1- -1,2,3-triazoles, photolysis

The second new synthesis of 7-methoxymitosene (3) was published by Wender and Cooper [34]. Based on a novel triazole photolysis, it affords the product in 8.3% yield after 12 steps. It is more efficient than the original synthesis of 3, but it requires more steps than the synthesis reported by Luly and Rapoport in 1984. As illustrated in Scheme 12, it began with 2,6-dimethoxytol-uene (91), which was converted into azido derivative 92 by the sequence of nitration, catalytic reduction, diazotization, and treatment with sodium azide. Dimroth condensation of 92 with ethyl 6-triisopropylsiloxy-3-oxohexanoate in... 

Intramolecular dipolar azide-olefin cycloaddition of 723 took place upon heating in benzene to afford 724 (83JA3273). An alternative rearrangement process can take place upon photolysis of 724 to give 725. Mesylation of 4-(3-hydroxypropyl)-2,4,6-trimethyl-2,5-cyclohexadiene-l-one (78JA4618) and subsequent treatment with sodium azide in DMF afforded the respective azide 726 which underwent intramolecular cycloaddition to afford the triazoline 727 (83JOC2432). Irradiation of 727 gave the triazole derivative 728 (Scheme 126). 

Photolysis of the azirines 68 in the presence of DEAZD gives 1,2,4-triazolines (69, R = Et) via cycloaddition to the nitrile ylid.114 The nitrile ylid generated thermally from 70 gives 1,2,4-triazolines (69, R = Me, R1 = R2 = CF3) (Scheme 7).11S The cycloadditions proceed in good yield, and the triazolines 69 are readily converted into aromatic 1,2,4-triazoles. 

Thermal decomposition of a doubly labelled azidotriazole gave rise to the open-chain triazine (117) in which the labels were not scrambled. Laser flash photolysis of the triazole leads to (117) within 20 ns with no observable intermediate. Ab initio calculations indicate that a dynamic equilibrium between the open-chain triazine (117) and a cyctic nitrene (118), as originally proposed, is highly disfavoured. 

Photolysis of 2-methyh5-phenyltetrazole gives 4,5-diphenyl-2-methyl-triazole. This may also be formed through a nitrilimine intermediate, although the mechanism has not been established. ... 

Photolysis and p5Tolysis of 4-phenyltriazole gives mainly phenyl-acetonitrile pyrolysis of 4,5-diphenyltriazole in solution gives 2,3,5,6-tetraphenylpyrazine, which is formally derived from the triazole by loss of nitrogen, dimerization, and oxidation (Scheme 58). ... 

Photolysis of azolediazonium tetrafluoroborates, by the method of Kirk and Cohen (73JA4619), is a general reaction in the pyrazole, imidazole, benzimidazole and triazole series and is of synthetic interest to obtain fluoroazoles. 

Thermolysis and photolysis of 28 (R = Ph) in cyclohexene gave a mixture of 3(2-, 3-, 4-cyclohexenyl)-5-phenyl-l,2,4-triazole... 

Several pyrazolo[5,l-c]-l,2,4-triazoles are obtained by thermal or photochemical isomerization of other heterocycles. Thus, pyrazolo [5,l-c]-l,2,4-triazoles (235) are prepared via photolysis of 234 (72JPR55). Thermolysis of 236 obtained by condensing 237 with ethyl acetoacetate, gives 238 (74TL23, 74JHC751). 

Irradiation is very effective in the extrusion of nitrogen from triazole and benzotriazoles. Many of the thermal reactions described in Section 4.01.5.1.2 also take place photochemically. The photolysis of benzotriazole in alcoholic glass at 254 nm is studied by UV absorption, emission, IR, and mass spectroscopies. Scission of the N—NH bond originates from the Si(7t,7t ) state to give the azo compound <87jpci793>. 

The photolysis of the meso-ionic 1,4-diphenyl-l,2,4-triazol-3-one (200, R = R = Ph, R = H) was stated to yield phenyl isocyanate (13%), 7V,iV -diphenylurea (23%), and the bicyclic compound 215 (49%). These results were interpreted in terms of the fragmentation of the photo-intermediate 213 yielding the iV-phenyldiazirine (214). A later publication by the same group reports different results. Photolysis of meso-ionic 1,4-diphenyl-l,2,4-triazol-3-one (200, R = R = Ph, R = H) was stated to yield phenylisocyanate and the bicyclic compound 215. Later studies have shown that the bicyclic compound... 

Photolysis of the diazocompound 171 caused ring contraction to the pyrazolo[3,2-c]-s-triazole (172) via rearrangement of the first-formed a-ketocarbene and hydrolysis of an intermediate ketene.161... 

Irradiation is very effective in promoting extrusion of nitrogen from triazole and benzotriazoles. For example, it is well-known that the photolysis of 1-arylbenzotriazoles afford a high yield of the corresponding carbazoles (see Scheme 2) (8lAHC(28)23l). 

On pyrolysis, 1-arylimidazoles rearrange to 2-arylimidazoles. In other systems pyrolysis causes more deep-seated changes. 1-Arylbenzotriazoles on pyrolysis or photolysis give carbazoles via intermediate nitrenes (see Section 3.4.1.2.1). 1-Phenyl-1,2,4-triazole (714) is pyrolyzed to isoindole... 

Nitrogen extrusion has been used to make fragile molecules 2-thiirene (99) has been obtained by matrix photolysis of 1,2,3-thiadiazole (98), and azirines from 4//-triazoles (100 — 101). 

Photolysis of 2,4,5-triphenyl-1,2,3-triazole 1-oxide gives the triphenyl-1,3,4,5-oxatriazine shown in Scheme 61 (80AJC2447). 

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