Studies toward Total Synthesis of Non-aromatic Erythring Alkaloids. (6). Synthesis of 8-OXO-γ-erythroidine and 8-Oxo-cycloerythroidine, Isomers of the Natural Alkaloids.

Abstract

A study directed to the total synthesis of β-erythroidine 1, a non-aromatic Erythrina alkaloid, was conducted based on a strategy involving construction of D-furanoerythrinan via Diels-Alder reaction of furodioxopyrroline and the conversion of the resulting furan to the δ-lactone via oxidative fission of the furan ring followed by one-carbon homologation. Oxidation of the furanoerythrinan 17 with N-bromoacetamide followed by treatment with Nafion-H gave the enol γ-lactone 27. Alkaline hydrolysis of 27 followed by methylation with diazomethane gave the keto-ester 31. Alkylation of 31a with dimethylsulfoxonium methylide gave 8-oxo-γ-erythrodine (5). One-carbon homologation of 31a by Yamakawa's method using chloromethyl phenyl sulfoxide resulted in the formation of 8-ozocycloerythroidine (6). Compounds 5 and 6 are structural isomers of natural 8-oxo-β-erthroidine (2).