Benzoxazoles 5-hydroxy- from

The precursor of poly(penzoxazole) (28), poly(o-hydroxy amide) (27) is amorphous and readily soluble in DMF, NMP, DMSO, pyridine, THF, and acetone.25 Transparent, flexible, and tough film of 27 can be obtained by casting. However, fluorine-containing poly(benzoxazole) (28) from 25 dissolves only in concentrated sulfuric acid and o-chlorophenol. 

Triazolo[3,4-6] 1,3-benzoxazole-3-thione (85) has been prepared by cyclizing 2-hydrazino-l,3-benzoxazole (83) with carbon disulfide (58USP2861076) or phenyl isothiocyanate (59JOC1478). The alternative approach (62TL1193) of fusing the 1,3-benzoxazole onto a 1,2,4-triazole was also used in the synthesis of 3-phenyl-1,2,4-triazolo[3,4- ]-benzoxazole (88) from 3-hydroxy-5-phenyl-4-(2-methoxyphenyl)-1,2,4-triazole (86). 

In both syntheses the benzoxazole synthon 176 was prepared from methyl 5-hydroxyanthranilate (177), the amino group of which was trifluoroacetylated to give 178. Nitration of 178 gave the 6-nitro derivative 179 as the major product (in a 2 1 mixture with the 4-nitro compound) catalytic reduction to the 6-amino-5-hydroxy compound 180 was followed by refluxing with acetyl chloride in xylene to afford the benzoxazole 181, N-methylation of which yielded 176, the overall yield from 177 being 60% (94). 

Figure 6.48 Arrhenius plot of the triplet state tautomerism of2-( 2 -hydroxy-4 -methylphenyl) benzoxazole (Me-BO, upper curve) and its deuterated analog (lower curve) dissolved in alkanes. The kinetic data were taken from Al-Soufi et al, [83]. The solid lines were calculated using the parameters listed in Table 6.4.

In benzoxazole, in view of the fact that a weak N—O bond does not exist, a stronger C O bond has to be broken. This, together with an H-atom shift from the carbon atom to the oxygen, initiates an isomerization process. With one transition state it forms o-hydroxy-wo-benzonitrile that further isomerizes to o-hydroxybenzonitrile [130]. This is shown in Figure 6.32. 

Methoxybenzoxazolin-2(3H)-one 752 (MBOA 6-methoxy-2-oxo-2,3-dihydro-benzoxazole) has been found in extracts of plant tissues from gramineous plants such as Coix lachrytna-jobi, wheat, and maize, and is implicated as a chemical resistance factor against fungi and insects. Because MBOA is only available in rather small amounts by plant extraction, several methods have been developed for its chemical synthesis. The most representative synthetic route involves the insertion of a C=0 unit between the amino and hydroxy groups of 2-amino-5-methoxy-phenol 751, a very easily oxidizable substance. 

Sun et al. reported a diversity-oriented tactic to access the benzo[d]oxazol-5-yl-lH-benzo[d]imidazole 64 on IL support (Scheme 24). The authors coupled 4-hydroxy-3-nitrobenzoic acid onto IL-immobilized o-phenylenediamine 58 that underwent ring closure to furnish benzimidazole derivatives 60. Further hydrogenation of the nitro group to an amine followed by the reaction with 1,1-thiocarbonyldiimidazols resulted into IL tagged-benzoxazol 63. In order to generate skeletal diversity, S-alkylation with alkyl and aryl bromides was carried out. Cleavage of 64 from IL support was done by treatment with sodium methoxide in methanol imder microwave irradiation [127]. 

Isonitrile 19 was detected in ultraviolet (UV) and infrared (IR) studies at low temperatures. They speculated that azirine intermediate 21 may precede the formation of 19 and, indeed, LFP studies on 18 performed by Richard and co-workers allowed observation of an intermediate which was assigned to 21 on the basis of the similarity of its UV absorption spectra to that of the cyclohexadienone chro-mophore. The reaction was not quenchable by piperylene and is presumed to occur via the singlet manifold. The authors did not put forward a mechanism for the formation of 21, although it may be formed by initial ESIPT from the phenol to the cyano group to give tautomer 22, which can then rearrange to give 21. Similar products (substituted benzoxazoles) were observed via presumed initial ESIPT in structurally related o-hydroxy substituted aromatic oximes and oxime ethers. ... 

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