Chemoenzymatic synthesis of α′- And α-acetoxylated cyclic ketones

Article abstract of DOI:10.1016/j.tetasy.2004.04.019

α,β-Unsaturated and saturated cyclic ketones were selectively oxidized at the α^′- and α-positions using Mn(OAc)₃ and Pb(OAc)₄, respectively, resulting in high chemical yields. The resultant racemic α^′- and α-acetoxylated substrates were resolved into corresponding enantiomerically enriched α^′- and α-hydroxylated and acetoxylated compounds with 96-98% ee via PLE hydrolysis. The absolute configurations of α^′-acetoxy-α,β-unsaturated cyclic ketones were determined by transforming them into the corresponding saturated α-acetoxy cyclic ketones of known absolute configuration.

Full text of DOI:10.1016/j.tetasy.2004.04.019

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1729–1733
Chemoenzymatic synthesis of a0- and a-acetoxylated cyclic ketones
*
_
Cihangir Tanyeli, Engin Turkut and Idris Mecidoglu Akhmedov
ꢀ
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 17 March 2004; accepted 16 April 2004
Abstract—a,b-Unsaturated and saturated cyclic ketones were selectively oxidized at the a0- and a-positions using Mn(OAc)3 and
Pb(OAc)4, respectively, resulting in high chemical yields. The resultant racemic a0- and a-acetoxylated substrates were resolved into
corresponding enantiomerically enriched a0- and a-hydroxylated and acetoxylated compounds with 96–98% ee via PLE hydrolysis.
The absolute configurations of a0-acetoxy-a,b-unsaturated cyclic ketones were determined by transforming them into the corre-
sponding saturated a-acetoxy cyclic ketones of known absolute configuration.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
consists of the asymmetric oxidation of enolates.13–15
Thus, by using enantiomerically pure N-sulfonyloxazir-
idines, Davis et al.13 were able to achieve good to
excellent enantioselectivities in reagent controlled
asymmetric oxidations of prochiral enolates. On the
other hand, in the chiral auxiliary approach, the dia-
stereoselective oxidation of chiral enolates has been
performed using oxidants such as achiral sulfonyloxaz-
iridines, dibenzyl peroxydicarbonate and Vedejs’
MoOPH reagent.14 Along these lines, an important
addition was devised by Sharpless et al.;15 indeed, these
authors have shown that a-hydroxy ketones in high
enantiomeric excess can be obtained by the well-estab-
lished osmium-catalyzed asymmetric dihydroxylation
(AD)16 of the corresponding enol ethers or silyl enol
ethers.15
Synthetic methods for the selective oxidation of com-
mon functional groups occupy a central position in the
syntheses of various complex natural products. In par-
ticular, the utilities of ( )-2-hydroxy and ( )-2-acetoxy
cyclic ketones are appealing synthons1 and several syn-
theses are already known.2 Furthermore, a0-acetoxy-a,b-
unsaturated cyclic ketones are also important in syn-
thetic methodologies. The acetate group can serve as a
useful protective group for the hydroxy functions in the
a-hydroxy ketones.3;4 The a-acetoxy ketones can be
prepared in various ways, which include the reaction of
a-bromo ketones with carboxylate ions,5 the oxidation
of morpholine enamine with thallium(III) triacetate,6
anodic oxidation of enol acetates in acetic acid,7
Cu(acac)2 catalyzed insertion reactions of a-diazo
ketones with carboxylic acids2e and the solvolytic reaction
of a-keto triflate in acetic acid or formic acid.8 Currently
there are only a few methods that deal with the direct
preparation of a-acetoxy ketones. These involve the
oxidation of ketones with lead(IV) tetraacetate,9 the
oxidation of ketones with manganese(III) triacetate in
acetic acid10 and the oxidation of aromatic ketones with
a hypervalent iodine reagent followed by solvolysis in
acetic acid in the presence of silver carbonate.11
The lack of known syntheses of enantiomerically
enriched a0-acetoxy-a,b-unsaturated cyclic ketones
prompted us towards the development of a new method.
