Chemoenzymatic synthesis of stegobinone and stegobiol, components of the natural sex pheromone of the drugstore beetle (Stegobium paniceum L.)

Article abstract of DOI:10.1002/ejoc.201101319

NADPH-dependent ketoreductases were used for the chemoenzymatic stereoselective synthesis of the two componentsof the natural sex pheromone of the drugstore beetle. The key step in the asymmetric synthesis was the enzymatic reduction of an α-methyl-1,3-di

Full text of DOI:10.1002/ejoc.201101319

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101319
Chemoenzymatic Synthesis of Stegobinone and Stegobiol, Components of the
Natural Sex Pheromone of the Drugstore Beetle (Stegobium paniceum L.)
Dimitris Kalaitzakis[a] and Ioulia Smonou*[a]
Keywords: Chemoenzymatic synthesis / Enzyme catalysis / Reduction / Pheromones
NADPH-dependent ketoreductases were used for the chemo-
enzymatic stereoselective synthesis of the two components
of the natural sex pheromone of the drugstore beetle. The
key step in the asymmetric synthesis was the enzymatic re-
duction of an α-methyl-1,3-diketone and an α-methyl-β-keto
ester, which finally led to the preparation of crystalline stego-
binone and stegobiol.
tum.[3] The minor component of the pheromone was iso-
lated by Kodama et al.[4] and was shown to be stegobiol (3,
Figure 1).
Introduction
There are many well known species of anobiid beetles
that cause extensive damage to wooden materials and
stored products. The drugstore beetle, Stegobium paniceum
(known in the United Kingdom as the biscuit beetle),[1] can
cause tremendous damage and economic losses to post-har-
vest and stored grains and packaged food products. The
drugstore beetle is a cosmopolitan species that resembles
the cigarette beetle and rivals it as a pantry pest. It is more
abundant in warmer climate regions.[1] It feeds on any of
the household foods and spices, as well as wool, hair, le-
ather, horn, museum specimens, and drugs. It has been
known to perforate books and even tin or aluminum foil
and lead.
One of the components of the female-produced sex pher-
omone of this species was isolated by Kuwahara et al.[2]
and called stegobinone. Stegobinone is the major crystalline
component of this pheromone and its proposed structure is
shown in Figure 1. Stegobinone has also been reported to
be an attractant of the furniture beetle, Anobium pouncta-
After the establishment of the absolute configuration of
these components,[5] several total syntheses were published.
The first asymmetric synthesis was reported by Mori et al.[6]
This method generated oily stegobinone (1), and it was re-
ported that the key cyclization reaction was capricious and
difficult to be reproduced. After the development of a more
efficient cyclization reaction by Oppolzer and Rodriguez,[7]
two asymmetric syntheses of enantiopure stegobinone (1)
and stegobiol (3) were developed.[8] However, one of the
two synthetic methods[8b] led to crystalline stegobiol (3) and
stegobinone (1) by using preparative TLC with a low total
yield and several synthetic steps. Another synthetic ap-
proach, which was accomplished by boronic ester chemis-
try, was reported by Matteson et al.[9] In this case, stegobiol
and stegobinone were produced in pure and crystalline
forms; however, many synthetic steps were required. Re-
cently, a new synthetic way for the synthesis of (Ϯ)-epi-ste-
gobinone was published that reported the use of silacyclo-
propanes as synthetic intermediates.[10] Presently, the lack
of an efficient, high-yielding synthesis of stegobinone re-
quires the development of a practical method to overcome
the above-mentioned disadvantages. Such
a synthetic
method could lead to the highly stereoselective formation
of the crystalline stegobinone product, which could be used
for the preparation of baited traps for beetles.[11]
Although stegobinone possess a simple structure, a facile
synthesis of this molecule seems to be elusive. This is due
to the fact that this compound readily epimerizes to 1Ј-epi-
mer 2 (Figure 1). The 1Ј-epimer of this compound is
strongly repellent to Stegobium paniceum even in trace
amounts. Racemic 1 is also repellent. Partially crystalline or
oily synthetic 1, which contains an amount of 2 and other
diastereomers, is unstable to prolonged storage. Therefore,
in order to synthesize stable, crystalline stegobinone, highly
Figure 1. Structures of stegobinone, epistegobinone and stegobiol.
