Enzyme-mediated synthesis of (R)- and (S)-α-ionone

Article abstract of DOI:10.1039/a806943c

Diastereoisomeric enrichment through fractional crystallisation of 4-nitrobenzoate derivatives of α-ionol, and enantioselective enzyme-mediated reactions of α-ionol and α-ionol acetate, were usefully combined to optimise two different procedures to enantiopure (S)- and (R)-ionone.

Full text of DOI:10.1039/a806943c

View Article Online / Journal Homepage / Table of Contents for this issue
Enzyme-mediated synthesis of (R)- and (S)-á-ionone
Elisabetta Brenna, Claudio Fuganti,* Piero Grasselli, Mara Redaelli and Stefano Serra
Dipartimento di Chimica del Politecnico, Centro CNR per lo Studio delle Sostanze Organiche
Naturali, Via Mancinelli, 7 I-20131 Milano, Italy
Received (in Cambridge) 7th September 1998, Accepted 27th October 1998
Diastereoisomeric enrichment through fractional crystallisation of 4-nitrobenzoate derivatives of α-ionol, and
enantioselective enzyme-mediated reactions of α-ionol and α-ionol acetate, were usefully combined to optimise
two different procedures to enantiopure (S)- and (R)-ionone.
Introduction
α-Ionone 1 entered the realm of flavours and fragrances over
one century ago, when, after its discovery as a fragrant com-
ponent of violet flowers by Tiemann,1 it became accessible on a
large scale by chemical synthesis from citral.2 Many years later
it was shown that in Viola odorata Linneaus α-ionone was pres-
ent in the (ϩ)-enantiomeric form,3 which was assigned the (R)
absolute configuration through correlation with manool.4 The
relative sensitivities for both enantiomers of α-ionone were
found to diverge widely for different flavorists.5,6 (R)- and (S)-1
show violet and, respectively, woody-like taste, whereas the
threshold values appeared in the 0.5–5 and, 20–40 ppb range,
respectively.6,7
Despite the marked difference in the flavour response of the
two enantiomers, α-ionone 1 is used in industrial formulations
in the racemic modification. This circumstance might also be
due to the fact that (R)-α-ionone is a rather inaccessible
material. The first historically important separation8 of the
enantiomers of α-ionone 1, applied also for the preparation of
enantiomer (R)-1 used as a starting material in the synthesis of
a variety of natural carotenoids,4 is based on a classical optical
resolution through fractional crystallisation of the l-menthyl-
hydrazide9 of the racemic ketone. Reportedly, over 20 recrystal-
lisations are required to obtain isomer (R)-1 (~1% yield) and
isolation of the (S) material is similarly laborious. A straight-
forward four-step entry to the enantiomers of α-ionone 1 from
(R)- and (S)-α-damascone in 48% yield through a new enone
transposition has been reported fairly recently.6 The starting
materials were produced, in turn, from racemic damascone via
enantioselective protonation of the corresponding enolate.10
Despite its elegance, the above method of synthesis requires
several steps and/or delicate reaction conditions starting from a
rather precious material such as racemic α-damascone. Even
more laborious is the procedure leading from (S)-(Ϫ)-4-
hydroxy-2,6,6-trimethylcyclohex-2-enone to isomer (R)-1 [85%
enantiomeric excess (ee)].11
In the light of the above considerations we thought it worth-
while to explore direct entries to ionones (R)- and (S)-1 through
enzymic kinetic resolution of the easily accessible α-ionol 2.
Indeed, the structural similarity between allylic alcohol 2 and
the aromatic allylic methyl alcohol recently resolved in the two
enantiomeric forms by lipase-mediated kinetic esterification12
induced us to explore the preparation, by these means, of the
enantiomers of the single diastereoisomers of compound 2, in
the expectation of subsequently obtaining from these latter
derivatives the target ionone enantiomers (R)- and (S)-1 by
MnO2 oxidation.
isomers. The same composition was obtained when α-ionone 1
was reduced either with LiAlH4 in THF at Ϫ70 ЊC or with
NaBH4 in propan-2-ol at Ϫ15 ЊC. The diastereoisomeric as well
as the enantiomeric composition of α-ionol 2a–d could be best
determined by gas-chromatographic analysis of the correspond-
ing acetate esters on a permethylated β-cyclodextrin-based
column. Under suitable conditions, four baseline-separated
peaks (Fig. 1a) were obtained for the two racemic diastereo-
isomers 3a–d (four stereoisomers).
Preliminary enzymic esterification experiments on com-
mercial α-ionol 2, in the presence of lipase PS (Pseudomonas
cepacia), using vinyl acetate as an acetate donor in tert-butyl
methyl ether (TBME) solution, provided a 1:1 mixture of
enantiomerically pure acetates 3b(S,R) and 3d(R,R) (GC analy-
sis, second and fourth peaks). Acetylation with Ac2O–pyridine
of the alcohol derivative, recovered from the enzymic esterifi-
We report herein on the results obtained.