In connection with our work on the development of
novel procedures for the direct oxidation of a,b-unsat-
17
urated and saturated cyclic ketones with Mn(OAc)3
and Pb(OAc)4, respectively, in the synthesis of ( )-5-
acetoxy-2-cyclopentenone 1a, ( )-6-acetoxy-2-cyclohex-
enone 1b and ( )-2-acetoxycyclopentanone 3a, ( )-2-
acetoxycyclohexanone 3b, we herein report the enzy-
matic resolution of them into enantiomerically enriched
forms. Over the course of our study on all biotransfor-
mations, screening reactions were first completed with
various hydrolases (i.e., PLE, CCL, HLE and PPL)
using a substrate:enzyme ratio from 1:1 to 1:0.5. Among
the hydrolases studied, PLE proved suitable for the
enantioselective hydrolysis of the substrates. The
In addition to this, several studies have also been
directed to the stereoselective synthesis of enantiomeri-
cally pure a-hydroxy ketones:12 One direct method
* Corresponding author. Tel.: +90-312-210-32-22; fax: +90-312-210-
12-80; e-mail: tanyeli@metu.edu.tr
0957-4166/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.04.019

1730
C. Tanyeli et al. / Tetrahedron: Asymmetry 15 (2004) 1729–1733
(pH 7.00, 50 mL), PLE (100 lL) was added in one por-
tion and the reaction mixture stirred at 20 °C in a pH
stat unit. The conversion was monitored by TLC. After
5 h, (+)-5-acetoxy-2-cyclopentenone 1a was obtained
with 96% ee in 45% chemical yield (Table 1, entry 1).
The next attempt involved substrate 1b using PLE under
the same conditions as above. ())-6-Acetoxy-2-cyclo-
hexenone 1b was obtained with 97% ee in 46% isolated
yield (entry 2). 5-Hydroxy-2-cyclopentenone 2a and
6-hydroxy-2-cyclohexenone 2b were not isolated in
enantiomerically enriched forms. This is presumably
due to the fast racemization and/or rearrangement of
a0-hydroxylated cyclic enones as observed in our
previous work.18
O
O
O
O
Mn(OAc)3
OAc
OH
OAc
PLE
PLE
+
benzene
reflux
( )n
( )n
( )n
( )n
(S)-1a-b
rac-1a-b
2a-b
a: n=1
b: n=2
O
O
O
O
Pb(OAc)4
OAc
OH
OAc
+
cyclohexane
reflux
( )n
( )n
( )n
(R)-4a-b
( )n
rac-3a-b
(S)-3a-b
Scheme 1.
observed preliminary results proved promising and
prompted us to undertake a thorough catalytic study.
PLE-catalyzed reactions of acetoxylated substrates
( )-1a and b and ( )-3a and b afforded hydroxylated
(R)-())-4a, (R)-(+)-4b and acetoxylated (S)-(+)-1a,
(S)-())-1b, (S)-(+)-3a, (S)-())-3b (Scheme 1).
The bioconversion of ( )-2-acetoxycyclopentanone 3a
afforded (+)-2-acetoxycyclopentanone 3a and ())-2-
hydroxycyclopentanone 4a with 98% and 99% ee in 47%
and 44% isolated yields, respectively (entry 3). In the last
example, we attempted to hydrolyze ( )-2-acetoxycy-
clohexanone 3b using the same enzyme, which afforded
())-2-acetoxycyclohexanone 3b with 96% ee and (+)-2-
hydroxycyclohexanone 4b with 99% ee (entry 4).
2. Results and discussion
2.1. Synthesis of racemic substrates 1a and b and 3a and b
2.3. Absolute configuration determinations
Racemic 1a and b were obtained in 65% and 72%
chemical yields, respectively, using a manganese(III)
acetate oxidation method by refluxing a,b-unsaturated
cyclic enones (1:1 molar ratio) in benzene under a Dean–
Stark trap for 8 h.17 The Mn(OAc)3 method did not
afford the desired a-acetoxylated products 3a and b with
saturated cyclic ketones. Thus, we improved the known
The absolute configurations of acetoxylated and
hydroxylated saturated cyclic ketones 3a and b and 4a
and b were determined by comparing the sign of their
specific rotations (at the same concentration and in the
same solvent) with those in the literature. According to
these results, ())-2-hydroxycyclopentanone 4a19a and
(+)-2-hydroxycyclohexanone 4b19b have (R)-configura-
tions, whereas (+)-2-acetoxycyclopentanone 3a19a and
())-2-acetoxycylohexanone 3b19c have (S)-configura-
tions.
