Carboxy mediated stereoselective reduction of ketones with sodium triacetoxyborohydride: synthesis of novel 3,4-fused tetrahydropyran and tetrahydrofuran prolines

In the presence of a carboxyl group positioned for participation, NaBH(OAc)3 will reduce the usually unreactive ketones in a stereoselective manner. This reaction was applied in a key step to prepare precursors 7 and 15 toward the title compounds 1 and 2. These results are consistent with the exchange of one of the acetoxy groups in the reducing agent with the carboxy group followed by intramolecular delivery of the hydride.