Results and discussion
Commercial α-ionol 2 is a 1:1 mixture of two racemic diastereo-
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134
4129

View Article Online
cation and separated from substrates 3b(S,R) and 3d(R,R) by
column chromatography, gave enantiomerically pure diastereo-
isomers 3a(S,S) and 3c(R,S) in a 1:1 ratio, as was shown by
GC analysis (first and third peaks). The assignment of the abso-
lute configuration to isomers 3b and 3d was performed through
the chemical correlations shown in Scheme 1, following the
Scheme 2 Separation of the two racemic diastereoisomers of nitro-
benzoate 6a–d.
Five crystallisations from hexane provided the racemic
diastereoisomers 6a,d(RS,RS) in ~15–18% yield. GC of the
corresponding acetate derivatives showed that these two
enantiomers corresponded to the first and last peak of the
series of four.
The other racemic diastereoisomer 6b,c(RS,SR) could not
be recovered in a pure state from the mother liquors of
the hexane crystallisations. Therefore, it was obtained from
diastereoisomeric racemate 6a,d(RS,RS) upon saponification
with methanolic potassium hydroxide, followed by Mitsunobu
esterification (Scheme 2) (second and third peak in the GC
analysis of the corresponding acetates).
Scheme 1 Determination of the absolute configuration of acetate
derivatives 3b and 3d obtained upon Lipase PS-catalysed acetylation of
compounds 2a–d.
same procedure as described for the structural determination of
(ϩ)-3-oxo-α-ionol isolated from tobacco leaves.13
First, in order to determine the absolute configuration of
C-9, enzyme-generated diastereoisomers 3b and 3d were con-
verted into the corresponding alcohols 2b and 2d, by treatment
with methanolic potassium hydroxide. A certain amount of the
mixture of alcohols 2b and 2d was submitted to benzoylation,
to give ester derivatives 4b and 4d. The mixture of these
latter substrates provided enantiomerically pure methyl (R)-O-
benzoyllactate 5 on ozonolysis, oxidative work-up and diazo-
methane treatment. This result allowed us to assign (9R) con-
figuration to both parent compounds 3b and 3d. The remaining
mixture of alcohols 2b and 2d was oxidised with MnO2 to
afford α-ionone 1 devoid of optical activity (1:1 mixture of (R)-
and (S)-enantiomers by GC analysis). These results clearly
indicated that the esterification of alcohols 2a–d, mediated by
lipase PS, had provided a 1:1 mixture of the two enantiomeri-
cally pure (9R) ionol acetates, showing opposite configurations
at C-6.
In the light of these observations, it appeared necessary to
separate the two racemic diastereoisomers of α-ionol, isomers
2a,d(RS,RS) and 2b,c(RS,SR), in order to obtain α-ionones
(R)- and (S)-1. Two procedures were devised to this end.
The first was based on the fractional crystallisation of ester
derivatives of α-ionol. Unfortunately, among the several known
esters of α-ionol (3,5-dinitrobenzoate and 4-bromobenzoate
derivatives) only the 4-nitrobenzoate derivative was found to be
a crystalline material.14 We thus relied on the fractional crystal-
lisation of the 4-nitrobenzoate esters 6a–d for the separation of
the two diastereosiomers of α-ionol (Scheme 2).
The material recovered from the mother liquors and enriched
in the more soluble diastereoisomer 6b,c(RS,SR) was not
wasted at all. It was hydrolysed with KOH–MeOH, the alcohol
was submitted to Mitsunobu esterification,15 and the product
was then crystallised from hexane to afford isomers 6a,d(RS,
1
RS). H NMR studies were particularly useful in determining
the diastereoisomeric composition of the esters during the
crystallisation sequence. Indeed, the two singlets, relative to
the methyl groups at position 1 of the framework of the two
diastereoisomers, appeared as baseline-separated peaks, thus
allowing a precise analysis.
The two diastereoisomers of α-ionol were recovered from
esters 6a,d(RS,RS) and 6b,c(RS,SR), respectively, upon treat-
ment with methanolic potassium hydroxide, and were submit-
ted separately to enzyme-mediated acetylation.
Lipase PS-mediated esterification of α-ionols 2a,d(RS,RS)
gave enantiomerically pure acetate 3d(R,R) (fourth peak in
GC analysis), and unchanged alcohol 2a(S,S), showing high
enantiomeric purity as was shown by GC analysis of the
corresponding acetate derivative (first peak).
