Sequential radical cyclization of β-functionalized allyl bromomethyldimethylsilyl ethers.: Application to the regio- and stereo-specific synthesis of an isoprostanoid precursor

The behaviour of allyl bromomethyldimethylsilyl ethers beta-substituted by various radical trapping functions (aldehyde, nitrile or acetylenic) is studied in tandem radical cyclizations. Only homopropargylic ethers (but-3-ynylic ethers) lead to the formation of cyclic compounds via a 5-exo-trig, 5-exo-dig or 5-exo-trig, 6-endo-dig mode. The influence of the TMS group located on the acetylenic moiety is shown to be determinant for the regio- and stereo-specific C5 ring closure (5-exo-dig mode).