Oxazole ring, condensed

The oxazolo[4,5-/z]quinolin-2(3//)-ones (89) and 2,5-disubstituted oxazolo[4,5-/i]quinolines (90) were prepared as new antiallergic agents by oxazole ring annelation conducted by condensing 7-amino-8-hydroxyquinoline intermediates with phosgene, o-ester, or imidate reagents (Scheme 7)... 

Carbon atom 2 of the oxazole ring is also supplied by aldehydes in their reaction with a- (hydroxylamino) ketones, which proceeds in the presence of sulfuric acid and acetic anhydride (equation 109). Three further oxazole syntheses involving incorporation of a C(2) fragment are the condensation of triethyl orthoformate with the hydrochlorides of a-aminoacetophenones (equation 110), the reaction of acyl chlorides with a-azido ketones or a-azido esters in the presence of triphenylphosphine (equation 111), and the preparation of 2-aminobenzoxazole and benzoxazoleimines from o-aminophenols and cyanogen bromide (equation 112). 

N—C—O + C—C. The construction of the oxazole ring by the condensation of a-halogeno ketones with primary amides (equation 122) is the Bliimlein-Lewy synthesis (1884/1888). The method succeeds best when the resulting oxazole contains one or more aryl substituents. The use of formamide leads to oxazoles with a free 2-position and in this case it is possible that the reaction proceeds as in equation (113). 2-Aminooxazoles are produced by the action of a-halogeno ketones on urea and its derivatives (equation 123) or on cyanamide (80ZOR2185). The mercury(II) sulfate-catalyzed condensation of alkynic alcohols or their esters with primary amides leads to trisubstituted oxazoles (equation 124). 

Reactions of oxazoles indicate that they are not fully aromatic substances and that the azadiene system present readily reacts with dienophiles by the Diels-Alder mechanism. The bicyclic adducts formed in these condensations undergo facile aromatization, particularly in acid media, by elimination of a molecule of water, an alcohol, hydrocyanic acid, a nitrile, or hydrogen, forming substituted pyridine bases. The course of the reaction is highly dependent on the substituents on the oxazole ring, the nature of the dienophile, and the reaction conditions. 

A is a dihydrophenanthrene group bonded in the 2,7 positions to the oxazole groups, optionally substituted by a alkyl or alkoxy-group or a halogen atom. At and A2 are independently benzene or naphthalene groups optionally substituted by non-chromophoric groups and which are condensed in the indicated manner with the oxazole ring... 

Ehrler and Farooq" prepared 1187 via a stepwise construction of the three thiazoline rings, introduction of the styryl group, and preparation of the oxazole ring (Scheme 1.333). Condensation of L-cysteine ethyl ester hydrochloride 1284 (R = C2H5) with an imino ether derived from cinnamamide was unsuccessful, owing to complications from Michael addition. This necessitated a strategy in which the styryl double bond was introduced after assembly of thiazoline rings. 

Without additional reagents Condensed oxazole ring closure... 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

Benzoxazoles and other condensed oxazoles are obtained by the oxidative ring-closure of phenolic Schiff s bases (equation 103) (78H(10)57). The formation of phenanthrooxazoles by the action of benzylamine or other amines on phenanthraquinone (equation 104) is a reaction of this type. 

The formation of oxazoles may begin to predominate if conditions in which the imidazole ring is formed preferentially (large excess of formamide at 180°-200°C, or passage of a stream of ammonia at 150°-175°C for 4-6 hours)66 are not maintained. This situation is also favored by the use of sulfuric acid as the condensing medium.67... 

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