Synthesis of both enantiomers of cis-α-irone and cis-γ-irone, principal constituents of iris oil, via resolution of (±)-2,2,4-trimethyl-3-cyclohexene-1-carboxylic acid

Article abstract of DOI:10.1016/S0957-4166(00)00345-1

The principal constituents of iris oil, (-)-cis-α-irone and (-)-cis-γ-irone, and their enantiomers, were synthesized from (-)- and (+)-2,2,4-trimethyl-3-cyclohexene-1-carboxylic acids. The racemic acid was resolved by recrystallization of its salt with a chiral amine, or by enzymatic hydrolysis of the corresponding alcohol. The fragrances of (-)-(1R,5S)-cis-α-irone and (-)-(1R,5S)-cis-γ-irone were superior to those of (+)-(1S,5R)-cis-α-irone and (+)-(1S,5R)-cis-γ-irone. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00345-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 3807–3818
Synthesis of both enantiomers of cis-a-irone and cis-g-irone,
principal constituents of iris oil, via resolution of
( )-2,2,4-trimethyl-3-cyclohexene-1-carboxylic acid
Takahiro Inoue, Hiromasa Kiyota* and Takayuki Oritani
Department of Applied Bioorganic Chemistry, Division of Life Science, Graduate School of Agricultural Science,
Tohoku University, 1-1 Tsutsumidori-Amamiya, Aoba-ku, Sendai 981-8555, Japan
Received 3 August 2000; accepted 24 August 2000
Abstract
The principal constituents of iris oil, (−)-cis-a-irone and (−)-cis-g-irone, and their enantiomers, were
synthesized from (−)- and (+)-2,2,4-trimethyl-3-cyclohexene-1-carboxylic acids. The racemic acid was
resolved by recrystallization of its salt with a chiral amine, or by enzymatic hydrolysis of the correspond-
ing alcohol. The fragrances of (−)-(1R,5S)-cis-a-irone and (−)-(1R,5S)-cis-g-irone were superior to those
of (+)-(1S,5R)-cis-a-irone and (+)-(1S,5R)-cis-g-irone. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
cis-a-Irone 1 and cis-g-irone 2 are the principal constituents of iris rhizome which is an
important perfume with its scent like violets (Fig. 1). Both enantiomers of these compounds exist
in a variety of natural iris oils.1 Although the olfactory activities of the enantiomers of chiral
cis-irones were investigated by GC–sniff analysis2 of the corresponding racemates separated with
chiral stationary phases,2,3 quite a few syntheses of single enantiomers of cis-irones have been
reported to date.4–6 However, comparison of the olfactory characteristics of the enantiomerically
pure forms have not yet been examined. For this purpose, we planned to synthesize both
enantiomers. Among the stereoselective syntheses of racemic cis-irones,7–9 we took notice of
( )-2,2,4-trimethyl-3-cyclohexene-1-methanol [( )-3],10,11 a key intermediate of Nussbaumer and
Fra´ter’s method,7 as an attractive substrate for resolution and total syntheses of our targets
(Scheme 1). Here we describe the resolution of ( )-3 and synthesis of both enantiomers of cis-a-
and cis-g-irones from homochiral 3.
* Corresponding author. Tel/fax: +81-22-717-8783; e-mail: kiyota@biochem.tohoku.ac.jp
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00345-1

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Figure 1.
Scheme 1. Synthetic plan for enantiomerically pure irones
2. Results and discussion
2.1. Asymmetric Diels–Alder reaction
Jitkow and Bogert prepared ( )-3 via a Diels–Alder reaction of 2,4-dimethyl-1,3-pentadiene 4
with acrolein 5.10 Therefore we first tried an application of this reaction under asymmetric
conditions (Scheme 2).12 Some asymmetric induction occurred in the presence of (R)-1,1%-
binaphthoxydichlorotitanium13 to give (−)-6,11,14,15 however, the enantiomeric excess of (−)-3 was
only 24% ee.
