Pilocarpine isopilocarpine

P. microphyllus (Stapf). Maranham jaborandi Pilocarpine isopilocarpine pilosine 0-70-0 78 0 45 ... 

A. Gomez-Gomar, M. Gonzalez-Aubert, and J. Costa-Segarva, HPLC method of pilocarpine, isopilocarpine, pilocarpic acid and isopipocarpic acid, J. Pharm. Biomed. Anal., 7 1729 (1989). 

Spectra for the diastereoisomeric pairs quinine-quinidine, cinchonine-cinchonidine alkaloids are mirror images of each other and mixtures have been determined using CD detection [57]. Spectra for the pilocarpine-isopilocarpine pair were such low quality that they could be used only for qualitative distinction. CD detection combined with UV detection was used to measure enantiomeric excesses in mixtures of L-hyoscyamine and atropine, i.e. racemic hyoscyamine. This subject is returned to in greater depth later. 

The 13C-NMR spectra of pilocarpine, isopilocarpine, and their methio-dides have been discussed (82). The spectral assignments reflect the differences between the cis and trans isomers. 

P. pennatifolius (Lemaire). Paraguay jaborandi Pilocarpine isopilocarpine 02-03 ... 

Pilocarpine, CiiHijOjNj. The base is a colourless oil, b.p. 260°/5 mm. (partially isomerised to iSopilocarpine on distillation), [a]n -f 100-5° (CHCI3), but has been crystallised, m.p. 34° it is freely soluble in water, alcohol or chloroform, but almost insoluble in ether. The salts with acids crystallise well the nitrate, B. HINO3, forms well-defined prisms, m.p. 178°, [a]r, 82-9° (HjO), and dissolves in 6-4 parts of water at 20° or... 

Constitvtion of Pilocarpine and isoPilocarpine. Though numerous interesting observations have been made on these two alkaloids by M. and M. Polonovski, knowledge of their constitution is due principally to the work of Jowett and of Pinner. ... 

Roylance showed that pilocarpidine on -methylation yielded two products, pilocarpine and neopilocarpine (see below), thus confirming Hamack s suggestion that pilocarpidine is the imino-base corresponding to pilocarpine, Spath and Kunz showed that pilocarpidine, on treatment with alcoholic sodium ethoxide, is converted into isopilocarpidine (nitrate, m.p. 109-111°), which, on quaternary methylation, yields isopilocarpine metho-salts (methopicrate, m.p. 119-120°). The confirmation of these observations by the synthesis of pilocarpidine and isopilocarpidine and their conversion into pilocarpine and isopilocarpine has been described already. 

Pilosine, CjgHjgOgNj, obtained by Pyman (and almost simultaneously by L ger and Roques, who named it carpidine) from mother liquors remaining after the isolation of pilocarpine and isopilocarpine from the total alkaloids of P. mi.crophyllus, crystallises from alcohol in large colourless plates, m.p. 187°, [a]n + 39-9° (EtOH), lasvorotatory in alkaline solution. The salts do not crystallise readily the sulphate,. H SO, forms clusters of plates, m.p. 194-5°, fajD + 21° (HgO) the acid tartrate, B. H2C4HiOg, has m.p. 135-6°, [a]n + 24-2°, and the aurichloride, B. HAuCb, m.p. 143-4°. 

Since pilosine can be recovered unchanged after boiling for a short time with dilute alkali, it is regarded as allied stereochemically to isopilocarpine rather than pilocarpine. 

Fig. 7.13. HO -Catalyzed ring opening of pilocarpine (7.76) and isopilocarpine (7.77) to pilocarpic acid and isopilocarpic acid, respectively, and proton-catalyzed lactonization of the two acids to the respective lactone. Note that pilocarpine and isopilocarpine interconvert by a base-catalyzed reaction of epimerization (Reaction a).

