2- benzoxazole, formation

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. 

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

Treatment of the monothione 1 with sodium ethoxide results in ring contraction with the formation of ethyl benzoxazole-2-acetate (2).37... 

Formation of the oxazole ring is not always the last step in synthesis of the brightener. Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkage can be prepared by the anil synthesis [51], in which a compound possessing an activated methyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustration of this method (Scheme 11.12). 

Cu-catalyzed domino reactions have been used for the synthesis of carbocycles, as well as for heterocycles such as indoles, benzoxazoles, and quinoxalines. A very useful process is also the combination of the formation of allyl vinyl ethers, followed by a Claisen rearrangement. 

An elegant approach toward formation of the central tetrahydropyrimidine ring has been reported by Tamura and co-workers who utilized an aza-Diels-Alder cyclization of ethyl ( )-3-aza-3-(l,3-benzoxazol-2-yl)propenoate 266 with a range of cinnamyl alcohols 267 catalyzed by l,l,3,3-tetra- -butyl-l,3-diisothiocyanatodistannoxane, and subsequent lactonization to generate the tricyclic core structure 268 (Scheme 18) <1998J(P1)3389>. 

It was interesting that 2,4-DIF-PMTAI and 2,4-DIF-PMI showed a two-step decomposition. The onset of decomposition at the second step of these two polymers were close to each other. More interesting, they were also close to the onset of decomposition of polybenzoxazole. We have confirmed that the decomposition of these two polymers at 400-550°C resulted in the formation of benzoxazole units on the backbone. ... 

An unexpected formation of a biguanide occurred 754) in the reaction of trans-l-amino-2-hydroxycyclohexane (LI) with cyanogen bromide in the presence of sodium acetate. In addition to the expected trans-2-amino-3a,4,5,6,7,7a-hexahydro-benzoxazole (LII), traces of its trimer having the biguanide structure (LIII) were obtained. 

This trimer was then synthesised by refluxing the benzoxazole (LII) with toluene-p-sulphonic acid in benzene. In contrast, the cis-isomer of (LII) is stable under these conditions. The mechanism of formation of the trimer (LIII) was discussed (754). 

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. 

Another reaction of such azides attached to the benzene part of benzo[7>]thiophene is the formation of benzoxazoles when heated in a mixture of carboxylic acid and PPA. The mechanism has been discussed in detail (79AG(E)900) and involves insertion of the singlet nitrene into the anhydride, followed by migration of the acyloxy group to the ortho position, and cyclization. The 5- and 6-azides thus given angularly fused benzoxazoles (Scheme 147). 

The formation of a benzoxazole ring under these conditions has not been hitherto reported and experimental conditions necessary for maximum yield were studied. 

Treatment of 2- or 4-(p-formylphenyl)-2/f-l,2,3-triazoles with p-tolyl-substituted heterocycles such as benzo[6]furans,58,63 benzol-thiophenes,56,63 benzoxazoles,63,64 benzisoxazoles,54 oxazoles,63 isoxa-zoles,63 oxadiazoles,63,65 benzotriazoles,63 and y-triazolo [ 1,5-a]-pyri-dines19 has led to the formation of a number of new stilbene derivatives some of which are shown in Table XI. 

Derivatives have also been obtained in which the 1,2,3-triazole ring is substituted by chlorine83 and/or methoxy groups,66 as, for example, in the formation of the stilbene 148 from 2-(jP-tolyl)-5,7-dimethyl-benzoxazole (146) and Schiff s base 147. 

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