9
literature Pb(OAc)4 procedure applied for saturated
ketones in which acetic acid or benzene is used as the
solvent with low chemical yield. In our modified pro-
cedure, saturated cyclic ketones were refluxed with
Pb(OAc)4 (1:2 molar ratio) in cyclohexane under an
argon atmosphere for 12 h to afford racemic 3a and b in
62% and 92% chemical yields, respectively.
In the absolute configuration determinations of a0-
acetoxylated cyclic enones 1a and b, they were trans-
formed into the corresponding saturated cyclic ketones
3a and b by H2, Pd(C). According to these transfor-
mations results, both (+)-5-acetoxy-2-cyclopentenone 1a
and ())-6-acetoxy-2-cyclohexenone 1b have (S)-config-
urations (Scheme 2).
2.2. Enzymatic hydrolyses of racemic substrates 1a and b
and 3a and b
Various lipases (PLE, CCL, HLE and PPL) were tested
with all racemic substrates 1a and b and 3a and b.
Among these lipases, PLE gave the best results whereas
the other lipases afforded poor chemical yields and ees
(<10% ee). The first bioconversion was performed using
PLE according to the following general procedure. To a
stirred solution of 1a (500 mg) in phosphate buffer
O
O
OAc
OAc
H2, Pd(C)
EtOAc
( )n
(S)-3a-b
( )n
(S)-1a-b
Scheme 2.
Table 1
20
D
20
D
Entry
Substrate
Time (h)
Acetoxy pdt. Yield (%)a ½aꢀ
Ee (%)b
Hydroxy pdt. Yield (%)a ½aꢀ
Ee (%)c
1
2
3
4
( )-1a
( )-1b
( )-3a
( )-3b
5
3
8
7
(S)-(+)-1a
(S)-())-1b
(S)-(+)-3a
(S)-())-3b
45
46
47
45
+60.3
)88.7
+61.6
)87.5
96
97
98
96
––
––
––
––
44
39
––
––
––
––
99
99
(R)-())-4a
(R)-(+)-4b
)42.2
+14.1
a Yields (%) are given as the isolated yields.
b Enantiomeric excess values are determined by the Chiralcel ODH chiral column HPLC analysis.
c Enantiomeric excess values are determined by the Phenomenex Chirex (S)-LEU and (R)-NEA chiral column HPLC analysis.

C. Tanyeli et al. / Tetrahedron: Asymmetry 15 (2004) 1729–1733
1731
3. Conclusion
CHaHbCHOAc), 2.66–2.73 (1H, ddd, J 3, 5, 19 Hz,
CHaHbCHOAc), 2.22 (3H, s, MeCO2); dC (100.6 MHz,
CDCl3) 202.5, 174.2, 169.1, 129.3, 75.2, 40.4, 21.2;
HRMS (EI): Mþ, found 140.0468. C7H8O3 requires
140.0473.
Herein, we have improved the direct regioselective oxi-
dations of a,b-unsaturated and saturated cyclic ketones
using Mn(OAc)3 and Pb(OAc)4, respectively, in good
yields and have shown that enantiomerically enriched
forms of (S)-1a–b, (S)-3a–b and (R)-4a–b can be
obtained through enzymatic hydrolyses of racemic
substrates 1a and b and 3a and b. Among the enzymes
used in hydrolysis conditions, PLE showed the best
enantioselectivity. The absolute configurations of both
1a and b were determined by transforming them into the
corresponding saturated derivatives 3a and b, respec-
tively, with H2/Pd(C) in EtOAc. Commercially available
and inexpensive enzyme PLE used in catalytic levels,
renders the process very attractive for large scale
preparations.
