Enzymatic desymmetrization of meso cis-2,6- and cis,cis-2,4,6-substituted piperidines. Chemoenzymatic synthesis of (5S,9S)-(+)-indolizidine 209D

Article abstract of DOI:10.1016/S0957-4166(99)00315-8

The stereoselective acylation of meso piperidines 3a,b by vinyl acetate (solvent and acyl donor) in the presence of Candida antarctica lipase gave the corresponding (2S,6R) and (2S,4R,6R) monoesters 2a,b in high enantiomeric purity. (5S,9S)-(+)-Indolizidine 209D was prepared in eight steps from (2S,6R)-2a.

Full text of DOI:10.1016/S0957-4166(99)00315-8

Pergamon
Tetrahedron: Asymmetry 10 (1999) 3117–3122
Enzymatic desymmetrization of meso cis-2,6- and
cis,cis-2,4,6-substituted piperidines. Chemoenzymatic synthesis
of (5S,9S)-(+)-indolizidine 209D
Robert Chênevert,∗ Ghodsi Mohammadi Ziarani, Marie Pascale Morin and Mohammed Dasser
Département de chimie, Faculté des sciences et de génie, Université Laval, Québec (QC), Canada, G1K 7P4
Received 8 June 1999; accepted 20 July 1999
Abstract
The stereoselective acylation of meso piperidines 3a,b by vinyl acetate (solvent and acyl donor) in the presence
of Candida antarctica lipase gave the corresponding (2S,6R) and (2S,4R,6R) monoesters 2a,b in high enantiomeric
purity. (5S,9S)-(+)-Indolizidine 209D was prepared in eight steps from (2S,6R)-2a. © 1999 Elsevier Science Ltd.
All rights reserved.
The piperidine ring is a widespread structural fragment of biologically active natural products. In
particular, the 2,6-disubstituted piperidine ring has been found in the skin secretion of neotropical frogs
of the family Dendrobatidae, and in some species of pines and fire ants. Also, the piperidine ring is a
common structural feature of synthetic pharmaceutical compounds. The interest in piperidine derivatives
is well displayed by the wealth of published material detailing their sources, synthesis, and biological
activities.1–8
Recently, we reported the desymmetrization of meso cis-2,6- and cis,cis-2,4,6-substituted piperidines
via enzymatic hydrolysis of diacetates 1a,b in phosphate buffer in the presence of Aspergillus niger
lipase9,10 (Scheme 1). These hydrolyses gave good to excellent results in terms of both chemical yield
and enantioselectivity but the reactions were very slow (several days). Furthermore, the addition of
7% acetonitrile to the reaction medium was necessary to suppress the growth of bacillus (probably
a spore contaminant in the commercial enzyme). We report here a novel, more convenient enzymatic
desymmetrization of these piperidine systems.
Known diols 3a,b were subjected to enzyme-catalyzed transesterification by treatment with Candida
antarctica lipase in vinyl acetate (solvent and acyl donor) to give optically active esters 2a,b (Table 1).
In general, the transesterification of meso diols and the hydrolysis of the corresponding meso diesters are
complementary and give the opposite enantiomers. As expected, monoesters (2S,6R)-2a and (2S,4R,6R)-
2b were obtained in good yields and high enantioselectivity.
∗
Corresponding author. E-mail: robert.chenevert@chm.ulaval.ca
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00315-8

