Biocatalytic asymmetric aldol addition into unactivated ketones

Aldolases are prodigious C-C bond forming enzymes, but their reactivity has only been extended past activated carbonyl electrophiles in special cases. We have used a pair of pyridoxal-phosphate-dependent aldolases to probe the mechanistic origins of this limitation. Our results reveal how aldolases are limited by thermodynamically favorable proton transfer with solvent, which undermines aldol addition into ketones. However, we show how a transaldolase can circumvent this limitation by protecting the enzyme-bound enolate from solvent protons and thereby enabling efficient addition into unactivated ketones. The resulting products are non-canonical amino acids with side chains that contain chiral tertiary alcohols. This study reveals the principles for extending aldolase catalysis beyond its previous limits and enables convergent, enantioselective C-C bond formation from simple starting materials.