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The congested nature of quaternary carbons hinders their preparation, most notably when stereocontrol is required. Here we report a biocatalytic method for the creation of quaternary carbon centers with broad substrate scope, leading to different compound classes bearing this structural feature. The key step comprises the aldol addition of 3,3-disubstituted 2-oxoacids to aldehydes catalyzed by metal dependent 3-methyl-2-oxobutanoate hydroxymethyltransferase from E. coli(KPHMT) and variants thereof. The 3,3,3-trisubstituted 2-oxoacids thus produced were converted into 2-oxolactones and 3-hydroxy acids and directly to ulosonic acid derivatives, all bearing gem-dialkyl, gem-cycloalkyl, and spirocyclic quaternary centers. In addition, some of these reactions use a single enantiomer from racemic nucleophiles to afford stereopure quaternary carbons. The notable substrate tolerance and stereocontrol of these enzymes are indicative of their potential for the synthesis of structurally intricate molecules.

Aldolase and transaminase coexpressed in Escherichia colicells and lyophilized (i.e., lyophilized whole-cell biocatalyst (LWCB)) were used as biocatalysts for the one-pot cascade synthesis of l-homoserine with substrate cycling. The kinetic analysis of enzymes within lyophilized cells was performed to evaluate the behavior of the system. The best result among the performed fed-batch reactor experiments achieved was 640.8 mM (76.3 g L–1) of l-homoserine with a volume productivity of 2.6 g L–1h–1. This is comparable with the results of the same cascade synthesis using cell-free extracts (CFEs) and significantly better than the reports in the literature applying fermentation technology. The approach applied here can serve as guidance for the design of microbial cells with an optimal ratio of expressed enzymes that act as biocatalysts in the cascade, resulting in lower biocatalyst cost, no need for the addition of expensive coenzymes, and enhanced enzyme stability as compared with cell-free extracts.