Enantiomerically pure acetate 3b(S,R) (second peak) and
alcohol 2c(R,S) (third peak in GC analysis of the acetate
derivative) were obtained upon Lipase PS-mediated esterifi-
cation of racemic diastereoisomer 2b,c. Basic hydrolysis of
enzyme-generated acetates 3d(R,R) and 3b(S,R) provided
4130
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134

View Article Online
Table 1 Qualitative studies of enzyme-mediated acetylation of ionols
2a–d
3b:3d ratio
(GC)
ee % 3b
(GC)
ee %3d
(GC)
Enzyme
PPL
CCL
Lipase PS
3:1
1:1.15
1:1
90
72
99
64
79
99
enantiomerically pure alcohols 2d(R,R) and 2b(S,R), which
16
were straightforwardly oxidised with freshly prepared MnO2
to (R)- and (S)-α-ionone 1, showing 98% and 97% ee (GC), and
[α]D20 420† and Ϫ418 (c 1, CHCl3) {lit.,6 (R)-ionone ee 98%, [α]D20
407, c 0.04, CHCl3; (S)-ionone ee > 99%, [α]D20 Ϫ431, c 0.035,
CHCl3} in ~35% yield from the single racemic diastereoisomer.
Oxidation of the alcohols recovered from the two enzymic
esterifications provided (S)- and (R)-1 ionones showing 87–
93% ee.
In the alternative procedure the separation of the two dia-
stereoisomers of α-ionol, to be used as precursors of enantiopure
α-ionone 1, was performed after the enantiomeric enrichment
of the same two stereoisomers by lipase-mediated reactions.
To this end, qualitative studies of enzyme-catalysed acetyl-
ation of alcohols 2a–d in TBME solution, in the presence of
vinyl acetate as an acylating agent, were performed also with
other lipases [porcine pancreas lipase (PPL) and Candida cylin-
dracea lipase (CCL)] (Table 1). PPL-mediated reaction gave a
3:1 mixture (de‡ = 52%, GC) of acetates 3b(S,R) (ee = 90%,
GC) and 3d(R,R) (ee = 64%, GC), while CCL catalysis afforded
the same two diastereoisomers in a low enantiomeric enrich-
ment (ee = 72% and 79% respectively) in a nearly 1:1 ratio [de
7% of 3d(R,R)]. On the other hand, as we have previously
described, Lipase PS led to total enantioselectivity with no
diastereoselectivity at all.
We tried to take advantage of the good diastereoselectivity
assured by PPL in the reverse reaction of hydrolysis of the two
racemic diastereoisomers of acetates 3a–d in water, keeping the
solution pH at 7.5 by means of the progressive addition of 0.5
M NaOH. The stereoisomer 3b(S,R) was preferentially hydro-
lysed under these reaction conditions to give alcohol 2b(S,R)
showing ee 98% and de 55% (GC). The gas chromatogram of
the surviving acetate (Fig. 1b) clearly showed the almost
complete consumption of the (S,R) stereoisomer (three peaks
survived). The alcohol, recovered from the saponification of
this residual acetate, was submitted to lipase PS-mediated
acetylation. The gas chromatogram of the resulting acetate is
shown in Fig. 1c (two peaks survived): the acetate 3d(R,R) was
thus obtained in 99% ee and 66% de. These two subsequent
enzyme-catalysed reactions afforded an enantiomerically pure
acetate 3d(R,R) which was to be brought to diastereoisomeric
purity. This goal was reached by means of two crystallisations
of the corresponding 4-nitrobenzoate derivative from hexane:
the gas chromatogram of the acetate prepared from the crystal-
line ester is shown in Fig. 1d (one peak survived). The oxidation
of the corresponding alcohol (6R,9R)-2 with MnO2 gave
enantiomerically pure (R)-ionone (GC).
Fig. 1 GC analyses of acetate derivatives 3a–d: (a) 1:1 mixture of the
two racemic diastereoisomers of compound 3; (b) mixture of stereo-
isomers recovered from PPL-mediated hydrolysis; (c) acetate derivatives
produced by Lipase PS-mediated acetylation of α-ionol recovered from
the residual acetate of the preceding hydrolysis; (d) acetate 3d(R,R)
after diastereoisomeric enrichment.
crystallisations, from hexane, of 4-nitrobenzoate derivatives of
α-ionol were successfully used in both synthetic paths to achieve
diastereoisomeric purity, while optical activation was assured
by enantioselective enzyme-mediated acetylation of α-ionol
and hydrolysis of α-ionol acetate.
Conclusions
Experimental
A successful combination of simple chemical and enzymic
methods allowed us to prepare extremely valuable (R)- and (S)-
α-ionone using inexpensive racemic α-ionone as a starting
material. Two different approaches have been devised by inter-
changing the application of traditional techniques of fractional
crystallisation and enzyme-mediated reactions. Fractional
The following enzymes were employed in this work: CCL
(Sigma, Type VII, 900 U mgϪ1), PPL (Sigma, Type II), and
Lipase PS (Pseudomonas cepacia) (Amano Pharmaceuticals
Co., Japan). Enantiomeric and diastereoisomeric excesses (ees
and des) were determined by chiral GC analysis on a Chirasil
DEX CB, 25 m × 0.25 mm (Chrompack), using a DANI HT
86.10 gas chromatograph; 70 ЊC (1 min)—3.5 ЊC minϪ1—140 ЊC
(6 min)—8 ЊC minϪ1—180 ЊC (1 min): (6S,9S)-2 tR 18.50 min,
(6S,9R)-2 tR 18.83 min, (6R,9S)-2 tR 19.09 min, (6R,9R)-2 tR
† [α]D-Values are given in units of 10Ϫ1 deg cm2 gϪ1
‡ de = diastereoisomeric excess.
