Oxazoles rearrangement

The isoxazole-to-oxazole rearrangement has been also used in the synthesis of benzocondensed derivatives. The irradiation of benzisoxazoles 81 in water (high-pressure Hg lamp) produces almost quantitative yields of the corresponding ben-zoxazoles 82 (Scheme 12.24) [53]. Benzoxazolin-2-ones 83 are similarly obtained [54], Furthermore, oxazolo[5,4-b]pyridines 84 and oxazolo[5,4-b]quinoline 85 are obtained by preparative-scale photolysis in ether with a high-pressure Hg lamp of the corresponding isoxazolo[5,4-b]pyridines or isoxazolo[5,4-b]quinoline, respectively (Scheme 12.24) [55]. 

In analogy to the isoxazole-to-oxazole rearrangement, thiazoles can be obtained photochemically from irradiation of the corresponding isothiazoles. In several cases, however, irradiations have been conducted for mechanistic purposes only, and mixtures of photoisomers arising from competing reaction mechanisms are often formed [4, 6]. 

The thermal rearrangement of 3-hydroxyindoxazene to benzoxazolin-2-one is temperature-dependent. The indoxazene is inert at 250°C but rearranges quantitatively at 450°C.78 3-Phenylindoxazene under flash vacuum pyrolysis conditions (800°C, 0.1-1 torr) rearranges to 3-phenylbenzoxazole in 80% yield.79 In both reactions a spiroazirine intermediate [59 or 60 (R = Ph)], analogous to those isolated in some isoxazole-oxazole rearrangements,80 is considered likely. 

Further studies of the photorearrangement of five-membered heterocycles have been described. The isoxazole-oxazole rearrangement has previously been shown... 

Vivona and co-workers also described an unusual base-catalyzed isoxazole-oxazole rearrangement. Here, treatment of 3-(aroylamino)-5-methylisoxazoles 122 with KOtBu/DMF effected rearrangement to 2-(aroylamino)-5-methyloxazoles 125 (Scheme 1.33). The authors proposed initial deprotonation of 122 to generate 123,... 

This type of rearrangement can also occur as a consequence of in situ formation of a suitable intermediate, such as in the case of tandem or cascade reactions. Examples of the isoxazole-to-oxazole rearrangement have been studied both computationally and experimentally, providing evidence of the involvement of a 1,2,4-oxadiazole intermediate (2009JOC351). Synthetically useful cascade rearrangements have also been reported for the one-pot synthesis of indazoles (2011SL3018) and a series of isoxazolo-pyrimidines (2011OL4749). 

Without additional reagents Cornforth oxazole rearrangement... 

Configuration, inversion of -Cope rearrangement Comforth oxazole rearrangements deamination-rearrangement homoallyl -migration... 

Copyrolysis 31,791 Corey oxidation 28,201 Comforth oxazole rearrangement 31, 318 Corrins... 

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