It was revealed that the Dieckmann type annulation of a half-thiol diester having latent σ- symmetry did not always proceed in the regiocontrolled fashion even though the reaction yielded the desired β-keto ester. Direction of the regioselectivity dramatically differed depending on the base employed, which was demonstrated by treating an optically pure half- thiol diester with lithium diisopropylamide, dimsylsodium, and dimsylpotassium, ...
