Synthetic studies towards naturally occurring γ-(Z)/(E)-alkylidenebutenolides through bimetallic cascade cyclization and an adventitious photoisomerization method

A general and flexible visible light-induced photoisomerization method of γ-(Z)-alkylidenebutenolides to their corresponding E-components was reported in this article. Initially, a series of naturally occurring enantiopure γ-(Z)-alkylidenebutenolides was synthesized by employing a "Pd-Cu" bimetallic cascade cyclization protocol. In the later part, the synthesized γ-(Z)-alkylidenebutenolides were photoisomerized in the presence of a triplet photosensitizer to γ-(E)-alkylidenebutenolides in reasonably acceptable yields. Total synthesis of goniobutenolides, hygrophorones, ramariolide D, melodorinols/acetyl-melodorinols, versicolactones, and phomopsolidones was achieved by employing the developed methods.