.
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134
4131

View Article Online
1
19.32 min, (S)-1 tR 17.55 min, (R)-1 tR 18.13 min. H and 13C
eluted fractions gave a 1:1 mixture of benzoates (6R,9R)-4 and
(6S,9R)-4 (2.40 g, 62%) nearly devoid of optical activity in
chloroform (Found: C, 80.6; H, 8.65. C20H26O2 requires C,
80.50; H, 8.78%); δH 7.96 (4H, d, J 7.9, 4 × Ar o-H), 7.47 (2H, t,
J 7.9, 2 × Ar p-H), 7.36 (4H, t, J 7.9, 4 × Ar m-H), 5.51 [6H, m,
2 × C(7)H, 2 × C(8)H ϩ 2 × CHOCOPh], 5.33 [2H, m, 2 ×
C(2)H], 2.04 [2H, d, J 7.3, 2 × C(6)H], 1.92 [4H, m, 4 × C(3)H],
1.57 [6H, m, 2 × C(1)Me], 1.37 [8H, 2 d, J 6.1, ϩm, 2 ×
CH3CHOCOPh ϩ 2 × C(4)H], 1.09 [2H, m, 2 × C(4)H], 0.82
[3H, s, C(5)Me], 0.80 [3H, s, C(5)Me], 0.73 [3H, s, C(5)Me] and
0.74 [3H, s, C(5)Me].
NMR spectra were recorded in CDCl3 solutions at rt unless
otherwise stated, on a Bruker AC-250 spectrometer (250 MHz
1H; 62.9 MHz 13C). The chemical-shift scale was based on
internal tetramethylsilane. J-Values are in Hz. Optical rotations
were measured on a JASCO DIP 181 digital polarimeter. TLC
analyses were performed on Merck Kieselgel 60 F254 plates. All
chromatographic separations were carried out on silica gel
columns.
(6RS,9RS)- and (6RS,9SR)-á-Ionol 2
A 1:1 mixture of the two racemic diastereoisomers of α-ionol 2
was obtained upon NaBH4 reduction of α-ionone 1 in propan-
2-ol at Ϫ15 ЊC (82% yield), and on treatment with LiAlH4 in
THF at Ϫ40 ЊC (75%) (Found: C, 80.3; H, 11.45. C13H22O
requires C, 80.36; H, 11.41%); δH 5.45 (6H, m, olefinic hydro-
gens), 4.30 (2H, quintet, J 6.1, 2 × CHOH), 2.08 [2H, d, J 8.3,
C(6)H], 1.99 [4H, m, 4 × C(3)H], 1.58 [6H, m, 2 × C(1)Me],
1.55–1.35 [2H, m, 2 × C(4)H], 1.27 (6H, m, 2 × CH3CHOH),
1.22–1.08 [2H, m, 2 × C(4)H], 0.88 [6H, s, 2 × C(5)Me], 0.80
[3H, s, C(5)Me of one diastereoisomer] and 0.82 [3H, s, C(5)Me
of other diastereoisomer].
Ozonisation, oxidative work-up and diazomethane treatment.
The two diastereoisomeric benzoyl derivatives (2.40 g, 8.05
mmol) were ozonised in methanol solution at Ϫ78 ЊC for 15
min. The reaction mixture was allowed to reach rt and the
solvent was removed under reduced pressure, to give a residue
to which was added a mixture of formic acid (20 cm3) and 35%
hydrogen peroxide (3 cm3). The reaction mixture was refluxed
for 30 min, concentrated in vacuo, diluted with water and
extracted with diethyl ether. The ethereal solution was treated
with diazomethane, and concentrated under reduced pressure
to give a residue, which was purified by column chrom-
atography with hexane as eluent, to give enantiopure methyl
(R)-O-benzoyllactate 5 (0.485 g, 29%) (Found: C, 63.6; H, 5.8.
C11H12O4 requires C, 63.46; H, 5.81%); [α]D20 Ϫ13.7 (c 4.5,
CHCl3); δH 8.10 (2H, d, J 7.5, Ar o-H), 7.59 (1H, t, J 7.5,
Ar p-H), 7.46 (2H, t, J 7.5, Ar m-H), 5.34 (1H, q, J 7.5,
CHOCOPh), 3.78 (3H, s, CO2CH3) and 1.64 (3H, d, J 7.5,
CH3CHOCOPh).