4.1.2. ( )-6-Acetoxy-2-cyclohexenone 1b. (0.77 g, 72%)
as a colourless oil; Rf (EtOAc/hexane 1:2) 0.26; mmax
(neat) 1732, 1677, 1608 cmꢁ1; dH (400 MHz, CDCl3)
6.87–6.92 (1H, m, CH@CHCO), 5.98–6.02 (1H, m,
CH@CHCO), 5.30 (1H, dd, J 5, 8 Hz, CHOAc), 2.47–
2.51 (2H, m, CH2CH@CH), 2.19–2.23 (1H, m,
CHaHbCHOAc), 2.11 (3H, s, MeCO2), 2.02–2.09 (1H,
m, CHaHbCHOAc); dC (100.6 MHz, CDCl3) 194.4,
170.5, 150.3, 128.9, 73.9, 28.9, 25.9, 21.2; HRMS (EI):
Mþ, found 154.0632. C8H10O3 requires 154.0630.
4.2. General procedure for the Pb(OAc)4 oxidation of
saturated ketones 3a and b
4. Experimental
A mixture of Pb(OAc)4 (5.00 g, 11.0 mmol) and cyclic
ketone (11.0 mmol) in cyclohexane (50 mL) was allowed
to reflux for 12 h and monitored by TLC. The reaction
mixture was diluted with cyclohexane (50 mL) and the
organic phase washed with water (100 mL), brine
(100 mL), saturated NaHCO3 (100 mL) and brine
(100 mL). The organic phase was dried over MgSO4 and
evaporated in vacuo. The crude product mixture was
separated by flash column chromatography using ethyl
acetate/hexane (1:2) as eluent to afford product 3a or b.
1H NMR spectra were recorded in CDCl3 on Bruker
Spectrospin Avance DPX 400 spectrometer. Chemical
shifts are given in ppm downfield from tetramethylsi-
lane. IR spectra were obtained from a Perkin–Elmer
Model 1600 series FT-IR spectrometer and are reported
in cmꢁ1. Optical rotations were measured in CHCl3 and
MeOH solution in a 1 dm cell using a Bellingham &
Stanley P20 polarimeter at 20 °C. Mass spectra were
recorded on a Varian MAT 212. PLE (Pig Liver Ester-
ase) and HLE (Horse Liver Esterase) were purchased
from Sigma as a suspension in ammonium sulfate
solution (3.2 mol/L) and as a powder, respectively. CCL
(Lipase, Type VII, from Candida Rugosa) and PPL
(Lipase, Type II, from Porcine Pancreas) were pur-
chased from Aldrich.
4.2.1. ( )-2-Acetoxycyclopentanone 3a. (0.96 g, 62%) as
a colourless oil; Rf (EtOAc/hexane 1:2) 0.47; mmax (neat)
1753, 1744 cmꢁ1; dH (400 MHz, CDCl3) 4.99 (1H, t, J
10 Hz, CHOAc), 2.23–2.37 (2H, m, CH2CO), 2.07–2.20
(2H, m, CH2CHOAc), 2.01 (3H, s, MeCO2), 1.69–1.88
(2H, m, CH2CH2CHOAc); dC (100.6 MHz, CDCl3)
212.6, 170.3, 75.7, 35.1, 28.4, 20.2, 17.4; HRMS (EI):
Mþ, found 141.0552. C7H9O3 requires 141.0552.
4.1. General procedure for the Mn(OAc)3 oxidation of
a,b-unsaturated ketones 1a and b
A mixture of Mn(OAc)3 (3.25 g, 14.0 mmol) in benzene
(150 mL) was refluxed for 45 min using a Dean–Stark
trap. The mixture was then cooled to room temperature
and enone (7.0 mmol) gradually added. The mixture was
allowed to reflux until the dark brown colour disap-
peared and also monitored by TLC. The reaction mix-
ture was diluted with ethyl acetate (150 mL) and the
organic phase washed with 1 M HCl (150 mL), saturated
NaHCO3 (150 mL) and brine (150 mL). The organic
phase was dried over MgSO4 and evaporated in vacuo.
The crude product mixture was separated by flash col-
umn chromatography using ethyl acetate/hexane (1:3) as
eluent to afford product 1a or b.