Scheme 2. Asymmetric Diels–Alder reaction
2.2. Enzymatic reaction
Next we examined the resolution of ( )-3 using hydrolytic enzymes.16 Transesterification was
carried out with vinyl acetate as an acyl donor. Table 1 summarizes the results of the enzyme

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T. Inoue et al. / Tetrahedron: Asymmetry 11 (2000) 3807–3818
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Scheme 3. Enzymatic resolution of ( )-3
screening: PPL (Sigma), lipase 2G (Nagase) and CHIRAZYME® L-9 exhibited moderate
selectivity. Table 2 also shows the results of hydrolysis of the corresponding acetate ( )-717
with pH 7 phosphate buffer. In this case, PPL (Sigma) selectively catalyzed the hydrolysis to
give (−)-(S)-3 (89% ee). To obtain the enantiomerically enriched compound, these reactions
were combined: the transesterification of ( )-3, followed by the hydrolysis of the intermediary
acetate (−)-7 (76% ee) gave (−)-(S)-3 (96% ee) (Scheme 3).
2.3. Resolution of chiral amine salt
Finally, resolution using a resolving agent was attempted. Esterification of ( )-3 with chiral
carboxylic acid, such as (S)-5-oxo-2-pyrrolidinecarboxylic acid, gave inseparable
diastereomers, and therefore the resolution as the corresponding carboxylic acid ( )-8 was
examined. The aldehyde ( )-4 was oxidized to give ( )-8.18–20 The resolution pathway is
summarized in Scheme 4. A solution of ( )-8 in 2-propanol was treated with 1 equiv. of
(−)-1-phenylethylamine [(−)-PA], and the resulting needles were recrystallized twice with 2-
propanol to give (−)-8·(−)-PA salt. This was treated with 1N HCl to give (−)-8 ([h]2D7 −108),
which was treated again with (−)-PA, recrystallized and acidified to afford enantiomerically
pure (−)-8 ([h]2D2 −140). The other diastereomer (+)-8·(−)-PA remaining in solution was aci-
dified to afford (+)-8, which was then treated with 1 equiv. of (+)-PA. The resulting needles
(+)-8·(+)-PA were recrystallized from 2-propanol and acidified to give (+)-8 ([h]2D3 +140). The
enantiomeric purity of this (+)-8 was determined by HPLC analysis (Chiralcel® OD) of the
corresponding p-bromophenacyl ester to be >99% ee. Both enantiomers were treated with
diazomethane to give 918 {(−)-9, [h]D22 −112 (c 1.00, CHCl3); (+)-9, [h]2D2 +113 (c 1.20,
CHCl3)}, followed by reduction with LiAlH4 to afford (−)-3 {[h]2D7 −45 (c 1.2, CHCl3); lit.,11
[h]2D5 −37.5 (c 1.18, CHCl3)} and (+)-3, respectively.
2.4. Synthesis of both enantiomers of cis-h-irone and cis-k-irone
The next stage was the conversion of (−)- and (+)-3 to homochiral cis-irones. The transfor-
mation to cis-a-irones was carried out according to the method reported by Nussbaumer and
Fra´ter7 (Scheme 5). Intramolecular ene reaction of the vinyl ether 10 derived from (−)-3 gave
cyclic ether (+)-11. After hydrolysis of the methyl ester, the resulting carboxylic acid (+)-12
was treated with LDA to give (−)-13. The newly formed hydroxy group was removed by
mesylation followed by reduction with zinc to afford (−)-14. Finally, the carboxy group was
converted to methyl ketone with MeLi to give (−)-cis-a-irone {(−)-1, [h]2D4 −119 (c 0.925,
CHCl3)}. Similarly, (+)-3 gave (+)-cis-a-irone {(+)-1, [h]1D9 +133 (c 1.42, CHCl3); lit.,21 [h]2D0
+109 (c 1.26, CHCl3)}.

