π-Selectivities of trans-2-heterobicyclo[4.4.0]decan-5-ones in reductions with NaBH < inf > 4 < /inf > and Na(CN)BH < inf > 3 < /inf >

Abstract

The axial selectivities of 2-heterobicyclo[4.4.0]decan-5-ones decrease from the N-benzyl-2-aza- to 2-oxa- to 2-thia-species. This does not follow the earlier proposed Cieplak's hypothesis of nheteroatom→σ *# interaction. It is rather dependent on the differential electron withdrawing abilities of the heteroatoms that translate further into differential σvicinal→π*C=O interactions in the ground states of the molecules. The heightened axial selectivity of the aza-species is probably due to hydrogen bonding of nitrogen with the solvent that enhances its electron withdrawing ability further.