Oxazolium salts rearrangement

Benzoxazoles are produced in high yield from a-acylphenol oximes by a Beckmann rearrangement using zeolite catalysts <95SC3315>. The reaction of the o-benzoquinone 40 with aromatic aldehyde oximes produces the benzoxazoles 41 <95ZOR1060>. The fused oxazolium salts 43 (R = Me, Et, Pr , or Ph R2 = Me or Pr ) are formed from tropone and nitrilium hexachloroantimonates 42 <96JPR598>. 

The reactions of nitrilium salts with nitrones usually give tars. However, pyridine-A -oxides afford oxazolium salts, which rearrange in solution to more stable amides (Scheme 105) <71TL1947,75JOC4l>. 

Nucleophilic substitution in the azine ring is a normal reaction. The fused oxazole ring, however, is susceptible to nucleophilic ring opening reactions, especially in oxazolium salts and dihydro derivatives. When fused to pyrimidine, substituents in the pyrimidine ring may promote a Dimroth rearrangement of the latter. 

The dihydro-oxazolium salt (375) gives the ring-expanded product (376) on treatment with sodium malononitrile. The oxazoline (377), prepared by the action of trimethyl phosphate on the imine F3CC(=CF2)N=CClPh, rearranges to the oxazole (378) in the presence of caesium fluoride. 3-... 

Oxazolium salts can undergo rearrangement in basic conditions to access... 

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