(6RS,9RS)- and (6RS,9SR)-á-Ionol acetate 3
Treatment of the above described mixture (0.01 mol) with acetic
anhydride (0.02 mol) in pyridine (20 cm3) gave a 1:1 mixture
of the two racemic diastereoisomers of α-ionol acetate 3
(Found: C, 76.3; H, 10.2. C15H24O2 requires C, 76.23; H,
10.23%); δH 5.6–5.2 (4H, m, olefinic hydrogens and CHOAc),
2.10–0.90 [6H, m ϩ s, C(6)H, 2 × C(3)H ϩ CH3CO2], 1.56 [3H,
m, C(1)Me], 1.38 [1H, m, C(4)H], 1.28 (3H, 2 d, J 6.1,
CH3CHOAc of both diastereoisomers), 1.15 [1H, m, C(4)H],
0.88 [3H, s, C(5)Me] and 0.81 [3H, s, C(5)Me].
First procedure to (R)- and (S)-á-ionone
(6RS,9RS)-á-Ionol 2a ؉2d. (a) Fractional crystallisation of
α-ionol 4-nitrobenzoate esters. A solution of α-ionol (50 g, 0.26
mol), 4-nitrobenzoyl chloride (70.50 g, 0.38 mol) and pyridine
(40 cm3) in methylene dichloride (400 cm3) was stirred at rt for
2 h. The reaction mixture was poured into ice and treated with
10% HCl; the organic layer was separated, washed first with
saturated aq. sodium hydrogen carbonate, then with water, and
dried over sodium sulfate. The solvent was removed under
reduced pressure and the residue was purified by column chrom-
atography with hexane as eluent. The first eluted fractions gave
a 1:1 mixture of (6RS,9RS)- and (6RS,9SR)-4-nitrobenzoate
esters 6 (63.32 g, 71%) (Found: C, 69.9; H, 7.4; N, 4.1.
C20H25NO4 requires C, 69.95; H, 7.34; N, 4.08%); δH 8.20
(8H, m, ArH), 5.60 [6H, m, 2 × C(7)H, 2 × C(8)H ϩ 2 ×
CHOCOAr], 5.40 [2H, m, 2 × C(2)H], 2.12 [2H, d, J 8.3,
2 × C(6)H], 2.00 [4H, m, 2 × C(3)H], 1.58 [3H, m, C(1)Me
of one diastereoisomer], 1.52 [3H, m, C(1)Me of the other
diastereoisomer], 1.42 (6H, 2 d, J 6.1, 2 × CH3CHOCOAr),
1.40–1.30 [2H, m, 2 × C(4)H], 1.05–1.20 [2H, m, 2 × C(4)H],
0.89 [3H, s, C(5)Me of one diastereoisomer], 0.87 [3H, s, C(5)Me
of the other diastereoisomer], 0.79 [3H, s, C(5)Me of one dia-
stereoisomer] and 0.81 [3H, s, C(5)Me of the other diastereo-
isomer].
Five crystallisations from hexane afforded (6RS,9RS)-α-
ionol 4-nitrobenzoate ester (6a and 6d) (16 g, 18%) with
de > 99% (NMR and GC of the corresponding acetate deriv-
atives): mp 65–67 ЊC; νmax/cmϪ1 1720 (CO); δH 8.23 (4H, m,
ArH), 5.60 [3H, m, C(7)H, C(8)H and CHOCOAr], 5.42 [1H,
m, C(2)H], 2.12 [1H, d, J 8.0, C(6)H], 1.99 [2H, m, 2 × C(3)H],
1.57 [3H, m, C(1)Me], 1.50–1.35 [4H, d, J 6.1, ϩdt, J 13.5 and
8.0, CH3CHOCOAr ϩ C(4)H], 1.17 [1H, dt, J 13.5 and 5.2,
C(4)H], 0.87 [3H, s, C(5)Me] and 0.79 [3H, s, C(5)Me]; δC 20.6,
22.8, 27.0, 27.4, 31.5, 32.0, 54.0, 73.2, 121.5, 123.5, 130.4, 130.6,
133.4, 135.2, 136.3, 150 and 164.
General procedure for enzyme-mediated acetylations
A 1:1 mixture of (6RS,9RS)- and (6RS,9SR)-2 (10 g, 0.04
mol), lipase PS (10 g), and vinyl acetate (40 cm3) in TBME (150
cm3) was stirred at rt for 72 h. The residue obtained upon evap-
oration of the filtered reaction mixture was chromatographed
with gradient elution, with hexane → hexane–ethyl acetate
(1:1). The first eluted fractions provided a 1:1 mixture (4.16 g,
44%) of (6S,9R)-3 (ee > 99%, GC) and (6R,9R)-3 (ee > 99%,
GC). The last eluted fractions gave a 1:1 mixture (3.20 g, 41%
recovery) of (6S,9S)-2 (ee > 99%, GC) and (6R,9S)-2 (ee >
99%, GC). The same procedure was followed for the prelimin-
ary studies of PPL- and CCL-mediated acetylations, to provide
acetates 3 showing the ees and des reported in the text.
