2- benzoxazole

The numbering 1,3- is omitted, as there is no other possibility for ring condensation. 

As in the case of benzo[Z ]furan, the signal for the proton on C-2 appears in the region of benze- 

Salt formation and quatemization of benzoxazoles occurs by analogy to oxazoles. In benzoxa-zoles, the electrophilically accessible atoms C-4 and C-5 of the oxazole ring are blocked. Nitrating acid causes substitution of the benzene ring in the 5- or 6-position. However, nucleophiles attack benzoxazoles, benzoxazolium salts and A-alkylbenzoxazolium salts in the 2-position, e.g.  

Nucleophilic substitution reactions of 2-halobenzoxazoles occur rapidly and A-alkyl-2-chlorobenz-oxazolium salts are even more reactive. They are efficient dehydrating agents [71], e.g. for the synthesis of alkynes from aryl ketones  

The low electron density on the C-2 atom, which is due to electron withdrawal by the N-atom, is responsible for the CH-acidity of 2-alkylbenzoxazoles. Thus 2-methylbenzoxazole undergoes a Claisen condensation, e.g.  

The numbering 1,3- is omitted, as there is no other possibility for ring condensation. As I I in the case of benzo[b]furan, the signal for the proton on C-2 appears in the region of - benzenoid protons in the H-NMR spectrum, with a small downfield shift. 

Salt formation and quaternization of benzoxazoles occur at the pyridine-like N-atom a 

Electrophilic substitutions (SEAr) like halogenation or nitration can be achieved only in the benzene ring for instance, HNO3/H2SO4 causes substitution in the 5- and 6-position, but not at C-2  

Nucleophilic displacement has been observed with 2-halogenobenzoxazoles and their corresponding N-quaternary salts (in analogy to oxazole), for example  

However, benzoxazoles and benzoxazolium or N-alkylbenzoxazolium salts are generally attacked by nucleophiles in the 2-position (i.e., without the presence of a leaving group). 

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Phenylstilben-4-yl)benzoxazoles are prepared by means of the anil synthesis from 2-(4-methylphenyl)benzoxazoles and 4-biphenylcarboxaldehyde anil, and used for brightening polyester fibers (24,25). An example is (3) [16143-18-3]. 

Bis(benZoxaZol-2-yl) Derivatives. Bis(benzoxazol-2-yl) derivatives (8) (Table 3) aie prepared in most cases by treatment of dicaiboxyhc acid derivatives of the central nucleus, eg, stilbene-4,4Cdicarboxyhc acid, naphthalene-l,4-dicarboxyhc acid, thiophene-2,5-dicarboxyhc acid, etc, with 2 moles of an appropriately substituted o-aminophenol, followed by a ring-closure reaction. These compounds are suitable for the brightening of plastics and synthetic fibers. 

A large variety of bisimides and polymers containing maleimide and citraconimide end groups have also been reported (21—26). Thus polymers based on bisimidobenzoxazoles from the reaction of maleic anhydride and citraconic anhydride with 5-aniino-2-(p-aniinophenyl)benzoxazole and 5-aniino-2(y -aniinophenyl)benzoxazole are found to be thermally stable up to 500°C in nitrogen. 

Benzoxazole dyes exhibit irreversible degradations that involve opening of the oxazole (66). Oxacarbocyanines, eg, 3,3 -dimethyloxacarbocyariine iodide [48198-86-3] (42), react most readily with aqueous acid, whereas benzoxazole merocarbocyanines (43) react with sulfite or hydroxide ion to produce ring-opened products such as (44). 

Azoles having heteroatoms in the 1,3-orlentatlon are more reactive than those in which the arrangement is 1,2. However, the magnitude of the factor varies. Thus oxazole is 68 times more reactive than Isoxazole, whereas benzoxazole quaternlzes 26 times faster than does 1,2-benzisoxazole (78AHC(22)71). 

Oxygen-containing azoles are readily reduced, usually with ring scission. Only acyclic products have been reported from the reductions with complex metal hydrides of oxazoles (e.g. 209 210), isoxazoles (e.g. 211 212), benzoxazoles (e.g. 213 214) and benzoxazolinones (e.g. 215, 216->214). Reductions of 1,2,4-oxadiazoles always involve ring scission. Lithium aluminum hydride breaks the C—O bond in the ring Scheme 19) 76AHC(20)65>. 

Electron impact fragmentation studies on 1,2-benzisoxazoles and benzoxazole indicate that isomerization takes place before degradation. Shape analysis and metastable ion abundances in the mass spectra indicate that isomerization to o-cyanophenols occurred prior to degradation by loss of CO or NCH (75BSB207). 

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). 

Salicylonitrile is believed to arise by direct cleavage with subsequent hydrogen transfer, while the benzoxazoles were produced by an isocyanide intermediate (73JA919, 74HCA376). Photolysis in D2O tends to confirm this possibility and rule out an azirine intermediate (39), due to deuterium corporation into the molecule (Scheme 10) (74HCA376). 

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

The sulfonate ester of o-hydroxyacetophenone oximes, when treated with pyridine, are similarly converted into a benzoxazole, but cyclize to a 1,2-benzisoxazole when treated with aqueous KOH <73JCS(P1)2220, 71T711). 

The reaction of vinylogous amides, or ketoaldehydes, with hydroxylamine produced 4,5,6,7-tetrahydro-l,2-benzisoxazole. A side product is the 2,1-benzisoxazole (Scheme 173) (67AHC(8)277). The ring system can also be prepared by the reaction of cyclohexanone enamines with nitrile oxides (Scheme 173) (78S43, 74KGS901). Base treatment produced ring fission products and photolysis resulted in isomerization to benzoxazoles (76JOC13). 

Acetanilides, benzoyl-colour couplers in colour photography, 1, 372 Acetanilides, pivaloyl-colour couplers in colour photography, 1, 372 Acetazolamide — see l,3,4-Thiadiazole-2-sulfonamide, 5-acetamido-Acetic acid, acetamidocyano-ethyl ester, 1, 307 Acetic acid, 2-acylphenyl-isochroman-3-one synthesis from, 3, 858 Acetic acid, 3-benzo[6]thiophenyl-biological activity, 4, 912 Acetic acid, l,2-benzoxazol-3-yl-electrophilic substitution, 6, 48... 

Benzoxazole, 2-carbonamidomethyl-5-methyl-, 1, 333 Benzoxazole, 2,3-dihydroring-chain tautomerism, 6, 186 structure, 6, 179 synthesis, 6, 227-228 Benzoxazole, 2-dimethylamino-synthesis, 5, 128 Benzoxazole, 2-mercapto-... 

Hydroxypbenyl)benzoxazole [835-64-3] M 211.2, m 127", b 338"/760mm. Recrystd several times from aqueous EtOH and by sublimation. [Itoh and Fujiwara J Am Chem Soc 107 1561 1985.]... 

V-Perfluorophenylben7armde readily undergoes intramolecular cyclization to produce the benzoxazole system shown in equation 9 [13]... 

Benzoxazol-4-fluoro-aniline 77 and benzthiazol-aniline 80 both provided the linear product in very good overall yield. This was considered to be the normal or... 

Other advances in the use of IR spectroscopy are (1) The substitution of sulfur by selenium, for comparison with the spectra of benzimidazole-, benzoxazole-, and benzothiazole-2-thiones 72 (80AJC279). (2) The use of IR, as a quantitative tool to determine the association (homo- and heterodimers) of thia- and oxa-diazolin-5-thiones and -5-ones 73 (80NJC527). 

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