Unexpected Formation of Thiophene in the Pyrrole Synthesis from Methoxyallene and Methyl Isothiocyanate

Abstract

The sequential reaction of lithiated methoxyallene, methyl isothiocyanate, and 2-(bromomethyl)-1,3-dioxolane in the presence of CuBr yields a 2,3-disubstituted thiophene instead of the expected pyrrole. The process is accomplished in one preparative stage and involves intramolecular cyclization of the in situ-generated lithium allenylimidothioate and N-alkylation of the resulting lithium thienylamide. Varying the reaction conditions does not affect its route: in all cases, the only product is N-(1,3-dioxolan-2-ylmethyl)-3-methoxy-N-methylthiophen-2-amine.