Anionic [4 + 2] cycloaddition with thiophthalides: an integrated approach to the synthesis of olivin and pillaromycinone

Abstract

A systematic study of the potential reactivity of the thiophthalides 4 and 18–20 as 1,4-dipolar synthons, has shown that 3-phenylthiothiophthalide 19 is the best annulating agent for the preparation of tricyclic intermediates related to olivin. The reagent 19 underwent anionic [4 + 2] cycloaddition with cyclohex-2-enone 5a in the presence of lithium tert-butoxide to give the anthracenones 24a and 25 in a combined yield of 90%.