Biocatalytic asymmetric aldol addition into unactivated ketones

Abstract

Enzymes are renowned for their catalytic efficiency and selectivity, but many classical transformations in organic synthesis have no biocatalytic counterpart. Aldolases are prodigious C–C bond-forming enzymes, but their reactivity has only been extended past activated carbonyl electrophiles in special cases. To probe the mechanistic origins of this limitation, we use a pair of aldolases whose activity is dependent on pyridoxal phosphate. Our results reveal how aldolases are limited by kinetically favourable proton transfer with solvent, which undermines aldol addition into ketones. We show how a transaldolase can circumvent this limitation, enabling efficient addition into unactivated ketones. The resulting products are highly sought non-canonical amino acids with side chains that contain chiral tertiary alcohols. Mechanistic analysis reveals that transaldolase activity is an intrinsic feature of pyridoxal phosphate chemistry and identifies principles for extending aldolase catalysis beyond its previous limits to enable convergent, enantioselective C–C bond formation from simple starting materials.