Sterically crowded heterocycles .7. Reduction of some (Z)-1,3-diphenyl-3-(2-phenylimidazo[1,2-a]pyridin-3-yl)prop-2-en-1-ones as their axial chirality probe

Borohydride reduction of titled ketones 1a-1g gave diastereoisomeric mixtures of (Z)-1,3-diphenyl-3-(2-phenylimidazo[ 1,2-a]pyridin-3-yl)prop-2-en-1-ols 2a-2g and 3a-3g in which the former ones prevailed. Only individual racemic products were obtained after borohydride reduction of (E)-1,3-diphenyl-3-(2-phenylimidazo[1,2-a]pyridin-3-yl)-prop-2-en-1-one 4 to corresponding 1-hydroxy derivative 5 and by conversion of (Z)-1-oxo derivative 1a to 1,3-diphenyl-3-(2-phenylimidazo[1,2-a]-pyridin-3-yl)propan-1-one (6) with sodium hydrogenselenide. Diastereoselectivity of the borohydride reduction is discussed using the PM3 calculations of the molecules 1a, 2a, 2b, 3a, 3b, 4, 5, and 6.