1.2.4- Oxadiazoles photolysis

A similar intramolecular trapping of the intermediate (511) from the photolysis of the corresponding methyl tetrazole-l,5-dicarboxylate (510) gave methyl 5-methoxy-l,2,4-oxadiazole-3-carboxylate (512). 

Co-photolysis of 2,5-di(trifluoromethyl)-l,3,4-oxadiazole with cyclotrisilane, which under these conditions decomposes to afford tetra-(/-butyl)disilene and di-(/-butyl)silylene, provides dihydrodioxadiazadisilocine and trihydro-oxazatrisiline derivatives (Scheme 13) <1996JOM355>. 

Acyl nitroso compounds react with 1, 3-dienes as N-O heterodienophiles to produce cycloadducts, which have found use in the total synthesis of a number of nitrogen-containing natural products [21]. The cycloadducts of acyl nitroso compounds and 9,10-dimethylanthracene (4, Scheme 7.3) undergo thermal decomposition through retro-Diels-Alder reactions to produce acyl nitroso compounds under non-oxidative conditions and at relatively mild temperatures (40-100°C) [11-14]. Decomposition of these compounds provides a particularly clean method for the formation of acyl nitroso compounds. Photolysis or thermolysis of 3, 5-diphenyl-l, 2, 4-oxadiazole-4-oxide (5) generates the aromatic acyl nitroso compound (6) and ben-zonitrile (Scheme 7.3) [22, 23]. Other reactions that generate acyl nitroso compounds include the treatment of 5 with a nitrile oxide [24], the addition of N-methyl morpholine N-oxide to nitrile oxides and the decomposition of N, O-diacylated or alkylated N-hydroxyarylsulfonamides [25-29]. 

The photolysis of 2-methyl-5-nitro-l//-imidazoles 132 in a water-containing solvent80 gives oxime 133, which is in turn hydrolysed to 134 followed by a dehydrative cyclization to the 1,2,4-oxadiazole 135. Light-induced hydrolysis of 135 gives 136 (equation 68). 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

Intermediate a-nitrosobenzaldehyde 36 was generated in solution by laser photolysis of 3,5-diphenyl-l,2,4-oxadiazole-4-oxide 37 and its time-resolved infrared (TRIR) spectroscopy has been recorded <2003JA1444>. The second-order rate constants for reaction with diethylamine and 1,3-cyclohexadiene were determined to be (1.3 0.5) x s ... 

An interesting preparation of aliphatic diazoalkanes (R R C = N2 R, R = alkyl) involves the photolysis of 2-alkoxy-2,5-dihydro-1,3.4-oxadiazoles (see Scheme 8.49). When the photolysis is carried out in the presence of an appropriate dipolarophUe, the diazo compounds can be intercepted (prior to their further photolysis) by a [3 + 2] cycloaddition reaction (54). As an example, 2-diazopropane was intercepted with A-phenylmaleimide (54) and norbornenes (55) to give the corresponding A -pyrazolines. 

Photolysis of 6-chloro-l,3-dimethyluracil (150) with the 1,3,4-oxadiazole 151 afforded the pyrazolo[3,4-d]pyrimidine 152, with photoelimination of benzoyl chloride (80CB2566). 

Substituted benzimidazoles have been reported to be readily obtained by photolysis of l,2,4-oxadiazol-5-ones. Thus, irradiation of dioxane solutions of 3,4-diphenyl-l,2,4-oxadiazol-.5-one (CCXLVII ) and of 3-carbalkoxy-2-phenyl-l, 2,4-oxadiazol-5-one (CCXLIX) with ultraviolet light yielded 2-phenyl- (CCLa),5 and 2-carbalkoxybenzimidazoles (CCLb), 6 respectively. 

The photolysis of acyl azides has also been studied, and in some respects these appear to behave analogously. Pivaloyl azide, for example, adds to cyclohexene to give a 26% yield of an aziridine [Eq. (85)],321 and the assumption is that this addition again occurs via a nitrene. The photodecomposition of acetyl azide (295) in benzo-nitrile and phenylacetylene, on the other hand, affords322 2-methyl-5-phenyl-l,3,4-oxadiazole (296) and 2-methyl-5-phenyloxazole (297),... 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

Complex products were also obtained from electron-poor dienes50. Photolysis of (t-Bu2Si)3 with tetrazine 8 gave a complex mixture of products, from which 9 and 10 could be isolated (equation 20). In 10, one of the CF3 groups has been completely degraded, with simultaneous rearrangement of the ring skeleton. The reaction with oxadiazole 11 also took an unexpected course (equation 21). 

UV irradiation of 2,5-diphenyloxadiazole (43) together with benzo[6]thiophene (42) yields an oxadiazepine (44) as the initial and major product, along with the 3-substituted benzo[6]thiophenes (45a) and (45b). These latter products are also obtained on photolysis of the oxadiazepine (Scheme 6). Prolonged irradiation gives products (45) and what is formally a trans [2 + 2] cycloadduct (46). This cycloadduct is also formed from oxadiazole (43) and benzo[6 jthiophene on irradiation with benzophenone as a sensitizer and the reaction presumably involves the triplet state of the oxadiazole. Somewhat similar photoreactions have been observed between oxadiazole (43) and methyIbenzo[6]thiophenes, indazoles, furan and indene (77BCJ3281). 

Nitrene-like intermediates can lead to CN bond formation and thus to imidazole derivatives. 1,5-Diphenyltetrazole 271 fragments to 272, which is trapped intramolecularly to form 273 (Scheme 152) (also see Section 3.4.1.2.1). Photolysis of the oxadiazole 274 also gives the intermediate 272 and thus 273 (Section 3.4.3.12.2). 

Cotter and Knight have reported the ionic and thermal fragmentation of 2,5-diphenyl-l,3,4-oxadiazole 24. The major fragmentation pathways for the molecule ion 8) and neutral 27) are shown in Eq. (13) and (14) respectively. The benzoyl cation mje 105) is certain to be relatively more stable than its neutral analog the reverse is true of the benzonitrile molecule. It seems likely that products which result from loss of H2, including benzoyl radicals will be obtained from 24 on photolysis. 

The 1,3,4-oxadiazole system is formed on cycloaddition of carbo-alkoxy nitrenes to nitriles. The photolysis of ethyl azidoformate in acetonitrile yielded 2-ethoxy-5-methyl-l,3,4-oxadiazole (210) (yields 52-60%) With benzonitriles yields were much lower . 

Irradiation of acetyl azide (98) in benzonitrile produced a small yield of the 1,3-cycloaddition product, 2-methyl-5-phenyl-l,3,4-oxadiazole (99) Photolysis of 98 in the presence of phenylacetyl-ene resulted in a small yield of the oxazole 101. While the tetrazole... 

Nitriles add carbonylnitrenes to form 1,3,4-oxadiazoles . Alkoxycarbonylnitrenes (made either by azide photolysis, azide thermolysis or by the a-elimination route) give the 2-alkoxy-1,3,4-... 

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