1.2.4- Triazolo triazines synthesis

Acenaphtheno[l,2-e][l,2,4]triazolo[4,3-h][l,2,4]triazine 747 was prepared (79AP147) by cyclizing 3-hydrazinoacenaphtheno[l,2-e][l,2,4]tria-zine 746 with formic acid. Reaction of 746 with sugars gave the hydrazones, which cyclized with iron(III) chloride to give 748 (93BCJ00). Similarly, the acetaldehyde derivative of 746 was cyclized to 748. The structure of 748 (R = Me) rather than 747 (R = Me) was deduced by unequivocal synthesis of the latter by condensation of acenaphthenequinone with 3,4-diamino[l,2,4]triazole (Scheme 155). 

Although, 3-hydrazino[l,2,4]triazines have been shown to be versatile precursors for the synthesis of [l,2,4]triazolo[4,3-6][l,2,4]triazines, 3-hydrazino-l, 2,4-triazine-l-oxide 821 cyclized with triethyl orthoformate to give [l,2,4]triazolo[3,4-c][l,2,4]triazine 822. Cyclization takes place at N-4 of 821 (80JOC5421) (Scheme 166). 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

The synthesis and structures of 6-amino-l,2,4-triazolo[3,4-/][l,2,4]triazin-8(7i/)-one derivatives <06MI169 06MI444> and of dihydrotetrazolo[5,l-c][l,2,4]triazines have been described <06H1595>. 

The synthesis and biological activity of l,2,4-triazolo[l,5-a][l,3,5]triazines (5-azapurines) has been reviewed <06H1723>. 

Four different ring systems belonging to this category have been published during the past period synthesis of [l,3,4]oxadiazolo[3,2-tf]triazines, [l,3,4]thiadiazolo[3,2-tf]triazines, and two [l,2,4]triazolo[l,3,5]triazines, the [1,5-a) and [4,3-a] fused ones, are discussed here (Scheme 31). 

When the synthesis of a heterocyclic system containing a diazole fused onto l,2,3-triazin-4-one (e.g., imidazo[4,5-i [l,2,3]tnazin-4-one (Section 10.13.9.1.l(i)) and pyrazolo[3,4-rf [l,2,3]triazin-4-one (Section 10.13.9.1.1(ii))) or 1,2,3-triazole fused onto pyrimidin-4(7)-(di)one (e.g., [l,2,3]triazolo[4,5-rf pyrimidin-4(7)-(di)one (Section 10.13.9.1.l(iv))) is required, the synthesis of the six-membered ring is the most reliable method. There are several recent accounts of six-membered annulations onto 1,2,3-triazoles to give [l,2,3]triazolo[4,5-r/ pyrimidines (Section 10.13.9.1.l(iv)), which are valuable alternatives to 1,2,3-triazole annulation using nitrosation techniques <1996CHEC-II(7)489>. 

Condensed thiophene systems, tetra- and pentacyclic, 32, 127 Condensed 1,2,4-triazines I. Fused to heterocycles with three-, four-, and five-membered rings, 59, 39 Condensed l,2,4-triazolo[3,4-z] heterocycles, synthesis, 49, 277... 

---