Photobiocatalytic Diastereo- and Enantioselective Synthesis of Non-Canonical Amino Acids Bearing Challenging Stereochemical Dyads and Triads

Abstract

Catalyst-controlled stereodivergent synthesis of acyclic molecules bearing multiple stereogenic centers remains a longstanding challenge in asymmetric catalysis, particularly through intermolecular radical C-C coupling. Here, we report the directed evolution of pyridoxal 5'-phosphate (PLP)-dependent biocatalysts for the stereodivergent synthesis of non canonical amino acids bearing either skipped , {gamma}-stereochemical dyads or contiguous , {beta}, {gamma}-stereochemical triads. Starting from a thermophilic tryptophan synthase {beta}-subunit, protein engineering generated complementary enzyme variants that selectively construct three of the four possible , {gamma}-stereoisomers through biocatalyst-controlled reversal of both remote {gamma}-stereochemistry and proximal -stereochemistry. These engineered enzymes catalyzed intermolecular radical C-C bond formation with a broad range of secondary alkyltrifluoroborates, affording , {gamma}-disubstituted amino acids in excellent yields and stereoselectivities. Furthermore, directed evolution enabled efficient {beta}-dehydroxylative radical coupling of the non-native threonine-derived aminoacrylate intermediate, providing amino acids bearing contiguous , {beta}, {gamma}-stereochemical triads with up to 96:4:<0.1:<0.1 d.r. and >99:1 e.r.. To further demonstrate the synthetic utility of this platform, the stereochemically enriched amino acids were transformed into valuable {gamma}-chiral amines through engineered PLP dependent tryptophan decarboxylases, establishing a dual PLP enzyme based biocatalytic cascade for the enantiodivergent synthesis of privileged amine scaffolds. Molecular dynamics simulations reveal that enzyme controlled {gamma}-stereoselectivity originates from active-site remodeling that modulates {pi}-{pi} interactions and steric contacts governing the facial selectivity of the radical addition step. This work expands the synthetic capabilities of radical biocatalysis by enabling catalyst-controlled stereodivergent assembly of complex acyclic stereochemical arrays and establishes engineered PLP enzymes as versatile catalysts for the synthesis of stereochemically complex amino acids and chiral amines.