4.2.2. ( )-2-Acetoxycyclohexanone 3b. (1.58 g, 92%) as a
colourless oil; Rf (EtOAc/hexane 1:2) 0.51; mmax (neat)
1750, 1720, 1230 cmꢁ1; dH (400 MHz, CDCl3) 5.05–5.14
(1H, m, CHOAc), 2.41–2.48 (1H, m, CHaHbCO), 2.27–
2.38 (1H, m, CHaHbCO), 2.19–2.26 (1H, m,
CHaHbCOAc), 2.08 (3H, s, MeCO2), 1.98–2.06 (1H, m,
CHaHbCOAc), 1.85–1.95 (1H, m, CHaHbCH2CO),
1.63–1.77 (2H, m, CH2CH2CHOAc), 1.48–1.62 (1H, m,
CHaHbCH2CO); dC (100.6 MHz, CDCl3) 204.8, 170.4,
76.9, 41.1, 33.4, 27.5, 24.1, 21.1; HRMS (EI): Mþ, found
156.0784. C8H12O3 requires 156.0786.
4.3. General procedure for enzymatic hydrolyses of 1a and
b and 3a and b
4.1.1. ( )-5-Acetoxy-2-cyclopentenone 1a. (0.64 g, 65%)
as a colourless oil; Rf (EtOAc/hexane 1:3) 0.38; mmax
(neat) 1743, 1635 cmꢁ1; dH (400 MHz, CDCl3) 7.76–7.78
(1H, m, CH@CHCO), 6.34–6.36 (1H, m, CH@CHCO),
5.20 (1H, dd, J 3, 7 Hz, CHOAc), 3.21–3.29 (1H, m,
To a stirred solution of 500 mg rac-1a–b or rac-3a–b in
50 mL pH 7.00 phosphate buffer, 100 lL PLE was added
in one portion and the reaction mixture stirred at 20 °C

1732
C. Tanyeli et al. / Tetrahedron: Asymmetry 15 (2004) 1729–1733
in a pH stat unit. The conversion was monitored by
TLC. The reaction mixture was extracted with ethyl
acetate, dried over MgSO4 and concentrated under
reduced pressure. The product was purified by flash
column chromatography (EtOAc/hexane 1:3).
Acknowledgements
We thank the Turkish Scientific and Technical Research
Council for a grant [no TBAG-2244(102T169)] and
Turkish State Planning Organization for a grant (no
BAP-07-02-DPT-2002 K120540-04).
4.3.1. (S)-(+)-5-Acetoxy-2-cyclopentenone (S)-(+)-1a.
20
D
(0.23 g, 45%) as a colourless oil; 96% ee ½aꢀ ¼ þ60:3
(c 0.2, CHCl3).
References and notes
1. (a) Brown, M. J.; Harrison, T.; Herrinton, P. M.;
Hopkins, M. H.; Hutchinson, K. D.; Overman, L. E.;
Mishra, P. J. Am. Chem. Soc. 1991, 113, 5365; (b) Ohno,
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R. J. Chem. Soc., Chem. Commun. 1992, 172; (c) Moriarty,
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Cookson, R. C.; Lane, R. M. J. Chem. Soc., Chem.
Commun. 1976, 804; (e) Shinada, T.; Kawakami, T.; Sakai,
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1983, 48, 2070; (g) Rubottom, G. M.; Mott, R. C.; Juve,
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44, 349; (i) Shono, T.; Okawa, M.; Nishiguchi, I. J. Am.
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1963; pp 537–591.
4.3.2. (S)-())-6-Acetoxy-2-cyclohexenone (S)-())-1b.
20
(0.23 g, 46%) as a colourless oil; 97% ee ½aꢀ ¼ ꢁ88:7
D
(c 0.5, MeOH).
4.3.3.
(S)-(+)-2-Acetoxycyclopentanone
(S)-(+)-3a.
20
D
(0.24 g, 47%) as a colourless oil; 98% ee ½aꢀ ¼ þ61:6
20
D
(c 1.57, CHCl3), lit.19a ½aꢀ ¼ þ61:0 (c 2.0, CHCl3).
4.3.4. (R)-())-2-Hydroxycyclopentanone (R)-())-4a.