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Scheme 1.
Table 1
Enzymatic acylation of diols by Candida antarctica lipase at rt
Biocatalytically generated compounds are valuable starting materials for the enantioselective synthesis
of natural products11 or pharmaceuticals.12 We used monoester 2a as a building block in the synthesis
of (5S,9S)-(+)-indolizidine 209D. This alkaloid has been isolated in minute quantities from a single
population of dendrobatid frogs of Central America and its absolute stereochemistry has been tentatively
assigned as (5R,9R).13 Enantioselective syntheses of all the four possible stereoisomers of indolizi-
dine 209D have been reported.14–23 These compounds are potent blockers for nicotinic acetylcholine
receptors.14,24
(5S,9S)-(+)-Indolizidine 209D (9) was prepared in 8 steps starting with (2S,6R)-2a (Scheme 2). Swern
oxidation of 2a gave the corresponding unstable aldehyde, which, after work up was immediately
added to the pentyl Wittig ylide to give the alkene 4 in a mixture of cis–trans isomers. The acetate
group was hydrolyzed in phosphate buffer in the presence of pig liver esterase to afford alcohol 5. A
second Swern oxidation of 5 gave the corresponding unstable aldehyde which was added to methyl-
(triphenylphosphoranylidene) acetate, and diene 6 was obtained as the sole trans isomer on the newly
formed double bond. The next step was accomplished by hydrogenation of 6 with Pearlman’s catalyst.
This reaction removed the N-Cbz protecting group as well as hydrogenating both carbon–carbon double
bonds to afford amino ester 7. Crude 7 was immediately treated with trimethylaluminum to give lactam

R. Chênevert et al. / Tetrahedron: Asymmetry 10 (1999) 3117–3122
3119
8. The final synthetic step was the reduction of lactam 8 with LiAlH4 to give (5S,9S)-(+)-indolizidine
209D (9); [α]25 +89.7 (c 0.36, CH2Cl2); lit.14: [α]25 +81.8 (c 0.540, CH2Cl2).
D
D
Scheme 2. Reagents: (a) i; Swern, ii; Ph3P(C5H11)Br, t-BuOK, C6H6. (b) PLE, phosphate buffer (pH=7). (c) i; Swern, ii;
Ph3PCHCO2Me, C6H6; (d) H2/Pd, EtOH. (e) Me3Al, C6H6. (f) LiAlH4, THF
1. Experimental
NMR spectra were recorded in CDCl3 solutions at 300 MHz (1H), 282 MHz (19F), 75 MHz (13C)
on a Bruker AC-300 instrument. Infrared spectra were recorded using a Bomem MB-100 (Fourier-
transform) spectrometer. Optical rotation values were obtained from a JASCO DIP-300 polarimeter.
Column purifications were conducted by flash chromatography on silica gel 60 (230–400 mesh). Pig
liver esterase was from Sigma or Amano. Candida antarctica lipase was purchased from Boehringer
Mannheim.
1.1. General procedure for enzymatic esterification of meso diol 3a and 3b
To a solution of diol 3a or 3b (0.19 mmol) in vinyl acetate (3 mL) was added 60 mg of Candida
antarctica lipase, and the mixture was stirred at rt. The progress of the reaction was monitored by TLC.
As the reaction proceeded, the amount of diacetate in the reaction mixture increased before the complete
disappearance of the starting material. The reaction was stopped when the trace of diacetate became as
visible as the trace of the starting diol. The mixture was filtered and concentrated. The crude product
was purified by flash chromatography (20% EtOAc and 80% petroleum ether to pure EtOAc for 3a, 50%
EtOAc and 50% petroleum ether for 3b) to give monoacetate 2a and 2b as colorless oils.
1.2. N-Benzyloxycarbonyl-cis-2(S)-(acetoxymethyl)-6(R)-(hydroxymethyl)piperidine 2a
Yield: 80%; [α]25 −5.0 (c 2.08, CHCl3); IR (neat) 3450, 2930, 1745, 1690 cm−1; 1H NMR (CDCl3)
D
7.35–7.25 (m, 5H), 5.12 (AB system, J=12.4 Hz, 2H), 4.48 (m, 1H), 4.31 (m, 1H), 4.13 (dd, J1=8.0 Hz,
J2=10.9 Hz, 1H), 3.95 (dd, J1=6.9 Hz, J2=10.9 Hz, 1H), 3.56 (d, J=7.6 Hz, 2H), 2.85 (br s, 1H), 1.92

