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.