2018 年 76 巻 3 号 p. 218-225
This paper describes racemic syntheses of naturally occurring lactams, epolactaene, berkeleyamide D, rubrobramide, flavipucine, and isoflavipucine. The key step is a regioselective Darzens reaction between glyoxals and α-bromo-β-ketoamides. (±)-Epolactaene was synthesized by a Darzens reaction between methylglyoxal and α-bromo-β-ketoamide. The synthesis of (±)-berkeleyamide D was accomplished in only four steps from the reported starting material. The key features of this synthesis include the construction of a spirocyclic ring system by a C-acylation reaction followed by an intramolecular spirocyclization via an epoxide-opening reaction. Following optical resolution by chiral HPLC, the absolute configurations of both enantiomers of berkeleyamide D were determined by the vibrational circular dichroism exciton chirality method. The construction of the core tricyclic ring system of (±)-rubrobramide was achieved by a cascade reaction in a single step from an α,β-epoxy-γ-lactam. The α,β-epoxy-γ-lactam was prepared by the Darzens reaction. The absolute configuration of naturally occurring (+)-rubrobramide was determined by vibrational circular dichroism exciton chirality method. (±)-Flavipucine and isoflavipucine were synthesized from an epoxyketone, which was prepared by reaction of isobutylglyoxal with a protected α-bromo-β-ketoamide. Removal of the protective groups in the epoxyketone, and formation of the pyridone ring gave (±)-flavipucine, which was converted into isoflavipucine by thermal isomerization.