A Bioinspired Synthesis of (±)-Rubrobramide, (±)-Flavipucine, and (±)-Isoflavipucine

A biomimetic synthesis of naturally occurring lactams rubrobramide, flavipucine, and isoflavipucine is described. The key step is a regioselective Darzens reaction between isobutyl glyoxal and an alpha-bromo-beta-ketoamide. The construction of the core tricyclic ring system of rubrobramide was achieved by a cascade reaction in a single step from an a, beta-epoxy-gamma-lactam. Furthermore, the absolute configuration of naturally occurring (+)-rubrobramide was determined by vibrational circular dichroism. (+/-)-Flavipucine and (+/-)-isoflavipucine were synthesized from an epoxyimide, which was prepared by reaction of isobutyl glyoxal with a protected a-bromo-b-ketoamide. Deprotection of the epoxyimide and formation of the pyridone ring gave (+/-)-flavipucine, which was converted into (+/-)-isoflavipucine by thermal isomerization.