Benzoxazoles oxazole

Ni(COD)2/IMes was demonstrated to catalyse C-H addition of heteroarenes to vinylarenes to produce 1-aryl-l-heteroarylethanes. A variety of heteroarenes including 1-methylindoles, benzimidazole, benzoxazole, oxazole,... 

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>. 

Oxazole. DaB nicht nur NH2-, sondem auch phenolische OH-Gruppen mit s-Triazin zu reagieren vermogen, laBt sich an dem zu Benzoxazol (XLI) ftihrenden RingschluB des o-Aminophenols demon-strieren (25). 

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). 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

Oxazoles and benzoxazoles are viable participants in the heteroaryl Heck reactions. In their monumental work published in 1992, Ohta and colleagues demonstrated that oxazoles and benzoxazoles, along with other rc-sufficient aromatic heterocycles such as furans, benzofurans, thiophenes, benzothiophenes, pyrroles, thiazole and imidazoles, are acceptable recipient partners for the heteroaryl Heck reactions of chloropyrazines [22b]. Therefore, treatment of 2-chloro-3,6-diethylpyrazine (27) with oxazole led to regioselective addition at C(5), giving rise to adduct 28. By contrast, a similar reaction between 2-chloro-3,6-diisobutylpyrazine (29) and benz[fc]oxazole took place at C(2) exclusively to afford pyrazinylbezoxazole 30. 

Similar results were obtained for the heteroaryl Heck reaction of iodobenzene or bromobenzene with oxazole and benzoxazole [23]. 

For the synthesis of base-sensitive oxazoles (Scheme 52), /3-acyloxyvinyl azides (130) react with triethyl phosphite via an aza-Wittig-cyclization. Phosphorimidate 131 is converted to 1,3-oxazole 132 with spontaneous elimination of triethyl phosphate (89JOC431). Benzoxazoles are also available by this synthetic route (71CC1608). 

Fio. 1. Bronsted plot for azoles reacting with methylating agents in quaternization reactions. Pyridine is the reference substrate. 1, 1-methylimidazole 2, 1-methylbenzimidazole 3, thiazole 4, 1-methylpyrazole 5, 2-methylindazole 6, benzo-thiazole 7, oxazole 8, 1-methylindazole 9, benzoxazole 10, 2,1-benzisothiazole 11, isothiazole 12, 2,1-benzisoxazole 13, isoxazole and 14, 1,2-benzisoxazole. 

Sections VI,A, B, D, and E give information about indazoles, oxazole, benzoxazole, isoxazole, benzisoxazoles, isothiazole, benziso-thiazoles, and benzothiazole. In addition, Sections II and VI,F,1 refer to quaternization reactions of isoxazole and benzisothiazoles, respectively. 

---