SYNTHESIS OF FUSED FURANS BY GAS-PHASE PYROLYSIS OF 2-ALLYLOXYARYLPROPENOIC ESTERS

Flash vacuum pyrolysis of 2-allyloxypropenoic esters (e.g. 7) gives be nzo[b]furans (e.g. 32) in synthetically useful yields by sequential ge neration of a phenoxyl radical, cyclisation and ejection of the carbox ylic ester function as a free radical leaving group, The method is com patible with a range of substituents on either the benzene ring or the propenoate chain, and is particularly effective for 2-substituted ben zo[b]furans. The natural products 5-methoxybenzo[b]furan 1 and angelic in 2 have been synthesised in three and four steps respectively from c ommercially available starting materials by this route, Related cyclis ations 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 yiel d of the required product could be optimised by the choice of the radi cal precursor, Annelation of a furan ring onto a thiophene is also pos sible by this method, but lower yields are obtained in such pyrolyses.