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Scheme 4. Resolution using (−)- and (+)-1-phenylethylamines
As cis-g-irones are the exo olefin isomers of cis-a-irones, we intended to prepare the
corresponding exo isomers of 11. Attempts of cyclization of 10 using other acids, such as
BF3·OEt2 and HBr gave exclusively the endo isomers.8 Thermal conditions over 270°C8 gave the
desired exo isomer 17, however, the yield was low (<23%). Migration of the endo double bond
of (+)-11 by epoxidation followed by base-catalyzed cleavage also failed. Finally, the shift of the
double bond was achieved in the following manner (Scheme 6). Bromination of (+)-11 with
N-bromosuccinimide (NBS) gave dibromide 15, which was treated with zinc to afford diene
(+)-16. Hydrogenation of the endo double bond was carried out using Adam’s catalyst to give
(+)-17 together with (+)-11. Conversion of (+)-17 to (−)-cis-g-irone (−)-2 was achieved by the
known procedure as shown in Scheme 6.8 (+)-cis-g-Irone (+)-2 was prepared from (+)-3 in the
same manner. Enantiomeric purity of (−)-2 was determined by HPLC analysis (Chiralcel® OD)

T. Inoue et al. / Tetrahedron: Asymmetry 11 (2000) 3807–3818
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Scheme 5. Synthesis of (−)- and (+)-a-irones. (a) N-Methylmorpholine, Et2O (quant.). (b) Methanesulfonic acid
(MsOH), Et2O (67%). (c) NaOH, MeOH (quant.). (d) LDA, THF. (e) (i) MsCl, Py, CH2Cl2; (ii) Zn, NaI, DME,
reflux [66% from (+)-12]. (f) MeLi, Et2O (68%)
Scheme 6. Synthesis of (−)- and (+)-g-irones. (a) NBS, CHCl3. (b) Zn, DME, reflux [78% from (+)-11]. (c) H2, PtO2,
MeOH [31% for (+)-17; 37% for (+)-11]. (d) NaOH, MeOH (quant.). (e) LDA, THF. (f) (i) MsCl, Py, CH2Cl2; (ii)
Zn, NaI, DME, reflux [40% from (+)-18]. (g) MeLi, Et2O (36%)

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Table 3
Olfactory evaluation of both enantiomers of cis-a-irone and cis-g-irone
Compound
Odor description
(+)-(1S,5R)-cis-a-Irone [(+)-l]
(−)-(1R,5S)-cis-a-Irone [(−)-l]
(+)-(1S,5R)-cis-g-Irone [(+)-2]
(−)-(1R,5S)-cis-g-Irone [(−)-2]
Green, light; weak
Single-floral, ionone-like
Floral, green; weak
Floral, sweet, ionone-like
to be >99% ee. Since the specific rotation value of chiral cis-g-irone was small (ꢀ[h]Dꢀ<2), the
absolute configuration of (−)-2 was confirmed from the CD spectrum.22
2.5. Odors of both enantiomers of cis-h-irone and cis-k-irone
The odors of the synthetic irones were tested, and the results are shown in Table 3. In both
a- and g-irones, the fragrances of (−)-(1R,5S)-isomers were superior to those of (+)-(1S,5R)-iso-
mers. The (1R,5S)-configuration rather than the geometry of the double bond was important for
the fragrant activity. These results were similar to those reported by Brenna et al.2,5
3. Conclusion
Synthesis of both enantiomers of cis-a-irone 1 and cis-g-irone 2 in enantiomerically pure
forms (>99% ee) was achieved via resolution of ( )-2,2,4-trimethyl-3-cyclohexene-1-carboxylic
acid 5 using homochiral 1-phenylethylamine as a resolving agent. Enzyme-catalyzed resolution
of the corresponding alcohol 3, transesterification followed by hydrolysis with PPL, also gave
(−)-3 (96% ee). The fragrances of (−)-(1R,5S)-1 and (−)-(1R,5S)-2 were superior to those of
(+)-(1S,5R)-1 and (+)-(1S,5R)-2.
