proline dl-form → 1-tert-butyl 2-methyl (2S,4EZ)-4-(methoxyimino)-1,2-pyrrolidinedicarboxylate

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A solution of the oximether- and/or olefin-type 2-pyrrolidinecarboxylic acid intermediate, e.g. (2S,4EZ)-1-(tert-butoxycarbonyl)-4-(methoxyimino)-2-pyrrolidinecarboxylic acid (0.648 g, 2.5 mmol), in a 1:1 mixture of methanol and toluene (35 ml) was made. Trimethylsilyl diazomethane (3.8 ml of a 2M solution in hexanes, 7.5 mmol) was then added dropwise to the stirred solution at room temperature under nitrogen. After completion of the evolution of nitrogen gas, the resulting yellow solution was evaporated in vacuo, and the residue filtered through a pad of silica gel, eluting with ethyl acetate. Removal of solvent from the filtrate gave the methylester product, e.g. 1-tert-butyl 2-methyl(2S,4EZ)-4-(methoxyimino)-1,2-pyrrolidinedicarboxylate, as a yellow oil (0.646 g, 95% yield).

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