Cross-acyloin condensation of aldehydes catalysed by transketolase variants for the synthesis of aliphatic α-hydroxyketones

We described a strategy for the enzymatic synthesis of 1-deoxy and 1,2-deoxyketoses from the aliphatic α-ketoacids, pyruvate and 2-oxobutyrate, as donors and natural aldoses of variable chain length as acceptors, catalyzed by thermostable transketolase variants from Geobacillus stearothermophilus (TKgst). Analytical studies have been carried out on a panel of TKgst variants with the appropriate substrates allowing to select the best combinations and to apply it to the preparative scale synthesis of 1-deoxy and 1,2-deoxyketoses obtained with good to excellent isolated yields (61%–86%). To optimize the strategy, and as a proof of principle, the α-ketoacids pyruvate and 2-oxobutyrate were generated in situ from the corresponding d-amino acids d-alanine and d-homoalanine respectively, using a thermostable d-amino acid oxidase dAAO4536 that was selected from a screening of 55 putative DAAOs provided by Prozomix Limited. Hence, a one-pot one step procedure was performed at 50°C by coupling dAAO4536 and the best TKgst variant H102L/L118I/ H474S in the presence of d-alanine or d-homoalanine as α-ketoacids precursors and d-erythrose as acceptor substrate. The corresponding 1-deoxy and 1,2-dideoxyketoses were isolated with good yields (64% and 72% respectively, out of two steps)