Methyl esters, synthesis, manganese dioxide

Activated manganese dioxide is a closely related oxidant to Ni02 but less commonly used. Meguro, Fujita, and co-workers described oxidation of oxazolines using Mn02 in their synthesis of antidiabetic agents. For example, the serine-derived P-hydroxyamide 21 was cyclized to the oxazoline 22 with polyphosphoric acid. Oxidation of 22 with activated manganese dioxide afforded 2-styryl-4-oxazolecarboxylic acid methyl ester, 23 (Scheme 1.4). 

Compound 136 can be transformed to the 3,4-trans isomer 138, which has the same configuration as ( + ) thienamycin, according to the procedure described by Chiba and coworkers in related structures [74]. The method involved oxidation of the hydroxyl group in 136 with activated manganese dioxide with concomitant isomerization at C-3, followed by K-selectride reduction of the resulting methyl ketone 137. Zinc enolates of iV,iV-disubstituted a-aminoacid esters have also been recently utilized for the synthesis of 3-amino P-lactams [75]. 

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