Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters

The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe₂, CHPh₂, c-C₆H₁₁ and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate bearing the OTBDMS group anti to the γ-benzyloxy group showed poor diastereoselectivity, but its diastereomer gave syn-adduct exclusively. The high syn-selectivitiy is referred to the H-atom transfer to the outside face of radical center in the sharply folded seven-membered chelate intermediate bearing the ethoxy group with Z-geometry. The corner flapping of the radical center atom of the global minimum energy conformer generates a local minimum conformer and the H-atom transfer to the outside face of the radical center of the newly formed structure gives the anti-adduct. The poor diastereoselectivity is due to the very small energy difference between the two conformers and consequently both the syn- and anti-adducts are yielded in nearly equal amounts.