Acetoxy group rearrangements

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

The reaction can also be done thermally. The stereochemistry of the thermal rearrangement of the acetoxy epoxides involves inversion at the carbon to which the acetoxy group migrates,143 and reaction probably proceeds through a cyclic TS. 

The presence of an acetoxy group is not a necessary condition for the methoxycarbonyl rearrangement. Thus, the pyrolysis of diene 334 (300-320 °C, 1 h, 97% conversion) gave the diester 335 as the major product (equation 119)174. Dicarboxylates like 334 were suggested to be the intermediates in pyrolysis (420 °C, flow system) of dimethyl and diethyl 2-acetoxycyclohex-3-ene 1,1-dicarboxylates175. 

Similarly, the bicyclic ketone 2 (a rearrangement product of germine) is reduced by NaBH4 exclusively to the endo-alcohol, and by NaBH(OAc)3 to the exo-alcohol in 85% yield. The stereoselectivity in the case of 1 is attributed to the presence of a nearby hydroxyl group at C14, which exchanges with one of the acetoxy groups of NaBH(OAc)3. The selectivity in the reduction of 2 is attributed to a similar complexation of the C7-hydroxyl group. 

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