Benzoxazoles Beckmann rearrangement

Benzoxazoles. Beckmann rearrangement of o-hydroxyacetophenone oximes proceeds on heating with HY zeolite at 160° (8 examples, 84-95%). 

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>. 

A simple montmorillonite K 10 clay surface is one among numerous acidic supports that have been explored for the Beckmann rearrangement of oximes (Scheme 6.27) [54]. However, the conditions are not adaptable for the aldoximes that are readily dehydrated to the corresponding nitriles under solventless conditions. Zinc chloride has been used in the above rearrangement for benzaldehyde and 2-hydroxyacetophe-none, the later being adapted for the synthesis of benzoxazoles. 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

Benzoxazoles.1 Benzoxazoles (2) are obtained in generally good yield by Beckmann rearrangement of the oxime of 2 -hydroxyacetophenones (1) using POCl3-DMA in CH3CN at 30° (equation I). The method is applicable to synthesis of hydroxy-substituted benzoxazoles, which are usually difficult to prepare. 

Thermal dehydration of o- (acylamino)phenols is the method of choice for the preparation of benzoxazoles (equation 96) and other annulated oxazoles. 0,iV-Diacyl derivatives of o-aminophenols cyclize at lower temperatures than do the monoacyl compounds. The synthesis is often carried out by heating the aminophenol with the carboxylic acid or a derivative, such as the acid chloride, anhydride, an ester, amide or nitrile. The Beckmann rearrangement of oximes of o-hydroxybenzophenones leads directly to benzoxazoles (equation 97). 

The configuration of the oxime is important to the course of the reaction. Thus, while the j8-oxime (11) gives the indoxazene or its degradation products, the a-oxime (12) undergoes a Beckmann rearrangement, finally yielding benzoxazole (13). This difference in reaction pathway between stereoisomeric oximes was used in the determination of their configurations.7,8 22,23... 

Again, the configuration of the oxime is of significance. The acetate (20, R = Ph) provides the indoxazene smoothly on pyrolysis, while its isomer (21) gives only a small yield of the same product.42 A Beckmann rearrangement diverts the pyrolysis of some of the oxime benzoate (22) to the formation of the benzoxazole (23).43... 

Benzoxazoles. - Both o- and m-halogenobenzanilides (413) are converted into the amidines (415) by potassium amide in liquid ammonia these are formed by aryne cyclization to 2-phenylbenzoxazole (414) and subsequent aminoly-sis." The conversion of acetophenone oxime into 2-methylbenzoxazole by the action of phosphorus oxychloride involves a Beckmann rearrangement. Pyrolysis of aryl azidoformates, ArOaCNa, gives benzoxazol-2-ones. The azide (416) is converted into the benzoxazole (417) on heating. The... 

A device which has been frequently used to produce a starting material with just one acyl group in place is to carry out a Beckmann rearrangement on an ortho-hydroxyaryl ketone, the Beckmann product cyclising in situ when the conditions are acidic a modern version of this is illustrated below." " An excellent route to mono-acylated precursors utilises mixed anhydrides.A very mild method for the dehydrative ring closure of or /io-hydroxyarylamino amides, employed in solid-supported benzoxazole syntheses, utilises typical Mitsunobu conditions - triphenyl-phosphine and diethyl azodicarboxylate." ... 

Several reagents have been shown to convert 2-hydroxyaryloximes into benzoxazoles through a Beckmann rearrangement. Zeolite catalysis in a continuous flow reactor has been shown to be successful in benzoxazole formation,as has diethyl chlorophosphate and POCI3. Rindhe and coworkers utilized POCI3 in the formation of pyrazolyl benzoxazoles for use as antimicrobial agents. 

Salicylaldoximes 38 and 40 underwent isomerization followed by Beckmann rearrangement at 200 °C in the presence of K-10 montmorillonite clay and silica-alumina to afford o-hydroxybenzamide 39 and benzoxazole 41, respectively. ... 

Thomas, B. Sugunan, S. Rare-earth (Ce, La, Sm, and RE ) exchanged Na-Y zeolites and K-10 clay as solid acid catalysts for the synthesis of benzoxazole via Beckmann rearrangement of salicylaldoxime. Micropor. Mesopor. Mater. 2006, 96, 55-64. 

It has also been demonstrated that o-hydroxyaryl-ketimines 127 tmdergo an iodine(in)-mediated Beckmann-type rearrangement to produce benzoxazoles 128 as outlined in Scheme 35 [29]. 

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