Unusual Stereoselectivity in Sialic Acid Aldolase-Catalyzed Aldol Condensations: Synthesis of Both Enantiomers of High-Carbon Monosaccharides

An inversion of stereoselectivity in aldol condensations catalyzed by sialic acid aldolase (from Escherichia coli, Shinko American Inc.) was observed when l-mannose, 6-deoxy-l-mannose, l-talose, 2-deoxy-l-glucose, 2-deoxy-l-rhamnose, N-acetyl-l-mannosamine, d-gulose, d-arabinose, and 2-azido-2-deoxy-l-mannose were used as acceptor substrates. In all substrates tested, except the last three, a complete inversion of stereoselectivity was observed; i.e., the C-nucleophile of pyruvate attacks there face of the acceptor carbonyl instead of the si face as in the normal case for the enantiomeric substrates. Examination of the product distribution during the course of enzymatic reactions indicates that the stereoselectivity is thermodynamically controlled in nature; i.e., attack on there face would take place if the resulting product would be more stable than the one from the si face attack. Both enantiomers of several high-carbon monosaccharides are now accessible via the aldolase reactions. A new practical procedure has also been developed for the preparation of the aldolase products where unreacted pyruvate (usually used in 7-fold excess to drive the reaction) is decomposed with pyruvate decarboxylase to simplify product isolation.