Repurposing an Aldolase for the Chemoenzymatic Synthesis of Substituted Quinolines

Quinoline derivatives are important natural products and pharmaceuticals, but their synthesis can be challenging due to poor yields, harsh reaction conditions, and instability of starting materials. Here we report the chemoenzymatic synthesis of quinaldic acids under mild conditions using an aldolase, trans-o-hydroxybenzylidenepyruvate hydratase-aldolase (NahE, or HBPA). A series of 2-aminobenzaldehydes derived from reduction of the corresponding nitro analogue were reacted with pyruvate in the presence of NahE to give substituted quinolines in up to 93% isolated yield. This reaction differs from the aldol condensation catalyzed by NahE in vivo, instead resembling the heterocycle formation catalyzed by its homologue, dihydrodipicolinate synthase.