Baker's yeast mediated biohydrogenation of unsaturated compounds containing a methylene group: Enantioselective preparation of 2-methyl alkanols from 2-substituted acrolein acetals

Article abstract of DOI:10.1016/S0957-4166(99)00256-6

The baker's yeast mediated biohydrogenation of unsaturated compounds containing a methylene group may constitute an enantioselective biocatalytic approach to the preparation of 2-methyl-1-alkanols, as exemplified by the reduction of the compounds 8a-d to 90-98% enantiomerically pure alcohols 2a-d.

Full text of DOI:10.1016/S0957-4166(99)00256-6

Pergamon
Tetrahedron: Asymmetry 10 (1999) 2639–2642
Baker’s yeast mediated biohydrogenation of unsaturated
compounds containing a methylene group: enantioselective
preparation of 2-methyl alkanols from 2-substituted acrolein
acetals
Patrizia Ferraboschi,a Shahrzad Reza-Elahi,a Elisa Verza a and Enzo Santaniello b,∗
aDipartmento di Chimica e Biochimica Medica, Università degli Studi di Milano, Italy
bDipartmento di Scienze Precliniche LITA Vialba, Università degli Studi di Milano, Italy
Received 11 June 1999; accepted 21 June 1999
Abstract
The baker’s yeast mediated biohydrogenation of unsaturated compounds containing a methylene group may
constitute an enantioselective biocatalytic approach to the preparation of 2-methyl-1-alkanols, as exemplified by
the reduction of the compounds 8a–d to 90–98% enantiomerically pure alcohols 2a–d. © 1999 Elsevier Science
Ltd. All rights reserved.
Among biocatalysts available for organic synthesis,1 a special role has been recognized for micro-
organisms that may be regarded as readily available reagents for the preparation of enantiomerically pure
compounds.2 Baker’s yeast (Saccharomyces cerevisiae) is well suited to this kind of application, since it
is commonly accessible and its utilization does not require any special care or skill in microbiology.3 The
reducing capabilities of baker’s yeast have been extensively exploited, the biohydrogenation of double
bonds constituting an interesting reaction that may proceed with high enantioselectivity.4 In this context,
we have recently shown that the baker’s yeast biohydrogenation of 2-substituted allyl alcohols 1 can be
used for the preparation of alcohols 2 of high enantiomeric excess (ee).5
The above results could be considered as a special example of the recognized capability of baker’s yeast
to mediate the biohydrogenation of other unsaturated alcohols such as compounds 3 to the corresponding
∗
Corresponding author. E-mail: enzo.santaniello@unimi.it
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00256-6

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saturated alcohols 4.6 It has been proposed that the real substrate for the addition of hydrogens is the
aldehyde 5 that can be also used as the substrate of the biohydrogenation,7 although sometimes a possible
inhibition of the enzymatic system(s) by the latter compounds may occur.8
A controlled release of aldehyde 5 into the reaction medium can be achieved using unsaturated acetals
6 as substrates, since these compounds are slowly hydrolyzed during the fermentation of the yeast.9 In this
way a convenient biohydrogenation to the final saturated alcohol 4 can be realized. Starting from all the
above considerations, we have extended our preliminary observations on the reduction of 2-substituted
allyl alcohols 1a–c5 and planned to use the unsaturated aldehydes 7 or the corresponding dimethyl acetals
810 as substrates for the baker’s yeast mediated biohydrogenation.
When aldehyde 7 (R=PhCH2) was the substrate, with a low yeast/substrate ratio (5 g/mmol) only the
unsaturated alcohol 1a was obtained. Using a 20 g/mmol ratio, modest yields of the nearly enantio-
merically pure 2-methyl alkanols 2a were obtained (20% yield, 98% ee) but a slow addition of an
ethanolic solution of the substrate was required (96 h). The product of the biohydrogenation was
accompanied by 10% of unsaturated alcohol 1a. Similar results were also obtained from the aldehydes
7b,c. For the acetals 8a–d a few experimental conditions were tested to establish the most satisfactory
protocol in terms of yields of saturated alcohol and enantioselectivity. A yeast/substrate ratio of 20
g/mmol gave the highest yield (60–83%) in the biohydrogenation products and minimized the formation
of unsaturated alcohols. For 8a and 8b a ratio of 95/5 and 82/18 of saturated 2a,b/unsaturated 1a,b was
obtained, whereas only traces of the unsaturated alcohol 1c were formed from 8c. In the above incubation
conditions, the formation of the saturated acids was minimized to about 5% in every case.11
The best experimental protocol consisted of the sequential addition of 5–6 portions of substrate to the
fixed amount of yeast at 5–10 hour intervals.12 As for the bioreduction of the unsaturated alcohols 1a–c,5
the configurations were established as R for the alcohols obtained from substrates 8a,b and S for the
alcohol 1c.13 Furthermore, the alcohol 2b showed a 90% ee and nearly enantiomerically pure 2a and 2c
(>98% ees) could be prepared in this way.14
The most interesting result was obtained from the acetal 8d that yielded in 48 hours the (R)-alcohol 2d
in 82% yield and >98% ee15 and only 2.5% of the unsaturated alcohol 1d. In this case, the slow release
of the intermediate aldehyde circumvented the problems associated with the direct biohydrogenation of

