1.2.3- Triazine 1-oxide, reduction

Four methods have been published for the introduction of hydrogen into the 1,2,4-triazine ring, i.e. reduction of halo-1,2,4-triazines, oxidation of hydrazino-l,2,4-triazines, treatment of sul-fonylhydrazino-l,2,4-triazines with base, and decarboxylation of 1,2,4-triazinecarboxylic acids. Decarboxylation of l,2,4-triazine-3-carboxylic acid has been used for the synthesis of the parent 1,2,4-triazine (l) 2 120 this method was also used for the synthesis of other 1,2,4-triazines.270 l,2,4-Benzotriazine-3-carboxylic acid was decarboxylated to give the parent 1,2,4-ben-zotriazine (2).151 271... 

Electrochemical reduction of various 3,4-disubstituted-l,2,5-thiadiazole 1,1-dioxides (3,4-diphenyl- 10, phenanthro[9,10]- 51, and acenaphtho[l,2]- 53) gave the corresponding thiadiazoline 1,1-dioxides <1999CJC511>. Voltammetric and bulk electrolysis electroreduction of 3,4-diphenyl-l,2,5-thiadiazole 1-oxide 9 at ca. —1.5 V, in acetonitrile, gave 3,4-diphenyl-l,2,5-thiadiazole 8 (50%) and 2,4,6-triphenyl-l,3,5-triazine 54 (30%) (Equation 3) <2000TL3531>. 

Amide reduction with lithium aluminum hydride, 39, 19 Amine oxide formation, 39, 40 Amine oxide pyrolysis, 39, 41, 42 -Aminoacetanilide, 39, 1 Amino adds, synthesis of, 30, 7 2-Amino-4-anilino-6-(chloro-METHYl) -S-TRIAZINE, 38, 1 -Aminobenzaldehyde, 31, 6 hydrazone, 31, 7 oxime, 31, 7 phenylhydrazone, 31, 7 > -Aminobenzoic add, 36, 95 2-Aminobenzophenone, 32, 8 c-Aminocaproic acid, 32, 13 6-Aminocaproic acid hydrochloride,... 

In a similar reaction, 2-(o-nitrophenyl)ethylamine (100) may be reduced in an ammonia buffer to the hydroxylamine and oxidized to the nitroso derivative, which condenses with the amino group to a dihydrocinnoline.160,161 It is also possible to prepare dihydrobenzo-l,2,3-triazinones on reduction of o-nitrobenzhydrazide, followed by oxidation of the hydroxylamine to a nitroso group, and 3-phenyldihydrobenzo-l,2,3-triazine (101) from N-phenyl-(V-(o-nitrobenzyl)hydrazine (100) in an analogous way161 [Eq. (78)]. 

Chemical reduction of azine //-oxides, depending on substrate structure, reductant and reaction conditions can proceed both with or without deoxygenation of the N-atom. Thus, 1,2,3-triazine 1-oxide (337) with NaBH4 gives 2,5-dihydro-1,2,3-triazine (336), suggesting that the N-oxide moiety back-donates electrons to the triazine ring. On the other hand, on reduction of the isomeric 2-oxide leading to tetrahydro derivatives (338) and (339) the N-oxide function is not touched (82H(17)317). 

Data on the structures of monocyclic dihydro- or hexahydro-1,2,3-triazines, on 1,2,3-triazine tV-oxides and 1,2,3-triazinones are not yet available. From studies on the one-electron reduction of the tetrahydro-l,2,3-triazinium salts (7) it was concluded that the heterocyclic ring is flexible and not planar (80LA285). No detailed information on the structure of 3-benzyl-l,5-diphenyl-l,2-dihydro-l,2,3-triazine-4,6(3//,5f/)-dione (8) or of the 6-hydroxy-4-oxo-l,4-dihydro-l,2,3-triazinium hydroxide inner salts (9) seems to be available. 

Triazine 4-oxides (440) react with ynamines (423) to give the same pyrimidine derivatives (424) which are formed from the reaction of the appropriate 1,2,4-triazine. It was shown that the first step in these reactions is the reduction of the Af-oxide (440) by the ynamine (78CB240). 

Anderson RF, ShindeSS, Elay MP, GamageSA, Denny WA (2003b) Activation of 3-amino-1,2,4-benzo-triazine 1,4-dioxide antitumor agents to oxidizing species following their one-electron reduction. J Am Chem Soc 125 748-756... 

