Preparation of 2-alkoxy-4-alkyl-5(4H)-oxazolones from mixed anhydrides of N-alkoxycarbonylamino acids.

A general procedure for the preparation of 2-alkoxy-4-alkyl-5(4H)-oxazolones (AlkOx's) from N-tert-butoxy-, N-benzyloxy- and N-(9-fluorenylmethoxy)-carbonylamino acids has been devised. Purified mixed anhydrides are prepared from the parent acid and isopropenyl chloroformate and left in chloroform (Boc) for 24 h or dimethylformamide (Z, Fmoc) for 0.5-2 h. Symmetrical anhydride that may originate from disproportionation or reaction of AlkOx with hydrolyzed product is partially removed by extracting the AlkOx into petroleum ether. The products contain 0.5-30% of symmetrical anhydride.