Thermal oxidation scheme irradiation

An interesting aspect of fused 1,2-thiazetidine A-oxides is their propensity for thermal isomerization. Thus, after heating the 1,2-thiazetidine 54 at 120 °C, the 1,4-thiazine A-oxide 55 was obtained as a pair of isomeric products (ratio 4 3, 78% yield) which differed only in the configuration of the A-oxide (Scheme 4). Irradiation of this 1,2-thiazetidine A-oxidc at A = 300 nm also effected isomerization affording the same ratio of products as that observed in the thermal process <1997SL167>. 

Thermal oxidation is also autocatalytic and considered as metal-catalyzed because it is very difficult to eliminate trace metals (from fats and oils or food) that act as catalysts and may occur as proposed in Equation 4. Redox metals of variable valency may also catalyze decomposition of hydroperoxides (Scheme 2, Equations [6] and [7]). Direct photooxidation is caused by free radicals produced by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and peroxides. This oxidation proceeds as a free radical chain reaction. Although there should be direct irradiation from ultraviolet light for the hpid substrate, which is usually uncommon under normal practices, the presence of metals and metal complexes of oxygen can become activated and generate free radicals or singlet oxygen. 

By using a multicomponent cascade reaction. Parsons et al. [88] achieved one-pot sequential [1+4] and [3+2] cycloadditions to synthesize highly substituted iso-xazolines via nitrile oxides (Scheme 11.28). These five-component reactions proceed by initial formation of isonitriles 109 that react with nitroalkenes 110 to form unstable N-(isoxazolylidene)alkylamines, which in turn fragment to generate the nitrile oxides 111. Cycloaddition then occurs with methyl acrylate, chosen for its expected reactivity with nitrile oxide dipoles, to generate the isoxazolines 112. Reactions using standard thermal conditions and microwave irradiation were com-... 

Nitrile oxides Nitrile oxides have been used in conjunction with microwaves in fullerene chemistry. For example, the 3 -(N-phenylpyrazolyl)isoxazolino[60]-fullerene dyad 38a was prepared in 22% yield from the corresponding nitrile oxide (Scheme 21.15) [49]. Longer reaction times afforded larger amounts of bis adducts. The same reactions under thermal conditions produced markedly lower yields (14-17%). A significant accelerating effect (10 min compared with 24 h) was observed on using microwave irradiation. 

Microbial oxidation of D-glucose gives calcium 2,5-diketo-D-gluconate (145) whose hydrazones (146) are converted to the betaines 148 by acid-catalyzed decarboxylation and cyclodehydralion (Scheme 6). ° Derivatives of these stable crystalline compounds have also been prepared by (i) methylation (Me2SO4) of 4-hydroxypyridazines (143) and (ii) thermal elimination of methyl iodide from 5-methoxy-l-methylpyridazinium iodides (144). 2 Irradiation of the mesomeric betaines (148) in ethanol (Hg arc lamp)... 

In contrast, in the photolysis of3,6-diphenylpyridazine-N-oxide 28 (R1 = R2 = Ph), a mixture of 3-benzoyl-5-phenylpyrazole 31 and 2,5-diphenylfuran 32 is formed. Diazoketone 29 (R1 = R2 = Ph) undergoes two competing reactions (i) thermal internal cyclization into the pyrazole 31 or (ii) photoinduced formation of a carbene leading to the final furan 32 (Scheme 12.9) therefore, the product distribution depends heavily on the reaction conditions. Thus, the formation of pyrazole 31 (75%) is favored over that of furan 32 (3%) by irradiation in acetone in a Rayonet reactor equipped with lamps irradiating at 350 nm. On the other hand, by irradiation with a Hanovia immersion lamp the yield of 31 decreases to 27%, whereas the yield of 32 increases to 67%. Moreover, the exclusive formation of the furan derivative 32 (43%) was observed by irradiation in the Rayonet reactor at low temperatures (—65 °C) [27]. 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

Diphenylpyridazine N-oxide is transformed upon irradiation into a mixture of 3-benzoyl-5-phenylpyrazole (307) and 2,5-diphenylfuran (308). A diazoketone (306) is proposed as intermediate for both products. In a low-energy photolysis experiment it was shown that the diazoketone reaches a maximum concentration after 20 /xsec, and in a nanosecond photolysis the diazoketone is formed as the primary photoproduct in less than 20 nsec. The diazoketone is able to decompose competitively by thermal (to 307) and photochemical (to 308) pathways. Similarly, irradiation of 3,4,5,6-tetraphenylpyridazine N-oxide results in the formation of a mixture of tetraphenylpyridazine (61) and a bicyclo-heptadienone (309) as the main products, together with compounds 310-312 7,598 an intermediate diazoketone (cf. 306) is proposed (Scheme 18). 3,4,6-Triphenylpyridazine 1-oxide gives upon irradiation almost equal... 

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