Pyridine vs DABCO vs TBAB in Annulations of δ-Acetoxy Allenoates with Thioamides Leading to Dihydrothiophene, Thiopyran, and Thiazole Scaffolds

Abstract

The same δ-acetoxy allenoates and thioamides, under DABCO, pyridine, or tetra-<i>n</i>-butyl ammonium bromide (TBAB) catalysis, undergo distinctly different annulations giving chemoselective routes to dihydrothiophene, thiopyran, or thiazole motifs. Thus, using pyridine in [3 + 2] annulation, dihydrothiophenes are obtained as <i>essentially single diastereomers</i>. By contrast, under DABCO catalysis, allenoates deliver thiopyran motifs in good to high yields through <i>6-exo-dig</i> cyclization. In the thiazole forming [3 + 2] annulation, tetra-<i>n</i>-butyl ammonium bromide (TBAB) facilitates addition-elimination and <i>5-exo-trig</i> cyclization in which β- and γ-carbons of allenoates participate to deliver thiazole cores exclusively with a <i>Z</i>-isomeric exocyclic double bond. A possible rationale for these observations is delved into.