[a] Department of Chemistry, University of Crete,
Heraklio 71003, Crete, Greece
Fax: +30-2810-545166
E-mail: smonou@chemistry.uoc.gr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101319.
Eur. J. Org. Chem. 2012, 43–46
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
43

D. Kalaitzakis, I. Smonou
SHORT COMMUNICATION
diastereo- and enantioselective reactions must be ac- hydroxy ketone 6 without the formation of any byproducts.
complished. This crucial observation led us to search for a Furthermore, this reaction was accomplished through dy-
synthetic approach[7,8b] in which the precursors of stego- namic kinetic resolution[17] of chiral diketone 5 to afford
binone, 6 and 11 (Figure 2), will be formed in high optical intermediate 6 with the desired absolute configuration
purity and in crystalline form.
(2S,3R enantiomer). Therefore, we applied this efficient bio-
catalytic procedure on a gram scale (1 g) for the synthesis
of hydroxy ketone 6 in high optical purity (Scheme 2). Only
two steps were utilized to synthesize 6 starting from com-
mercially available 3-pentanone (4). The easy conversion of
diketone 4 into diketone 5 followed by enzymatic reduction
catalyzed by KRED-102 gave keto alcohol 6 in 78% overall
yield and high optical purity (Scheme 2).
Figure 2. Intermediate compounds for the synthesis of stegobiol
and stegobinone.
Results and Discussion
Ketoreductases have proved to be very powerful catalysts
for the stereoselective reduction of carbonyl compounds.[12]
We have recently reported the stereoselective preparation of
α-alkyl-β-hydroxy ketones[13] and α-alkyl-β-hydroxy esters
by using these enzymes.[14] Two natural pheromones, sito-
philure[15] and sitophilate,[16] have also been synthesized
chemoenzymatically by using ketoreductases. In this paper,
we present the stereoselective synthesis of the female-pro-
duced pheromone of Stegobium paniceum by utilizing com-
mercially available ketoreductases.
The stereoselective enzymatic reduction of 2-alkyl-1,3-di-
ketones can provide optically pure α-alkyl-β-hydroxy
ketones (Figure 3). We applied this biocatalytic method for
the stereoselective conversion of 3-methyl-2,4-hexanodione
(5) into intermediate compound 6. As published before,[13]
compound 6 can be easily produced quantitatively on a
small scale with excellent optical purity (Scheme 1).[13] This
reduction was carried out in a phosphate buffer (200 mm,
pH 6.5) in the presence of ketoreductases, NADPH, and
glucose/glucose dehydrogenase (GDH) for cofactor recycl-
ing.
Scheme 2. Chemoenzymatic synthesis of (2S,3R)-2-hydroxy-3-
methyl-4-hexanone. Reagents and conditions: (i)1. LDA,
CH3CHO, dry THF, –78 °C, 99% yield; 2. Jones oxidation, ace-
tone, 0 °C, 90% yield, (89% from 3-pentanone); (ii) KRED-102,
NADPH, r.t., 88% yield, Ͼ99%de, Ͼ99%ee, overall yield 78%.
Furthermore, the stereoselective reduction of α-alkyl-β-
keto esters can provide optically pure α-alkyl-β-hydroxy es-
ters.[16] This result prompted us to apply this biocatalytic
method for the reduction of ester 8 to hydroxy ester 9, a
potential precursor of target intermediate 11. In particular,
KRED-B1E and KRED-B1B catalyzed effectively the re-
duction of chiral substrate 8 to afford desired product 9
with Ͼ99% diastereomeric and enantiomeric excess in only
24 h. This reduction also was accomplished by dynamic ki-
netic resolution, providing the 2S,3S stereoisomer[16] with
100% conversion (Scheme 3).
Figure 3. Enzymatic reduction of α-alkyl-1,3-diketones.
