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Astemizole is used for preventing and treating severe seasonal and chronic allergic rhinitis, allergic conjunctivitis, hives, Quinke's edema, other allergic conditions and dermatitis. Synonyms of this drug are hismanal, histazol, and others.

Diphenylpyraline: Diphenylpyraline, 4-diphenylmethoxy-1-methylpiperidine (16.1.32), is synthesized by alkylating 4-hydroxy-1-methylpiperidine with benzhydrylbromide

Diphenylpyraline is an antihistamine drug with anticholinergic and sedative action.

It is intended for symptomatic treatment of seasonal allergies and allergic reactions as well as an adjuvant drug in anaphylactic reaction therapy. Synonyms of this drug are arbid, timpil, histryl, hispril, and others.

H2-receptor antagonists almost completely block secretion of hydrochloric acid in the stomach in response to most stimuli. These drugs play a major role in treating stomach ulcers associated with hypersecretion, because they have the ability to reduce both the volume of stomach secretion and overall acidity as well as pepsin activity.

Drugs of this kind are used for treating stomach and duodenum ulcers and hypersecre-tive conditions.

Traditional or H1 antihistamine drugs block many effects caused by histamine; however, it turns out that they are not able to withstand events mediated by H2 receptors, in particular excess gastric juice secretion. In 1977 an H2-receptor antagonist, cimetidine, was proposed, which revolutionized stomach ulcer treatment. Later on, ranitidine was proposed, followed by drugs with minor structural and pharmacological differences such as famoti-dine and nizatidine.

16.2 H2RECEPTOR ANTAGONISTS

H2-receptor antagonists reversibly and competitively inhibit histamine action on H2 receptors. They are pure antagonists since they do not affect receptors, /¡-adrenorecep-tors, or muscarinic receptors.

Moreover, they do not have a significant effect on the synthesis, release, and biotransformation of histamine.

The structure of cimitidine is synthesized up of a methylimidazol ring with a sulfur-containing side chain with a cyanoguanidine group. It seemed that the presence of an imidazol ring in cimetidine contained in the structure of histamine should be the determining factor in the exhibition of H2 blocking activity; however, the formation of ranitidine, famotidine, and nizatidine, which contain furane and thiazol rings in place of an imidazole ring showed the incorrectness of this suggestion.

Cimetidine: Cimetidine, 1-cyano-2-methyl-3-[2-[[5-[[methylimidazol-4-yl)methyl]thio] ethyl] guanidine (16.2.5), is synthesized in the following manner. Reacting 2-chloroace-toacetic ether with two moles of formamide gives 4-carbethoxy-5-methylimidazol (16.2.1). Reduction of the carbethoxy group of this produced with sodium in liquid ammonia gives 4-hydroxymethyl-5-methylimidazol (16.2.2). The hydrochloride of the resulting alcohol is reacted with 2-mercaptoethylamine hydrochloride to produce 4-(2-aminomethyl)-thiomethyl-5-methylimidazol dihydrochloride (16.2.3). This is reacted with N-cyanimido-S,S-dimethyldithiocarbonate to give a thiourea derivative (16.2.4), which upon reaction with methylamine turns into cimetidine (16.2.5) [51-58].

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