Determination of the absolute configuration at C-9 in acetate
derivatives 3 obtained from alcohols 2 by Lipase PS-mediated
acetylation
The 1:1 mixture of the two diastereoisomeric acetates obtained
in lipase-mediated acetylation of alcohol 2 (4.0 g, 0.017 mmol)
was saponified with potassium hydroxide (1.14 g, 0.020 mol)
in refluxing methanol (40 cm3). A portion of this alcohol was
oxidised with manganese() oxide in methylene dichloride
solution, to give racemic α-ionone (GC). The remaining alcohol
was submitted to the following sequence of chemical corre-
lation to determine the configuration at C-9.
Benzoylation. The alcohol 2 (2.50 g, 0.013 mol) was treated
with a solution of benzoyl chloride (2.67 g, 0.019 mol) in pyri-
dine (20 cm3). The reaction mixture was poured into ice, and
treated with 10% HCl. The organic phase was separated,
washed first with saturated aq. sodium hydrogen carbonate,
then with water, and dried over sodium sulfate. The solvent was
removed under reduced pressure and the residue was purified
by column chromatography with hexane as eluent. The first
(b) Saponification. (6RS,9RS)-α-Ionol 4-nitrobenzoate ester
6a/d (16.0 g, 0.047 mol) was saponified with potassium hydrox-
ide (3.14 g, 0.056 mol) in refluxing methanol (100 cm3). After
4132
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134

View Article Online
the usual work-up, ionol (6RS,9RS)-2 (7.11 g, 78%) was
obtained: de > 99% (GC analysis of the corresponding acet-
ates: first and fourth peak); δH 5.45 (3H, m, olefinic hydrogens),
4.30 (1H, quintet, J 6.1, CHOH), 2.08 [1H, d, J 8.3, C(6)H],
1.99 [2H, m, 2 × C(3)H], 1.57 [3H, m, C(1)Me], 1.42 [1H, dt,
J 13.1 and 8, C(4)H], 1.27 (3H, d, J 6.1, CH3CHOH), 1.16 [1H,
dt, J 13.1 and 5, C(4)H], 0.89 [3H, s, C(5)Me] and 0.82 [3H, s,
C(5)Me].
reduced pressure. The residue was chromatographed and eluted
with (gradient elution) hexane → hexane–ethyl acetate (1:1).
The first eluted fractions gave acetate (6S,9R)-3 (3b) showing ee
97% (GC, second peak) (2.56 g, 42%); [α]D20 Ϫ305 (c 1, CHCl3);
δH 5.60–5.20 (4H, m, olefinic hydrogens and CHOAc), 2.15–
1.95 [6H, m ϩ s, C(6)H, 2 × C(3)H ϩ CH3CO2], 1.56 [3H,
m, C(1)Me], 1.39 [1H, dt, J 13.5 and 8.0, C(4)H], 1.30 (3H, d,
J 6.1, CH3CHOAc), 0.88 [3H, s, C(5)Me] and 0.79 [3H, s,
C(5)Me].
(6RS,9SR)-á-Ionol 2b ؉ 2c. (a) Esterification of (6RS,9RS)-
2 under Mitsunobu conditions. A solution of ionol (6RS,9RS)-2
(5.0 g, 0.026 mmol) and triphenylphosphine (4.45 g, 0.017 mol)
in THF (45 cm3) was dropped into a solution of diisopropyl
azodicarboxylate (3.43 g, 0.017 mmol) and 4-nitrobenzoic acid
(2.84 g, 0.017 mol) in THF (30 cm3). The reaction mixture was
stirred at rt for 12 h, then was concentrated under reduced pres-
sure to give a residue, which was chromatographed with gradi-
ent elution with hexane → hexane–ethyl acetate (9:1). The
product obtained from the first eluted fractions was (6RS,9SR)-
α-ionol 4-nitrobenzoate 6b and 6c (de > 99%, NMR and GC of
the corresponding acetate derivatives) (6.33 g, 71%): mp 62–
64 ЊC; νmax(Nujol)/cmϪ1 1720 (CO); δH 8.22 (4H, m, ArH), 5.60
[3H, m, C(7)H, C(8)H ϩ CHOCOAr], 5.42 [1H, m, C(2)H],
2.12 [1H, d, J 8.3, C(6)H], 1.99 [2H, m, 2 × C(3)H], 1.56 [3H,
m, C(1)Me], 1.47 (3H, d, J 6.1, CH3CHOCOAr), 1.39 [1H, dt,
J 13.5 and 8.0, C(4)H], 1.17 [1H, dt, J 13.5 and 5.2, C(4)H],
0.89 [3H, s, C(5)Me] and 0.81 [3H, s, C(5)Me]; δC 20.6, 23.1,
27.0, 27.4, 31.6, 32.0, 54.0, 73.1, 121.5, 123.5, 130.6, 130.7,
133.4, 134.8, 136.4, 150.0 and 164.
Saponification with potassium hydroxide in methanol gave
ionol (6S,9R)-2 (2b) (1.59 g, 75%); [α]D20 Ϫ290 (c 1, CHCl3).
