Oxazoles radicals from

Violene Radicals from Oxazoles, Thiazoles, and Selenazoles... 

No spectral determination of a true oxazole radical has been reported. A radical produced from irradiation of oxazole and sodium in an argon matrix was studied. The ESR spectrum indicated rupture of the CO bond, with a radical at what had been C2. Species formed from attack of the hydroxyl, sulfate, and phosphate radicals on oxazole, 4-methyloxazole 2, 5-methyloxazole 9, and other heterocycles have been analyzed by ESR (Fig. 2.26). In all cases, a dihydrooxazole radical at C4 is produced by addition of the radical to the double bond at C5 g for these species varies between 2.00251 and 2.(X)258. 

The flash vacuum pyrolysis of 1-acyltriazoles at 520 °C gave a mixture of oxazole (16%) and 68 in 33% yield to which was tentatively assigned the isoindolo[l,2-4][l,2,3,5]benzotetraazepine structure, based only on mass and NMR spectrometric evidence and its acidity. Benzotetrazepine 68 is believed to be formed by radical recombination of the first formed diradical from 1-acyltriazoles, and followed by addition of benzotriazole, itself formed by fragmentation of 1-acyltriazole (Equation 12) <1999AJC775>. 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

A wide variety of a-trifluoromethyl-substituted amino acids are now available from the reaction of 5-fluoro-4-trifluoromethyl-l,3-oxazoles with allylic alcohols and benzyl alcohols. The reaction sequence involves a low-temperature Claisen rearrangement or a radical 1,3-benzyl shift from oxygen to carbon, respectively [88AG(E)848 89S850] (Scheme 86). 

Anion-radicals generated from oxazole and isoxazole in an argon matrix were shown, like their counterpart from furan, to have undergone ring scission. Consistent with this, various attempts to reduce isoxazoles polarographically in both protic and aprotic media have also resulted in the loss of ring integrity, and indeed the reduction process has always involved two electrons. However, electrochemical reduction of... 

The oxidative transformation of dihydrooxazoles to oxazoles has also been achieved using the Kharasch-Sosnovsky reaction <94TL6803>. The first step is a hydrogen atom abstraction by a species generated from a copper-ion catalyzed (CuBr) decomposition of a peroxyester (Bu OOC(O)Ph). Yields are better when a copper(II) species is added, perhaps because of faster reaction with the radical intermediate (135) (Scheme 61). 

Isoxazole (1,2-oxazole) contains a pyridine-like N-atom, but differs from oxazole by the presence of an N-0 bond. The bond energy of such a cr-bond amounts only to 200 kJ mol", much lower than that of N-C or O-C bonds. The univalent radical is known as isoxazolyl. 

Alkyl-substituted oxazoles can also give benzylic cleavage products from the initial radical cation (Fig. 2.17). With substituents at C2, cleavage with McLafferty rearrangement is often noted (Fig. 2.18). 

The fused oxazole derivative 100 was synthesized from 3-azido-3-deoxy-l,2 5,6-di-O-isopropylidene-a-D-allofuranose, the key final step being the reaction of methyl 3-amino-3-deoxy-5,6-0-isopropylidene-p-D-alloside with DMF dimethyl acetal. The annelated pyranoside 101 was obtained by cyclization of a branched chain hexosulose derivative, and the fused triazole-piperidinoses such as 102 were made by a radical cyclization of a 3-pyrazolo-6-iodo-sugar derivative. The spiro-isoxazohne 103 and related isomers have been synthesized by dipolar cycloadditions of mesitonitrile oxide to 2-deoxy-2-C-methyl-ene-pentonic acids, themselves available in five steps from D-mannitol. Intra-... 

Nicolaides observed that the reflux of oxime 19 in PhMe formed oxazole 4 in 37% yield, and proposed its formation by a sequence of reactions as follows. A homolytic reaction between oxime 19 and PhMe led originally to the formation of radicals 20 and 21 from oxime 19 and PhMe, respectively then the combination of the two radicals formed intermediate 22. After loss of one molecule of MeOH from 22, o-quinone-imine 23 was formed. o-Quinone-imine 23 tautomerized to hydroxyl-imine 24, which after cyclization and loss of of one molecule of H2 formed oxazole 4 in 37% yield. Oxime 19 could also react with other methyl containing aromatics and heteroaromatics to form oxazoles (19 25). 

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