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Part-1Review on Antiulcer LiteratureNNSNNSHNNNNNYJA20379-4 34YJA20379-2 35Yoon, et al., 75 have replaced the conventional benzimidazole ring system with thebioisosteric benzothiazolidine ring system. They have reported the synthesis ofderivatives of 2-[(3,5-dimethyl-4-methoxypyridylalkyl]-benzothiazolidine 36 which werefound to be more potent in vitro inhibitors of H + /K + -ATPase. The methylsufinyl linkagehas also been replaced by methylene linkages.OR 3 R 2SNnR 136NHN-alkylation/acylation of the benzimidazole ring nitrogen leads to the biolabile N-substituted benzimidazole derivatives (prodrugs) of timoprazole. The parent N-Hcompound is liberated either by in vivo esterase hydrolysis or requires an acidicenvironment. N-(acyloxy)alkyl-substituted benzimidazoles showed improved chemicalstability of which 37 proved twice potent as omeprazole. Similarly 38 was found to betwice active as timoprazole. 76NOONOSSNNOO37N38ONFusion of one more ring on the benzimidazole nucleus has been shown to be beneficial.Sigrist-Nelson et al 77 ., have reported the synthesis and evaluation of 5,7-dihydro-2{[(4-methoxy-3-methyl-2-pyridyl)methyl]sulfinyl}-5,5,7,7-tetramethylindeno[5,6-d]imidazol-6-(1H)-one (Ro 18-5364) 39 as an extremely effective inhibiting agent. Ro 18-5364produced almost complete inhibition of the H + /K + -ATPase activity, as well as, associated27