The last eluted fractions gave ionol (6R,9S)-2 (2c) (1.76 g,
35%) showing ee 93% (GC of the corresponding acetate deriv-
ative, third peak); [α]D20 ϩ269 (c 1, CHCl3). E (enantiomeric
ratio) = 222, c (conversion) = 48%.17
(R)- and (S)-á-Ionone 1 and 2. Oxidation of the various
stereoisomers of α-ionol to enantioenriched ionones was per-
formed in methylene dichloride solution in the presence of
manganese() oxide in 69–76% yield. The following results
were obtained:
Compound (6R,9R)-2 (2d) gave (R)-ionone showing ee 98%
and [α]D20 ϩ420 (c 1, CHCl3);
Compound (6S,9S)-2 (2a) gave (S)-ionone showing ee 87%
and [α]D20 Ϫ364 (c 1, CHCl3);
Compound (6S,9R)-2 (2b) gave (S)-ionone showing ee 97%
and [α]D20 Ϫ418 (c 1, CHCl3);
Compound (6R,9S)-2 (2c) gave (R)-ionone showing ee 93%
and [α]D20 ϩ389 (c 1, CHCl3).
(b) Saponification. (6RS,9SR)-α-Ionol 4-nitrobenzoate ester
6b/c (6.30 g, 0.018 mol) was saponified with potassium hydrox-
ide (1.21 g, 0.002 mol) in refluxing methanol (50 cm3). After the
usual work-up, ionol (6RS,9SR)-2 (2.62 g, 75%) was obtained:
de > 99% (GC analysis of the corresponding acetates: second
and third peak); δH 5.48 (3H, m, olefinic hydrogens), 4.30
(1H, quintet, J 6.1, CHOH), 2.08 [1H, d, J 8.0, C(6)H], 1.99
[2H, m, 2 × C(3)H], 1.59 [3H, m, C(1)Me], 1.42 [1H, dt, J 13.5
and 8.0, C(4)H], 1.27 (3H, d, J 6.1, CH3CHOH), 1.17 [1H, dt,
J 13.5 and 5.2, C(4)H], 0.88 [3H, s, C(5)Me] and 0.80 [3H, s,
C(5)Me].
Second procedure to (R)-á-ionone R-(؊)-1
Enzymic hydrolysis of á-ionol acetate 3 and Lipase PS-
mediated acetylation of á-ionol recovered from residual acetate.
A 1:1 mixture of the two diastereoisomers of acetate derivative
3 (10.0 g, 0.042 mol) was suspended in water (150 cm3): the
solution pH was brought to 7.5 by adding NaOH 0.5 M. PPL
(10.0 g) was added and the reaction mixture was stirred for 72 h,
with the solution pH kept constant by progressive addition of
NaOH 0.5 M. After treatment of the reaction mixture with
ethyl acetate, the enzyme was filtered off, and the organic phase
was separated, and dried on Na2SO4. The solvent was removed
under reduced pressure to give a residue which was chromato-
graphed (gradient elution) with hexane → hexane–ethyl
acetate (1:1). The first eluted fractions gave a 1.6:1 mixture
(de = 0.21) (5.79 g, 58%) of acetates (6S,9S)-3 (ee 11%) and
(6R,9S)-3 (ee 73%) (GC analysis, see Fig. 1b).
(6S,9S)- and (6R,9R)-á-Ionol 2a and 2d. Ionol (6RS,9RS)-2
(5.0 g, 0.026 mol) was submitted to enzyme-mediated acetyl-
ation in tert-butyl methyl ether solution (100 cm3) in the pres-
ence of Lipase PS (5.0 g), using vinyl acetate as acylating agent
(20 cm3). After stirring of the mixture for 72 h at rt, the enzyme
was removed by filtration and the solvent was evaporated off
under reduced pressure. The residue was chromatographed with
gradient elution with hexane → hexane ethyl acetate (1:1).
The first eluted fractions gave acetate (6R,9R)-3 showing
enantiomeric purity (GC, fourth peak) (2.27 g, 37%); [α]D20 352
(c 1, CHCl3); δH 5.60–5.20 (4H, m, olefinic hydrogens and
CHOAc), 2.10–1.90 [6H, m ϩ s, C(6)H, 2 × C(3)H ϩ CH3CO2],
1.55 [3H, m, C(1)Me], 1.40 [1H, dt, J 8.0 and 13.1, C(4)H], 1.29
(3H, d, J 6.1, CH3CHOAc), 1.15 [1H, dt, J 5.2 and 13.1,
C(4)H], 0.87 [3H, s, C(5)Me] and 0.79 [3H s, C(5)Me].
Saponification with potassium hydroxide in methanol gave
ionol (6R,9R)-2 (2d) (1.43 g, 77%) showing ee 98%; [α]D20 338 (c 1,
CHCl3).
The last eluted fractions gave ionol (6S,9R)-2 (2b) (1.40 g,
17%) showing ee 98% and de 50% (GC).
