Facile synthesis of aliphatic isothiocyanates and thioureas on solid phase using peptide coupling reagents

Article abstract of DOI:10.1016/j.tetlet.2003.10.182

Peptide coupling reagents can be used as versatile reagents for the formation of aliphatic isothiocyanates and thioureas on solid phase from the corresponding solid-phase anchored aliphatic primary amines. The formation of the thioureas is fast and highly

Full text of DOI:10.1016/j.tetlet.2003.10.182

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 269–272
Facile synthesis of aliphatic isothiocyanates and thioureas
on solid phase using peptide coupling reagentsq
Ulrik Boas,a,* Heidi Gertz,a Jørn B. Christensenb and Peter M. H. Heegaarda
aDepartment of Immunology and Biochemistry, The Danish Veterinary Institute, B€ulowsvej 27, DK-1790 Copenhagen, Denmark
bDepartment of General and Organic Chemistry, The H. C. Ørsted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Received 29August 2003; revised 21 October 2003; accepted 30 October 2003
Abstract—Peptide coupling reagents can be used as versatile reagents for the formation of aliphatic isothiocyanates and thioureas
on solid phase from the corresponding solid-phase anchored aliphatic primary amines. The formation of the thioureas is fast and
highly chemoselective, and proceeds via formation of the intermediate isothiocyanate. The isothiocyanate and subsequent thiourea
formation take place under standard peptide coupling conditions using carbon disulfide as the Ôamino acidÕ. The thioureas are
released from the resin and isolated in moderate to high yields.
Ó 2003 Elsevier Ltd. All rights reserved.
Isothiocyanates and thioureas are valuable functional
groups in organic chemistry, and have found wide use
in, for example, bioconjugate1 and heterocyclic chemis-
try.2 For this reason, much effort has been devoted to
develop efficient routes for the synthesis of these classes
of compounds. The traditional synthetic pathway to
intermediate isothiocyanates, utilises thiocarbonyl
insertion reagents, for example, thiophosgene3 or
dipyridyl thionocarbonate.4 Other routes utilise electro-
philic phosphorous reagents such as triphenylphosphine
dibromide and proceed via intermediate iminophos-
phorane formation, followed by reaction with carbon
disulfide.5 However, these highly reactive reagents suffer
from low chemoselectivity and high toxicity. Alternative
routes to isothiocyanates proceed via intermediate for-
mation of the dithiocarbamate from the corresponding
amine and carbon disulfide. The dithiocarbamate can be
desulfurised by a variety of reagents.6 In these proce-
dures it is of crucial importance to ensure complete
formation of the dithiocarbamate to prevent reaction
between the electrophilic desulfurising reagent and the
amine. Previously, we found that phosphonium- and
uronium-based peptide coupling reagents (BOP and
TFFH, respectively)7 together with DIPEA or triethyl-
amine act as mild and chemoselective desulfurising
agents for dithiocarbamates in solution.8;9 The peptide
coupling reagents did not react with the precursor
amine. The one-pot reaction between the amine, carbon
disulfide and the peptide coupling agent gave direct and
efficient conversion to the corresponding isothiocyanate,
which could then be reacted with, for example, amines
for direct formation of thioureas.
Due to the similarity with conventional peptide meth-
odology on solid phase, this method should be easily
adapted for solid-phase synthesis (SPS) of isothiocya-
nates and thioureas. The majority of reports on SPS of
thioureas employ the isothiocyanate as the solution re-
agent reacting with a solid-phase bound amine.10
However, there are synthetic demands for the formation
of the isothiocyanate on solid phase and employment of
the amine in solution, especially if the amine is poly-
functional, leading to side reactions upon formation of
the corresponding isothiocyanate in solution.
Only very few reports employ SPS of isothiocyanates
and derivatives by this Ôreversed pathwayÕ. Zaragosa and
co-workers synthesised isothiocyanates on solid phase,
as precursors for thiophene derivatives, employing tosyl
chloride as the desulfurising reagent for the dithiocar-
bamate.11 This procedure depends on full conversion of
the amine to the dithiocarbamate, before reaction with
Keywords: Peptide coupling reagents; Isothiocyanates; Thioureas;
Solid-phase synthesis; Combinatorial chemistry.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2003.10.182.
