Dynamic kinetic resolution of tert-butyl 4-methyl-3,5-dioxohexanoate through enzymatic reduction

Abstract

An entirely new method for the dynamic kinetic resolution of a racemic, 2-methyl substituted, unsymmetrical 1,3-diketone via enzymatic reduction to give an enantiomerically pure compound is introduced.

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The dynamic kinetic resolution of α-substituted β-keto esters by chemical¹ or biocatalytical² reduction is particularly useful due to the simultaneous introduction of two stereogenic centres into the molecule in combinationwith a theoretical maximum yield of 100%. Although this method has proven broad applicability in stereoselective synthesis, the corresponding dynamic kinetic resolution of 2-substituted 1,3-diketones is rarely found in the literature.³ Our aim is directed toward extending dynamic kinetic resolution to enantio- and regioselective reduction of alkyl-substituted 3,5-dioxoesters, which would enable the introduction of up to four stereogenic centers by two consecutive reduction steps.

The attempted enantioselective ketone reduction of 3,5-dioxohexanoate esters by chemical methods⁴ or biotransformation⁵ usually results in complex mixtures of several stereo- and regioisomeric products with one or both keto groups reduced. We figured out that this difficult transformation can be accomplished by using isolated enzymes to afford optically pure 5-hydroxy-3-oxohexanoates in high yield.⁶ Herein we wish to report in preliminary form on the first enantio- and regioselective enzymatic reduction of 4-alkyl-3,5-dioxohexanoates resulting in formation of one out of a total of 8 monoreduction and 8 bisreduction products.

tert-Butyl 4-methyl-3,5-dioxohexanoate (1) was prepared by acylation of the bisenolate of tert-butyl 3-oxovalerate with commercially available Weinreb acetamide.⁷ For the enzymatic reduction recombinant alcohol dehydrogenase from Lactobacillus brevis (recLBADH) was chosen, which has been cloned and overexpressed in E. coli.⁸ recLBADH exhibits a broad substrate range and considerable stability even towards highly reactive compounds like 6-chloro-3,5-dioxohexanoates.^6,8 Cofactor (NADPH) regeneration succeeds via a coupled-substrate process. Propan-2-ol (200 mM) was applied in excess to the reaction mixture as an auxiliary substrate in order to shift the equilibrium of the reaction towards the desired direction (Scheme 1).⁹

Scheme 1 Reagents and conditions: i, 1 (20 mM), propan-2-ol (200 mM), NADP⁺ (1 mM), pH 6.5, recLBADH (360 U), 23 h, rt (66%).

NMR data of the major product (4S,5R)-2 which was obtained in 66% isolated yield, clearly proved the regioselective monoreduction of the keto group at C-5. Additionally, from GC-MS data of the crude product after derivatisation with (F₃CCO)₂O, pyridine, no evidence could be found for the reduction of the keto group at C-3. In order to verify the proposed absolute configuration and to enable precise determination of the enantiomeric excess, (4S,5R)-2 was transformed through sodium borohydride reduction into lactone 4via diol (3RS,4S,5R)-3. Lactonisation and dehydration gave the unsaturated lactone (5R,6R)-4 which is known in racemic form¹⁰ (Scheme 2).

Scheme 2 Reagents and conditions: i, NaBH₄, EtOH, 0 °C; ii, cat. TsOH, toluene, reflux, 2 h (60% over two steps).

As a standard a racemic 1:1 mixture of syn- and anti-lactonerac-4 was synthesised from keto ester 5 by sodium borohydride reduction, subsequent chain elongation, and, finally, lactone formation as described above (Scheme 3). The four stereoisomers of syn/anti-rac-4, which were formed in equal amounts, can be separated by HPLC on chiral stationary phase (Daicel Chiracel OB).

Scheme 3 Reagents and conditions: i, NaBH₄, EtOH, 0 °C (83%); ii, CH₂[double bond, length half m-dash]C(OLi)OtBu, THF, −30 °C (53%); iii, NaBH₄, EtOH, 0 °C; iv, cat. TsOH, toluene, reflux, 2 h (60% over two steps).

An authentic sample of the enantiomeric syn-lactone (5S,6S)-4 was synthesised by the same sequence starting from bakers’ yeast reduction of 5via the known¹¹ ethyl (2R,3S)-2-methyl-3-hydroxybutyrate (2R,3S)-6 (Scheme 4).

Scheme 4 Reagents and conditions: i, bakers’ yeast, 10% aq. EtOH, (50%); ii, CH₂[double bond, length half m-dash]C(OLi)OtBu, THF, −30 °C; iii, NaBH₄, EtOH, 0 °C; iv, cat. TsOH, toluene, reflux, 2 h (53% over three steps).

The spectroscopic data (¹H-NMR, ¹³C-NMR, MS) of (5S,6S)-4 and of (5R,6R)-4, produced via enzymatic (recLBADH) reduction of 1, are identical. Comparison of the CSP-HPLC data of both lactones revealed the (4S,5R)-absolute configuration for the product 2 of the recLBADH reduction. This product is formed in almost enantiomerically pure form (99.2% ee, HPLC data); the diastereomeric ratio of syn∶anti 97∶3 is likewise very high (NMR and HPLC data).

In summary, we have shown the regio- and enantioselective reduction of tert-butyl 4-methyl-3,5-dioxohexanoate via dynamic kinetic resolution to give an almost enantiomerically and diastereomerically pure compound introducing two stereogenic centers can be done efficiently by enzyme-catalysed reduction. This method represents a novel entry into the chemistry of polypropionates based on a biomimetic approach via polyketides. This method should be extendable towards dynamic kinetic resolution of other 2-alkyl-substituted unsymmetrical 1,3-diketones.

Acknowledgements

A. Ji is the recipient of a Chinese Government Fellowship. The skilful technical assistance of Mrs Silke Bode is gratefully acknowledged. We thank the Deutsche Forschungsgemeinschaft (SFB 380) for financial support.

Notes and references

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