt/ice
Figure 15. Downfield region for the 400 MHz proton NMR spectrum for
a,c-biladiene 38. The four nonequivalent NH protons produce a series of
four overlapping doublets between 12.9 and 13.5 ppm. The two none-
quivalent methine protons give triplets (3JNHZ5 Hz) at 6.98 and 7.07 ppm.
Scheme 8.

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11593
Figure 16. 400 MHz proton NMR spectra of per-15N tetrahydrobenzoporphyrin 8. (A). Free base in CDCl3. The NH protons appear as a quintet at K3.8 ppm
from coupling (1JNHZK23 Hz) from all four nitrogen-15’s due to rapid intramolecular exchange within the macrocyclic cavity. The meso-protons are all
nonequivalent and produce two sets of two overlapping triplets at 10.0 and 10.1 ppm, respectively. (B). Dication in TFA–CDCl3. The meso-protons again
appear as two sets of two overlapping while the four nonequivalent internal NH protons give rise three overlapping nitrogen-15 coupled doublets between
K3.7 and K4.2 ppm.
bath. Benzoyl chloride (1 mL) was added dropwise while
the temperature was maintained below 0 8C. When the
addition was complete, the salt/ice bath was removed and
the mixture was stirred for an additional 15 min to allow the
temperature to gradually rise to room temperature. Toluene
(10 mL) was added and the resulting mixture was cooled on
the salt/ice bath and stirred for 1 h. The imine salt was
filtered and washed with cold toluene (10 mL). The solid
and sodium carbonate (1.00 g) were dissolved in 50%
ethanol/water (16 mL), and the resulting solution stirred on
a boiling water bath for 15 min. Water was then added to the
mixture and the solution was either cooled on an ice bath or
left at room temperature overnight. The resulting precipitate
was filtered and recrystallized from ethanol/water to give

11594
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
Figure 17. 100 MHz carbon-13 NMR spectrum of per-15N tetrahydrobenzoporphyrin 8 in TFA–CDCl3. Due to the lack of symmetry in 8, there are four meso-
resonances (observed between 98 and 99 ppm). In principle, there are 16 different pyrrolic carbons all of which may be coupled to the nitrogen-15 nuclei, but
the region between 137 and 145 ppm is too complex for analysis. The alkyl group region from 11 to 24 ppm is also only partially resolved.
the pyrrole aldehyde as light yellow crystals (290 mg;
mixture between 20 and 30 8C throughout. The reaction
flask was placed in an oil bath that had been preheated to
75 8C, keeping the liquid level in the flask slightly above the
level of the oil, and the reaction mixture was stirred under
reflux for 16 h. The mixture was diluted with chloroform
(25 mL) and washed with 5% HCl (25 mL). The aqueous
phase was extracted with chloroform and the combined
organic phases evaporated under reduced pressure. The
residue was chromatographed on silica gel, eluting with
toluene, and the product fractions crystallized from hexane
to yield the title pyrrole (1.370 g; 6.55 mmol; 36% from
glycine) as pale yellow crystals, mp 33.5–35 8C. IR (nujol
1
1.92 mmol; 75%), mp 101–102 8C (lit.39 mp 106 8C). H
NMR (CDCl3): d 1.06 (3H, t, CH2CH3), 2.26 (6H, s, CH3),
2.42 (2H, q, CH2CH3), 9.40 (1H, s, CHO), 9.62 (1H, br s,
1
NH). 15N labeled 13. H NMR (CDCl3): d 1.05 (3H, t,
3JHHZ7.5 Hz), 2.25–2.27 (6H, overlapping singlet and
3
3
doublet), 2.38 (2H, q, JHHZ7.5 Hz), 9.43 (1H, d, JNH
Z
2.4 Hz), 9.89 (1H, d, 1JNHZK97 Hz); 13C NMR (CDCl3): d
2
3
8.9, 11.7 (d, JNCZ1.9 Hz), 15.2, 17.1 (d, JNCZ1.6 Hz),
125.1 (d, 2JNCZ4 Hz), 128.0 (d, 1JNCZ14.5 Hz), 132.4 (br),
1
136.0 (d, JNCZ14.4 Hz), 175.6; MS (EI, 70 eV): m/z (%):
152 (41) [MC], 151 (6.5), 137 (51) ([MKCH3]C), 135 (4).
1
mull): n 3322 (st, br, NH), 1672 cmK1 (st, sh, C]O); H
NMR (CDCl3): d 0.97 (3H, t), 1.13 (3H, t), 1.47 (2H, m),
1.72 (2H, m), 2.04 (3H, s), 2.76 (2H, q), 4.27 (2H, t), 6.66
(1H, d, 3JHHZ3 Hz), 8.65 (1H, br s); 13C NMR (CDCl3): d
9.7, 13.7, 15.2, 18.3, 19.4, 31.0, 63.8, 118.9, 120.0, 120.7,
133.2, 162.2. Anal. Calcd for C12H19NO2: C, 68.86; H, 9.15;
4.2.1.5. Butyl 3-ethyl-4-methylpyrrole-2-carboxylate
(23). DBU (3.5 g) was added dropwise to a stirred solution
of 2-acetoxy-3-nitropentane35 (29) (2.536 g; 14.5 mmol)
and n-butyl isocyanoacetate17 (28) (1.584 g; 11.2 mmol) in
THF (10 mL), maintaining the temperature of the reaction

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11595
1
N, 6.69. Found: C, 68.47; H, 8.24; N, 6.72. 15N labeled 23.
