Narasaka reduction mechanism where a beta hydroxyketone is chelated first with a boron based bidentate Lewis acid. The chelated structure prefers to adopt a halfchair sixmembered ring conformation. Attack of an external borohydride nucleophile is from the face of the carbonyl that leads to a chairlike transition state (rather than a twist boat) to the 1,3syn diol product. The reaction is very diastereoselective.