4. Experimental
4.1. General
Melting point values are uncorrected. Optical rotations were measured on HORIBA SEPA-
300. CD spectrum was recorded on JASCO J-720WI. IR spectra were recorded on JASCO IR
1
Report-100. H and 13C NMR spectra were recorded on Varian GEMINI 2000/300 (300 MHz
1
for H) and Varian Unity INOVA 500 (125 MHz for 13C) in CDCl3 using Me4Si as an internal
standard. Mass spectra were recorded on a Jeol JMS-700. HPLC analyses were done with a
HITACHI L-6000 pump and an L-4200H UV–vis detector. Column chromatography was
performed with Merck Silica gel 60, mesh size 0.063–0.200 mm. Preparative TLC plates (0.75
mm thickness) were made of Merck silica gel 60 PF254
.

T. Inoue et al. / Tetrahedron: Asymmetry 11 (2000) 3807–3818
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4.2. (−)-(S)-2,2,4-Trimethyl-3-cyclohexene-1-carbaldehyde (−)-6: asymmetric Diels–Alder reac-
tion
To a suspension of powdered MS4A (2.0 g) in dry toluene (10 ml) were added (+)-(R)-1,1%-bi-
2-naphthol (100 mg, 0.35 mmol) and TiCl2(Oi-Pr)2 in toluene (0.88 M, 0.4 ml, 0.35 mmol). The
resulting mixture was stirred at room temperature under N2 for 1 h and then cooled to −70°C.
Then acrolein (4, 0.5 ml, 7.5 mmol) followed by a solution of 2,4-dimethyl-1,3-pentadiene (5,
350 mg, 3.7 mmol) in dry toluene (1 ml) was added dropwise to the mixture. The resulting
mixture was stirred at −70°C for 20 min and then warmed up to 5°C during 4 h. The reaction
mixture was poured into aq. 5% NaHCO3 soln (100 ml) and then filtered through a Celite pad.
The filtrate was extracted with ether (50 ml×3) and washed with brine. The organic extract was
dried over MgSO4 and concentrated in vacuo. The residue was chromatographed on silica gel
(hexane:ethyl acetate=10/1) to afford (−)-6 (297 mg, 1.95 mmol, 53%) as a colorless oil. Spectra
of this compound were identical with those reported,11,14,15 [h]D24 −16 (c 1.6, CHCl3). This
aldehyde 6 was reduced with LiAlH4 (82 mg) in ether to give alcohol (−)-3. The enantiomeric
excess was determined to be 24% ee from comparison of the specific rotation value {[h]2D3 −11 (c
1.5, CHCl3)} with that of an enantiomerically pure sample.
4.3. General procedure of enzymatic transesterification of ( )-3
A suspension of ( )-3 (200 mg, 1.3 mmol), vinyl acetate (3.7 ml), diisopropyl ether (25 ml),
hexane (25 ml)) and enzyme (20–50 mg) was stirred vigorously. The reaction was monitored by
TLC. The suspension was filtered through a Celite pad and the filtrate was concentrated in
vacuo. Purification by silica gel column chromatography (hexane:ethyl acetate=10/1–5/1)
afforded acetate 7 and recovered 3.
4.4. General procedure for enzymatic hydrolysis of ( )-7
A suspension of ( )-7 (0.2 g, 1.0 mmol) and enzyme (20–50 mg) in toluene (0.5 ml) and 0.1
M phosphate buffer (pH 7.0, 4 ml) was stirred vigorously. The reaction was monitored by TLC.
The suspension was filtered through a Celite pad. The filtrate was extracted with ether (20 ml×3)
and washed with brine, dried over MgSO4 and concentrated in vacuo. Purification by silica gel
column chromatography (hexane:ethyl acetate=10/1–5/1) afforded 3 and recovered 7.
