Sodium borohydride-manganese chloride

The synthesis of 3-hydroxy-2-methyl esters and amides in a stereoselective manner is a challenge which needs to be met due to the prevalence of these moieties in natural products201. This has been accomplished by using sodium borohydride, catalyzed by manganese(II) chloride or by tetrabutylammonium borohydride (equation 52)202,203. 

Acetylation of solafloridine (41) with acetic anhydride-zinc chloride-acetic acid or photolysis of iV-nitrosodihydrosolafloridine diacetate (142) furnished the diacetate 143. Subsequent oxidation with manganese dioxide in CHCI3 at room temperature yielded ketone 144, which was reduced with sodium borohydride, without purification, to the piperidinol 145 (yield from 143 55%). As a by-product of the Mn02 oxidation of 143, the pyridyl steroid 146 was isolated. 

Berkes, D., Kolarovic, A., Povazanec, R (2000). Stereoselective sodium borohydride reduction, catalyzed by manganese(II) chloride, of y-oxo-a-amino acids. A practical approach to si/n-y-hydroxy-a-amino acids. Tetrahedron Lett., 41,5257-5260. 

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