Scheme 3. KRED-B1E-mediated reduction of keto ester 8 by dy-
namic kinetic resolution.
Therefore, we applied this biocatalytic method to a larger
scale (632 mg) to synthesize intermediate 11. Its synthesis
started from commercially available ethyl 3-oxopentanoate
(7), which was converted quantitatively into keto ester 8.
Hydroxy ester 9 was produced by enzymatic reduction with
Scheme 1. Enzymatic preparation of hydroxy ketone 6 through dy-
namic kinetic resolution of diketone 5.
As shown in Scheme 1, KRED-102 is an excellent cata- KRED-B1E. Finally, intermediate 11 was derived by pro-
lyst for the regio- and stereoselective transformation of di- tection with TBDMSCl followed by hydrolysis. The overall
ketone 5, providing only one of the four stereoisomers of yield was 73% starting from keto ester 7 (Scheme 4).
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Eur. J. Org. Chem. 2012, 43–46

Chemoenzymatic Synthesis of Stegobinone and Stegobiol
Thus, crystalline stegobinone was obtained without detec-
tion of the 1Ј-epimer of this compound. It was very stable
after storage at –20 °C for more than one year. The key step
for this synthesis was the high stereoselective reduction of
α-alkyl-1,3-diketones and α-alkyl-β-keto esters catalyzed by
ketoreductases.
Experimental Section
Scheme 4. Chemoenzymatic synthesis of intermediate 11. Reagents
and conditions: (i) K2CO3, MeI, dry acetone, reflux, Ͼ99% yield;
(ii) KRED-B1E, NADPH, r.t., 99% yield, Ͼ99%de, Ͼ99%ee;
(iii) TBDMSCl, imidazole, DMF, r.t., 85% yield; (iv) NaOH,
MeOH/H2O (1:1), r.t., 87%, overall yield 73%.
(2S,3R)-2,3-Dihydro-6-[(1ЈS,2ЈS)-2Ј-hydroxy-1Ј-methylbutyl]-2,3,5-
trimethyl-4H-pyran-4-one (3): To
a solution of 13 (248 mg,
0.72 mmol) in CH3CN was added 40% HF (0.1 mL, 1.8 mmol) at
0 °C. The mixture was stirred at 0 °C for 10 h, then diluted with
Et2O and a satd. NaHCO3 solution and separated. The aqueous
layer was extracted with Et2O. The combined organic layers were
washed with a satd. NaHCO3 solution and brine, dried with
MgSO4, and concentrated in vacuo. The residue was purified by
silica gel chromatography (hexane/EtOAc, 5:1). Yield: 80%, 130 mg
In the final step, compounds 6 and 11 were coupled to
give ester 12, which was easily transformed into TBDMS-
protected compound 13 by using titanium tetrachloride
(5 equiv.) and ethyldiisopropylamine (8 equiv.) in dry
CH2Cl2. In contrast to the fact that no reproducible results
was previously reported,[6] this cyclization was very efficient
and easily reproduced and gave 67% yield. Deprotection of
compound 13 provided stegobiol (3) and after oxidation of
3 with Jones reagent followed by quick purification, crystal-
line stegobinone (1) was isolated in pure form. The absolute
configurations of synthesized compounds 1 and 3 were in
very good agreement with the previously published da-
ta.[6,8b,9] Crystallization was achieved after storage at
–20 °C. It is interesting to note that the stegobinone sample,
1
(oil). H NMR (500 MHz, CDCl3): δ = 4.46–4.53 (m, 1 H), 3.54–
3.61 (m, 1 H), 2.82–2.88 (m, 1 H), 2.35–2.42 (m, 1 H), 1.90 (d, J
= 7.0 Hz, 1 H), 1.75 (s, 3 H), 1.53–1.63 (m, 1 H), 1.37–1.46 (m, 1
H), 1.32 (d, J = 6.5 Hz, 3 H), 1.18 (d, J = 7.0 Hz, 3 H), 1.04 (d, J
= 7.5 Hz, 3 H), 1.00 (t, J = 7.5 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 9.2, 9.4, 10.0, 14.6, 15.8, 28.2, 40.9, 43.6, 75.2, 76.6,
109.2, 172.7, 197.0 ppm. MS: m/z (%) = 226.34 (11.1), 168 (72.1),
141 (20.6), 139 (13.4), 125 (9.7), 124 (9.3), 112 (100). [α]2D5
=
–112Ϯ3 (c = 0.26, CHCl3) {ref.[6b] [α]1D9 = –110Ϯ6 (c = 0.42,
CHCl3)}.
