Synthesis of Erythrina and Related Alkaloids.XXIV. Total Synthesis of Erysotrine from 1, 7-Cycloerythrinan Derivatives by the Use of a New 1, 2-Carbonyl Transposition Method

Abstract

Treatment of 2, 8-dioxo-1, 7-cycloerythrinans with phenylselenenyl chloride in the presence of BF₃·Et₂O as a catalyst gave 3-chloro-3-phenylselenenyl derivaties through the 3-phenylselenenyl derivative, which changed into the Δ³-3-phenylselenenyl derivative on further reaction.Both the 3-phenylselenenyl and 3-chloro-3-phenylselenenyl derivatives gave the 3, 3-dimethoxy derivative on treatment with mercury (II) perchlorate (MPC) in methanol, thus providing a new method for introduction of a masked carbonyl group at the α-position to the original carbonyl group.Thus, the reaction of 1 with phenylselenenyl chloride under acidic conditions followed by MPC treatment in methanol and borohydride reduction gave the 2α-hydroxy-3, 3-dimethoxy derivative in 57% yield.This was converted to the conjugated ketone in four steps (72%).The carbomethoxy group of this compound was removed by the CaCl₂-dimethyl sulfoxide-3-ethylpentane-3-thiol method to give the enone 4(70-80%) which isomerized to the conjugated ketone 25c(100%).This ws converted to a natural Erythrina alkaloid, (±)-erysotramidine (5), in four steps (31%), and thence to (±)-erosotrine (6).