Rearrangements of substituted propargyl vinyl sulfides: Formation of allenic dithioesters and their conversion into derivatives of 2H‐thiopyran and thiopene

Abstract

Abstract Compounds of the general formula R 1 CH=C(SC 2 H 5 )S‐CH 2 C 2 ≡CR 2 (II) (with R 1 =H, alkyl or aryl and R 2 =H or CH 3 ) prepared by metallation of dithioesters R 1 CH 2 C(=S)SC 2 H 5 (I) with potassium amide in liquid ammonia and subsequent reaction with R 2 C≡CCH 2 Br, rearrange at elevated temperatures. The first product is the allenic dithioester CH 2 =C=C(R 2 )‐CH(R 1 )C(=S)SC 2 H 5 (III), which can undergo cyclisation either to a 2 H ‐thiopyran derivative (IV, see reaction scheme) or to a substituted thiophene (V, see scheme I). The thiopyran formation is strongly accelerated by triethylamine, whereas (traces of) acids favour the cyclisation to thiophenes. Energies of activation for the conversion of II to III have been determined.