Radical cyclization of homopropargylic bromomethyldimethylsilyl ethers. Evidence for a hetero-trans-cycloheptene intermediate

This communication describes our preliminary results on the radical cyclization of homopropargylic bromomethyldimethylsilyl ethers, an unknown reaction. With precursor 5a, only the very rare 7-endo-dig process was observed. The reduction of the resulting vinyl radical is not stereoselective and gives a cis/trans, 1.35:1 mixture of silaoxepan derivatives, as determined after a methyllithium treatment yielding olefins 11 and 12. This represents a new example of formation of a trans heterocyclic cycloheptene. With disubstituted precursor 5b, the system is less reactive and still no 6-exo-dig radical cyclization has been observed.