P. Ferraboschi et al. / Tetrahedron: Asymmetry 10 (1999) 2639–2642
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the unsaturated alcohol 1d that could not be transformed into the corresponding saturated alkanol 2d.5a
It should also be noted that compound 2d could not be obtained in an enantiomerically pure form by the
enzymatic resolution of (RS)-1d. In fact, the Pseudomonas cepacia lipase-catalyzed transesterification
afforded a nearly racemic product under the same conditions that allowed a highly enantioselective
resolution of a long series of similar 2-methyl-1-alkanols.16 In conclusion, we have shown that substrates
containing a methylene moiety such as the acetals 8a–c are nicely biohydrogenated to the corresponding
saturated compounds, i.e. 2-methyl alkanols 2a–c, faster than the alcohols (2–4 versus 14 days). Finally,
results from the acetal 8d show that the biotransformation may be successful even in the case where the
corresponding alcohol fails to react.
Acknowledgements
This work has been financially supported by Ministero dell’Università e della Ricerca Scientifica
(Fondi MURST ex-40%) and Consiglio Nazionale delle Ricerche (CNR, Target Project in Bio-
technology).
References
1. (a) Azerad, R. Bull. Soc. Chim. Fr. 1995, 132, 17–51. (b) Roberts, S. M. J. Chem. Soc., Perkin Trans. 1 1998, 157–169.
2. Microbial Reagents in Organic Synthesis; Servi, S., Ed.; Kluwer Academic: Dordrecht, 1992; Vol. 381.
3. For recent reviews on synthetic capacities of baker’s yeast, see: D’Arrigo, P.; Högberg, H.-E.; Pedrocchi-Fantoni, G.; Servi,
S. Biocatalysis 1994, 9, 299–312.
4. D’Arrigo, P.; Pedrocchi-Fantoni, G.; Servi, S. Advances in Applied Microbiology 1997, 44, 81–123.
5. (a) Ferraboschi, P.; Casati, S.; Santaniello, E. Tetrahedron: Asymmetry 1994, 5, 19–20. (b) Ferraboschi, P.; Reza-Elahi, S.;
Verza, E.; Meroni Rivolta, F.; Santaniello, E. Synlett 1996, 1176–1178.
6. (a) Gramatica, P.; Manitto, P.; Ranzi, B. M.; Delbianco, A.; Francavilla, M. Experientia 1982, 38, 775–777. (b) Fuganti,
C.; Serra, S.; Duilio, A. J. Chem. Soc., Perkin Trans. 1 1999, 279–282.
7. Fuganti, C.; Grasselli, P.; Seneci, P. F.; Casati, P. J. Chem. Soc., Perkin Trans. 1 1988, 3061–3065.
8. Ferraboschi, P.; Grisenti, P.; Casati, R.; Fiecchi, A.; Santaniello, E. J. Chem. Soc., Perkin Trans. 1 1987, 1743–1748.
9. (a) Leuenberger, H. G. W.; Boguth, W.; Barner, R.; Schmid, M.; Zell, R. Helv. Chim. Acta 1979, 455–463. (b) Ferraboschi,
P.; Fiecchi, A.; Grisenti, P.; Santaniello, E. J. Chem. Soc., Perkin Trans. 1 1987, 1749–1752.
10. The alcohols 1a and 1b were prepared according to Ref. 5a; the synthesis of alcohol 1c was carried out as described in
Ref. 5b. Oxidation to the aldehydes 7a–c with MnO2 in chloroform at room temperature (16 h) gave 55–65% of isolated
products. Reaction of the aldehydes with trimethyl orthoformate in methanol in the presence of NH4Cl at reflux overnight
afforded the acetals 8a–c (73–83% of isolated products).
11. The biohydrogenation of aldehydes 7 and acetals 8 constantly afforded variable amounts of the unsaturated alcohols 1 that
are the products of bioreduction of the substrates and require longer times (14 days, see Ref. 5a) for their biohydrogenation
to the alcohols 2. Prolonged incubation with fermenting baker’s yeast leads also to the formation of the saturated acids.
This oxidative mechanism of baker’s yeast has already been observed for other substrates, see: Sato, T.; Hanayama, K.;
Fujisawa, T. Tetrahedron Lett. 1988, 29, 2197–2200.
12. Typically, to a solution of sucrose (15.4 g) in water (280 ml) baker’s yeast (31 g) was added. The suspension was kept
at 30°C, under vigorous stirring (0.5 h), then a solution of 8a (0.3 g, 1.56 mmol) in ethanol (3 ml) was added over 3
days (five additions). The reaction progress was monitored by GLC (HP-5, oven temperature 130°C). The mixture was
filtered through a Celite pad; the aqueous solution was extracted with diethyl ether (3×100 ml) and after usual work-up,
the alcohol 2a (0.14 g, 60%) was obtained from silica gel column chromatography (1/20) by elution with hexane/ethyl
acetate (7/3).
13. The stereochemical outcome is the same for all three substrates, the difference in the configuration being only a
consequence of the priorities of the groups.

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14. The enantiomeric excess of the alcohols 2a–c was established by 1H NMR (500 MHz) analysis of the corresponding (R)-
MTPA esters, comparing the resonances of the MTPA ester from the (RS)-alcohol and the same derivative of the products
from the biotransformation, as described in Ref. 5a and b.
15. The R configuration of (+)-alcohol 2d was established by comparison with the published values of specific rotation: Suzuki,
K.; Katayama, E.; Matsumoto, T.; Tsuchihashi, G. Tetrahedron Lett. 1984, 25, 3715–3718. The ee was determined by 1H
NMR (500 MHz) analysis of (R)-MTPA esters of (RS)-2d (obtained by NaBH4 reduction of 2-propionaldehyde) and of
(R)-2d. Only the decoupling technique allowed a significant spectrum to be obtained: by irradiation of methyl group signal
(a doublet centered at 1.268 ppm) of the (RS)-derivative the proton at C-2 showed two triplets centered at 3.157 and 3.174
ppm. In the same derivative from the (R)-alcohol 2d only the triplet at 3.157 was present.
16. Santaniello, E.; Ferraboschi, P. In Advances in Asymmetric Synthesis; Hassner, A., Ed.; JAI Press: New York, 1997; Vol.
2, pp. 237–283.