Synthesis and Rhizopus oryzae mediated enantioselective hydrolysis of α-acetoxy aryl alkyl ketones

Article abstract of DOI:10.1016/S0957-4166(98)00165-7

Mn(OAc)₃ oxidation of aromatic ketones afforded the α-acetoxy ketones in good yield. Selective hydrolysis of the acetoxy ketones by the fungus Rhizopus oryzae yields (R)-hydroxy ketones in high enantiomeric excess.

Full text of DOI:10.1016/S0957-4166(98)00165-7

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1673–1677
Synthesis and Rhizopus oryzae mediated enantioselective
hydrolysis of α-acetoxy aryl alkyl ketones
Ayhan S. Demir,a,b,∗ Haluk Hamamci,b,c Cihangir Tanyeli,a,b Idris M. Akhmedova and
Fatos Doganel b
aDepartment of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
bDepartment of Biotechnology, Middle East Technical University, 06531 Ankara, Turkey
cDepartment of Food Engineering, Middle East Technical University, 06531 Ankara, Turkey
Received 18 February 1998; accepted 24 April 1998
Abstract
Mn(OAc)3 oxidation of aromatic ketones afforded the α-acetoxy ketones in good yield. Selective hydrolysis
of the acetoxy ketones by the fungus Rhizopus oryzae yields (R)-hydroxy ketones in high enantiomeric excess.
© 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
The α-hydroxy ketones are important structural units in many biologically active natural products
and are also important synthons for the asymmetric synthesis of natural products.1 Several methods
have been developed for the preparation of optically active α-hydroxy ketones. For example, the
stereoselective oxidation of optically active enolates,2 oxidation of prochiral enolates using optically
active oxaziridines,3 selective oxidation of chiral titanium enolates and asymmetric oxidation of silyl
enol ethers.4 As an alternative to chemical methods, optically active α-hydroxy ketones are prepared
enzymatically by reduction of α-diketones5 and by kinetic resolution of racemic α-hydroxy and α-
acetoxy ketones.6
Recently we have reported the synthesis of optically active α-hydroxy enones by lipase-catalyzed
hydrolysis of α-acetoxy enones.7 During the course of our studies on the biotransformation of acetoxy
ketones to hydroxy ketones using different enzymes, we observed that Rhizopus oryzae has the versatility
for the enantioselective hydrolysis of acetoxy ketones.
In this paper we present the enantioselective synthesis of hydroxy ketones from ketones via Mn(OAc)3
mediated acetoxylation followed by enantioselective ester hydrolysis utilizing Rhizopus oryzae.
∗
Corresponding author. E-mail: asdemi1@pop.uky.edu
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00165-7

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2. Results and discussion
In an initial reaction shown in Scheme 1, the oxidation of aromatic ketones 1 with four equivalents of
manganese(III) acetate furnished the desired α-acetoxy ketones 2 in good yields (Table 1). In connection
with ongoing research8a–h we found that the source of manganese(III) acetate is very important for the
high yields of the reactions. The anhydrous manganese(III) acetate used in this oxidation was prepared
from manganese(II) nitrate and acetic anhydride and dried using phosphorus pentoxide under vacuum
prior to use.8i–k
Scheme 1.
The first bioconversion was performed with tetralone in EtOH and the fungus was incubated in the
presence of tetralone at 25°C. The conversion was monitored by TLC using authentic hydroxy ketone as
a reference. After 26 h about 40–45% conversion of the product was observed. The product was separated
using flash column chromatography and hydroxy ketone 3a was isolated in 38% yield and in 35% ee. The
configuration of the keto alcohol was assessed as (R) by comparison of its specific rotation with literature
data.3c,6a,9 Under similar conditions, neutralization of the reaction medium during the conversion with
CaCO3 solution increases the ee to 94%. Changing the reaction temperature and ethanol solvent for THF
or DMSO did not improve the results. The use of excess CaCO3 decreases the enantiomeric excesses.
A variety of cyclic and acyclic aromatic acetoxy ketones are employed as substrates since the absolute
stereochemistry of the resulting hydroxy ketones has been previously established. In an initial survey,
Rizhopus oryzae appears to contain an esterase that is able to yield alcohols with configurations that
exhibit a consistent pattern. The hydrolysis of aromatic acetoxy ketones with different R1 and R2 are
examined. The results are given in Table 1 and show that the enantiomeric excess (ee) varied from 66%
to 96%. It is apparent that higher ees are found in those alcohols which show the differences in the
relative sizes of R1 (aryl) and R2 (alkyl), i.e. phenyl, methoxyphenyl, methoxynaphthyl groups versus
methyl substituent (3c, 3d, 3f). The relatively smaller ee values are obtained with phenyl, methoxyphenyl
versus phenyl and isopropyl (3e, 3g). From these observations there is a change in the observed ees on
changing alkyl groups from methyl to isopropyl and phenyl versus aryl. The control experiment with
authentic optically active acetates under the conditions of the bioconversion reaction proved them to be
configurationally stable and racemization is not responsible for their lower enantiomeric excesses. The
enantiomeric excesses of recovered acetoxy ketones varied from 24 to 86% and were determined via their
hydroxy ketones.9
The results show that Rhisopus oryzae mediated hydrolysis of acetoxy ketones provides hydroxy
ketones with high enantiomeric excesses and enantiomerically enriched acetoxy ketones in good chem-
ical yields. The enzymes favor the (R)-enantiomers in accordance with the suggested mechanism of
hydrolase-catalyzed enantioselective hydrolysis of the acetoxy group to the hydroxy group.10–14

