Allenic Compounds and Isothiocyanates as Key Building Units in the Synthesis of Heterocycles

Reaction between lithiated allenic compounds and isothiocyanates, R1N=C=S, in most cases gives exclusively thioimidates with the allenic structure, C=C=CC(SLi)=NR1. These intermediates have been used in novel approaches to 2,3-dihydropyridines, pyrroles, quinolines, cyclobutanopyrrolines, and thiophene or dihydrothiophene derivatives. The procedures leading to the heterocycles with a nitrogen atom in the ring involve S-alkylation followed by simple heating or by treatment with copper(I) halide. 2-Aminothiophenes and 2-imino-2,5-dihydrothiophenes are formed by intramolecular nucleophilic attack of the thiolate moiety on the allenic system and subsequent addition of methyl iodide or protonation.