Dextro-isomers

The dextro-isomer may be separated via the dextro[Pg.456]

The free base of either optical isomer may be obtained by addition to the d-tartrate in the case of the levo isomer and the d-bitartrate in the case of the dextro isomer of alkali in excess, as, for example, by the addition of an aqueous solution of caustic soda, which will cause the base to separate as an oil which may be recovered and purified by any well-known procedure. The base is exactly neutralized with sulfuric acid to give the sulfate. 

They rotate the plane of polarized light in opposite directions, though in equal amounts. The isomer that rotates the plane to the left (counterclockwise) is called the levo isomer and is designated ( —), while the one that rotates the plane to the right (clockwise) is called the dextro isomer and is designated (-t-). Because they differ in this property they are often called optical antipodes. 

Shannon 1983). and PCP-mimetic activity is exhibited stereo -specifically by the dextro isomer (Brady et al. 1982). 

As shown from the above formula, the two hydrogen atoms (H1 and H2) in propionic acid are identical. There is, therefore, an equal chance of either H atom being replaced by the Cl. atom. Hence there results equal number of molecules of laevo and dextro isomers and the product is a racemic mixture. It may be noted that the two chloropropionic acids are the mirror images of each other. 

Pentazocine has been successfully used to relieve labour pain [201] and its obstetric use in place of pethidine is favoured by,its apparent inferior ability to pass the placental barrier [206]. A clinical trial of (+)- and (-)-pentazocine adds to the rare number of examples in which optical enantiomorphs have been evaluated [207]. In post-operative patients, response to 60 mg of the dextro isomer was less than that to 5 mg of morphine, while 25—29 mg of (-)-pentazocine was as effective as 10 mg of morphine. Hence most of the activity of the race-mate resides in the laevo isomer, as anticipated from results in animals [208]. Several studies of the distribution, excretion and metabolism of pentazocine have been made. Peak levels of the tritium-labelled drug (and its c/s-3-chloroallyl analogue) were present in the C.N.S. of a cat within 40 minutes of intramuscular administration [209], the comparable figure for morphine being 2 hours [210]. 

Quinidine Quinidine, (5-vinyl-2-quinychdinyl)-(6-methyoxy-4-quinolyl)-methanol (18.1.1) is the dextro-isomer of the alkaloid quinine and is one of the four most important alkaloids, which are isolated from the bark of the cinchona tree [1-3]. Quinidine is a secondary alcohol. 

Mechanism of Action A propylami ne derivative that competes with histamine for H, -receptor sites on effector cells in the gastrointestinal (GI) tract, bloodvessels, and respiratory tract. Dexchlorpheniramine is the dextro-isomer of chlorpheniramine and is approximately two times more active. Therapeutic Effect Prevents allergic response, produces mild bronchodilation, blocks histamine-induced bronchitis. Pharmacokinetics ... 

It is dextro isomer of propoxyphene which is an analgesic and possesses antitussive property. It has low analgesic activity even half of codeine. It is metabolized in Uver. Side effects include vomiting, epigastric distress and sedation. The demethylated metabolite of propoxyphene is cardiotoxic. It is used in the treatment of mild type of pain. 

It is a synthetic compound with structural similarity to ephedrine and is available in racemic and dextro isomers. It increases the systolic and diastolic blood pressure. Amphetamine is a potent CNS stimulant and causes alertness, insomnia, increased concentration, euphoria or dysphoria and increased work capacity. Amphetamine produces wakefulness and improved physical performance. It contracts the sphincter of the bladder and relaxes the bronchial smooth muscle in large doses. Amphetamines are drugs of abuse and can produce behavioural abnormalities and can precipitate psychosis. It can produce psychological but no physical dependence. 

It is an alkaloid obtained from the bark of cinchona and is a dextro isomer of anti-malarial drug quinine. Its sodium channel blocking property results in an increased threshold for excitability and decreased automaticity. As a consequence of its potassium channel blocking properties, it prolongs action potential in most cardiac cells. 

Ethambutol is a synthetic, water-soluble, heat-stable compound, the dextro-isomer of the structure shown below, dispensed as the dihydrochloride salt. 

Infestations with parasitic worms and flukes are widespread both in humans and in animals, and their treatment requires drugs that act in a different manner from antibacterial and antiprotozoal agents. It is desirable for worms to be expelled from the body intact since the presence of dead worms in the tissues can provoke severe reactions. Such reactions are seen when filarial worms which circulate in the blood and lymph are killed by diethylcar-bamazine (264). Intestinal worms may be expelled when they are paralyzed by neuromuscular blockers such as piperazine citrate or pyrantel (265), or their metabolism may be disrupted by the anthelmintic drugs tetramisole and thiabendazole (266) which inhibit fumarate reductase, or mebendazole (267) which prevents glucose uptake by the worms. The anthelmintic activity of tetramisole is due to its laevo isomer levamisole (186). The dextro isomer has antidepressant activity. 

Drug absorption from the oral cavity occurs through the passive diffusion of the nonionized form from an aqueous phase to one that is lipid in nature. In addition, there is also evidence for the carrier-mediated transport of drugs, whereby the levo isomers, but not the dextro isomers, of many drugs are absorbed. 

Splitting off the d-camphorsulfonic acid by using ammonium hydroxide and conversion of the desired a-dextro isomer to the hydrochloride only the dextro isomer is active as an analgesic... 

Esterification of the a-dextro isomer with propionic anhydride... 

While the alanine obtained by hydrolysis of proteins is the dextro- isomer, synthetic methods, of course, yield the dl mixture, from which the d- isomer may be separated as given below. 

From the viewpoint of preferred conformation a stereochemical correlation between the more potent antipodal forms of /3-prodine and the a-2,5-dimethyl derivative (a-promedol) is not immediately apparent. In the latter compound the dextro isomer, depicted in its axial 4-phenyl chair form (preferred for a-4-piperidinol base, X-ray evidence)/38 is about 20 times more active than the desmethyl parent while the levo form is inactive (see 22).chair form of (+)-a-promedol (23) clearly reveals a stereochemical kinship with (+)-/3-prodine in respect to C-3(5) geometry, and furthermore points to the activity-raising role of axial methyl adjacent to nitrogen when... 

Agonist activity was considerably reduced in the isomorphan analogs of Nos 2 (NMe) and 8 (NCBM), most dextro isomers had very low potencies in either test Versus oxymorhponc in mouse Straub tail test (OMST) d Mouse writhing lest... 

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