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R. Chênevert et al. / Tetrahedron: Asymmetry 10 (1999) 3117–3122
(s, 3H), 1.81–1.45 (m, 6H); 13C NMR (CDCl3) 170.60, 156.79, 136.43, 128.36, 127.89, 127.72, 67.28,
64.38, 64.02, 51.71, 48.45, 24.84, 24.32, 20.50, 14.47.
1.3. N-Benzyloxycarbonyl-cis,cis-2(S)-(acetoxymethyl)-4(R)-[(methyloxy)methoxy]-6(R)-
(hydroxymethyl)piperidine 2b
Yield: 83%; [α]25 +3.6 (c 1.08, CHCl3); IR (neat) 3450, 3035, 2920, 1740, 1690, 1410 cm−1; H
1
D
NMR (CDCl3) 7.26–7.19 (m, 5H), 5.05 (AB system, J=12.5 Hz, 2H), 4.52 (m, 3H), 4.30 (m, 1H), 4.18
(AB system, J=11.9 Hz, 2H), 3.82 (m, 1H), 3.71 (m, 2H), 3.24 (s, 3H), 3.06 (br s, 1H), 1.80 (m, 4H),
1.81 (s, 3H); 13C NMR (CDCl3) 170.6, 156.5, 136.3, 128.3, 127.8, 127.7, 94.6, 68.3, 67.2, 66.4, 65.1,
55.2, 51.1, 47.8, 29.3, 28.3, 20.5.
1.4. N-Benzyloxycarbonyl-cis-2(R)-(1-hexenyl)-6(S)-(acetoxymethyl)piperidine 4
To a stirred solution of oxalyl chloride (0.13 mL, 1.57 mmol) in 2.5 mL of anhydrous CH2Cl2 at −78°C
under N2, was added DMSO (0.167 mL, 2.36 mmol) in 0.78 mL of anhydrous CH2Cl2 dropwise and the
mixture allowed to react for 5 min at −78°C. Alcohol 2a (253.5 mg, 0.79 mmol) in 0.78 mL of anhydrous
CH2Cl2 was added, and the reaction mixture was stirred for 1 h at −78°C. On addition of anhydrous Et3N
(0.44 mL, 3.15 mmol), the dry ice/acetone bath was removed, and the reaction temperature was left to
go to rt. The reaction was diluted with 4 mL of CH2Cl2 and then poured into 10 mL of CH2Cl2 and 5
mL of 10% NH4OH solution. The aqueous phase was extracted twice with CH2Cl2 and the combined
CH2Cl2 fractions were dried and evaporated. The residue was dissolved in ether and filtered through a
MgSO4 pad and the ether evaporated. The crude aldehyde was used immediately in the next step (Wittig
reaction) of the synthesis.
Anhydrous benzene (6 mL) and t-BuOK (265 mg, 2.36 mmol) were mixed at rt under N2, and Ph3P
(C5H11) (Br) (971 mg, 2.35 mmol) was added to the mixture. After the mixture was stirred for 1.5 h,
the crude aldehyde (0.79 mmol, assuming 100% yield from Swern oxidation) was added with 3 mL
of anhydrous benzene to the reaction mixture, and it was refluxed for 3 h. The reaction mixture was
then partitioned between 20 mL of EtOAc and 10 mL of 10% Na2S2O3 solution, and the aqueous
phase was extracted 3 times with EtOAc. The organic fractions were dried and evaporated. The crude
product was purified by flash chromatography using pure CH2Cl2 to 5% EtOAc/95% CH2Cl2 to yield
compound 4 as a colorless oil (280 mg, 95% yield). Data for cis-4 obtained pure from fractions of the
chromatography: [α]25 −69.6 (c 0.86, CHCl3); IR (neat) 2953, 1745, 1697 cm−1; H NMR (CDCl3)
1
D
7.34–7.28 (m, 5H), 5.53–5.35 (m, 2H), 5.16–5.06 (m, 3H), 4.50–4.48 (m, 1H), 4.29–4.22 (dd, J1=8.8 Hz,
J2=10.7 Hz, 1H), 4.00–3.95 (dd, J1=6.1 Hz, J2=10.7 Hz, 1H), 2.08–1.95 (m, 2H), 1.97 (s, 3H), 1.75–1.49
(m, 6H), 1.24–1.16 (m, 4H), 0.86–0.82 (t, J=6.9 Hz, 3H); 13C NMR (CDCl3) 170.66, 155.65, 136.66,
132.37, 129.39, 128.26, 127.76, 67.06, 64.28, 48.75, 47.42, 31.62, 30.18, 26.97, 24.92, 22.20, 20.67,
14.58, 13.82; HRMS (CI, NH3) calcd for C22H32NO4 (MH+) 374.2331, found 374.2318ꢀ0.0011.
1.5. N-Benzyloxycarbonyl-cis-2(R)-(1-hexenyl)-6(S)-(hydroxymethyl)piperidine 5
The ester 4 (42 mg, 0.12 mmol) was suspended in phosphate buffer (pH 7.0, 3.5 mL), pig liver
esterase (40 mg) was added, and the mixture was stirred at rt. The pH of the solution was maintained
at its initial value by addition of 0.1 M aqueous NaOH. After the addition of 1 equiv. of base, the
aqueous layer was extracted 3 times with ethyl acetate. The combined organic fractions were dried and
evaporated. The crude product was purified by flash chromatography (pure CH2Cl2 to 20% EtOAc and

