Synthesis of 4-Alkoxy-1,3-oxazol-5(2H)-ones, Precursors of 1-Alkoxy-Substituted Nitrile Ylides

Article abstract of DOI:10.1002/hlca.19890720424

4-Alkoxy-1,3-oxazol-5(2H)-ones of type 4 and 7 were synthesized by two different methods: oxidation of the 4-(phenylthio)-1,3-oxazol-5(2H)-one 2a with m-chloroperbenzoic acid in the presence of an alcohol gave the corresponding 4-alkoxy derivatives 4, presumably via nucleophilic substitution of an intermediate sulfoxide (Scheme 2).The second approach is the BF3-catalyzed condensation of imino-acetates of type 6 and ketones (Scheme 3).The yields of this more straightforward method were modest due to the competitive formation of 1,3,5-triazine tricarboxylate 8.At 155 deg C, 1,3-oxazol-5(2H)-one 7b underwent decarboxylation leading to an alkoxy-substituted nitrile ylide which was trapped in a 1,3-dipolar cycloaddition by trifluoro-acetophenone to give the dihydro-oxazoles cis- and trans-9 (Scheme 4).In the absence of a dipolarophile, 1,5-dipolar cyclization of the intermediate nitrile ylide yielded isoindole derivatives 10 (Schemes 4 and 5).