Use of 2,3-dichloropropene and 1,3-dichlorobut-2-ene as synthons for heterocyclic compounds: Synthesis of 2-methylbenzo[b]furans, 2-methylbenzo[b]thiophens, and 4-methyl-2H-chromen

The aryl 2-chloroprop-2-enyl ethers (1a-g), prepared from the appropriate phenols by treatment with 2,3-dichloropropene, underwent smooth Claisen rearrangement to the 2-(2-chloroprop-2-enyl)phenols (2a-g), acid-catalysed cyclization of which afforded the 2-methylbenzo[b]furans (4a-g). The chloropropenyl thioethers (5a-e) yielded the 2-methylbenzo[b]thiophens (6a-e) directly when heated at 200°C in NN-diethylaniline. Attempts to prepare 2-methylindoles by this method failed. 3-Chloro-1-phenoxybut-2-ene (7a) cyclized thermally to give 4-methyl-2H-chromen (9a). Similar attempts to prepare 1,2-dihydro-4-methylquinoline (12) and 4-methyl-2H-thiochromen (18) failed.