Alkaloid mixture, preparative separation

Alkaloid mixture, preparative separation by high-speed countercurrent diroinatpgraphy, 426-434... 

Initial methods for the isolation of vinblastine from the periwinkle plants (vinca rosea) had been described (5,7,23-25) and well documented in several texts including the previous profile of vinblastine sulfate (12). Isolation of vinblastine and vincristine from Catharanthus roseus continues to receive attention, and several procedures have been reported (mainly in the patent literature) for the isolation and separation of these alkaloids (24-29). Extracts of Catharanthus roseus have been found to contain N-demethylvinblastine and this can be used to prepare vincristine by formylating the alkaloid mixture before separation and purification (30). 

Preparative Separation of Complex Alkaloid Mixture by High-Speed Countercurrent Chromatography... 

The combination of preparative high-speed countercurrent chromatography with other separation methods, such as HPLC, and TLC, will enable chemists to isolate minor components of complex alkaloid mixtures more efficiently. This technique is not limited to alkaloid separations and, in theory, other complex mixtures of compounds having only minor differences in their partition coefficients should be efficiently separated by high-speed countercurrent chromatography. 

Numerous methods for high-performance Uquid chromatography (HPLC) of tropane alkaloids have been developed. The chromatographic conditions depend on the variability of the analyzed matrices (extracts from different plant tissues, pharmaceutical preparations, clinical, and forensic probes) and analytes (pure compounds or alkaloid mixtures with different composition). Most often, columns packed with reverse-phase Cl 8 stationary phase are used for the separation of tropane alkaloids. Gradient or isocratic elution generally involves buffered mixtures at the acidic pH of water—acetonitrile or acetonitrile—methanol, such as acetonitrile-triethylammonium phosphate buffer (25 75) at pH 6.2 [65] acetonitrile-50 mM phosphate buffer at pH 2.95 (10 90 and 20 80) [66], methanol-0.05 M... 

Since N-l-glycosides of EA can be considered as nucleosides, some interesting activities can be expected. / -N-l-Ribofnranosides of clavine alkaloids were prepared by the reaction of N-l-trimethylsilyl derivatives with l-O-acetyl-2, 3, 5tri-0-benzoyl- -D-ribofuranose in dichloromethane under SnCh catalysis in yields of 20-40% (Kfen et al., 1997a). Similarly a mixture of a and anomers of N-l-deoxyribofuranosides of clavine alkaloids was prepared by reaction with l-chloro-2-deoxy-3, 5-di-O-toluoyl-a-D-ribofuranose in acetonitrile. The two anomers were separated by preparative chromatography (Kfen et ai, 1997b). 

The synthesis of dextromethorphan is an outgrowth of early efforts to synthesize the morphine skeleton. /V-Methy1morphinan(40) was synthesized in 1946 (58,59). The 3-hydroxyl and the 3-methoxy analogues were prepared by the same method. Whereas the natural alkaloids of opium are optically active, ie, only one optical isomer can be isolated, synthetic routes to the morphine skeleton provide racemic mixtures, ie, both optical isomers, which can be separated, tested, and compared pharmacologically. In the case of 3-methoxy-/V-methylmorphinan, the levorotatory isomer levorphanol [77-07-6] (levorphan) was found to possess both analgesic and antitussive activity whereas the dextrorotatory isomer, dextromethorphan (39), possessed only antitussive activity. Dextromethorphan, unlike most narcotics, does not depress ciUary activity, secretion of respiratory tract fluid, or respiration. 

Truxillines, CggH4jOgN2. In 1887 Hesse isolated from Peruvian coca leaves an amorphous alkaloid which he named cocamine a year later Liebermann examined this material, and by fractioimtion of its solutions by addition of petroleum proved it to be a mixture of at least two isomeric bases, which he named a- and jS-truxillines. The pure alkaloids have not been obtained from coca leaves owing to the difficulty of separating them, but each has been prepared synthetically. ... 