The residual acetate was saponified (5.79 g, 0.024 mol) with
potassium hydroxide (1.62 g, 0.029 mol) in methanol (40 cm3)
and the corresponding α-ionol was submitted to Lipase PS
acetylation according to the usual procedure. After 72 h, the
enzyme was filtered off and the solvent was removed under
reduced pressure to give a residue, which was chromatographed
(gradient elution with hexane → hexane–ethyl acetate (1:1).
The first eluted fractions gave acetate (6R,9R)-3 (3d) (1.30 g,
23%) (ee > 99%, de 66%; GC see Fig. 1c).
(6R,9R)-á-Ionol 2d. Acetate (6R,9R)-3 (3d) (ee > 99%, de
66%) (1.30 g, 5.52 mmol) was saponified with potassium
hydroxide (0.371 g, 6.62 mmol) in refluxing methanol (15 cm3)
(85% yield) and the alcohol 2d thus obtained was converted into
the corresponding 4-nitrobenzoate ester (76%) according to the
usual procedure. Two subsequent crystallisations from hexane
(39%), and saponification (84%), allowed us to recover com-
pound (6R,9R)-2 (2d) (0.225 g, 21%) showing diastereoisomeric
purity (GC analysis of the corresponding acetate, see Fig. 1d);
[α]D20 335 (c 1, CHCl3).
The last eluted fractions gave ionol (6S,9S)-2 (2a) (1.96 g,
39%) showing ee 87% (GC of the corresponding acetate deriv-
ative, first peak); [α]D20 Ϫ293 (c 1, CHCl3). E (Enantiomeric
ratio) = 506, c (conversion) = 47%.17
(6S,9R)- and (6R,9S)-á-Ionol 2b and 2c. Ionol (6RS,9SR)-2
(2b/c) (5.0 g, 0.026 mol) was submitted to enzyme-mediated
acetylation in TBME solution (100 cm3) in the presence of
Lipase PS (5.0 g), using vinyl acetate as acylating agent (20
cm3). After stirring of the mixture for 72 h at rt, the enzyme was
removed by filtration and the solvent was evaporated off under
(R)-á-Ionone (R)-1. Ionol (6R,9R)-2 2d (0.220 g, 1.13 mmol)
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134
4133

View Article Online
9 R. B. Woodward, T. P. Kohman and G. C. Harris, J. Am. Chem. Soc.,
1941, 63, 120.
10 C. Fehr and J. Galindo, J. Am. Chem. Soc., 1988, 110, 6909.
11 H. Pfander and P. A. Semadeni, Aust. J. Chem., 1995, 48, 145.
12 E. Brenna, N. Caraccia, C. Fuganti, D. Fuganti and P. Grasselli,
Tetrahedron: Asymmetry, 1997, 8, 3801.
13 A. J. Aasen, B. Kimland and C. R. Enzell, Acta Chem. Scand., Ser. B,
1973, 27, 2107.
14 L. Palfray, S. Sabetay and J. Kandel, C.R. Hebd. Seances Acad. Sci.,
1936, 203, 1376.
15 O. Mitsunobu, Synthesis, 1981, 1.
(ee > 99%, de > 99%) was treated with manganese() oxide in
methylene dichloride solution to give (R)-α-ionone (0.171 g,
79%) (ee > 99%); [α]D20 ϩ422 (c 1, CHCl3).
References
1 F. Tiemann and F. Krueger, Ber. Dtsch. Chem. Ges., 1893, 26, 2675.
2 F. Tiemann, Ber. Dtsch. Chem. Ges., 1898, 31, 808.
3 G. Uhde and G. Ohloff, Helv. Chim. Acta, 1972, 55, 2621.
4 R. Buchecker, R. Egli, H. Regel-Wild, C. Tscharner, C. H. Eugster,
G. Uhde and G. Ohloff, Helv. Chim. Acta, 1973, 56, 2548.
5 E. H. Polak, A. M. Forobon, C. Tilquin and P. H. Punter, Behav.
Brain Res., 1989, 31, 199.
16 J. Attenburrow, A. F. B. Cameron, J. H. Chapman, R. M. Evans,
B. A. Hems, A. B. A. Jansen and T. Walker, J. Chem. Soc., 1952,
1094.
6 C. Fehr and O. Guntern, Helv. Chim. Acta, 1992, 75, 1023.
7 P. Werkhoff, W. Bretschneider, M. Guentert, R. Hopp and
H. Surburg, Flavor Science and Technology, ed. Y. Bessiere and
A. F. Thomas, J. Wiley & Sons, New York, 1990, p. 33.
8 H. Sobotka, E. Bloch, H. Cahnmann, E. Feldbau and E. Rosen,
J. Am. Chem. Soc., 1943, 65, 2061; Y. R. Naves, Helv. Chim. Acta,
1947, 30, 769.
17 C. S. Chen, Y. Fujimoto, G. Girdaukas and C. J. Sih, J. Am. Chem.
Soc., 1982, 104, 7294.
Paper 8/06943C
4134
J. Chem. Soc., Perkin Trans. 1, 1998, 4129–4134