Benzoxazoles, 2-amino

Unlike the products of condensation of 1,2-diaminoethane and 1-amino-2-hydroxyethane with aminobutynones, which exist almost exclusively in the enole form, benzoxazoles 359,361 and benzothiazoles 360,362 are present in the solution as a tautomeric mixture of the ketone (359, 360) and enole (361,362) forms. This seems to be related to weakening of the imine nitrogen basicity due to adjacent oxygen and sulfur atoms. 

Benzoxazole reagieren mit Diboran in 1,2-Dimethoxy-athan bei 25° unter Amin-Boran-Bildung, nachfolgender Umlagerung und Verseifung der 2,3-Dihydro-(benzo-1,3,2-oxazaborole), mit Salzsaure zu den entsprechenden o-Amino-phenolen2 ... 

As in the case of benzimidazole, a parallel synthesis of benzoxazoles was described. The authors report that mixing directly differently substituted o-amino phenols 193 with acylating agents 194 and heating at 200 °C for 10-15 min under microwave irradiation, a collection of benzoxazoles 195 was obtained (Scheme 70). With this reaction, a 48-member library of benzoxazoles with different substituents on the aromatic rings was obtained [125]. 

This Compound is a benzoxazole with a disconnection to an ortho amino phenol (54) and an acid derivative. Further disconnections on (54) are straightforward. 

A direct catalytic conversion of esters, lactones, and carboxylic acids to oxazolines was efficiently achieved by treatment with amino alcohols in the presence of the tetranuclear zinc cluster Zn4(0C0CF3)60 as catalyst, essential for condensation and cyclodehydration reactions. For example, the use of (5)-valinol allowed the easy synthesis of oxazolines 125 and 126 in satisfactory yields <06CC2711>. A one-pot direct preparation of various 2-substituted oxazolines (as well as benzoxazoles and oxadiazoles) was also performed from carboxylic acids and amino alcohols (or aminophenols or benzhydrazide) using Deoxo-Fluor reagent <06TL6497>. 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

High-molecular-weight fluorine-containing aromatic poly(benzoxazole)s have not been obtained either by the direct solution polycondensation in PPA at 200°C or by the low-temperature solution polycondensation in DMAc at 0 to 5°C from 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoropropane and aromatic diacid derivatives because the fluorine-containing monomer has low nucleophilicity owing to the presence of the electron-withdrawing hexafluoroiso-propylidene unit. 

Mamyama et al.25 have obtained high-molecular-weight poly(benzoxazole)s by the low-temperature solution polycondensation of A,A 0,0 -tetrais(trimethyl-silyl)-substituted 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoro-propane (25) with aromatic diacids and subsequent thermal cyclodehydration of the resulting poly(o-hydroxy amide)s in vacuo. In this method, aromatic diamines with low nucleophilicity are activated more positively through the conversion to the /V-silylated diamines, and the nucleophilicity of the fluorine-containing bis(o-aminophenol) can be improved by silylation. 

Carbodiimides are rapidly and conveniently obtained from thioureas (Table 10.14) [19], whereas 2-amino- and 2-hydroxyphenylthioureas are oxidatively cyclized to benzimidazoles and benzoxazoles, respectively (Scheme 10.2). 

The condensation of 5-chloro-2-amino-benzothiazoles and -benzoxazoles with a-bromoketones, PhCOCH(Br)R (R = H, Me, Et, 4-C6H4S02Me), produces a range of fused heterocyclesthe mechanisms involved have been investigated by isotopic labelling. 

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. 

Beispielsweise liefert die Kupplung von Phenylamino-methansulfonsaure mit diazotiertem 2-(4-Amino-phenyl)-l, 3-benzoxazol einen Monoazofarbstoff, der hydrolysiert und mit ein-em aromatischen Aldehyd umgesetzt wird. Dabei entstehen typische Farbstoffe fur Flus-sigkristalle vom Guest-Host-Typ3 ... 

Unter ahnlichen Bedingungen werden 1,2- und 1,3-Oxazolc zu 3- bzw. 2-Amino-alko-holen gespalten, z. B. 2,1-Benzoxazol zu 2-Amino-benzylalkohol (65%)2 ... 

The effect of amino, hydroxy or mercapto substituents is to increase hydrogen bonding properties. However, if stable hydrogen bonds are formed in the crystal, then this can decrease their solubility in water (63PMH(l)177), e.g. indazole and benzimidazole are less soluble than benzoxazole. 

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-amino-2-(/>-aminophenyl)benzoxazole and 5-amino-2(w-aminophenyl)benzoxazole are found to be thermally stable up to 500°C in nitrogen. 

Nair 482 reacted benzoxazole with a molar quantity of DM AD at room temperature for 10 days, then he heated the product for 6 hours at 100°, and obtained 3-methoxycarbonylmethylene-2-oxo-3,4-dihydro-2H-1,4-benzoxazine (18), identical with that obtained from o-amino-phenol and the acetylenic ester his benzoxazole may have hydrolyzed before or during reaction. 

The tetrahydro-derivatives of the oxazole and isoxazole system are unstable. As a consequence, only acyclic products have been reported from the reductions with complex metal hydrides. 2,5-Diphenyl-oxazole (119) gave 2-benzylamino-l-phenylethanol (120),141 and 3,5-diphenyl-2-isoxazoline (121) was converted to 3-amino-l,3-diphenylpropanol (122)142 on reduction with lithium aluminum hydride. 3-Phenylbenzisoxazole was resistant to reduction with lithium aluminum hydride and sodium borohydride,143 but benz-oxazole (123), benzoxazol-2-one (124), and benzoxazol-2-thione (125) have been reported 141 to yield 2-methylaminophenol (126) on reduction with lithium aluminum hydride. 

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). 

The 2- and 5-aminooxazoles afford normal acyl and arenesulfonyl derivatives but attempts to diazotize the amino group have been unsuccessful. Oxazoles with a free amino group in position 4 are unknown, as are hydroxyoxazoles. 2-Methylthiooxazoles are produced when oxazole-2-thiones are methylated in alkaline solution. For the alkylation and benzoylation of benzoxazole-2-thione, see Section 4.18.3.1.4(ii). 

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