1.2.3- Triazolo pyridine 3-oxides

Another dipolar cycloaddition is that between the sulfimide 46 and p-toluonitrile oxide, giving the triazolopyridine 3-oxide 47, which can be deoxygenated by phosphorus trichloride to give the 2-(p-tolyl)triazolo-pyridine.61... 

The ring-chain isomerisation of phosphino-substituted triazolo-pyridines was found to depend on the nature of the phosphine substituent. Conversion of the phosphines to their selenides resulted in a complete shift of the equilibrium towards electron-acceptor structure D (Scheme 10). Acenaphthene and acenaphthylene were converted to polycyclic phosphole derivatives via Ti(II)-mediated cyclization of the corresponding dialkynylated arenas. The related phosphole oxides were stable species (Scheme 11). Asymmetric lithiation of dimethyl-tert-butylphosphine sulfide and trimethylsilylation gave an intermediate that was subjected to a five-step reaction sequence including metallation, P-functionalization (in three steps) and removal of the silyl group to yield the precursor of Mini-PHOS (Scheme 12). ... 

HOAt, 7-aza-l-hydroxybenzotriazole HATU (CAS Registry No. 148893-10-1), A-[(dimethylamino) (3//-1,2,3-triazolo(4,5- )pyridin-3-yloxy)methylene]-A-methyl-methanaminium hexafluorophosphate, previously known as G-(7-azabenzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate. [Note Assignment of structure to HATU as a guanidinium species rather than as a uronium species, i.e., attachment of the (Mc2NC=NMe2) unit to N3 of 7-azabenzotriazole 1-A-oxide instead of to the O, is based on X-ray analysis (ref. 33b)]. 

Oxidation of 5-arylazo-6-aminoquinoline 146 with copper sulfate in pyridine gave the corresponding 2-aryltriazolo[4,5-/]quinolines 147. Condensation of halo-genated nitrobenzenes with triazolo[4,5-/]quinoline 145 yielded the appropriate 2H- and 3//-aryl derivatives. The nitration of 3-phenyl-3//-triazolo[4,5-/]quino-line 147 occurred at position 4 of the phenyl ring (Scheme 46) (73T221). 

The only examples of ring oxidation are the one-electron anodic oxidation of Nl-aryl[l,2,4]triazolo[4,3-a]pyridines such as compound 143 to give quaternary salts (88ZC187), and the voltammetric oxidation of the anti-depressant Trazodone (Section V.A) (87MI1). 

Reactions of 2-nitrosopyridine with nitrile oxides afford, depending on structure of the latter, either l,2,4-triazolo[l,5-a]pyridine 1,3-di-N-oxides (370) or the corresponding 1.2.4-triazolo, 5-a Ipyridine 3-oxides (371) (419). 

Cycloaddition of pyridine Ar-oxides (see Section 11.13.2) led to careful examination of thermodynamic aspects, though no experimental measurement was provided. Thermodynamic profile for the ring-chain isomerization of [l,2,3]triazolo[l,5- ]pyridines via a ring-opening pathway (Equation 2) was calculated. Based on this computational study, a multistep mechanism was proposed <20050BC3905>. 

Table 2 Syntheses of [1,2,4]triazolo[4,3-a]pyridines by means of hydrazone oxidation ...

HATU /V-[(dimethylamino)-l//-l,2,3-triazolo[4,5-/ ]pyridin-l-ylmethylene] -A-methylmethanaminium hexafluorophosphate N-oxide (also known as <9-(7-azabenzotriazol-l-yl)-l, 1,3,3-tetramethyluronium hexafluorophosphate). 

The oxidation of A -phenylhydrazones in the presence of pyridine leads to the formation of 5-triazolo[4,3a]pyridinium salts hy attack of pyridine as a nucleophile on the nitrilimine intermediate (Scheme 54) [77]. 

In 1981 we published the first paper [22] on the synthesis of s-triazolo[4,3-a]pyridinium salts, 4, by the anodic oxidation of hydrazones 3 in the presence of pyridine (Scheme 5). In our working mechanistic scheme we proposed nitrilimine as the possible intermediate and pointed out that this reaction opens the door to a wide range of heterocyclic systems via anodic oxidation of aldehyde hydrazones through 1,3-dipolar cycloaddition reactions of the nitrilimine involved. 

Several heterocyclic hydrazones 32 have been oxidized in CH3CN-O.I mol/1 Et4C104 solution with addition of 60% HCIO4 to s-triazolo[4,3-a]pyridine derivatives 33 in yields ranging from 55 to 92% [56] (Scheme 20). 

Zinc chloride-doped natural phosphate was shown to have catalytic behavior in the 1,3-dipolar cycloadditions of nucleoside acetylenes with azides to form triazolonucleosides <99SC1057>. A soluble polymer-supported 1,3-dipolar cycloaddition of carbohydrate-derived 1,2,3-triazoles has been reported <99H(51)1807>. 2-Styrylchromones and sodium azide were employed in the synthesis of 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles <99H(51)481>. Lead(IV) acetate oxidation of mixed bis-aroyl hydrazones of biacetyl led to l-(a-aroyloxyarylideneamino)-3,5-dimethyl-l,2,3-triazoles <99H(51)599>. Reaction of 1-amino-3-methylbenzimidazolium chloride with lead(fV) acetate afforded l-methyl-l/f-benzotriazole <99BML961>. Hydrogenation reactions of some [l,2,3]triazolo[l,5-a]pyridines, [l,2,3]triazolo[l,5-a]quinolines, and [l,2,3]triazolo[l,5-a]isoquinolines were studied <99T12881>. 

The only alkyl group reported as reductively removed is the di-phenylmethyl substituent from N3 in compound 165.153 Deoxygenation of N-oxides can be done thermally (heating in toluene with oxygen transfer to solvent) for compounds of type 166,44 or more commonly by use of phosphorus derivatives, as described for compounds 32, 33,45 166,44 and 167.61 Halogens can be reductively removed from [l,2,3]triazolo-[4,5-6]pyridines 160,146 168,220 and 169,142 and from pentachloro derivative 164, giving dichloro compound 170.208 Removal of thiol groups by reduction... 

Several heterocyclic hydrazones (22) have been oxidized in CH3CN-Et4NC104 solution with addition of 60% HC104 to 5-triazolo [4,3-a]pyridine (23) derivatives in yields ranging from 55 to 92%73 [Eq. (31)]. 

The oxidative cyclization is presumed to occur after the second electron transfer and to involve a nitrilimine as an intermediate. When benzylidene 2-pyridylhydrazone (24) was oxidized, 3-phenyl-3-triazolo[4,3-a]pyridine... 

---