R. Chênevert et al. / Tetrahedron: Asymmetry 10 (1999) 3117–3122
3121
80% CH2Cl2) to give 5 (35 mg, 88%) as a colorless oil. Data for the cis-5 obtained pure from fractions
of the chromatography: [α]25 −65.3 (c 1.61, CHCl3); IR (neat) 3439, 2932, 1691, 1671 cm−1; 1H NMR
D
(CDCl3) 7.34–7.28 (m, 5H), 5.54 (dd, J=9.5 Hz, J=10.7 Hz, 1H), 5.43–5.34 (m, 1H), 5.16–5.05 (m, 3H),
4.38–4.34 (m, 1H), 3.61–3.37 (m, 2H), 2.20 (s, 1H), 2.16–1.99 (m, 2H), 1.79–1.46 (m, 6H), 1.24–1.22
(m, 4H), 0.85–0.80 (t, J=6.7 Hz, 3H); 13C NMR (CDCl3) 156.77, 136.53, 132.14. 129.72, 128.31, 127.82,
127.79, 67.25, 64.47, 52.10, 47.64, 31.62, 30.26, 26.92, 24.65, 22.21, 14.84, 13.82; HRMS (CI, NH3)
calcd for C20H30NO3 (MH+) 332.2226, found 332.2221ꢀ0.0010.
1.6. N-Benzyloxycarbonyl-cis-2(S)-(trans-methoxycarbonylethylene)-6(R)-(1-hexenyl)piperidine 6
The Swern oxidation of 5 (262 mg, 0.79 mmol) was performed in the same manner as described above.
Methyl(triphenylphosphoranylidene) acetate (792 mg, 2.37 mmol) was added to a solution of the crude
aldehyde (0.79 mmol, assuming 100% yield from the Swern oxidation) in anhydrous benzene (8 mL),
and the mixture was refluxed for 3 h. After cooling to rt, the mixture was partitioned between 20 mL
CH2Cl3 and 10 mL of 10% Na2S2O3. The aqueous layer was extracted with CH2Cl2 and the combined
organic fractions were dried and evaporated. The crude product was purified by flash chromatography
(pure CH2Cl2 to 5% EtOAc and 95% CH2Cl2) to give 6 (280 mg, 92%) as a colorless oil. [α]25 −146.31
D
(c 0.526, CHCl3); IR (neat) 2953, 1723, 1694, 1645, 1396, 1309 cm−1; 1H NMR (CDCl3) 7.26–7.23 (m,
5H), 6.98–6.91 (dd, J1=5.2 Hz, J2=16.0 Hz, 1H), 5.86–5.80 (dd, J1=15.9 Hz, J2=1.7 Hz, 1H), 5.51–5.44
(m, 1H), 5.34–5.25 (m, 1H), 5.10–5.01 (m, 3H), 4.99–4.90 (m, 1H), 3.66 (s, 3H), 2.17–1.84 (m, 2H),
1.70–1.38 (m, 6H), 1.18 (m, 4H), 0.79–0.75 (t, J=6.9 Hz, 3H); 13C NMR (CDCl3) 166.69, 155.43,
149.41, 136.45, 131.78, 129.04, 128.31, 127.92, 127.87, 121.01, 67.29, 51.42, 50.75, 47.89, 31.60,
30.34, 27.53, 26.82, 22.19, 14.94, 13.80; HRMS (CI, NH3) calcd for C23H32NO4 (MH+) 386.2331, found
386.2320ꢀ0.0011.
1.7. (5S,9S)-5-Hexylindolizidin-3-one 8
Palladium hydroxide (20 mg) was added to a solution of 6 (157 mg, 0.407 mmol) in absolute EtOH (10
mL) and the mixture was stirred under a hydrogen atmosphere (40 psi) for 15 h. The insoluble materials
were removed by filtration on Celite, and the filtrate was concentrated under reduced pressure to give the
crude amino ester 7. To a solution of amino ester 7 (87 mg, 0.407 mmol) in CH2Cl2 (4 mL) was added
dropwise a solution of trimethylaluminum in toluene (2.0 M, 0.245 mL, 0.49 mmol). The solution was
stirred for 2 h at rt and then refluxed overnight. The mixture was cooled to rt and then quenched with 1%
HCl. The aqueous phase was extracted 3 times with CH2Cl2. The combined organic fractions were dried
and concentrated under reduced pressure. The crude product was purified by flash chromatography (25%
EtOAc and 75% hexane to pure EtOAc) to give lactam 8 as an oil (79 mg, 87%). [α]25 +20.8 (c 0.678,
D
CHCl3); lit.17: [α]D −19.8 (c 1.15, CDCl3) for the (R,R) enantiomer; IR (neat) 2928, 2855, 1692, 1422
cm−1; 1H NMR (CDCl3) 3.40–3.31 (m, 1H), 3.18–3.09 (m, 1H), 2.39–2.21 (m, 3H), 2.12–2.01 (m, 1H),
1.86–1.60 (m, 4H), 1.55–1.18 (m, 12H), 0.84 (t, J=6.5 Hz, 3H); 13C NMR (CDCl3) 174.19, 59.53, 57.47,
32.30, 31.81, 31.70, 29.38, 29.21, 26.78, 24.98, 22.57, 22.51, 13.92.
1.8. (5S,9S)-(+)-Indolizidine 209D 9
To a solution of 8 (80 mg, 0.36 mmol) in ether (4 mL) was added LiAlH4 (27 mg, 0.71 mmol), and
the mixture was heated at reflux for 3 h. After cooling to rt, water (0.03 mL), 20% NaOH (0.02 mL)
and water (0.1 mL) were added, and the mixture was dried (Na2SO4) and filtered on Celite. The organic