4.5. Resolution of ( )-8 with 1-phenylethylamine
Both ( )-8 (12 g, 71 mmol) and (S)-(−)-1-phenylethylamine [(−)-PA, 8.6 g, 71 mmol] were
dissolved in 2-propanol (100 ml) and the resulting precipitates were recrystallized three times
with 2-propanol to afford (−)-8·(−)-PA. This salt was dissolved in ether and washed with 1N
HCl three times. The organic layer was washed with brine, dried over MgSO4, and concentrated
in vacuo to give (−)-8 as white plates, [h]D27 −108 (c 1.70, CHCl3). The enantiomeric purity was
75% ee by HPLC analysis of the corresponding p-bromophenacyl ester (Chiralcel® OD, 4.6×250
mm; hexane:2-propanol=200/1, 0.5 ml/min; 254 nm; tR=59, 63 min). These plates were purified
by repeating the same procedure as described above to give enantiomerically pure (−)-8 (1.5 g,
8.8 mmol, 12%, >99% ee), [h]2D2 −140 (c 1.60, CHCl3).

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The enantiomer (+)-8 was obtained from the mother liquor, and purified with the similar
manner as for (−)-8 using (+)-PA. (+)-8 (2.0 g, 12 mmol, 17%, >99% ee); mp 58.9–59.9°C, [h]D23
+140 (c 1.60, CHCl3) {lit.,20 [h]D +52.8 (c 12.5, EtOH), ee unknown}.
4.6. (−)-(S)-2,2,4-Trimethyl-3-cyclohexene-1-methanol (−)-3
To a suspension of LiAlH4 (350 mg, 9.1 mmol) in dry ether (30 ml) was added dropwise (−)-9
(2.2 g, 12 mmol) in dry ether (12 ml) at 0°C. The resulting mixture was stirred under N2 at room
temperature for 20 min and quenched with EtOH, water and 1N HCl. The mixture was
extracted with ether (50 ml×3), washed with brine, dried over MgSO4, filtered and concentrated
in vacuo. Purification by silica gel column chromatography (hexane:ethyl acetate=2/1) afforded
(−)-3 (1.90 g (12 mmol, quant.) as a colorless oil, [h]2D7 −45 (c 1.2, CHCl3) {lit.,11 [h]D25 −37.5 (c
1.18, CHCl3)}. The spectral data were identical with those reported.11
For (+)-3, [h]2D4 +46 (c 1.6, CHCl3).
4.7. Methyl (1S,2R,5S)-(9,9-dimethyl-8-methylene-3-oxabicyclo[3.3.1]non-6-en-2-yl)acetate:
(+)-16
To a solution of (+)-11 (394 mg, 1.65 mmol) in CHCl3 (20 ml) was added N-bromosuccin-
imide (619 mg, 3.48 mmol) and stirred at room temperature for 12 h. To the mixture was added
additional N-bromosuccinimide (350 mg, 1.97 mmol) and stirred at room temperature for 5 h.
