Enzymatic diastereo- and enantioselective synthesis of α-alkyl-α,β-dihydroxyketones

Abstract

An enzymatic strategy for the preparation of optically pure α-alkyl-α,β-dihydroxyketones is reported. Homo- and cross-coupling reactions of α-diketones catalyzed by acetylacetoin synthase (AAS) produce a set of α-alkyl-α-hydroxy-β-diketones (30-60%, ee 67-90%), which in turn are reduced regio-, diastereo-, and enantioselectively to the corresponding chiral α-alkyl-α,β-dihydroxyketones (60-70%, ee >95%) using acetylacetoin reductase (AAR) as catalyst. Both enzymes are obtained from Bacillus licheniformis and used in a crude form. The relative syn stereochemistry of the enantiopure α,β-dihydroxy products is assigned by NOE experiments, whereas their absolute configuration is determined by conversion of the selected 3,4-dihydroxy-3-methyl-pentan-2-one to the natural product (+)-citreodiol.

An enzymatic approach to the synthesis of optically pure (3R)- and (3S)-enantiomers of green tea flavor compound 3-hydroxy-3-methylnonane-2,4-dione

“…Information on the gene sequence and protein structure is not available for AAS and AAR of B. licheniformis but only for enzymes from bacteria of the same genus [27,31,32]. We recently found, however, that the combined use of AAS and AAR from B. licheniformis allows to prepare optically pure ␣-alkyl-␣,␤-dihydroxy ketones from 1,2-dione substrates by a two-step coupling/reduction sequence [26]. This finding prompted us to investigate the enantioselective synthesis of the flavor compound 3 by a similar approach.…”

“…Chiral GC-MS analysis of pure 3 revealed it as a scalemic mixture (ee 44%), whose major enantiomer was tentatively assigned the (3R) stereochemistry. At this stage of the work, stereochemical assignment was based on analogy to other ␣-hydroxy-1,3-diketones prepared by the same AAS-based methodology [26], but it was later confirmed through NMR investigations (vide infra).…”

“…reported, in fact, that this NADH-dependent dehydrogenase in its crude form is capable to catalyze the regio-and diastereo-selective reduction of the methyl ketone group of shorter homologues of 3 to give exclusively the corresponding syn 1,2-diols [26]. Thus, we attempted to take advantage of the same procedure for the kinetic resolution of 3.…”

“…Egg white lysozyme (10 mg) was added and the suspension was gently shaken at 30 • C for 2 h. After centrifugation (11,800 × g, 20 min, 10 • C) the supernatant was used without further purification. Cell-free extract obtained by high pressure treatment (French-press) showed similar specific activity [26].…”

“…The supernatant was used without further purification. Cell-free extract obtained using high pressure treatment (French-press) showed similar specific activity [26].…”