* Corresponding author. Tel.: +45-35-300215; fax: +45-35-300120;
e-mail: ubo@vetinst.dk
0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.182

270
U. Boas et al. / Tetrahedron Letters 45 (2004) 269–272
Table 1
tosyl chloride and base in order to avoid formation of
the sulfonamide via a side reaction. Di-(2-pyridyl)thiono-
carbonate has very recently been applied in the synthesis
of isothiocyanates on solid phase in the synthesis of
1,3,4-oxadiazoles12 and 2-amino-1,3,4-thiadiazines,13
however, longer reaction times were required. Here, we
introduce a rapid method for SPS of isothiocyanates
and thioureas using peptide coupling reagents. The
formation of the isothiocyanates and thioureas proceeds
under traditional peptide coupling conditions, making
this new method useful in combinatorial synthesis.
Compound18
Yield19 (%)
HBTU/PyBOP
60/70
quant/98
A high loading (1.2–1.9mmol/g) of 2-chlorotrityl deri-
vatised polystyrene resin was utilised, as the products
can be released from the resin under very mild acidic
conditions using 20% 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP) in dichloromethane.14 This cleavage method has
the advantage that the products can easily be isolated by
evaporation of the volatile cleavage mixture.15
58/71
quant/76
Initially, the Fmoc-amino acid (tyrosine) was loaded
onto the trityl resin and the Fmoc group removed
according to literature procedures.16 The solid-phase
bound amine was then reacted with carbon disulfide,
DIPEA and the peptide coupling reagents HBTU,
HATU, TFFH, DIPCDI, BOP, PyBOP, PyBrOP and
BOP-Cl (Fig. 1) under various conditions. Initially, the
corresponding isothiocyanate on solid phase was not
isolated, but reacted directly with benzylamine to fur-
nish the thiourea.
quant/quant
95/quant
N
N
N
N
N
PF6
N
N
N
N
N
N
N
PF6
N
F
N=C=N
DIPCDI
O
PF6
O
HATU
TFFH
HBTU
47/81
60/73
N
N
O
O
N
N
N
O
PF6
Br
N
N
N
N
P
N
N
N
N
N
N
O
P
Cl
P
O
O
N
P
N
PF6
PF6
O
BOP
BOP-Cl
PyBOP
PyBrOP
N
N
N
OH
HOBt
quant/quant
Figure 1.
HPLC-MS analysis showed that these peptide coupling
reagents were capable of fast and complete formation of
thiourea 1 (Table 1) on solid phase. The phosphorus-
based PyBrOP and BOP-Cl gave slightly slower (2 h
reaction time) conversion to the intermediate isothio-
cyanate in comparison with the 30 min reaction time for
the remaining coupling reagents. With all coupling re-
agents it was found that a large excess (50–100 fold) of
carbon disulfide was needed for fast and efficient for-
mation of the isothiocyanate. The reaction can be car-
ried out in apolar solvents like dichloromethane or in
polar solvents like DMF or NMP. As the reaction does
not depend on the full formation of the dithiocarbamate
the reactants can be mixed together in a one-pot reac-
tion to furnish the aliphatic isothiocyanate within
30 min, for subsequent reaction with an amine or other
nucleophile (Scheme 1).17
Solid-phase infrared analysis (KBr) of the intermediate
isothiocyanate showed two bands at approximately
2100 cmꢀ1 after reaction between the solid-phase bound
amine, carbon disulfide, DIPEA and coupling agent

U. Boas et al. / Tetrahedron Letters 45 (2004) 269–272
271
CS2/Coupling agent
DIPEA
combinatorial chemistry. The thioureas were isolated in
good yields and high crude purity. The intermediate
solid-phase bound isothiocyanates can furthermore serve
as useful intermediates in the formation of hetero-
cycles3;12;13 or thioamides11 offering a wide scope for this
new method in solid-phase organic chemistry.
R1
O
R1
O
H2N
S=C=N
O
O
DMF
S
S
R2 NH2
DMF
R1
O
R2
R2
R1 OH
O
20% HFIP
DCM
N
N
N
N
H
H
H
H
O
R1, R2= Alkyl or benzyl
Acknowledgements
Scheme 1.
The authors would like to thank Theis Brock-Nannes-
tad for assistance in performing the IR measurements,
and the Danish Agricultural and Veterinary Research
Council for financial support.
