2 -oxazolonium salts

German workers reported that 5-acyloxy-4(5/i/)-oxazolonium salts 24 are readily prepared by reaction of acyl cyanides and acid anhydrides in the presence of a strong acid (Scheme 6.8). Analogues like 24 are not available using the traditional methods (see above). 

Condensation products of 4(5//)-oxazolonium salts with aldehydes and orthoesters are the subject of a series of papers by Kosulina and co-workers Reaction of 2-methyl-4(5//)-oxazolonium perchlorates 44 with an ortho ester gives rise to an enol ether, which reacts with furanamides to afford the frani-eneamides 45 (Scheme 6.14). " Using electron deficient anilines in a three component condensation affords either 46 or 47 in 64-80% and 78-84% yields, respectively, depending on whether the reaction is performed in acetic acid or acetic anhydride. Electron-rich anilines are unreactive since they are merely protonated by the perchloric acid present in the reaction medium. ... 

TABLE 6.2. 2-[2-(a-HETEROARYL)ETHENYL]-4(5//)-OXAZOLONIUM PERCHLORATES FROM OXAZOLONIUM SALTS AND ALDEHYDES ... 

---