IR (nujol mull): n 3315 (st, br, NH), 1673 cmK1 (st, sh,
mp O300 8C; H NMR (CDCl3): d K3.77 (2H, quintet,
1JNHZK24 Hz), 1.87 (12H, t, JHHZ7.6 Hz), 3.65 (12H,
3
1
3
C]O); H NMR (CDCl3): d 0.96 (3H, t, JHHZ7.3 Hz),
s), 4.10 (8H, q, 3JHHZ7.6 Hz), 10.09 (4H, t, 3JNHZ4.4 Hz);
3
1
1.12 (3H, t, JHHZ7.5 Hz), 1.39–1.52 (2H, m), 1.66–1.75
1H NMR (TFA–CDCl3): d K3.84 (4H, d, JNHZK
4
3
3
(2H, m), 2.03 (3H, d, JNHZ0.6 Hz), 2.75 (2H, q, JHH
Z
-
94.4 Hz), 1.75 (12H, t, JHHZ7.8 Hz), 3.67 (12H, s), 4.15
3
2
3
3
7.5 Hz), 4.26 (2H, t, JHHZ6.6 Hz), 6.65 (1H, ‘t’, JNH
(8H, q, JHHZ7.7 Hz), 10.65 (4H, t, JNHZ4.8 Hz); 13C
NMR (TFA–CDCl3): d 12.0, 16.6, 20.3, 98.5, 137.9 (br),
Z3JHHZ3 Hz, pyrrole-H), 8.68 (1H, dd, JNHZK98 Hz,
3JHHZ3 Hz, NH); 13C NMR (CDCl3): d 9.7, 13.7, 15.2,
1
1
1
141.4 (d, JNCZ12.9 Hz), 142.3 (d, JNCZ13.3 Hz), 144.6
(br).
18.3, 19.4, 31.0, 63.8, 118.9 (1JNCZ15 Hz), 120.0 (2JNC
Z
1
4 Hz), 120.7 (d, JNCZ15 Hz), 133.2, 162.2.
4.2.3. Synthesis of etioporphyrin-II.
4.2.1.6. Nitrogen-15 labeled tert-butyl 4-ethyl-3,5-
dimethylpyrrole-2-carboxylate (10). A solution of 15N
labeled sodium nitrite (1.00 g; 14.3 mmol) in water
(2.0 mL) was added dropwise to a stirred solution of tert-
butyl acetoacetate (1.80 g; 11.4 mmol) in acetic acid (2 mL)
while maintaining the temperature of the reaction mixture
below 15 8C with the aid of a salt/ice bath. The resulting
crude oxime solution was stirred at room temperature
overnight. A solution of 3-ethyl-2,4-pentanedione (1.50 g;
11.7 mmol) in acetic acid (3.0 mL) was heated to 70 8C on a
water bath. The crude oxime solution was then added
dropwise over a period of 30 min while simultaneously
adding a mixture of zinc powder (2.0 g) and anhydrous
sodium acetate (1.0 g) in small portions and maintaining the
temperature of the reaction mixture between 80 and 90 8C
throughout. Once the addition was completed, the mixture
was stirred on a boiling water bath for 2 h. The mixture was
poured into ice/water (100 mL) and the resulting precipitate
suction filtered and washed with liberal amounts of water.
Recrystallization from ethanol–water gave the pyrrole
(1.21 g; 5.40 mmol; 38% from NaNO2; 47% from tert-
butyl acetoacetate) as white crystals, mp 134–135 8C (lit.
4.2.3.1. Diethyl 3,30-diethyl-4,40-dimethyl-2,20-dipyr-
rylmethane-5,50-dicarboxylate (21). Concentrated hydro-
chloric acid (16 drops) was added to ethyl 5-acetoxymethyl-
4-ethyl-3-methylpyrrole-2-carboxylate (20) (494 mg;
1.95 mmol) in methanol (8 mL) and the mixture was heated
under reflux for 3 h. The mixture was cooled and stored in a
freezer overnight. The resulting crystals were collected by
suction filtration. Recrystallization from absolute ethanol
gave the title compound (297 mg; 0.794 mmol; 81%) as
1
white needles, mp 126.5 8C (lit.38 mp 126 8C). H NMR
(CDCl3): d 1.05 (6H, t), 1.33 (6H, t), 2.30 (6H, s), 2.40 (4H,
q), 3.88 (2H, s), 4.27 (4H, q), 8.50 (2H, br s). 15N labeled 21.
1H NMR (CDCl3): d 1.04 (6H, t, 3JHHZ7.4 Hz), 1.31 (6H, t,
3
3JHHZ7.2 Hz), 2.28 (6H, s), 2.41 (4H, q, JHHZ7.4 Hz),
3
3
3.86 (2H, t, JNHZ2.4 Hz), 4.25 (4H, q, JHHZ7.1 Hz),
1
8.75 (2H, d, JNHZK96.8 Hz); 13C NMR (CDCl3): d 10.7
(d, 3JNCZ0.7 Hz), 14.7, 15.7, 17.4 (d, JNCZ1.5 Hz), 23.2
2
(t, JNCZ1.8 Hz), 60.0, 118.1 (1JNCZ15.1 Hz), 124.5 (d,
2
2JNCZ4.1 Hz), 127.1 (d, JNCZ4.9 Hz), 128.8 (d, JNC
14.8 Hz), 162.0.
Z
2
1
4.2.3.2.
2,8,12,18-Tetraethyl-1,3,7,13,17,19-
mp40 134–135 8C); 1H NMR (CDCl3): d 1.04 (3H, t, 3JHH
7.5 Hz), 1.55 (9H, s), 2.19 (3H, d, 3JNHZ2.8 Hz), 2.25 (3H,
Z
hexamethyl-10,23-dihydrobilin dihydrobromide (22).