HBTU or PyBOP.20 These bands were absent in the
amino functionalised resin, and can be assigned to the
asymmetrical vibration of the corresponding isothio-
cyanate.21 In order to elucidate the reaction pathway
further, the filtrate from the reaction between carbon
disulfide, the solid-phase bound amine, coupling reagent
and DIPEA was analysed by HPLC-MS. Analysis of the
HBTU mediated isothiocyanate formation showed that
the coupling agent acted as a desulfurising agent creat-
ing the corresponding tetramethyl thiourea as well as
HOBt as byproducts (Scheme 2). However, in the
analysis of the PyBOP mediated formation of the iso-
thiocyanate, only the phosphine oxide was found in the
mixture, with no traces of phosphine sulfide. In addition
almost no HOBt was present in the filtrate. 31P NMR
analysis of the BOP mediated formation of isothiocya-
nates in solution, similarly showed no formation of the
P@S compound during the reaction. Also here, the
phosphine oxide (HMPA) was the only byproduct,
suggesting that the formation of isothiocyanates by
phosphonium based coupling reagents may proceed via
another reaction pathway.
References and Notes
1. For a general review on isothiocyanates and thioureas, see:
The Chemistry of Cyanates and their Thioderivatives; Patai,
S., Ed.; Wiley: Chichester, 1977; Applications of isothio-
cyanates as useful intermediates in bioconjugate chemis-
ꢀ
try, see for example, Fernandez, J. M. G.; Mellet, C. O.;
Blanco, J. L. J.; Mota, J. F.; Gadelle, A.; Coste-Sarguet,
A.; Defaye, J. Carbohydr. Res. 1995, 268, 57–71.
2. For a review in the use of isothiocyanates in the synthesis
of heterocycles, see: Mukerjee, A. K.; Ashare, R. Chem.
Rev. 1991, 91, 1–24.
3. Rathke, A. Chem. Ber. 1872, 5, 799; Dyson, G. M.;
George, H. J. J. Chem. Soc. 1924, 125, 1702–1708.
4. Kim, S.; Yi, K. Y. Tetrahedron Lett. 1985, 26, 1661–1664.
5. Molina, P.; Alajarin, M.; Tamiaki, H. Synthesis 1982,
596–597.
6. See, for example, Jochims, J. C.; Seeliger, A. Angew.
Chem. 1967, 79, 151.
7. Abbreviations used, BOP: (Benzotriazol-1-yloxy)tris(di-
methylamino)phosphonium hexafluorophosphate; BOP-Cl:
Bis(2-oxo-3-oxazolidinyl) phosphinic chloride; DIPCDI:
N,N0-Diisopropylcarbodiimide; Fmoc: Fluorenyloxy-
carbonyl; HATU: O-(7-Azabenzotriazol-1-yl)-N,N,N0,
N0-tetramethyluronium hexafluorophosphate; HBTU:
N-[(1H-benzotriazol-1-yl)(dimethylamino)methylene]-N-
methyl-methanaminium hexafluorophosphate N-oxide;
HOBt: 1-hydroxybenzotriazole; PyBOP: (Benzotriazol-1-yl-
oxy)tris(pyrrolidino)phosphonium hexafluorophosphate;
PyBrOP: Bromo-tris(pyrrolidino)phosphonium hexaflu-
orophosphate; TFFH: Fluoro-N,N,N0,N0-tetramethyl-
formamidinium hexafluorophosphate.
S
N
S
HBTU/
DIPEA
R1
O
R1
O
R1
O
CS2
H2N
HS
O
N
N
S
N
H
H
O
O
O
N
S
N
S
N
R1
R1
O
+
S=C=N
N
S
N
H
O
O
N
Scheme 2.
8. Boas, U.; Jakobsen, M. H. J. Chem. Soc., Chem. Commun.
1995, 1995–1996.
9. Boas, U.; Pedersen, B.; Christensen, J. B. Synth. Commun.
1998, 28, 1223–1231.
Amino acids were easily converted to the corresponding
isothiocyanates and thioureas, and in some cases the
isothiocyanate could be released from the resin and
isolated.22 However, attempts to convert longer peptides
on solid phase to the corresponding thioureas or iso-
thiocyanates resulted in complex mixtures. It has been
reported that thiohydantoin formation is a major side
reaction in the formation of peptide isothiocyanates in
solution.23;24
10. See, for example, Makino, S.; Suzuki, N.; Nakanishi, E.;
Tsuji, T. Tetrahedron Lett. 2000, 41, 8333–8337; Li, M.;
Wilson, L. J.; Portlock, D. E. Tetrahedron Lett. 2001, 42,
2273–2275; Kesarwani, A. P.; Srivastava, G. K.; Rastogi,
S. K.; Kundu, B. Tetrahedron Lett. 2002, 43, 5579–5581.
11. Stephensen, H.; Zaragosa, F. J. Org. Chem. 1997, 62,
6096–6097.
