New cascade synthesis of pyrido[2',3':4,5]thieno- [2,3-b]pyridine derivatives

Abstract

In previous work [1][2][3][4][5], we showed that the triple cyclocondensation of partially hydrogenated 3-cyano-2-pyridinethiolates with acetone and malononitrile leads to the formation of derivatives of pyrido [2',3':4,5]thieno [2,3-b]pyridine. The mechanism of this unusual reaction involves the oxidation of the starting thiolates by atmospheric oxygen to give bis(2-pyridyl) disulfides, subsequent nucleophilic cleavage of the S-S bond by the isopropylidenemalononitrile anion, and cascade heterocyclization [6]. As a rule, the partially hydrogenated pyridine system remains unchanged during the reaction although, in some cases, the formation of oxidation products was noted [4,5]. In a continuation of our investigation, we decided to study the behavior of readily available 5-acetyl-4-aryl-3-cyano-6-methyl-3,4-dihydropyridine-2(1H)-thiones 1a and 1b [7][8][9][10][11] under conditions for cyclocondensation with acetone and malononitrile. N N Me Me O N H Ar O Me Me S N N S N O Me Ar Me Me NH 2 N 1a,b + 2a,b B, EtOH, t o В = N-methylmorpholine; a Ar = 2-ClC 6 H 4 ; b Ar = 4-O 2 NC 6 H 4We have found that dipyridothiophene derivatives are also formed during this reaction but, in contrast to other tetrahydropyridine derivatives [1][2][3], cyclocondensation in this case is accompanied by aromatization of the tetrahydropyridine ring. The structure of dipyridothiophenes 2a and 2b was supported by the spectroscopic data. We should also note that these products have a highly pronounced blue-green UV photoluminescence.