Dipyrrylmethane 21 (150 mg; 0.40 mmol) was heated with
sodium hydroxide (1.00 g) and ethylene glycol (10 mL)
under reflux for 1 h. The mixture was diluted with water and
extracted with hexane. The hexane solution was dried over
sodium sulfate and the solvent evaporated under reduced
pressure to give a dark oil 14. A solution of 4-ethyl-3,5-
3
1
s), 2.37 (2H, q, JHHZ7.5 Hz), 8.42 (1H, d, JNH
Z
Z
K96.8 Hz); 13C NMR (CDCl3): d 10.7, 11.5 (d, JNC
2
3
1.5 Hz), 15.6, 17.4 (d, JNCZ1.5 Hz), 28.8, 80.2, 118.2
(1JNCZ14.8 Hz), 123.9 (d, 2JNCZ3.8 Hz), 126.1 (d, 2JNC
Z
1
4 Hz), 128.4 (d, JNCZ14.4 Hz), 161.5; MS (EI, 70 eV):
m/z (%): 224 (35) [MC], 168 (80) ([MKCH2]C(CH3)2]C),
153 (100) ([MKCH2]C(CH3)2 and CH3]C), 151 (26)
([MKOt-Bu]C), 150 (15) ([MKt-BuOH]C), 135 (65)
([MKCH3 and t-BuOH]C).
dimethylpyrrole-2-carboxaldehyde
(13)
(110 mg;
0.73 mmol) in 1.7 mL methanol was added, and residual
material was washed into the reaction flask with additional
methanol (0.7 mL). Hydrogen bromide in acetic acid (30%,
0.6 mL) was immediately added and the mixture was stirred
for 30 min at room temperature. Anhydrous ether (17 mL)
was added dropwise after which the mixture was stirred at
room temperature overnight. The resulting precipitate was
filtered and rinsed with anhydrous ether to give the title a,c-
biladiene dihydrobromide as a purple-red solid (162 mg;
2.46 mmol; 67%), mp 227 8C, dec. UV–vis (CHCl3): lmax
4.2.2. Synthesis of nitrogen-15 labeled 2,7,12,17-tetra-
ethyl-3,8,13,18-tetramethylporphyrin
porphyrin-I, 5). Per-15N-etioporphyrin-I was prepared
using the method previously described by Smith.22,23
(15N4-etio-
A
solution of bromine (2.17 g) in acetic acid (3.0 mL) was
added to a stirred solution of 15N-labeled tert-butyl 4-ethyl-
3,5-dimethylpyrrole-2-carboxylate (10; 1.00 g; 4.46 mmol)
in acetic acid (3.5 mL) and the mixture stirred at room
temperature overnight. The dark precipitate was filtered,
washed with petroleum ether (60–908) and dried in vacuo.
The resulting red-brown pyrromethene (0.92 g) was heated
under reflux with formic acid (2.2 mL) for 3 h. The excess
formic acid was removed on a rotary evaporator and the
residue dissolved in chloroform and washed with water, 5%
aqueous sodium bicarbonate solution and water. The solvent
was evaporated under reduced pressure and the residue
recrystallized from chloroform–methanol to give the
porphyrin (111 mg; 0.23 mmol; 21%) as purple crystals,
374, 458, 528 nm; 1H NMR (CDCl3): d 0.63 (6H, t, 3JHH
Z
7.6 Hz), 1.09 (6H, t, 3JHHZ7.6 Hz), 2.22 (6H, s), 2.29 (6H,
s), 2.44 (4H, q, 3JHHZ7.6 Hz), 2.55 (4H, q, 3JHHZ7.5 Hz),
2.71 (6H, s), 5.19 (2H, s), 7.09 (2H, s), 13.20 (2H, s), 13.27
(2H, s). Anal. Calcd for C33H46N4Br2$1⁄2 H2O: C, 59.37; H,
7.09; N, 8.39. Found: C, 59.39; H, 6.89; N, 8.36. per-15N
labeled 22. Nitrogen-15 labeled dipyrrylmethane 21
(85 mg; 0.226 mmol) and 4-ethyl-3,5-dimethylpyrrole-2-
carboxaldehyde (13; 63 mg; 0.42 mmol) gave the 15N4-a,c-
biladiene (106 mg; 0.16 mmol; 76%). 1H NMR (CDCl3): d
0.64 (6H, t, 3JHHZ7.4 Hz), 1.09 (6H, t, 3JHHZ7.4 Hz), 2.22
3
(6H, s), 2.29 (6H, s), 2.45 (4H, q, JHHZ7.5 Hz), 2.55

11596
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
3
3
(4H, q, JHHZ7.3 Hz), 2.71 (6H, d, JNHZ2.8 Hz), 5.19
(2H, m), 1.63–1.71 (2H, m), 1.97 (3H, s), 2.29 (3H, s), 2.40
3
3
3
(2H, broad unresolved triplet), 7.09 (2H, t, JNHZ5.2 Hz),
13.20 (2H, d, JNHZK95.2 Hz), 13.28 (2H, d, JNHZK
94.8 Hz).
(2H, q, JHHZ7.4 Hz), 2.73 (2H, q, JHHZ7.6 Hz), 3.85
1
1
3
3
(2H, s), 4.21 (2H, t, JHHZ6.6 Hz), 4.27 (2H, q, JHHZ
7 Hz), 8.55 (1H, br s), 8.58 (1H, br s); 13C NMR (CDCl3): d
8.9, 10.8, 13.9, 14.7, 15.4, 15.7, 17.5, 18.8, 19.5, 23.1, 31.1,
60.1, 64.0, 116.5, 117.2, 118.0, 124.3, 127.2, 129.6, 130.2,
134.2, 162.4. Anal. Calcd for C23H34N2O4: C, 68.62; H,
8.51; N, 6.96. Found: C, 68.32; H, 8.16; N, 6.79. 15N labeled
4.2.3.3. 3,7,13,17-Tetraethyl-2,8,12,18-tetramethyl-
porphyrin (etioporphyrin-II; 6). a,c-Biladiene 22
(62 mg; 0.094 mmol) was added to a stirred solution of
copper(II) chloride (197 mg) in DMF (34 mL). The flask
was covered with aluminum foil and the solution stirred at
room temperature for 4 h. The solution was diluted with
chloroform (30 mL) and washed with water (3!30 mL).
