Thermal lactonization of brominated alkylidenemalonates: Synthesis of 2‐buten‐4‐olides

Abstract
Heating of dialkyl 2‐bromoalkylidenemalonates in boiling xylene resulted in lactonization with formation of 2‐alkoxycarbonyl‐2‐buten‐4‐olides. Bromination of dimethyl 2‐phenylpropylidenemalonate with NBS in CCI4 gave the corresponding butenolide. However, heating of the 2‐bromocyclohexylidenemalonate only afforded the dehydrobromination product.