Floripavidine, CjjHjgOjN. This occurs in the crude mixture of non-phenolic bases, and is separated from fioribundine by repeated crystallisation of the mixed hydrochlorides from water. The base crystallises from alcohol in prisms, has m.p. 241-2°, and [a]n — 156-25° (MeOH) the hydrochloride, m.p. 209-10°, hydriodide, and methiodide, m.p. 228-30°, were prepared. The alkaloid contains no hydroxyl group (Zerewitinoff), but a methoxyl, a dioxymethylene, and a methylimino group are present. 

The alkaloid-modified catalyst can be easily prepared either by stirring the metal catalyst with a solution of the alkaloid in air and subsequent separation by decantation, as described by Orito and coworkers [103], or by in situ addition of alkaloid to the reactant mixture [226], Good OYs are achieved with both methods. Reactions are generally carried out at room temperature, or slightly above, and at hydrogen pressures in the range 1 to 10 MPa. The best solvent is AcOH. Under optimal reaction conditions the decrease in e.e. can be ascribed to the hydrogenation of the modifier. 

The Pictet-Spengler route to tetrahydro-P-carbolines is frequently used in indole alkaloid synthesis, and much attention has been devoted in recent years to the development of enantiospecific Pictet-Spengler reactions and the factors which influence the diastereochemistry at C-l and C-3 (c.f 1). The diester la was prepared from 2 as 1 1 mixture of diastereomers and heated in 2% ethanolic hydrogen chloride for 3 hours in an attempt to effect epimerisation at C-l and increase the amount of trans isomer. The product, however, was 2, which was isolated in 76% yield. Heating of the cis and trans diesters 1 separately in ethanolic hydrogen chloride for 3 hours also gave 2. By comparison, when a cis, trans mixture (39 61) of lb was stirred in a mixture of methylene chloride and trifluoroacetic acid at room temperature for 90 minutes the trans diester was obtained in 96% yield. 

The solvent extracts can be cleaned up by traditional column chromatography or by solid-phase extraction cartridges. This is a common cleanup method that is widely used in biological, clinical, and environmental sample preparation. More details are presented in Chapter 2. Some examples include the cleanup of pesticide residues and chlorinated hydrocarbons, the separation of nitrogen compounds from hydrocarbons, the separation of aromatic compounds from an aliphatic-aromatic mixture, and similar applications for use with fats, oils, and waxes. This approach provides efficient cleanup of steroids, esters, ketones, glycerides, alkaloids, and carbohydrates as well. Cations, anions, metals, and inorganic compounds are also candidates for this method [7],... 

The methods employed for isolation of the alkaloids depend on the nature of the compounds, and specific conditions have frequently been devised for the selective isolation of particular types of compounds. Usually, fresh or dried plant material is extracted with dilute acid solution or with alcohol, and the extract obtained is further fractionated by extraction into organic solvents with variation of pH. Extraction columns (288), membrane processes (425), and ion-exchange materials (288-290) may be particularly useful for subfractionation or isolation procedures. For further identification and isolation of separate compounds, preparative thin-layer chromatography, (288, 291, 292, 426), liquid chromatography (293, 294), or gas chromatography may be used (202, 296, 297). Because some of the products reviewed in this chapter occur naturally in very small amounts, they have not been isolated in crystalline form. Gas chromatography-mass spectrometry (87, 213, 299), mass fragmentography (192), and mass spectrometry-mass spectrometry (301, 359) have proved to be particularly useful techniques for identification of trace alkaloids in complex mixtures. 

Several syntheses of 1-hydroxymethylpyrrolizidines have been reported. Borch and Ho1 have utilized a reductive cyclization method for their synthesis of ( )-isoretronecanol (6) and ( )-trachelanthamidine (7). The cycloheptenone ester (1), prepared by a novel route (Scheme 1), was reductively aminated to give a mixture of the diastereoisomeric amino-esters (2) and (3) in 48% yield. These esters could not be separated. Oxidative cleavage of the double bond of the esters, followed by reductive cyclization, gave a 35% yield of the pyrrolizidine esters (4) and (5). Separation of these compounds was achieved by preparative t.l.c., and a final reduction step afforded the racemic alkaloids (6) and (7). The second reductive amination process was stereoselective, because reduction of the unseparated ester mixture (4) and (5) gave a 1 2 ratio (g.l.c.) of the 1-hydroxymethylpyrrolizidines. 

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