Pyridines pyrido-fused system

Various pyrido-fused systems have shown good antibacterial properties. For example, two types of analogs of linezolid, six imidazo[l,2-a]pyridines and six [l,2,4]triazolo[l,3-a]pyridines, demonstrated good in vitro and in vivo (mouse) potential against methicillin-resistant Staphylococcus aureus and other antibiotic-resistant bacterial strains (13MCL1074). 

Whereas in all previously mentioned inverse cycloaddition reactions [h]-fused pyrido annelated systems are formed, some reactions are described which lead to [c]-pyridine annelated bicyclic systems. 5-(Butynylthio)pyrimidines (R = Ph, NHCOCH3) give on heating at 180°C in nitrobenzene 5-R-2,3-dihydrothieno[2,3-c]pyridines (89T803). 5-Propynyloxymethylpyrimidines also readily undergo cycloaddition into l,3-dihydrofuro[3,4-c]pyridines (89T5151) (Scheme 39). Considerable rate enhancements were observed with quaternized pyrimidinium salts. Whereas... 

Compounds 39 and 40, in which a pyrazole ring is fused to a pyrido[2,3-2,]pyrazinc, are produced as a 3 2 mixture by reaction of pyridine-2,3-diamine with 3-methyl-l-phenylpyrazoline-4,5-dione (Equation 9) <1999T8475>, and the synthesis of compound 41 (Scheme 11) exemplifies a different approach to the same ring system <1997JCM318, 1997JRM2026>. 

Pyrido[2,3- pyrimidine bicyclic systems represent interesting fused heterocyclic compounds having pharmacological and biological properties. Their syntheses are well documented in the literature. The pyridine or pyrimidine rings have been used as precursors for constructing the second heterocyclic ring. 

The novel fused heterocyclic system pyrazolo[4, 3 5,6]pyrido-[2,3-b]-1,5-benzoxepine (53) and its benzothiazepine analogue have been obtained by cyclisation of a substituted pyrazolo[3,4-b]-pyridine derivative. 

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