Each of the aqueous solutions were back extracted with
chloroform (30 mL each). The combined organic layers
were dried over sodium sulfate, filtered and the solvent
evaporated on a rotatory evaporator, first under water
aspirator pressure and then using a vacuum pump. The dark
purple residue was taken up in 15% (v/v) sulfuric acid/
trifluoroacetic acid (6 mL), and stirred at room temperature
for 45 min. The solution was diluted with water, extracted
with chloroform and washed with 5% aqueous sodium
bicarbonate solution, and the solvent evaporated under
reduced pressure to give a dark solid. The residue was
chromatographed on grade III alumina, eluting with
dichloromethane, and recrystallized from chloroform/
methanol to give the title porphyrin (14 mg; 0.029 mmol;
1
3
24. H NMR (CDCl3): d 0.92 (3H, t, JHHZ7.4 Hz), 1.03
(3H, t, 3JHHZ7.8 Hz), 1.10 (3H, t, 3JHHZ7.6 Hz), 1.31 (3H,
3
t, JHHZ7.2 Hz), 1.35–1.45 (2H, m), 1.62–1.69 (2H, m),
1.97 (3H, s), 2.28 (3H, s), 2.41 (2H, q, 3JHHZ7.5 Hz), 2.72
3
3
(2H, q, JHHZ7.5 Hz), 3.86 (2H, t, JNHZ2.6 Hz), 4.20
3
3
(2H, t, JHHZ6.6 Hz), 4.26 (2H, q, JHHZ7.2 Hz), 8.90
(1H, d, 1JNHZK97 Hz), 8.95 (1H, d, 1JNHZK97 Hz); 13
NMR (CDCl3): d 8.8, 10.7, 13.9, 14.7, 15.4, 15.7, 17.5 (d,
C
3JNCZ1.1 Hz), 18.8, 19.5, 23.3 (t, JNCZ1.5 Hz), 31.1,
2
2
1
60.1, 64.0, 116.7 (d, JNCZ4.2 Hz), 117.4 (d, JNC
Z
Z
1
2
15.1 Hz), 118.1 (d, JNCZ15.2 Hz), 124.4 (d, JNC
3.8 Hz), 127.1 (2JNCZ4 Hz), 129.0 (d, JNCZ15.2 Hz),
1
129.7 (d, JNCZ15.2 Hz), 134.2 (2JNCZ4.5 Hz), 162.2.
1
4.2.4.2.
2,7,12,18-Tetraethyl-1,3,8,13,17,19-
hexamethyl-10,23-dihydrobilin dihydrobromide (32).
Dipyrrylmethane 24 (146 mg; 0.363 mmol) was heated
with sodium hydroxide (1.00 g) and ethylene glycol
(10 mL) under reflux conditions for 1 h. The mixture was
diluted with water and extracted with hexane. The hexane
solution was dried over sodium sulfate and the solvent
evaporated under reduced pressure to give 31 as a dark oil.
A solution of 4-ethyl 3,5-dimethylpyrrole-2-carboxalde-
hyde (13) (0.118 g, 0.78 mmol) in methanol (2.0 mL) was
added and any residual material was washed into the
reaction flask with additional methanol (2.0 mL). Hydrogen
bromide in acetic acid (30%, 0.8 mL) was immediately
added and the mixture was stirred for 30 min at room
temperature. Anhydrous ether (17 mL) was added dropwise
after which the mixture was stirred at room temperature for
a further 2 h. The resulting precipitate was filtered and
rinsed with anhydrous ether to give the title a,c-biladiene
dihydrobromide 32 as a brick red solid (123 mg, 0.19 mmol;
51%), mp 237 8C dec. UV–is (CHCl3): lmax 374, 458,
1
31%) as a purple solid, mpO300 8C. H NMR (CDCl3): d
K3.75 (2H, s, NH), 1.87 (12H, t), 3.65 (12H, s), 4.10 (8H,
q), 10.09 (2H, s), 10.10 (2H, s); EI MS: m/z (rel int.) 480 (6),
479 (36), 478 (100) (MC), 464 (11), 463 (31), 448 (7), 240
(6), 239 (18), 224 (10), 217 (6). 15N4-etioporphyrin-II (6).
Using the same procedure, 70 mg (0.10 mmol) of nitrogen-
15 labeled a,c-biladiene dibromide 22 gave 23 mg
(0.048 mmol; 48%) of the title porphyrin. 1H NMR
1
(CDCl3): d K3.77 (2H, quintet, JNHZK23.2 Hz), 1.88
3
3
(12H, t, JHHZ7.8 Hz), 3.64 (12H, s), 4.10 (8H, q, JHH
Z
Z
3
3
7.7 Hz), 10.09 (2H, t, JNHZ4.8 Hz), 10.11 (2H, t, JNH
4.6 Hz); 1H NMR (TFA–CDCl3): d K3.43 (2H, d,
1JNHZ K94 Hz), 1.73 (12H, t, 3JHHZ7.8 Hz), 3.65 (12H,
3
s), 4.14 (8H, q, JHHZ7.7 Hz), 10.58–10.62 (4H, two
overlapping triplets, 3JNHZ5 Hz); 13C NMR (TFA–CDCl3):
d 12.0, 16.7, 20.3, 98.2, 98.6, 137.4 (d, 2JNCZ2.6 Hz), 141.5
(d, 1JNCZ13.3 Hz), 142.3 (d, 1JNCZ13.3 Hz), 144.0 (2JNC
Z
528 nm; H NMR (CDCl3): d 0.63 (6H, t, JHHZ7.4 Hz),
1.06–1.12 (6H, two overlapping triplets), 1.93 (3H, s), 2.24
(3H, s), 2.29 (6H, s), 2.41–2.51 (6H, three overlapping
1
3
1.9 Hz); EI MS m/e (rel int.): 484 (6), 483 (35), 482 (100)
(MC), 481 (9), 468 (9), 467 (27), 452 (6), 241 (21), 234 (5),
226 (10), 219 (5).
