Analgesics reactions

Phenyllithium can be used in Grignard-type reactions involving attachment of phenyl group, eg, in the preparation of analgesics and other chemotherapeutic agents (qv). It also may be used in metal—metal interconversion reactions leading, eg, to phenyl-substituted siUcon and tin organics. 

Aluminum flufenamate, tris-[2-(3-trifluoromethylphenyl)aminoben2oate]aluminum, is a safer and more effective analgesic than aspirin (94). The dihydroxyalurninum flufenamate is made by reaction of flufenamic acid with aqueous caustic, followed by addition of aluminum chloride with stirring at 42°C for 15 min to give 99% yield (95). Both forms are less irritating and less toxic than the parent acid or aspirin (94,95). 

Compounds of type (42) are widely used in the dye industry (see Azo dyes). The Mannich reaction also takes place at C, as does halogenation and nitration. The important analgesic aminoantipyrine [83-07-8] (43) on photolysis in methanol undergoes ring fission to yield (44) (27). 

A large number of salts of sahcyhc acid have been prepared and evaluated for therapeutic or other commercial use. Table 7 hsts those most frequently referenced. Sodium sahcylate has analgesic, antiinflammatory, and antipyretic activities and was used extensively in the sixteenth and seventeenth centuries as a remedy, prepared from natural sources, for arthritis and rheumatism. In the 1990s the salt can be obtained directly from Kolbe-Schmitt carboxylation or by the reaction of sahcyhc acid with either aqueous sodium bicarbonate or sodium carbonate. The resulting mixture is heated until effervescence stops the salt is then isolated by filtration and evaporation to dryness at low temperatures. Generally, the solution must be kept slightly acidic so that a white product is obtained if the mixture is basic, a colored product results. The USP product contains 99.5—100.5% NaC H O (anhydrous). The May 1996 price was 8.15/kg (18). 

Sahcylamide [65-45-2] is prepared by the reaction of methyl sahcylate with ammonia. Sahcylamide has mild analgesic, antiinflammatory, and antipyretic properties. Sahcylamide is unlike other sahcylates in that it causes sedation and central nervous system depression. Sahcylamide is not hydroly2ed to sahcylate and its action depends on the entire molecule. Sahcylamide has been useful for protection against mil dew and fungus in a variety of soaps, salves, lotions, and oils. The May 1996 price was 8.00/kg (18). 

According to Eddy, as quoted by Small, the analgesic action of neopine, n omorphine, 6-acetylneomorphine or 3 6-diacetylneomorphine (p. 218) is definitely less than that of morphine and its corresponding analogues. The first two are about half as toxic as codeine and morphine respectively, and the second pair are more toxic than their morphine analogues. None of the four shows the Straub reaction and the convulsant action is less marked. 

Similarly, W-methyl-D-aspartate (NMDA) antagonists 32 with analgesic activity were prepared, again using the Meth-Cohn quinoline synthesis as the key entry reaction, subsequent functional group manipulation giving the desired target compound. 

Bromination of the methyl group of (249) with A -bromosuccinimide, followed by reaction with excess secondary amine gave (250) which shows combined analgesic and antitussive properties. The Reformatsky reaction has also been used for the preparation of 2-amino-ethyl 3,3-diaryl-3-hydroxypropanates (251) as well as their dehydration products. The propene amides (252) have also been prepared for pharmacological evaluation. In l-methyl-3-bis (2-thienyl)-... 

Omission of the side chain methyl group also leads to an active analgesic, the potency of which is somewhat less than half that of the parent. Alkylation of the familiar nitrile with N (2-chioroethyl)dimethylamine gives the amine, 126. Reaction with... 

Replacement of the ketone by an amide leads to Increased potency. Hydrolysis of nitrile, 133 (obtained by alkylation of diphenylacetonitrile with the morpholine analog of the chloro-amine used in the original preparation of methadone), affords acid, 134. Conversion to the acid chloride followed by reaction with pyrrolidine affords racemoramide (135) Separation of the (+) isomer by optical resolution gives dextromoramide, an analgesic an order of magnitude more potent than methadone. 

In an unusual variant on the Chichibabin reaction, treatment of 3-hydroxypyridine with sodium amide at 200° affords 2,6-di-aminopyridine (21). Coupling of the product with benzenediazo-nium chloride gives phenazopyridine (22). This drug is used as an analgesic for the urinary tract in conjunction with antibacterial agents for treatment of urinary infections. 

Catalytic reduction of codeine (2) affords the analgesic dihydrocodeine (7) Oxidation of the alcohol at 6 by means of the Oppenauer reaction gives hydrocodone (9)an agent once used extensively as an antitussive. It is of note that treatment of codeine under strongly acidic conditions similarly affords hydrocodone by a very unusual rearrangement of an allyl alcohol to the corresponding enol, followed by ketonization. 

The ring-contracted analog of alphaprodine is prepared by a variation of the scheme above. Alkylation of 109 with ethyl bromoacetate affords the lower homolog diester (115). Dieckmann cyclization followed by saponification-decarboxylation yields the pyrrolidine (116). Reaction with phenylmagnesium bromide leads to the condensation product (117) acylation with propionic anhydride gives the analgesic agent prolidine (118)... 

The majority of analgesics can be classified as either central or peripheral on the basis of their mode of action. Structural characteristics usually follow the same divisions the former show some relation to the opioids while the latter can be recognized as NSAlD s. The triamino pyridine 17 is an analgesic which does not seem to belong stmcturally to either class. Reaction of substituted pyridine 13 (obtainable from 12 by nitration ) with benzylamine 14 leads to the product from replacement of the methoxyl group (15). The reaction probably proceeds by the addition elimination sequence characteristic of heterocyclic nucleophilic displacements. Reduction of the nitro group with Raney nickel gives triamine 16. Acylation of the product with ethyl chlorofor-mate produces flupirtine (17) [4]. 

The search for opioid analgesics which show reduced addiction liability ha.s centered largely on benzomorphan and morphinan derivatives. Some research has, however, been devoted to derivatives of the structurally simpler meperidine series. The preparation of one such compound, picenadol (59), starts with the reaction of N-methyl-4-piperidone with the lithium derivative from m-methoxybromobenzene. Dehydration of the first formed carbinol 51 gives the intermediate 52. Deprotonation by means of butyl lithium gives an anion which can be depicted in the ambident form 53. In the event, treatment of the anion with propyl bromide gives the product 54 from reaction of the benzylic anion. Treatment of that product, which now contains an eneamine function. 

Conversion of Acid Anhydrides into Amides Acetic anhydride is also commonly used to prepare iV-substituted acetamides from amines. For example, acetaminophen, a drug used in over-the-counter analgesics such as Tylenol, is prepared by reaction of p-hydroxyaniline with acetic anhydride. Note that the more nucleophilic -NH2 group reacts rather than the less nucleophilic -OH group. 

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