(2S,3R)-2,3-Dihydro-6-[(1ЈR)-1Ј-methyl-2Ј-oxobutyl]-2,3,5-tri-
free of byproducts, was crystallized easily because this new methyl-4H-pyran-4-one (1): To a stirred and cooled solution of 3
(130 mg, 0.57 mmol) in acetone (15 mL) was slowly added Jones
reagent at 0 °C over 40 s. Then the mixture was diluted with Et2O
and a satd. aqueous sodium thiosulfate, stirred over 5 min and the
organic layer was separated. The aqueous layer was extracted with
Et2O. The combined organic layers were washed with water, a satd.
NaHCO3 solution, and brine, dried with MgSO4, and concentrated
in vacuo. The residue was purified by silica gel chromatography
(hexane/EtOAc, 4:1) to give pure and crystalline stegobinone.
Yield: 90%, 115 mg. Stegobionone was crystallized when left over-
synthetic methodology afforded products of high enantio-
and diastereoselectivities. The key reaction steps were the
high stereoselective transformations provided by the ketore-
ductases. The overall yield of the final stage was 40% start-
ing from intermediate 11 (Scheme 5).
1
night in a refrigerator at –20 °C. H NMR (500 MHz, CDCl3): δ =
4.43–4.47 (m, 1 H), 3.63 (q, J = 7.0 Hz, 1 H), 2.35–2.52 (m, 3 H),
1.79 (s, 1 H), 1.30 (d, J = 7.0 Hz, 3 H), 1.28 (d, J = 7.0 Hz, 3 H),
1.06 (t, J = 7.0 Hz, 3 H), 1.03 (d, J = 7.5 Hz, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 7.9, 9.36, 9.39, 12.8, 15.7, 33.9, 43.7, 49.2,
77.2, 109.4, 169.0, 197.0, 207.5 ppm. MS: m/z (%) = 224.18 (8.6),
168 (44.5), 113 (28.4), 83 (22.9), 57 (100). M.p. 52.0–53.0 °C (ref.[2b]
52.5–53.5 °C). [α]2D5 = –285Ϯ6 (c = 0.21 CHCl3) {ref.[8b] [α]2D4
–283 (c = 0.07 CHCl3)}.
=
Scheme 5. Final steps for the synthesis of stegobiol and stegobi-
none. Reagents and conditions: (i) 1. 2,6-Dichlorobenzoyl chloride,
Et3N, THF, r.t.; 2. 6, DMAP, C6H6, 0 °C to r.t., 83% yield;
(ii) TiCl4 (5 equiv.), iPr2NEt (8 equiv.), CH2Cl2, –78 to –15 °C, 67%
yield; (iii) HF aq., MeCN, 80% yield; (iv) Jones reagent, acetone,
0 °C, 90% yield.
Supporting Information (see footnote on the first page of this arti-
cle): Additional experimental procedures and characterization data,
1
GC traces, and copies of selected H and 13C NMR spectra.
Acknowledgments
The financial support from ELKE, University of Crete (RG2752)
is acknowledged. D.K. thanks the Greek National Scholarships
Foundation (IKY).
Conclusions
In conclusion, commercially available ketoreductases
were used for the synthesis of the female-produced sex
pheromone of the drugstore beetle, Stegobium paniceum.
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Eur. J. Org. Chem. 2012, 43–46
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D. Kalaitzakis, I. Smonou
SHORT COMMUNICATION
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Published Online: November 2, 2011
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