12. Kilburn, J. P.; Lau, J.; Jones, R. C. F. Tetrahedron Lett.
2001, 42, 2583–2586.
13. Kilburn, J. P.; Lau, J.; Jones, R. C. F. Tetrahedron Lett.
2002, 43, 3309–3311.
14. Bollhagen, R.; Schmiedberger, M.; Barlos, K.; Grell, E.
J. Chem. Soc., Chem. Commun. 1994, 2559–2560.
15. Bp (HFIP): 58 °C Purcell, K. F.; Stikeleather, J. A.;
Brunk, S. D. J. Am. Chem. Soc. 1969, 91, 4019–4027.
In summary, a new facile route for the formation of
thioureas and isothiocyanates on solid phase has been
developed. The method employs standard peptide cou-
pling conditions, and may thus be of interest to chemists
working in the area of solid-phase organic chemistry and

272
U. Boas et al. / Tetrahedron Letters 45 (2004) 269–272
16. Barlos, K.; Gatos, D.; Kallitis, J.; Papaphotiu, G.; Sotiriu,
P.; Yao, W. Q.; Schafer, W. Tetrahedron Lett. 1989, 3943–
3946.
(101 MHz, d6-DMSO): d 28.3, 39.0, 40.2, 77.8, 127.1,
129.3, 129.5, 155.7, 167.2; HPLC (220 nm): tR
¼
18:061 min; 99% crude purity; HR-MS (TOF-ESþ) calcd
for C16H24N3O4S (MHþ) 354.1487 (mono-isotopic);
found. 354.1492 (7): 1H NMR (400 MHz, d6-DMSO): d
2.45 (t, poorly resolved, 2H), 3.49(br t, 2H), 4.75 (br s,
2H), 7.38 (d, J ¼ 8:4 Hz, 2H), 7.59(br s, 1H), 7.88 (d,
J ¼ 6:6 Hz, 2H), 8.05 (br s, 1H). 13C (101 MHz, d6-
DMSO): d 38.9, 40.2, 59.6, 127.2, 129.3, 132.9, 142.1,
167.3, 170.1; HPLC (220 nm): tR ¼ 15:201 min; 99% crude
purity; HR-MS (TOF-ESþ) calcd for C11H15N2O3S
(MHþ) 255.0803 (mono-isotopic); found. 255.0825 (8):
1H NMR (400 MHz, d6-DMSO): d 1.50–1.70 (m, 10H),
1.75 (m, 3H), 4.68 (br d, J ¼ 5:3 Hz, 2H), 7.17 (br s, 1H),
7.30 (d, J ¼ 8:2 Hz, 2H), 7.75 (br t, J ¼ 5:4 Hz, 1H), 7.82
(d, J ¼ 8:2 Hz, 2H); 13C (101 MHz, d6-DMSO): d 28.4,
29.0, 35.2, 36.0, 38.6, 40.6, 41.2, 42.0, 46.2, 49.2, 50.8, 52.9,
126.9, 127.0, 129.3, 143.7, 167.8, 181.5; HPLC (220 nm):
tR ¼ 19:457 min; 93% crude purity; HR-MS (TOF-ESþ)
calcd for C19H25N2O2S (MHþ) 345.1637 (mono-isotopic);
found. 345.1680 (9): 1H NMR (400 MHz, d6-DMSO): d
3.15 (d, J ¼ 5:3 Hz, 2H), 3.50 (s, 3H), 5.12 (q, J ¼ 5:9Hz,
1H), 7.11–7.35 (m, 6H), 8.03 (d, J ¼ 8:6 Hz, 1H), 12.72 (br
s, 1H); 13C (101 MHz, d6-DMSO): d 53.7, 57.1, 63.1, 128.2,
17. SPS of thioureas, general procedure: Deprotected amino
acid derivatised 2-chlorotrityl resin (0.10 g, 0.19mmol,
theoretical loading: 1.9mmol/g) was suspended in DMF
(0.50 mL). Carbon disulfide (1.00 mL) was added followed
by HBTU (0.29g, 0.76 mmol) or PyBOP (0.40 g,
0.76 mmol) and DIPEA (0.39mL, 2.28 mmol). The sus-
pension was shaken for 30 min at rt. The solvent was
removed by suction, and the resin was washed with DCM
(10 times), DMF (5 times), and air was flushed through for
10 min to remove residual carbon disulfide. The deriva-
tised resin was swelled in DMF (1.5 mL) and amine
(5 equiv, 0.95 mmol) was added. If the amine was a
hydrochloride, DIPEA (0.33 mL, 1.9mmol) was added
and the mixture was shaken for 2 h at rt. The solvent was
removed by suction and the resin was washed with DMF
(5 times) and DCM (5 times). The thiourea compound was
released from the resin using 20% HFIP (2.5 mL) for
30 min, the product was collected by suction and the
cleavage mixture was removed in vacuo.
