Radical Cyclization in Heterocycle Synthesis. 12. Sulfanyl Radical Addition-Addition-Cyclization (SRAAC) of Unbranched Diynes and Its Application to the Synthesis of A-Ring Fragment of 1.ALPHA.,25-Dihydroxyvitamin D3.

Sulfanyl radical addition-addition-cyclization (SRAAC) of unbranched diynes proceeded smoothly to give cyclized exo-olefins, while the sulfanyl radical addition-cyclization-addition (SRACA) of diynes having a quaternary carbon gave cyclized endo-olefins. This method was successfully applied to the synthesis of A-ring fragment of 1alpha,25-dihydroxyvitamin D3.