A hydroxyl-directed Evans–Saksena reduction gives the required anti-1,3-diol. A suitable reagent is tetramethylammonium triacetoxyborohydride in an acidic medium, for example acetic acid/acetonitrile. Ligand exchange between the pre-existing alcohol and an acetoxyborohydride group tethers the reducing reagent to the substrate. Intramolecular hydride delivery to the ketone then proceeds through a six-membered arrangement, converting C=O to an alkoxide while conserving all existing stereocentres. Hydrolysis releases the second hydroxyl group.
The preferred directed transition-state conformation minimizes crowding by the larger carbon-chain substituents. It delivers the new C–H bond on the face that leaves the two hydroxyl groups anti, as in B: with the original hydroxyl drawn as a wedge in the printed zigzag, the new hydroxyl is dashed. The intervening methyl stereocentre is retained. Choosing an undirected borohydride reduction would not by itself rationalize this strong substrate-controlled relationship.
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