3
quartets), 2.60 (2H, q, JHHZ7.6 Hz), 2.71 (6H, s), 5.19
(2H, s), 7.06 (1H, s), 7.09 (1H, s), 13.16 (1H, s), 13.18 (1H,
s), 13.24 (1H, s), 13.30 (1H, s). Anal. Calcd for
C33H46N4Br2$1⁄2 H2O: C, 59.37; H, 7.09; N, 8.39. Found:
C, 59.34; H, 6.81; N, 8.33. 15N labeled 32. Nitrogen-15
labeled dipyrrylmethane 24 (100 mg; 0.25 mmol) and
pyrrole aldehyde 13 (81 mg; 0.53 mmol) gave the required
4.2.4. Synthesis of etioporphyrin-III.
4.2.4.1. Butyl 50-Ethoxycarboxyl-30,4-diethyl-3,40-
dimethyl-2,20-dipyrrylmethane-5-carboxylate (24). Mon-
tmorillonite clay (K-10; 2.70 g) was added to a stirred
solution of acetoxymethylpyrrole 20 (0.253 g; 1.00 mmol)
and a-free pyrrole 23 (0.209 g, 1.00 mmol) in dichloro-
methane (40 mL). The mixture was stirred vigorously
overnight at room temperature. The clay was removed by
suction filtration and the solvent evaporated under reduced
pressure to give a solid. The residue was purified by flash
chromatography, eluting with 5% acetone/hexane and
recrystallized from 95% methanol to give the title
dipyrrylmethane (0.325 g; 0.81 mmol; 81%) as white
crystals, mp 105.5–106.5 8C. 1H NMR (CDCl3): d 0.93
(3H, t, 3JHHZ7.4 Hz), 1.03 (3H, t, 3JHHZ7.4 Hz), 1.11 (3H,
1
labeled a,c-biladiene (107 mg; 0.16 mmol; 65%): H NMR
(CDCl3): d 0.65 (6H, t), 1.09 (6H, t), 1.91 (3H, s), 2.21 (3H,
s), 2.27 (6H, s), 2.42 (6H, m), 2.52 (2H, q), 2.73 (6H, s), 5.19
(2H, br), 7.09 (2H, br), 13.24 (2H, d, 1JNHZK96 Hz), 13.26
(1H, d, 1JNHZK96 Hz), 13.34 (1H, d, JZK96 Hz).
4.2.4.3. 3,8,13,17-Tetraethyl-2,7,12,18-tetramethyl-
porphyrin (etioporphyrin-III, 3). a,c-Biladiene 32
(104 mg; 0.158 mmol) was added to a stirred solution of
copper(II) chloride (0.26 g) in dimethylformamide (45 mL).
The flask was covered with aluminum foil and the solution
3
3
t, JHHZ7.4 Hz), 1.32 (3H, t, JHHZ7.0 Hz), 1.38–1.47

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11597
stirred at room temperature for 2.5 h. The solution was
diluted with chloroform (50 mL) and washed with water
(3!50 mL). Each of the aqueous solutions was back
extracted with chloroform (50 mL each). The combined
organic layers were dried over sodium sulfate, filtered and
the solvent evaporated on a rotatory evaporator, first under
water aspirator pressure and then using a vacuum pump. The
dark purple residue was taken up in 15% (v/v) sulfuric acid/
trifluoroacetic acid (6 mL) and stirred at room temperature
for 45 min. The solution was diluted with water, extracted
with chloroform and washed with 5% aqueous sodium
bicarbonate solution, and the solvent evaporated under
reduced pressure to give a dark solid. The residue was
chromatographed on grade III alumina, eluting with
dichloromethane, and recrystallized from chloroform/
methanol to give the title porphyrin (40 mg; 0.084 mmol;
under reduced pressure to give 34 as a dark brown oil. A
solution of 4-ethyl-3,5-dimethylpyrrole-2-carboxaldehyde
(13) (81 mg; 0.54 mmol) in 1 mL of methanol was added,
and residual material was washed into the reaction flask
with additional methanol (1 mL). Hydrogen bromide in
acetic acid (30%, 0.8 mL) was immediately added and the
mixture was stirred for 30 min at room temperature.
Anhydrous ether (17 mL) was added dropwise after which
the mixture was stirred at room temperature overnight. The
resulting precipitate was filtered and rinsed with anhydrous
ether to give the title a,c-biladiene dihydrobromide as a
purple-red solid (93 mg; 0.14 mmol; 52%), mp 227 8C dec.
UV–vis (CHCl3): lmax (log103) 372, 458, 528 nm; 1H NMR
(CDCl3): d 1.08 (6H, t), 1.13 (6H, t), 1.89 (6H, s), 2.29 (6H,
s), 2.44 (4H, q), 2.61 (4H, q), 2.70 (6H, s), 5.19 (2H, s), 7.06
(2H, s), 13.14 (2H, br s), 13.26 (2H, br s). 15N4-labeled 35.
Using the same procedure, 120 mg (0.28 mmol) of nitrogen-
15 labeled dipyrrylmethane 33 and 83 mg (0.55 mmol) of
15N pyrrole aldehyde 13 gave the a,c-biladiene (103 mg;
0.155 mmol; 56%) as a red solid. 1H NMR (CDCl3): d 1.08
(6H, t, 3JHHZ7.6 Hz), 1.13 (6H, t, 3JHHZ7.6 Hz), 1.89 (6H,
1
53%) as a purple solid, mp O300 8C. H NMR (CDCl3): d
K3.75 (2H, s), 1.85 (12H, t), 3.65 (12H, s), 4.10 (8H, m),
10.09 (4H, s); EI MS m/z (rel int.): 480 (4), 479 (37), 478
(100) (MC), 463 (24), 448 (6), 240 (7), 240 (7), 239 (20),
1
224 (9). 15N4-etioporphyrin-III (3). H NMR (CDCl3): d
1
3
K3.77 (2H, quintet, JNHZK24 Hz), 1.85 (12H, two
overlapping triplets), 3.64 (6H, s), 3.65 (6H, s), 4.10 (8H,
s), 2.29 (6H, s), 2.44 (4H, q, JHHZ7.5 Hz), 2.62 (4H, q,
3JHHZ7.6 Hz), 2.70 (6H, d, 3JNHZ2.4 Hz), 5.19 (2H, broad
3
3
q, JHHZ7.6 Hz), 10.08–10.12 (4H, m); 1H NMR
unresolved triplet), 7.06 (2H, t, JNHZ5.0 Hz), 13.14 (2H,
1
1
1
(TFA–CDCl3): d K3.86 (1H, d, JNHZK95 Hz), K3.84
d, JNHZK95.2 Hz), 13.26 (2H, d, JNHZK94.8 Hz).
