Stereoselective Formation of Eight-Membered Rings by Radical Cyclization of Silylenedioxy-Tethered Bis-methacrylate Derivatives

Radical-initiated addition of CCl4, Cl3CBr,
PhSH, and (TMS)3SiH to (bisisopropyl)­silylene­dioxy-tethered
bis-methacrylate derivatives gives the corresponding eight-membered
ring cyclic adducts stereoselectively. Hydrolysis of halo-substituted
cyclic adducts with HCl in methanol affords the corresponding valerolactones,
and the stereochemistry was determined by the X-ray crystallography
on a dibromobenzoate derivative. DFT calculation on the eight-membered
radical intermediate offers a plausible rationale for the stereoselectivity
of the reaction.