Stereoselective amino acid synthesis by photobiocatalytic oxidative coupling
Photobiocatalysis—where light is used to expand the reactivity of an enzyme—has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts. So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes. Here we report photobiocatalytic asymmetric sp3–sp3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5′-phosphate-dependent enzymes, threonine aldolases, for the α-C–H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox–pyridoxal biocatalysis provides a platform for sp3–sp3 oxidative coupling, permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology.
