Unusual oxidation in thionyl chloride: novel synthesis of methyl 3-alkoxy-1,4-dioxo-1,2,3,4-tetrahydroisoquinoline-3-carboxylates

Abstract

Oxidation of methyl 1,4-dioxo-1,2,3,4-tetrahydroisoquinoline-3-carboxylate 1 with thionyl chloride gives an unstable intermediate that reacts with alcohols to give 3-alkoxy-1,4-dioxo-1,2,3,4-tetrahydro-isoquinoline-3-carboxylates (e.g. 6) and that is reduced with N-acetylcysteine to regenerate 1. In this system thionyl chloride is a convenient alternative oxidant to lead tetraacetate for the preparation of these masked equivalents of the unusual and reactive dienophile methyl 1,4-dioxo-1,4-dihydroisoquinoline-3-carboxylate 7.