(3H, d, 1JNHZK94 Hz), 1.71–1.77 (12H, m), 3.67 (9H, s),
3.68 (3H, s), 4.12–4.18 (8H, m), 10.63–10.68 (4H, m); 13C
NMR (TFA–CDCl3): d 12.0, 16.6 (3), 20.3, 98.2, 98.4, 98.6,
98.8, 138 (unresolved multiplet), 141.3–141.6 (m), 142.1–
142.5 (m), 144.4–144.6 (m); EI MS m/e (rel int.): 484 (6),
483 (35), 482 (100) (MC), 481 (6), 467 (25), 452 (6), 241
(18), 226 (8).
4.2.5.3. 2,8,13,17-Tetraethyl-3,7,12,18-tetramethyl-
porphyrin (etioporphyrin-IV; 7). a,c-Biladiene dihydro-
bromide 35 (70 mg; 0.11 mmol) was added to a stirred
solution of copper(II) chloride (150 mg) in DMF (26 mL).
The flask was covered with aluminum foil and the solution
stirred at room temperature for 4 h. The solution was diluted
with chloroform (30 mL) and washed with water (3!
30 mL). Each of the aqueous solutions were back extracted
with chloroform (30 mL each). The combined organic
layers were dried over sodium sulfate, filtered and solvent
evaporated on a rotatory evaporator, first under aspirator
pressure and then using a vacuum pump. The purple residue
was taken up in 15% (v/v) sulfuric acid/trifluoroacetic acid
(5 mL) and stirred at room temperature for 45 min. The
solution was diluted with water, extracted with chloroform
and washed with 5% aqueous sodium bicarbonate solution,
and the solvent evaporated under reduced pressure to give a
dark solid. The residue was chromatographed on grade III
alumina, eluting with dichloromethane, and recrystallized
from chloroform /methanol to give the title porphyrin
(17 mg; 0.036 mmol; 32%) as a purple solid, mp O300 8C.
1H NMR (CDCl3): d K3.75 (2H, s, NH), 1.87 (12H, t), 3.64
(12H, s), 4.10 (8H, q), 10.09 (4H, s); EI MS m/z (rel int.):
480 (6), 479 (35), 478 (100) (MC), 464 (9), 463 (25), 448
(6), 239 (21), 224 (9), 217 (5). per-15N-etioporphyrin-IV.
Using the same procedure, 92 mg (0.14 mmol) of nitrogen-
15 labeled a,c-biladiene 35 gave 15N4-etioporphyrin-IV
4.2.5. Synthesis of etioporphyrin-IV.
4.2.5.1. Dibutyl 4,40-diethyl-3,30-dimethyl-2,20-dipyr-
rylmethane-5,50-dicarboxylate (33). Butyl 3-methyl-4-
ethylpyrrole-2-carboxylate (23) (0.23 g; 1.10 mmol) and
paraformaldehyde (0.13 g, 4.3 mmol) were dissolved in
1.25 mL ethanol in a 10 mL round bottom flask. Concen-
trated hydrochloric acid (0.025 mL) was added, and the
mixture heated under reflux for 30 min under nitrogen. After
it had cooled to room temperature, the reaction flask was
kept in the freezer overnight. The resulting crystals were
filtered off and recrystallized from absolute ethanol to give
the required dipyrrylmethane (148 mg; 0.344 mmol; 63%)
as white crystals, mp 113–114 8C. 1H NMR (CDCl3): d 0.87
3
3
(6H, t, JHHZ7.4 Hz), 1.11 (6H, t, JHHZ7.4 Hz), 1.33–
1.42 (4H, m), 1.59–1.66 (4H, m), 1.99 (6H, s), 2.72 (4H, q,
3
3JHHZ7.4 Hz), 3.87 (2H, s), 4.19 (4H, t, JHHZ6.6 Hz),
9.58 (2H, br s); 13C NMR (CDCl3): d 8.8, 13.8, 15.3, 18.8,
19.5, 23.1, 31.0, 64.0, 116.5, 117.1, 130.4, 134.2, 162.5. 15N
labeled 33. H NMR (CDCl3): d 0.94 (6H, t), 1.12 (6H, t),
1
1.40 (4H, m), 1.65 (4H, m), 1.97 (6H, s), 2.71 (4H, q), 3.83
Z
3
1
1
(2H, t, JNHZ3 Hz), 4.19 (4H, t), 8.48 (2H, d, JNH
K96 Hz).
(27 mg; 0.056 mmol; 40%). H NMR (CDCl3): d K3.77
Z
1
3
(2H, br quintet, JNHZK23 Hz), 1.88 (6H, t, JHH
7.6 Hz), 1.89 (6H, t, 3JHHZ7.8 Hz), 3.64 (6H, s), 3.65 (6H,
s), 4.07–4.14 (8H, two overlapping quartets), 10.09 (4H, br
4.2.5.2.
2,7,13,18-Tetraethyl-1,3,8,12,17,19-
3
1
hexamethyl-10,23-dihydrobilin dihydrobromide (35).
