The analgesic action of fentanyl surpasses that of morphine by approximately 100-fold. It has a suppressive action on the respiratory center and slows heart rate. Fentanyl is used in anesthesiology both independently and in combination with droperidol for neuroleptanalgesia, and in preanesthetic medication, in different forms of narcosis, and in post-operational anesthesia. Unlike morphine, it does not cause a release of histamines. It is used only in specialized hospital conditions. Synonyms for this drug are fentanest, leprofen, and others.

Alfentanil: Alfentanil, N-[1-(2-(4-ethyl-4,5-dihydro-5-oxy-1#-tetrazol-1-yl)ethyl]-4-(methoxymethyl)-4-piperidine]anilinopropionamide (3.1.71), is the next representative of drugs of the class of anilidopiperidines, which differ from fentanyl in that they have a second substituent on the fourth position of the piperidine ring, and also in the replacement of the phenyl group in the arylethyl substituent on the nitrogen atom of the piperidine ring for an aromatic heterocycle, tetrazolone. The synthesis of alfentanil consists in the alkylation of separately prepared N-(4-methoxymethyl)-4-piperidyl)-propionanilide (3.1.68) using 1-(4-ethyl-4,5-dihydro-5-oxo-1#-tetrazol-1-yl)ethyl-2-chloride (3.1.66).

N-(4-Methoxymethyl)-4-piperidyl)-propionanilide (3.1.68) is synthesized from 1-benzylpiperidine-4-one (3.1.48) by means of condensation with aniline in the presence of hydrogen cyanide. The resulting 4-anilino-4-cyano-1-benzylpiperidine (3.1.64) undergoes ethanolysis, forming 4-anilino-4-carboethoxy-1-benzylpiperidine (3.1.65), which is reduced by lithium aluminum hydride into 4-anilino-4-hydroxymethyl-1-benzylpiperidine, which is methylated by methyl iodide to give 4-anilino-4-methoxymethyl-1-benzylpiperidine (3.1.66). The resulting product is acylated using propionic anhydride to give 1-benzyl-4-methoxymethyl-4-N-propionyl-anilinopiperidine (3.1.67), which undergoes debenzylation by hydrogen using a palladium on carbon catalyst to give 4-methoxymethyl-4-N-propionyl-anilinopiperidine (3. 1.68). 1-(4-Ethyl-4,5-dihydro-5-oxy- 1H-tetrazol-1-yl)ethyl-2-chloride (3.1.70) is synthesized starting with ethyl isocyanate and sodium azide. The product resulting from 2+3 cycloaddition to give 4-ethyl-4,5,dihydro-5-oxy-1H-tetrazol (3.1.69) undergoes further alkylation using 1-bromo-2-chloroethane, forming 1-(4-ethyl-4,5-dihydro-5-oxy-1H-tetrazol-1-yl)ethyl-2-chloride (3.1.70), which reacts with 4-methoxymethyl-4-N-propionyl-anilinopiperidine (3.1.68), to give alfentanil (3.1.71) [41-43].

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