Cycloaddition Approach to Substituted Thiophenes

Significance

Reported is a metal-free annulation protocol for accessing functionalized thiophenes by reaction of dialkyl (Me and Et) acetylene dicarboxylates with β-keto dithioesters. The latter were synthesized from the corresponding aryl ethan­ones with trithiocarbonate (R. Samuel et al. ­Tetrahedron Lett. 2007, 48, 8376) generally in ˜50% yields. The reaction itself is mediated by DMAP and is exceedingly rapid (3-5 min). DABCO and DBU were able to effect the reaction, albeit with longer reaction times (DABCO) or slightly lower yield (90% DMAP vs. 78% DBU). Use of a catalytic amount of base required longer reaction times (40% yield after 30 min using 50 mol% DMAP), which was not studied further to ascertain whether the reaction goes to completion. The substrate scope was well studied and the yields are generally good.