Dipyrrylmethane 33 (116 mg; 0.27 mmol) was heated with
sodium hydroxide (1.00 g) and ethylene glycol (10 mL)
under reflux conditions for 1 h. The mixture was diluted
with water and extracted with hexane. The hexane solution
was dried over sodium sulfate and the solvent evaporated
t, JNHZ4 Hz); H NMR (TFA–CDCl3): d K3.70 (2H, d,
1JNHZK94 Hz), K3.68 (2H, d, 1JNHZK94 Hz), 1.73 (6H,
3
3
t, JHHZ7.6 Hz), 1.74 (6H, t, JHHZ7.6 Hz), 3.66 (6H, s),
3.67 (6H, s), 4.11–4.18 (8H, two overlapping quartets),
10.60–10.66 (4H, m); 13C NMR (TFA–CDCl3): d 11.9–12.1
(m), 16.6, 16.7, 20.3, 20.4, 98.1, 98.5, 98.9, 137.7–137.9

11598
T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
1
1
(m), 141.5 (d, JNCZ13.3 Hz), 141.6 (d, JNCZ13.3 Hz),
142.3 (d, 1JNCZ13.2 Hz), 142.4 (d, 1JNCZ13.3 Hz), 144.3–
144.4 (m); EI MS: m/z (rel int.): 484 (6), 483 (34), 482 (100)
(MC), 468 (8), 467 (25), 452 (6), 241 (22), 226 (9), 219 (6).
C34H46N4Br2$H2O: C, 59.30; H, 7.02; N, 8.14. Found: C,
59.22; H, 6.77; N, 8.10. 15N labeled 38. Using the same
procedure, 160 mg (0.38 mmol) of labeled dipyrrylmethane
36 and 108 mg (0.71 mmol) of labeled pyrrole aldehyde 13
gave the required a,c-biladiene (186 mg; 0.276 mmol; 77%)
as a brick red solid. 1H NMR (CDCl3): d 0.70 (3H, t, 3JHH
Z
7.4 Hz), 1.08 (6H, two overlapping triplets), 1.68 (4H, m),
4.2.6. Synthesis of butanoporphyrin 8.
4.2.6.1. Butyl 3,4-butano-50-ethoxycarbonyl-40-ethyl-
30-methyl-2,20-dipyrrylmethane-5-carboxylate (36).
Montmorillonite clay (K-10, 2.70 g) was added to a stirred
solution of acetoxymethylpyrrole 20 (253 mg; 1.00 mmol)
and n-butyl 4,5,6,7-tetrahydro-2H-isoindole-1-carboxylate
(25)17 (221 mg, 1.00 mmol) in dichloromethane (40 mL).
The mixture was stirred vigorously overnight at room
temperature. The clay was removed by suction filtration and
the solvent evaporated over reduced pressure to give a pink
solid. The residue was purified by flash chromatography
eluting with 5% ethyl acetate/toluene and recrystallized
from ethanol to give the title dipyrrylmethane (260 mg;
2.25 (3H, s), 2.26 (3H, s), 2.29 (3H, s), 2.41–2.49 (4H, m),
Z
3
2.66 (2H, m), 2.70 (6H, two overlapping doublets, JNH
3 Hz), 5.15 (2H, s), 6.98 (1H, t, 3JNHZ5.2 Hz), 7.07 (1H, t,
3JNHZ5.0 Hz), 13.06 (1H, d, 1JNHZK94.8 Hz), 13.14 (1H,
1
1
d, JNHZK94.8 Hz), 13.23 (1H, d, JNHZK95.2 Hz),
1
13.33 (1H, d, JNHZ K94.4 Hz).
4.2.6.3. 2,3-Butano-7,13,17-triethyl-8,12,18-trimethyl-
porphyrin (8). a,c-Biladiene dihydrobromide (38) (70 mg;
0.104 mmol) was added to a stirred solution of copper(II)
chloride (0.15 g) in DMF (45 mL). The flask was covered
with aluminum foil and the solution stirred at room
temperature for 2.5 h. The solution was diluted with
chloroform (50 mL) and washed with water (3!50 mL).
The combined organic layers were then dried over sodium
sulfate, filtered and the solvent evaporated under vacuum.
The dark purple residue was taken up in 15% (v/v) sulfuric
acid/trifluroacetic acid (6 mL) and stirred at room tempera-
ture for 45 min. The solution was diluted with water,
extracted with chloroform and washed with 5% aqueous
sodium bicarbonate solution, and the solvent evaporated
under reduced pressure to give a dark solid. The residue was
chromatographed on grade III alumina, eluting with
dichloromethane, and recrystallized from chloroform/
methanol to give the tetrahydrobenzoporphyrin (20 mg;
0.041 mmol; 40%) as a purple solid, mp O300 8C. 1H NMR
(CDCl3): d K3.76 (2H, s), 1.84–1.90 (9H, three overlapping
triplets), 2.53 (4H, m), 3.62 (3H, s), 3.64 (6H, s), 4.05–4.14
(6H, three overlapping quartets), 4.16 (4H, m), 9.98 (1H, s),
9.99 (1H, s), 10.09 (1H, s), 10.11 (1H, s); 1H NMR
(TFA–CDCl3): d K3.89 (1H, br s), K3.84 (2H, br s),
K3.73 (1H, br s), 1.70–1.76 (9H, three overlapping triplets),
2.56 (4H, m), 3.66 (3H, s), 3.67 (6H, s), 4.10–4.18 (10H, m),
10.56 (2H, s), 10.65 (1H, s), 10.66 (1H, s); 9.98 (1H, s), 9.99
(1H, s), 10.09 (1H, s), 10.11 (1H, s); 13C NMR
(TFA–CDCl3): d 12.0, 16.6 (2), 16.7, 20.3 (2), 22.7, 23.5,
98.1, 98.2, 98.4, 99.0, 137.6, 137.7, 138.0, 141.2 (2), 141.3,
141.7, 141.8 (2), 142.0, 142.1, 142.5, 144.2, 144.5; EI MS:
m/z (rel int.) 492 (7), 491 (38), 490 (100) (MC), 478 (5), 476
(8), 475 (22), 246 (10), 245 (27), 230 (6), 223 (5), 216 (5).
