Reaction of stabilised arsonium ylides with acetylenic esters: convenient ring synthesis of a tetrasubstituted benzene

Reaction of stabilised arsonium ylides such as 24 with methyl and ethyl propiolate in benzene gives the beta,gamma-unsaturated arsonium ylides 25 and 27 resulting from net insertion of the alkyne fragment into the C=As bond, but the isomeric ylides 26 and 28 corresponding to net insertion into the C-H bond when the reaction is carried out in methanol, The assignment of the structures is confirmed in the case of 25 by an X-ray structure determination. The corresponding phosphonium ylide 29 reacts with methyl propiolate to give 31 in either benzene or methanol in contrast to an earlier report. Further reaction of 25 with DMAD proceeds with net insertion of the alkyne into the C=C double bond and spontaneous intramolecular cyclisation to give the tetrasubstituted benzene derivative 39.