The resulting mixture was poured into water and extracted with ether (50 ml×3). The organic
layer was washed with brine and dried over MgSO4. Then to this was added a small amount of
cyclohexene to remove bromine. The resulting suspension was then filtered and concentrated in
vacuo to give 15. To a solution of this crude dibromide 15 in 1,2-dimethoxyethane (DME, 50
ml) was added zinc powder (7.9 g, 12 mmol) and the mixture was refluxed for 2 h. After being
cooled to room temperature, the suspension was filtered through a Celite pad, and the remaining
solid was washed with ether and water. The filtrate was extracted with ether (50 ml×3). The
organic layer was washed with brine, dried over MgSO4 and concentrated in vacuo. Purification
by silica gel column chromatography (hexane:ethyl acetate=20/1–10/1) and preparative TLC
(hexane:ethyl acetate=5/1) afforded (+)-16 [305 mg, 1.29 mmol, 78% from (+)-11] as a colorless
oil; [h]2D3 +94 (c 1.5, CHCl3). IR (film) wmax: 3070, 3020, 2880, 1730, 1630, 1590, 1170, 1090, 1000,
1
890 cm−1. H NMR (300 MHz) l: 0.95 (s, 3H), 1.26 (s, 3H), 1.8–1.9 (2H), 2.31 (dd, 1H, J=5.4,
15.8 Hz), 2.46 (dd, 1H, J=8.0, 15.9 Hz), 3.40 (dd, 1H, J=1.4, 11.5 Hz), 3.68 (s, 3H), 4.07 (dd,
1H, J=1.8, 11.4 Hz), 4.39 (ddd, 1H, J=2.0, 5.6, 7.7 Hz), 4.69 (s, 1H), 5.09 (d, 1H, J=1.9 Hz),
5.74 (dd, 1H, J=6.7, 9.6 Hz), 6.34 (d, 1H, J=9.6 Hz). 13C NMR (125 MHz) l: 24.2, 28.4, 32.6,
39.2, 41.5, 50.7, 51.6, 64.6, 70.6, 115.6, 129.3, 131.7, 142.6, 172.2. HREIMS m/z (M+): calcd for
C14H20O3: 236.1412; found: 236.1414.
4.8. Methyl (1S,2R,5S)-(9,9-dimethyl-8-methylene-3-oxabicyclo[3.3.1]non-2-yl)acetate: (+)-17
A suspension of (+)-16 (248 mg, 1.05 mmol) and PtO2·(H2O)1–3 (0.04 g) in MeOH (20 ml) was
stirred under H2 at 27°C until 36 ml of H2 (1.5 mmol) was absorbed. The resulting suspension
was filtered through a Celite pad and the filtrate was concentrated in vacuo. Purification by
silica gel column chromatography (hexane:ethyl acetate=20/1) and preparative TLC (hex-
ane:ethyl acetate=5/1) afforded (+)-17 {78 mg, 0.33 mmol, 31%; [h]2D4 +62 (c 1.6, CHCl3)} and

T. Inoue et al. / Tetrahedron: Asymmetry 11 (2000) 3807–3818
3817
(+)-11 (98 mg, 0.39 mmol, 37%) as colorless oils. The spectral data were identical with those
reported.8
4.9. HPLC analysis of cis-k-irones
Column, Daicel Chiralcel® OD 4.6×250 mm; solvent, hexane:2-propanol=500/1; flow rate,
1.0 ml/min; detect, 254 nm; tR=8.4 [(+)-2] and 10.0 [(−)-2] min.
4.10. CD spectrum of (+)-cis-k-irone (+)-2
umax (Dm) 329 (+0.08), 224 (+2.3), 210 (+1.9); 2.3 mmol/l in CH3CN {lit.22 umax (Dm) 372
(+0.05), 354 (+0.15), 340 (+0.22), 326 (+0.21), 315 (+0.15), 223 (+3.86); 5.2 mmol/l in
CH3CN}.
4.11. Optical rotation values of other compounds reported as racemates7,8
Compound (−)-10, [h]2D4 −18 (c 1.8, CHCl3); (+)-11, [h]2D3 +31 (c 1.8, CHCl3); (−)-14, [h]2D4
−114 (c 3.28, CHCl3); (−)-20, [h]2D0 −10 (c 1.4, CHCl3).
Acknowledgements
We thank Nagase & Co., Ltd, Roche Diagnostics K. K., Meito Sangyo Co., Ltd, Toyobo
Co., Ltd and Amano Pharmaceutical Co., Ltd for the gifts of various lipases. We also thank
Dr. Akira Tanaka and the late Takeyoshi Sugiyama for helpful discussion, and Mrs. Teiko
Yamada and Mrs. Yuhko Sugiyama for technical assistance.
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