1
0.63 mmol; 63%) as white crystals, mp 114–115 8C. H
NMR (CDCl3): d 0.90 (3H, t), 1.03 (3H, t), 1.32 (3H, t), 1.41
(2H, m), 1.66 (2H, m), 1.72 (4H, m), 2.28 (3H, s), 2.40 (2H,
m), 2.77 (2H, q), 3.82 (2H, s), 4.23 (4H, m), 8.75 (2H, br s);
13C NMR (CDCl3): d 10.7, 13.9, 14.7, 15.7, 17.5, 19.5, 21.5,
23.0, 23.4, 23.5, 23.6, 31.1, 60.1, 64.0, 116.7, 118.0, 119.2,
124.3, 127.1, 129.2, 129.3, 129.4, 162.2, 162.5. Anal. Calcd
for C24H34N2O4: C, 69.47; H, 8.26; N, 6.75. Found: C,
69.35; H, 7.77; N, 6.67. 15N labeled 36. 1H NMR (CDCl3): d
0.93 (3H, t, 3JHHZ7.2 Hz), 1.03 (3H, t, 3JHHZ7.4 Hz), 1.32
3
(3H, t, JHHZ7.2 Hz), 1.36–1.47 (2H, m), 1.64–1.71 (2H,
m), 1.72–1.76 (4H, m), 2.28 (3H, s), 2.37–2.43 (4H, m),
3
2.77 (2H, br t), 3.82 (2H, t, JNHZ2.6 Hz), 4.21 (2H, t,
3JHHZ6.6 Hz), 4.27 (2H, q, JHHZ7.2 Hz), 8.59 (1H, d,
3
1JNHZK97 Hz), 8.62 (1H, d, JNHZK97 Hz); 13C NMR
(CDCl3): d 10.7, 13.9, 14.7, 15.7, 17.5, 19.5, 21.5, 22.9,
23.5, 23.6, 31.1, 60.1, 64.0, 116.7 (d, 1JNCZ15.5 Hz), 118.0
1
2
(d, JNCZ14.8 Hz), 119.1 (d, JNCZ3.8 Hz), 124.2 (d,
2JNCZ3.8 Hz), 127.1 (d, JNCZ4.6 Hz), 129.3–129.5 (m),
162.3, 162.6.
2
4.2.6.2. 7,8-Butano-3,12,17-triethyl-1,2,13,18,19-
pentamethyl-10,23-dihydrobilin dihydrobromide (38).
Dipyrrylmethane (36) (91 mg; 0.22 mmol) was heated
with sodium hydroxide (1.00 g) and ethylene glycol
(10 mL) under reflux conditions for 30 min. The mixture
was diluted with water and extracted with hexane. The
hexane solution was dried over sodium sulfate and the
solvent evaporated under reduced pressure to give 37 as a
dark oil. A solution of 4-ethyl-3,5-dimethylpyrrole-2-
carboxaldehyde (13) (66 mg; 0.44 mmol) in methanol
(1.0 mL) was added and all residual material was washed
into the reaction flask with additional methanol (0.7 mL).
Hydrogen bromide in acetic acid (30%, 0.3 mL) was
immediately added and the mixture was stirred for 30 min
at room temperature. Anhydrous ether (15 mL) was added
dropwise after which the mixture was stirred at room
temperature for 2 h. The resulting precipitate was filtered
and rinsed with anhydrous ether to give the title a,c-
biladiene dihydrobromide as a brick red solid (96 mg;
0.143 mmol; 65%), mp 237 8C, dec. UV–vis: lmax: 374,
458, 528 nm; 1H NMR (CDCl3): d 0.71 (3H, t), 1.09 (6H, t),
1.69 (4H, m), 2.25–2.31 (11H, m), 2.46 (4H, m), 2.71 (6H,
s), 5.15 (2H, s), 7.00 (1H, s), 7.11 (1H, s), 13.04 (1H, s),
13.12 (1H, s), 13.20 (1H, s), 13.31 (1H, s). Anal. Calcd for
1
15N labeled 8. H NMR (CDCl3): d K3.79 (2H, quintet,
1JNHZK23 Hz), 1.83–1.89 (9H, three overlapping triplets),
2.53 (4H, m), 3.62 (3H, s), 3.64 (6H, s), 4.05–4.14 (6H, m),
4.16 (4H, m), 9.97–10.01 (2H, two overlapping triplets,
3JNHZ4 Hz), 10.07–10.13 (2H, two overlapping triplets,
1
3JNHZ4 Hz); H NMR (TFA–CDCl3): d K4.03 (2H, d,
1
1JNHZK94.8 Hz), K3.98 (1H, d, JNHZK94.0 Hz),
1
K3.87 (1H, d, JNHZ K94.0 Hz), 1.70–1.76 (9H, three
overlapping triplets), 2.56 (4H, m), 3.66 (3H, s), 3.67 (6H,
3
s), 4.11–4.19 (10H, m), 10.57 (1H, t, JNHZ5 Hz), 10.58
3
3
(1H, t, JNHZ4.8 Hz), 10.67 (1H, t, JNHZ5.2 Hz), 10.68
(1H, t, 3JNHZ5 Hz); 13C NMR (TFA–CDCl3): d 12.0, 16.6
(2), 16.7, 20.3 (2), 22.7, 23.5, 98.1, 98.3, 98.5, 99.0, 137.9,
1
138.0, 138.2, 141.0–142.1 (m), 142.5 (d, JNCZ13.2 Hz),
144.5, 144.8; EI MS: m/z (rel int.) 496 (7), 495 (36), 494

T. D. Lash, S. Chen / Tetrahedron 61 (2005) 11577–11600
11599
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Acknowledgements
This work was supported by the National Science
Foundation under Grant no. CHE-0134472 and the
Petroleum Research Fund, administered by the American
Chemical Society.
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