3- - triazolo rearrangements

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

Amino-l,2,4-triazole undergoes a cyclocondensation with 3-etlioxyacrolein (7) to form 1,2,4-triazolo[l,5-a]pyrimidine (3) or its [4,3-u] isomer (5), according to whether it reacts as IH or 4H tautomer 2 or 4. Moreover, the pyrimidines 3 and 5 can interconvert by a Dimroth rearrangement. Since the H NMR spectrum 30a does not enable a clear distinction to be made AMX systems for... 

Assignment of the l,2,4-triazolo[l,5-c]pyrimidine structures to the products obtained from the previously described cyclizations and not the alternative [4,3-c] structures has been rationalized and corroborated on the basis of (a) preference of cyclization at the more nucleophilic triazole ring N2 rather than at its less nucleophilic N4 (65JOC3601 88JMC1014), (b) inability of the obtained products to undergo acid- or base-catalyzed Dimroth rearrangement, a property characteristic of the thermodynamically less stable [4,3-c] isomers (91JMC281), (c) comparison with unequivocally prepared... 

Tlie thermodynamically more stable l,2,4-triazolo[l,5-c]pyrimidines 23 were frequently prepared by Dimroth rearrangement of their thermody-... 

In all syntheses of [ 1,2,4]triazolo[4,3-a]pyridines it should be remembered that electron withdrawing substituents on the pyridine ring can cause Dimroth rearrangement of the initially formed compounds into derivatives of [l,2,4]tria-zolo[l,5-fl]pyridines (see Section B.2.c). 

Rearrangement of oxadiazole bearing quinoline 772 gave the triazolo-quinolinyl ketoxime 773 (93H1577) (Seheme 135). 

The reaction of several substituted imidazo[4,5-c/]-, pyrazolo[3,4-r/]- and triazolo[4,5-zf]pyrid-azines 3 with ynamines, in competition with [4 + 2] cycloaddition, leads to [2 + 2] derivatives 4, which rearrange to l,2-diazocines5.7 8 The reaction seems to be sensitive to the substituents, as replacement of the electron-withdrawing group R on the pyridazine ring of the pyrazolo compound (A = N, B = CH) by chlorine completely inhibits both the [4 + 2] and [2 + 2] cycloaddition reactions. The X-ray structure of the imidazo derivative 5 (R = Ms, A = CH, B = N) reveals a tub conformation of the eight-membered ring. 

Diazotization of 5-amino[l, 2,3]triazole 692 afforded (88BSB179) triaz-olo[l, 5-i>][l, 2,4]triazine 694 as a result of a Dimroth rearrangement of the initially formed isomeric structure triazolo[5,l-c][l,2,4]triazine 693. Molecular structure of 694 was determined by single X-ray diffraction (Scheme 146). 

Treatment of a solution of 2-benzothiazolylthioacetyl hydrazide 386 in ethanol with carbon disulfide in the presence of potassium hydroxide gives the rearrangement product [l,2,4]triazolo[3,4- ]benzothiazole-3-thiol 387 (unreported yield) (Scheme 43). The structure of this compound was confirmed by its analytical and spectroscopic data, and confirmed by unequivocal synthesis from 2-benzothiazolhydrazine 388 under the same experimental conditions <2002MI3, 2002MI4>. 

X-Ray analyses of azapentalenes have been used to determine the structure of products from reactions with ambiguous reaction sites or with concomitant rearrangements. Thus, the X-ray structure of the /3-D-[l,2,4]triazolo[4,3-3][l,2,4]triazole 2 establishes that glycosylation has occurred at the N-l position furthermore, it also shows that the compound is the ft-anomer. In addition to intermolecular hydrogen bonds in the crystal structure an intramolecular hydrogen bond is extended from OH-5 to N-7 <1984NN187>. The X-ray structure of the inner salt of compound 1 has been determined... 

On heating, 6-aryl-2-methyl-7/f-[l,2,4]triazolo[4,3-3][l,2,4]triazole-3(2//)-thiones 46 undergo N-to-S migration of the methyl group, yielding the rearranged products 47 (Equation 6) <1985BCJ735>. 

By analogy with the conversion of 1,2-dialkylpyridinium salts into 2-alkylaminopyridines known as the Kost-Sagitullin rearrangement, the pyridinium salt 252 was treated with two different hydrazides (Equation 33). The [l,2,4]triazolo[4,3-tf]-pyridines 253 with Y = 4-pyridyl and NH2 were obtained with 35% and 38% yields, respectively <2003CHE275>. 

It is important to note that besides these synthetic pathways a very important access to [ 1,2,4]triazolo[ 1,5 z] pyrimidine derivatives is the Dimroth rearrangement of [l,2,4]triazolo[4,3-c]pyrimidine compounds. This type of ring transformation is specifically discussed in Section 11.16.5.2 these possibilities are also reviewed in Section 11.16.7. As these isomerizations always take place into the direction of the [l,2,4]triazolo[l,5-c]pyrimidine ring, in several studies only these products are described without special (or any) note of the primarily formed [l,2,4]triazolo[4,3-c]pyrim-idine ring. Table 17 contains the stmctures of some [l,2,4]triazolopyrimidines and benzologues with a fusion site of the triazole ring that have been formed via transformation of the isomeric [ 1,2,4] triazolo[4,3-f]-pyrimidine compounds with or without isolation of these intermediates. 

Another interesting rearrangement, involving a pyrimidine-pyridine ring transformation combined with a Dimroth rearrangement, is observed when 6-nitro[l,2,4-triazolo][l,5- ]pyrimidine (74) reacts with ethyl cyanoacetate. 

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