Enzymatic Chemoselective Aldehyde–Ketone Cross‐Couplings through the Polarity Reversal of Methylacetoin

Abstract

The thiamine diphosphate (ThDP) dependent enzyme acetoin:dichlorophenolindophenol oxidoreductase (Ao:DCPIP OR) from Bacillus licheniformis was cloned and overexpressed in Escherichia coli. The recombinant enzyme shared close similarities with the acetylacetoin synthase (AAS) partially purified from Bacillus licheniformis suggesting that they could be the same enzyme. The product scope of the recombinant Ao:DCPIP OR was expanded to chiral tertiary α‐hydroxy ketones through the rare aldehyde–ketone cross‐carboligation reaction. Unprecedented is the use of methylacetoin as the acetyl anion donor in combination with a range of strongly to weakly activated ketones. In some cases, Ao:DCPIP OR produced the desired tertiary alcohols with stereochemistry opposite to that obtained with other ThDP‐dependent enzymes. The combination of methylacetoin as acyl anion synthon and novel ThDP‐dependent enzymes considerably expands the available range of CC bond formations in asymmetric synthesis.

(S)‐Selectivity in Phenylacetyl Carbinol Synthesis Using the Wild‐Type Enzyme Acetoin:Dichlorophenolindophenol Oxidoreductase from Bacillus licheniformis

“…We have recently demonstrated that Ao:DCPIP OR from Bacillus licheniformis, cloned and overexpressed in E. coli, is also capable to mediate the non-physiological 1,2-addition of methylacetoin (donor) to activated ketones (acceptors) yielding chiral tertiary α-hydroxy ketones with high efficiency (Scheme 1, route b). [24] Complete control of chemoselectivity could be achieved in this transformation due to the hitherto unreported use of methylacetoin, whose activation occurs with elimination of unreactive acetone. In addition, some of the reaction products were formed with opposite stereochemistry compared to that obtained using other ThDP-dependent enzymes.…”

“…[24] As a logical extension of that study on the formal aldehyde-ketone cross-carboligation reaction, we planned to examine the efficiency of the Ao:DCPIP OR-methylacetoin enzyme-substrate pair in the mixed benzoin-like reaction with aromatic aldehydes to access the class of valuable phenylacetyl carbinols, eventually with unusual (S)-configuration. Gratifyingly, condensation of methylacetoin (3) with benzaldehyde (5) under similar conditions to those previously described for the synthesis of chiral tertiary alcohols 4 [24] [3 (20 mM), 5 (20 mM), phosphate buffer pH 6.5 (50 mM), DMSO (10% v/v), MgSO 4 (0.9 mM), ThDP (0.4 mM), purified and lyophilized Ao:DCPIP OR 0.5 mg mL −1 , 30 °C, 12 h], afforded the enantioenriched (S)-1-hydroxy-1- The (S)-configuration of 6 was further confirmed by comparison with an authentic sample of (R)-PAC prepared using the highly (R)-selective cyclohexane-1,2-dione hydrolase (CDH). [25] Hence, the first important result of this explorative study was the confirmation of Ao:DCPIP OR peculiarity to promote, in some occasions, carboligations with opposite stereochemical outcome compared to other wt ThDPdependent enzymes.…”

“…[25,29] The donor substrate range of the Ao:DCPIP ORcatalyzed cross-benzoin reaction was briefly investigated in an explorative study on the carboligation of benzaldehyde ( 5) with 3,4hexanedione (30) for the challenging synthesis of 1hydroxy-1-phenylbutan-2-one (phenylpropionyl carbinol, PPC) 31 with (S)-selectivity (Scheme 4). [30] It is important to remember that alkyl α-diketones are highly reactive donors in Ao:DCPIP OR catalysis; [24,31] however, their utilization in mixed condensations with carbonyl acceptors is complicated by the occurrence of the α-diketone homocoupling side-reaction, which reduces the chemoselectivity of the coupling process. Nevertheless, the crossbenzoin-type reaction of 30 and 5 was attempted to gain information about the Ao:DCPIP OR capability to promote the (S)-selective synthesis of PPC derivatives.…”

“…Indeed, the carboligation of methylacetoin (3) (acetyl anion donor) with differently substituted benzaldehydes afforded the target α-hydroxy ketone derivatives with good levels of conversion efficiency and enantioselectivity and, significantly, with almost complete chemoselectivity. In anticipation of a future improvement based on the use of the higher homolog of methylacetoin, the propionyl anion transfer on benzaldehyde was attempted in this study employing the α-diketone 3,4-hexanedione (30) OR belongs to the narrow group of ThDP-dependent enzymes capable to promote asymmetric aldehydeketone cross-couplings yielding chiral tertiary alcohols; [24] this constitutes a further evidence of the tremendous catalytic potential of Ao:DCPIP OR in asymmetric carboligation reactions. The straightforward production, robustness and facile immobilization of Ao:DCPIP OR [31b] are additional values of this enzyme for the development of novel synthetic applications and technological improvements.…”