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Table 1
Synthesis and Rhizopus oryzae mediated enantioselective hydrolysis of α-hydroxy aryl alkyl ketones
3. Experimental15,16
3.1. General procedure for the preparation of α-acetoxy ketones
A mixture of 10 mmol of manganese(III) acetate and 2.5 mmol of ketone in 50 ml of benzene was
refluxed (the reaction was monitored by TLC) under a Dean–Stark trap. The mixture was cooled to

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RT, diluted with ethyl acetate, was washed successively with 1 M aqueous HCl solution, saturated
aqueous NaHCO3 solution and brine, and dried over anhydrous MgSO4. The crude products were
chromatographed on flash silica gel in ethyl acetate:hexane (1:3).
3.2. General procedure for the preparation of α-hydroxy ketones
Rhizopus oryzae (NRRL 395) was used for the experiments. It was cultivated on boiled rice and the
fungal spores were transferred by loopfuls into the sterile flasks containing the medium and grown in a
rotary shaker at 30°C for two days. The medium for fungal growth includes 15.0 g glucose syrup, 1.0
g (NH4)2SO4, 0.30 g KH2PO4, 0.12 g MgSO4 and 0.02 g ZnSO4 diluted to 500 ml by distilled water.
The medium was divided into five portions and sterilized in the autoclave for 15 minutes. Spores from
the main plate were transferred into an Erlenmeyer flask containing 100 ml sterile medium. The fungus
was inoculated at 30°C for two days in the rotary shaker and the substrate (1.7 mmol) dissolved in 2 ml
EtOH. Shaking was resumed until approximately 40–45% of the racemic acetate had been hydrolyzed.
During the hydrolysis the medium was neutralized with 10% CaCO3 solution. The fungus was filtered
out, washed with distilled water, and the combined aqueous phases extracted with ethyl acetate and the
alcohol and unhydrolyzed acetate separated by flash column chromatography (EtOAc:hexane, 1:2).
3.3. 2-Acetoxy-1-(4-methoxyphenyl)-1-propanone 2d
1
Colorless semisolid. FR-IR: 1735, 1670, 1595 cm−1. H NMR (CDCl3): δ 1.42 (d, J=7.0 Hz, 3H,
CH3), 2.12 (s, 3H, CH3), 3.86 (s, 3H, OCH3), 8.72 (dd, J=14.0 and 6.8 Hz, 1H, CH), 6.94 (m, 2H, C6H4),
7.86 (m, 2H, C6H4). Anal. calcd for C12H14O4 (222.24): C, 64.85; H, 6.34. Found: C, 65.12; H, 6.43.
3.4. 2-Acetoxy-1-(4-methoxyphenyl)-3-methyl-1-butanone 2e
1
Colorless semisolid. FT-IR: 1740, 1665, 1510 cm−1. H NMR (CDCl3): δ 0.62 (d, J=7.0 Hz, 3H,
CH3), 1.14 (d, J=6.9 Hz, 3H, CH3), 2.11 (m, 1H, CH), 2.14 (s, 3H, CH3), 3.88 (s, 3H, OCH3), 5.92 (dd,
J=13.7 Hz and 7.0 Hz, 1H, CH), 6.92 (m, 2H, C6H4), 7.88 (m, 2H, C6H4). Anal. calcd for C14H18O4
(250.29): C, 67.18; H, 7.24. Found: C, 67.38; H, 7.33.
3.5. 2-Acetoxy-1-(6-methoxy-2-naphthyl)-1-propanone 2f
Colorless solid, m.p. 112–114°C. IR (KBr) 1720, 1680, 1610 cm−1. 1H NMR(CDCl3): δ 1.53 (d, J=6.8
Hz, 3H, CH3), 2.10 (s, 3H, CH3), 3.96 (s, 3H, OCH3), 5.88 (dd, J=14 Hz and 7.0 Hz, 1H, CH), 7.18–7.32
and 7.62–8.39 (m, 6H, C10H6). Anal. calcd for C16H16O4 (272.31): C, 70.57; H, 5.92. Found: C, 70.23;
H, 5.71.
Acknowledgements
We thank the Middle East Technical University for grant (AFP-1997) and the Turkish State Planning
Organization for grant (DPT-1997).

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