in Wiley Online Library (wileyonlinelibrary.com). The procedures for the synthesis of substituted pyrazolo[3,4-b]pyridine-6-ones and some of their properties are reviewed.

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1 [4 Aminoarylpyrazolo[3,4 b]pyridin 5 yl]pyridinium chlorides undergo cyclization un der reflux in tert butanol in the presence of an excess of potassium tert butoxide to form tetracyclic derivatives of pyrazolo[3,4 b]pyrido[1´,2´:1,2]imidazo[4,5 d]pyridine. The reac tion scheme of the processes is proposed. The structures of the reaction products were con firmed by physicochemical methods. Key words: 2 chloroacetamido 4 cyanopyrazole, 1 [4 aminopyrazolo[3,4 b]pyridin 5 yl]pyridinium chloride, pyrazolo[3,4 b]pyrido[1´,2´:1,2]imidazo[4,5 d]pyridine, 4,5 diamino pyrazolo[3,4 b]pyridine.

Starting from the substituted 4,5 diaminopyrazolo [3,4 b]pyridine, derivatives of a number of tricyclic systems, viz., imidazo[4,5 d]pyrazolo [3,4 b]pyridine, pyrazolo[3,4 b][1,2,5]thia diazolo[3,4 d]pyridine, pyrazolo[3,4 b][1,2,3]triazolo[4,5 d]pyridine, and [1,3]oxazo lo[5,4 b]pyrazolo[4,3 e]pyridine, were synthesized and reaction schemes for the processes were proposed. Key words: pyrazolo[3,4 b]pyridin 6(7H ) one, dihydroimidazo[4,5 d]pyrazolo[3,4 b]pyri din 4(3H ) one, diazepines, pyrazolo[3,4 b][1,2,5]thiadiazolo[3,4 d]pyridin 4(3H ) one, [1,3]oxazolo[5,4 b]pyrazolo[4,3 e]pyridine, dimethylformamide diethyl acetal.One of the most fruitful trends in the development of chemistry of heterocyclic compounds is a creation of new approaches to the synthesis of fused heterocycles, the detailed analysis of properties of which has both the theo retical and practical importance. Many compounds, con taining pyrazole and pyridine rings (often the fused ones), are of interest as the prospective biologically active sub stances. Thus, the new nonglycoside antiherpetic com pounds, 1,2 nonpeptide corticotropin releasing factor in hibitors, active against depressions, anxiety, and other stress pathologies, 3-5 as well as selective GAMK recep tor modulators, showing strong sedative effect and con siderable analgesic activity, were discovered among pyrazolopyridines. 6 Earlier, 7 we synthesized 4,5 diamino 1 phenyl 1H pyrazolo[3,4 b]pyridin 6(7H ) one hydrochloride (1). The presence of two amino groups, located at the adjacent positions of pyrazolopyridine (4 and 5), opens wide possi bilities for the application of this compound in the heterocyclization reaction for the synthesis of tricyclic systems, including pyrazolo[3,4 b]pyridine fragment.We showed that the reflux of diamine 1 with acetyl acetone in ethanol for 2 h (Scheme 1) leads to the imid azole ring closure, resulting in 69% isolated yield of 2 methyl 6 phenyl 5,6 dihydroimidazo[4,5 d]pyrazo lo[3,4 b]pyridin 4(3H ) one hydrochloride (3). TLC analysis of the reaction mixture showed the presence of insignificant admixture of diazepine 2, which upon fur ther heating was completely converted to the final tri cyclic compound 4. Undoubtedly, the initial condensa tion of diketone at one of the amino groups proceeds to form azomethines А and (or) B. Further, the addition of the free amino group at the azomethine double bond takes place to form imidazoline ring, the aromatization of which proceeds with elimination of acetone and formation of tricycle 3.Based on the data in papers, 8-10 it can be assumed that during the course of the reaction, an equilibrium mixture of the starting compound 1, the intermediates А-C, and diazepine 2 is formed, which further slowly and irreversibly is converted to hydrochloride 3. Imidazo pyrazolopyridine 4 can be further liberated from the latter. In order to confirm our suggestions on the course of cyclization under consideration, we carried out the reac tion of the base of the starting diamine 1 with acetylacetone in benzene with the removal ...

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