Synthesis of fused furans by gas-phase pyrolysis of2-allyloxyarylpropenoic esters1

Flash vacuum pyrolysis of 2-allyloxypropenoic esters (e.g. 7) gives benzo[b]furans (e.g. 32) in synthetically useful yields by sequential generation of a phenoxyl radical, cyclisation and ejection of the carboxylic ester function as a free radical leaving group. The method is compatible with a range of substituents on either the benzene ring or the propenoate chain, and is particularly effective for 2-substituted benzo[b]furans. The natural products 5-methoxybenzo[b]furan 1 and angelicin 2 have been synthesised in three and four steps respectively from commercially available starting materials by this route. Related cyclisations to give naphtho[2,1-b]furan 40 were complicated by competitive formation of naphtho[2,1-b]pyran-3-ones (e.g. 41 and 42), but the yield of the required product could be optimised by the choice of the radical precursor. Annelation of a furan ring onto a thiophene is also possible by this method, but lower yields are obtained in such pyrolyses.