18. Analytical data for compounds 1–9: (1): 1H NMR
(250 MHz, d6-DMSO): d 1.27 (s, 9H), 2.93 (m, 2H), 4.64
(d, J ¼ 4:9Hz, 2H), 5.15 (br q, 1H), 6.85 (d, J ¼ 8:4 Hz,
2H), 7.10 (d, J ¼ 8:4 Hz, 2H), 7.23 (m, 5H), 7.58 (d,
J ¼ 7:7 Hz, 1H), 8.12 (t, J ¼ 5:5 Hz, 1H); 13C (63 MHz,
d6-DMSO): d 28.6, 36.6, 47.1, 57.8, 77.8, 123.4, 123.6,
126.9, 127.0, 127.3, 128.2, 128.3, 129.8, 130.3, 131.7, 139.1,
153.7, 173.2, 182.9. HPLC (220 nm): tR ¼ 22:549min; 97%
crude purity. HR-MS (TOF-ESþ) calcd for C21H27N2O3S
(MHþ) 387.1739(mono-isotopic); found. 387.1739( 2): 1H
NMR (250 MHz, d6-DMSO): d 4.62 (br s, 2H), 4.75 (br s,
2H), 7.25–7.40 (m, 7H), 7.90 (d, J ¼ 7:3 Hz, 2H), 8.05 (br
s, 2H), 12.71 (br s, 1H). 13C (63 MHz, d6-DMSO): d 46.6,
47.0, 126.9, 127.1, 127.3, 128.3, 129.4, 144.7, 167.3; HPLC
(220 nm): tR ¼ 18:468 min; 98% crude purity; HR-MS
(TOF-ESþ) calcd for C16H17N2O2S (MHþ) 301.1037
(mono-isotopic); found. 301.1073 (3): 1H NMR (250 MHz,
d6-DMSO): d 1.27 (qn, J ¼ 6:6 Hz, 2H), 1.50 (m, 4H), 2.20
(t, J ¼ 7:3 Hz, 2H), 4.64 (br s, 2H), 7.15–7.40 (m, 5H), 7.50
(br s, 1H), 7.80 (br s, 1H); 13C (63 MHz, d6-DMSO): d
24.3, 26.0, 28.6, 33.8, 43.5, 46.9, 126.8, 127.3, 128.3, 139.9,
174.6; HPLC (220 nm): tR ¼ 18:633 min; 98% crude purity;
HR-MS (TOF-ESþ) calcd for C14H21N2O2S (MHþ)
129.2,
137.6,
172.7,
179.5;
HPLC
(220 nm):
tR ¼ 16:572 min; 95% crude purity; HR-MS (TOF-ESþ)
calcd for C11H15N2O3S (MHþ) 255.0803 (mono-isotopic);
found. 255.0797.
19. The yields are based on the amount of amino acid coupled
to the resin. The actual loading of amino acid was
determined by Fmoc quantification: 5 mg resin (ca.
10 lmol theoretical loading) was swelled in 20% piper-
idine/DMF (25 mL), the mixture was shaken for 30 min
and the absorption (290 nm) was measured. A 20%
piperidine/DMF solution was used as reference.
20. IR-spectral data can be found in the supporting information.
21. Dolphin, D.; Wick, A. Tabulation of Infrared Spectral
Data; Wiley & Sons, 1977; p 412.