20
(0.15 g, 44%) as a colourless oil; 99% ee ½aꢀ ¼ ꢁ42:2 (c
D
20
D
1.2, CHCl3), lit.19a ½aꢀ ¼ ꢁ38:4 (c 1.2, CHCl3); mmax
(neat) 3153, 1790, 1721 cmꢁ1; dH (400 MHz, CDCl3) 3.99
(1H, t, J 10 Hz, CHOH), 3.12–3.16 (1H, br s, OH), 2.26–
2.40 (2H, m, CH2CO), 2.06–2.18 (1H, m,
CHaHbCHOH), 1.91–2.05 (1H, m, CHaHbCHOH),
1.56–1.83 (2H, m, CH2CH2CO); dC (100.6 MHz, CDCl3)
218.8, 76.2, 34.4, 31.0, 16.7; HRMS (EI): Mþ, found
100.0520. C5H8O2 requires 100.0524.
4.3.5.
(S)-())-2-Acetoxycyclohexanone
(S)-())-3b.
20
D
4. (a) Green, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis; Wiley: New York, 1991, pp 87–104; (b)
Kocienski, P. Protecting Groups; Thieme: Stuttgart, 1994.
pp 22–29.
(0.23 g, 45%) as a colourless oil; 96% ee ½aꢀ ¼ ꢁ87:5 (c
20
D
0.2, MeOH), lit.19c for (R)-(+)-3b ½aꢀ ¼ þ89:3 (c 1.0,
MeOH).
5. (a) Levine, P. A.; Walti, A. Org. Synth., Coll. Vol. II, 1943,
4843; (b) Reid, E. B.; Fortenbauch, R. B.; Patterson, H. R.
J. Org. Chem. 1950, 15, 572.
6. Kuehne, M. E.; Giacobbe, T. J. J. Org. Chem. 1968, 33,
3359.
4.3.6. (R)-(+)-2-Hydroxycyclohexanone (R)-(+)-4b.
20
D
(0.14 g, 39%) as a colourless oil; 99% ee ½aꢀ ¼ þ14:1 (c
20
0.5, CHCl3), lit.19d for (R)-(+)-3b ½aꢀ ¼ þ13:4 (c 0.5,
D
CHCl3); mmax (neat) 3146, 1790, 1716 cmꢁ1
;
dH
7. Shono, T.; Matsumura, Y.; Nakagawa, Y. J. Am. Chem.
Soc. 1975, 97, 6144.
(400 MHz, CDCl3) 4.00–4.15 (1H, m, CHOH), 2.8 (1H,
br s, OH), 2.46–2.55 (1H, m, CHaHbCO), 2.35–2.45 (1H,
m, CHaHbCO), 2.24–2.34 (1H, m, CHaHbCHOH), 2.01–
2.10 (1H, m, CHaHbCHOH), 1.35–1.88 (4H, m,
CH2CH2); dC (100.6 MHz, CDCl3) 211.7, 75.7, 39.8,
37.1, 27.9, 23.8; HRMS (EI): Mþ, found 114.0680.
C6H10O2 requires 114.0681.
8. Creary, X. J. Am. Chem. Soc. 1984, 106, 5568.
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4426; (b) Criegee, R.; Klonk, K. Liebigs Ann. Chem. 1949,
1, 564; (c) May, G. L.; Pinhey, J. T. Aust. J. Chem. 1982,
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Lohnay, B. B.; Enders, D. Helv. Chim. Acta 1989, 72, 980.
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Reddy, R. T.; Chen, B.-C. J. Org. Chem. 1992, 57, 7274;
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S. J. Am. Chem. Soc. 1990, 112, 6679.
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Dorrow, R. L. J. Am. Chem. Soc. 1985, 107, 4346; (c)
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4.4. Hydrogenation of (S)-1a and (S)-1b
To a stirred solution of (S)-1a (0.20 g) in EtOAc
(20 mL), Pd(C) (10 mg) was added and stirred at room
temperature under a hydrogen atmosphere for 3 h. The
filtration of the mixture followed by evaporation of
the solvent in vacuo quantitatively afforded (S)-3a. The
same procedure was applied for the transformation of
(S)-1b into (S)-3b. All spectroscopic data of the products
are in accordance with (S)-3a and (S)-3b, respectively.

C. Tanyeli et al. / Tetrahedron: Asymmetry 15 (2004) 1729–1733
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