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R. Chênevert et al. / Tetrahedron: Asymmetry 10 (1999) 3117–3122
phase was evaporated and the crude product was purified by chromatography on neutral alumina (pure
hexane to 10% ether and 90% hexane) to give (5S,9S)-(+)-indolizidine 209D (9) as a volatile oil (64 mg,
85%). [α]25 +89.7 (c 0.36, CH2Cl2); lit.14: [α]25 +81.8 (c 0.540, CH2Cl2); IR (neat) 2927, 2857, 2780,
D
D
1457, 1380 cm−1; 1H NMR (CDCl3) 3.27 (m, 1H), 2.0–1.2 (m, 23H), 0.84 (t, J=6.5 Hz, 3H); 13C NMR
(CDCl3) 65.22, 63.97, 51.29, 34.16, 31.67, 31.10, 30.18, 30.60, 29.50, 25.68, 24.40, 22.47, 20.17, 13.93.
Acknowledgements
We acknowledge the financial support of this work by the Natural Sciences and Engineering Research
Council of Canada (NSERC). G.M.Z. is grateful to the Ministry of Culture and Higher Education of Iran
and Al-Zahra University for a fellowship. M.P.M. would like to thank NSERC and ‘le Fonds pour les
chercheurs et l’aide à la recherche (FCAR), Québec’, for postgraduate scholarship.
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