Spectrophotometric techniques combined with flow injection analysis (FIA) and on-line preconcentration can meet the required detection limits for natural Fe concentrations in aquatic systems (Table 7.2) by also using very specific and sensitive ligands, such as ferrozine [3-(2-bipyridyl)-5,6-bis(4-phenylsulfonic acid)-l,2,4-triazine], that selectively bind Fe(II). Determining Fe(II) as well as the total Fe after on-line reduction of Fe(III) to Fe(II) with ascorbic acid allows a kind of speciation.37 A drawback is that the selective complexing agents can shift the iron redox speciation in the sample. For example, several researchers have reported a tendency for ferrozine to reduce Fe(III) to Fe(II) under certain conditions.76 Most ferrozine methods involve sample acidification, which may also promote reduction of Fe(III) in the sample. Fe(II) is a transient species in most seawater environments and is rapidly oxidized to Fe(III) therefore, unacidified samples are required in order to maintain redox integrity.8 An alternative is to couple FIA with a chemiluminescence reaction.77-78... 

Dipolar cycloaddition of nitrile oxides to olefins and acetylenes is among the most widely exploited synthetic routes towards isoxazoles and isoxazolines. It is well-known that microwave irradiation in cycloaddition reactions considerably reduces reaction times. Indeed, the use of dielectric heating (microwave-heated reactions were performed in a flask with a reflux condenser mounted outside the apparatus) allowed for a remarkable reduction of the cycloaddition reaction time from 6-12 hours to merely 3 minutes [69]. Simple aqueous workup provided the target isoxazoles and isoxazolines. The requisite nitrile oxides for the cycloaddition reaction were generated in situ from the corresponding nitroalkanes, 4-(4,6-dimethoxy [1,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) and 4-dimethylaminopyridine (DMAP) (Scheme 22). 

Conversion of the triazine 1-oxide (54a) to 3-aminopyrido[2,3-e]-l,2,4-triazine (58) can be achieved by reduction to the dihydro triazine (57) followed by oxidation using K3Fe(CN)6. 3-Aminopyrido[2,3-e]-l,2,4-triazine (58) is obtained in an overall yield of 57% (Scheme 2) <62JOC2504>. 

Photo reduction of 3-aminopyridine[4,3-e]-1,2,4-triazine 1-oxide (83) at 254 nm in either aqueous methanol or HC1 delivered the pyrido-l,2,4-triazine (92) subsequent photolysis in anhydrous methanol gave the methoxymethyl derivative (93) <76MI 717-03). 

Electrochemical reduction of pyrano-l,2,4-triazin-3-ones (132) and (148) delivered the corresponding dihydro derivatives (134) and (153) in yields of 81% and 85%, respectively. Further electrochemical treatment of the compounds (134) and (153) provoked a ring contraction, leading to the pyranoimidazol-2-ones (154a,b) in 83% and 68% yield, respectively. Unexpectedly, attempted reaction of compounds (134) and (153) with NaBH4 in ethanol resulted in oxidation to the triazin-3-ones (132) and (148) (Scheme 7) (86JHC491). 

Eight examples of 3-aryl pyrido[3,2-e]-1,2,4-triazines were prepared via a reductive cyclization route. Thus, 3-phenylpyrido[3,2-e]-l,2,4-triazine (65) was prepared from the aminopyridine (234) by cyclization followed by oxidation <76CR(C)487>. In another study, seven 3-substituted pyrido-1,2,4-triazines were prepared via an analogous route <76MI 717-04). This work mirrored the earlier investigations of Lewis and Shepherd <7lJHC4l>. 

At pH <1.5, however, A-oxides such as those of cinnoline,44 3,6-diphenylpyridazine,94 and pyrazine,254 give a two-electron polarographic wave followed by the reduction waves of the parent compounds. An attempt based on this to reduce a benzo-1,2,3-triazine-3-oxide to the parent compound failed as the compound was hydrolyzed too rapidly in strongly acid solution. 

The [Fe(terpy)2] cation is low-spin, as demonstrated by Mossbauer, electronic, H NMR, and resonance Raman spectroscopy and magnetic measurements (20,184,187, 228, 266). Similarly, spectroscopic studies of the iron(III) cation have indicated a low-spin ( B) ground term (382). There have been numerous electrochemical studies of the bis complexes (177, 200, 256, 298, 332, 344, 373, 378, 379, 397, 398). Ligand-centered reductions to formal oxidation states of iron(I), iron(O), and iron( — 1) and oxidations to iron(III) are observed. The complex [Fe(terpy)L][C104]2 [L = tris(2 -pyridyl)l,3,5-triazine (Fig. 15)] has been prepared (399, 442). 

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