Transition state (TS) structures for the reduction of 2-methoxy, 2-methylthio, and 2-methylselenocyclohexanone by LiA1H(4) were optimized by density functional theory (B3LYP/6-31G(d,p)). Four transition state structures corresponding to the axial and equatorial attacks by LiA1H(4) were located for each ketone conformer. Electronic potential maps were used to investigate the electronic effect of the substituent group on the stabilization of transition states (TS). Furthermore, it was analyzed the uneven carbonyl orbital distribution in LUMO (pi*). Reduction stereoselectivity showed to be dependent on both ketone conformational ratio and reaction transition state. (c) 2005 Elsevier B.V. All rights reserved.