Phosphorus in organic synthesis. IX. On the mechanism of esterification of malonic acid half esters by diphenyl phosphorazidate.

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The mechanism of esterification of malonic acid half esters by diphenyl phosphorazidate (DPPA) was investigated by treatment of some possible intermediates, such as mixed carboxylic phosphoric anhydrides and carboxylic acid azides, under similar reaction conditions to esterification, which revealed these are really intermediates of the esterification. Ethyl hydrogen (3, 4-methylenedioxybenzyl) methylmalonate (XI), which has no α-hydrogen to ester group, smoothly underwent the Crutius rearrangement instead of esterification under usual reaction conditions for the modified Curtius reaction by DPPA. This demonstrates that esterification takes place if substrates have an acidic hydrogen at the α-position to electron-withdrawing groups, allowing the formation of ketene intermediates. The overall mechanism is summarized in Chart 3.

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