Triazolo pyridine

When the six-membered ring is strongly activated, e.g. by a 6-nitro substituent, nucleophilic attack by alcoholic alkali may lead to rearrangement as in Scheme 15 (81T3423). These reactions are analogous to those occurring in the corresponding 1,2,4-triazolo-pyridine and -pyrimidine systems. 

Flash vacuum pyrolysis of 5-(2-pyridyl)tetrazoIe (25) gives triazolo-pyridine 1 in low yield, presumably via the diazoalkane.35... 

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... 

There is a single example of formation of the l,8a-bond. The pyridinium salts 66 can be cyclized by chloranil or by DDQ to give 3-substituted triazolo-pyridines.94... 

A similar series of nucleophilic substitutions has been performed on 4-chloro[l,2,3]triazolo[4,5-c]pyridine (246) and some of its simple substituted derivatives. On compound 246 itself formic acid gives the triazolo-pyridin-4-one,192 ammonia gives the 4-amino compound,192 and hydrazine the 4-hydrazino derivative 245 hydrosulfide gives the 4-thione192 and thiourea in propanol, at first reported151 to give the 4-thione, has subsequently been... 

There are virtually no reports of homolytic reactions on the triazolo-pyridines. Unsuccessful attempts have been made to treat triazolopyridine (1) with methyl radicals,25 and a free radical mechanism is suggested as a possibility in the replacement of the methylthio group by chlorine (Section IV,C).208... 

While there are substantial differences between the chemistry of benzotriazoles and triazolo-pyridines, the systems also have some similar reactivity profiles. As shown in Scheme 9, reductive... 

The synthesis of the triazolo-pyridine-fused 1,3-diazocine 20a was achieved starting from 56a, which by de-O-benzoylation followed by reaction with NaN3 in DMF afforded the triazolo-pyridine 57. The latter by treatment with a mixture of acetone and 2,2-dimethoxypropane in the presence of acid afforded the O-isopropylidene derivative 60. The Mitsunobu reaction led to the diazocine 19a, which, as mentioned in Section 14.05.2.5 (Scheme 2), afforded 20a in 19% overall yield (Scheme 11) <2005JME6454>. 

When 56a was directly treated with NaN3, followed by de-O-debenzoylation with methoxide ion, a mixture of 57 and diazocine 20a was obtained. In this reaction, 57 was formed by solvolysis of 58. Reaction of 56 with NaN3 in DMF at 90 °C afforded only triazolo-pyridine 58 (84%). Increasing either the temperature and the reaction time led, in 60% yield, to the 1,3-diazocine 23, which was O-debenzoylated to give 20a (Scheme 11). This method proved the most convenient to get 20a (47% overall yield) <2004JME6100, 2005JME6454>. 

The formation of thioethers as by-products occurs because the triazolo-pyridines are in equilibrium with a diazo form, and the position of the equilibrium depends on the substitution pattern on triazolopyridine (050BC3905). The presence of an electron-withdrawing sulphoxide in position 6, as is the case in 98 or 99, shifts the equUibrium to the diazo form. The formation of thioethers 104 and 105 from the diazo form, can be explained on the assumption that a carbene intermediate formed by extrusion of nitrogen (07ARK297) is trapped by an oxygen transfer from the corresponding sulphoxide (67TL2363), which is reduced to a thioether (Scheme 21). 

Triazolo-pyridine 26 is structurally related to 2-pyridones and also... 

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). ... 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Triazolo[4,3-a]pyridine, 3-amino-diazonium reactions, 5, 863 Dimroth rearrangement, 5, 861 Sandmeyer reaction, 5, 862 synthesis, 5, 883... 

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