22. 4-Isothiocyanatomethylbenzoic acid: Deprotected amino
acid derivatised 2-chlorotrityl resin (0.10 g, 0.19mmol)
was suspended in DMF (0.50 mL). Carbon disulfide
(1.00 mL) was added followed by HBTU (0.29g,
0.76 mmol) or PyBOP (0.40 g, 0.76 mmol) and DIPEA
(0.39mL, 2.28 mmol). The suspension was shaken for
30 min at rt. The solvent was removed by suction, and the
resin was washed with DCM (10 times), DMF (5 times),
and air was flushed through for 10 min to remove residual
carbon disulfide. The isothiocyanate was released from the
resin by 20% HFIP (2.5 mL) for 30 min, the product was
collected by suction and the cleavage mixture was removed
in vacuo, the product was obtained as an off-white
powder. Yield (HBTU): 13.6 mg (76%); Yield (PyBOP):
13.8 mg (77%). 1H NMR (400 MHz, CDCl3): d 4.77 (s,
1
281.1324 (mono-isotopic); found. 281.1319( 4): H NMR
(400 MHz, d6-DMSO): d 1.30 (d, J2 ¼ 7:1 Hz, 3H), 1.40 (s,
9H), 2.95 (dd, J1 ¼ 13:7 Hz, J2 ¼ 6:4 Hz, 2H), 4.60 (qn,
J ¼ 7:1 Hz, 1H), 4.95 (br q, 1H), 7.20–7.30 (m, 5H), 7.65
(br d, J ¼ 7:1 Hz, 1H), 8.00 (br d, J ¼ 6:2 Hz, 1H), 10.20
(br s, 1H); 13C (101 MHz, d6-DMSO): d 27.7, 31.4, 37.2,
52.7, 57.7, 80.5, 126.4, 128.1, 129.4, 137.2, 171.9, 172.8,
182.1; HPLC (220 nm): tR ¼ 22:632 min; 99% crude purity
HR-MS (TOF-ESþ) calcd for C17H25N2O4S (MHþ)
2H), 7.38 (d, J ¼ 8:1 Hz, 2H), 8.07 (d, J ¼ 8:2 Hz, 2H); 13
C
1
353.1535 (mono-isotopic); found. 353.1547 (5): H NMR
(400 MHz, d6-DMSO):
(101 MHz, CDCl3): d 47.4, 125.7, 128.4, 129.8, 130.0 (low
intensity); 139.0, 152.4; HPLC (220 nm): tR ¼ 21.066 min;
97% crude purity; MS (FABþ) calcd for C9H7NO2S (m=z)
193.23; found. 216.96 (M+Naþ).
d 2.80–3.05 (m, 4H), 4.11
(d, J ¼ 10:1 Hz, 2H), 4.40 (m, 2H), 4.92 (br s, 2H), 7.15–
7.30 (m, 10H), 7.80 (br s, 1H), 7.92 (br s, 1H), 8.17 (br d,
1H), 10.42 (br s, 1H); 13C (101 MHz, d6-DMSO): d 42.3,
47.0, 58.3, 59.8, 67.3, 126.3, 126.9, 127.8, 128.3, 129.2,
129.3, 129.5, 130.2, 135.2, 137.5, 137.8, 138.1, 165.3, 168.4,
172.6, 182.3; HPLC (220 nm): tR ¼ 20:824 min; 90% crude
purity; HR-MS (TOF-ESþ) calcd for C21H25N4O4S
(MHþ) 429.1596 (mono-isotopic); found. 429.1606 (6):
1H NMR (400 MHz, d6-DMSO): d 1.38 (s, 9H), 3.08 (q,
J ¼ 5:9Hz, 2H), 3.48 (br s, 2H), 4.73 (br s, 2H), 6.85 (br s,
1H), 7.38 (d, J ¼ 8:4 Hz, 2H), 7.60 (br t, 1H), 7.88 (d,
J ¼ 8:4 Hz, 2H), 8.05 (br s, 1H), 12.60 (br s, 1H); 13C
23. Nowick, J.; Holmes, D. L.; Noronba, G.; Smith, E. M.;
Nguyen, T. M.; Huang, S.-L. J. Org. Chem. 1996, 61,
3929–3934.
24. A way to circumvent the formation of thiohydantoins
could be to introduce 2-hydroxy-4-methoxybenzyl (Hmb)
protection of the last backbone amide at the N-terminal.
The Hmb group has found wide use for preventing peptide
aggregation during Fmoc solid-phase peptide synthesis,
see for example, Johnson, T.; Quibell, M. Tetrahedron
Lett. 1994, 35, 463–466.