Heterocycles. XXV Sodium borohydride reduction of flavanonols

Solvent effects on the stereochemistry in the sodium borohydride reduction of (±)-flavanonols have been examined. The effects observed for the (±)-flavanonols with 5-OMe in 2-propanol, dioxane and methanol are explainable by the differences between the steric interactions inherent in the product-like transition states A and B. It has been also found that 5-OAc peculiarly affects the stereochemistry in the reduction to produce the (±)-catechin-type compounds in a one-pot process. The solvent and temperature effects are examined using a model analogous to the above.