The reverse reaction of lactonization of pilocarpic acid is proton-catalyzed and, hence, favored at low pH. Thus, the pilocarpine/pilocarpic acid ratio was 1.0, 1.6, 2.7, 4.0, 5.1, and 6.7 at pH values of 6.0, 5.6, 5.2, 4.8, 4.4, and 4.0, respectively [161]. Interestingly, the lactonization of isopilocarpic acid is ca. 17-fold faster than the lactonization of pilocarpic acid, since the second-order rate constants of proton-catalyzed lactonization at 25° were 0.96 and 0.055 M-1 min-1 for isopilocarpine and pilocarpine, respectively [162], A lack of planarity in the lactone ring of pilocarpine, and a more lac-tone-like planarity in isopilocarpine, appear to account for this difference in reactivity between the two epimers. 

Pilocarpine (P), a drug used in treating glaucoma, can potentially contain its epimer, isopilocarpine (I) as an impurity. In a study it was not possible to completely separate pilocarpine and isopilocarpine by variation of the pH of the running buffer. The optimal pH for separation should be 6.9 where both compounds are ca 50% ionised but even at this pH separation was incomplete.- ... 

Separation of pilocarpine and isopilocarpine by inclusion of P-cyclodextrin, the running buffer. (A) With addition of cyclodextrin. (B) Without addition of cyclodextrin. 

In order to avoid the undesirable isomerization to isopilocarpine (Section III,D,3) and, probably, to avoid enzymatic degradation, the Jaborandi leaves are processed as rapidly as possible. Acidification converts the alkaloids to their water-soluble salts, which allows defatting of the leaves with, for example, petroleum ether. Alkalinization and extraction with a suitable lipid-extracting solvent or solvent mixture gives the crude alkaloid mixture. (+)-Pilocarpine (mp 34°C) crystallizes only with difficulty it can be obtained readily as the nitrate salt (mp 178°C) in the absence of isopilocarpine. The separation of isopilocarpine nitrate from pilocarpine nitrate by crystallization is often difficult because mixtures may give products of constant melting points the same holds for the hydrochlorides. 

It was found long ago that pilocarpine (7) isomerizes to isopilocarpine (8) under a wide variety of conditions. In 1880 it already was observed that pilocarpine undergoes a change when heated alone or with hydrochloric acid (37). Other investigators noted a change when the alkaloid was heated with aqueous sodium hydroxide. This isomerization is discussed in Section III,D,3. 

In 1900 Jowett (38) suggested that the differences between pilocarpine and isopilocarpine had their origin in the steric structures. This was in contrast to the opinion of Pinner et al. who assumed the differences to be caused by N-methylation at different sites in the imidazole ring (39). Lan-genbeck in 1924 assumed on the basis of chemical reactions, such as the formation of quaternary salts and ozonolysis, that the differences were of a steric nature (40,41). Preobrazhenski et al. in 1936 (42) supposed on the basis of different stability and optical rotation that pilocarpine and isopilocarpine should possess the cis and trans configuration, respectively. An identical conclusion was drawn by Zavyalov (43) in 1952, on the basis of... 

The phenomenon of isomerization of pilocarpine to isopilocarpine was already discussed in 1880. The change takes place under a wide variety of conditions, which were reviewed earlier (7). Dopke and d Heureuse (54) in 1968 proved that the isomerization (inter alia an undesirable side reaction occurring during the isolation of pilocarpine from plant material) is not caused by an opening of the lactone ring, but proceeds via epimerization at... 

Scheme 1. Pilocarpine (7)-isopilocarpine (8) isomerization according to Dopke and d Heureuse (54).

Nunes and Brochmann-Hanssen et al. (55,56) proposed a reaction scheme for the hydroxide ion-catalyzed hydrolysis of pilocarpine (Scheme 2) based on the theory of Dopke and d Heureuse. Two main pathways may be distinguished (1) hydrolysis of pilocarpine (7) to the pilocarpate anion (7 ) and (2) epimerization of pilocarpine (7) to isopilocarpine (8), following to the isopilocarpate anion (8°). These two pathways appeared to be competing pseudo-first-order reactions. 

The kinetics of the hydroxide ion-catalyzed epimerization of pilocarpine to isopilocarpine and of its hydrolysis to pilocarpic acid have been studied (56). Both forms of degradation lead to loss of pharmacological activity. The importance of possible inactivation by epimerization during thermal sterilization of ophthalmic preparations of pilocarpine was pointed out. It was also considered that some epimerization would always occur during the extraction of pilocarpine from Jaborandi leaves, and that isopilocarpine might, therefore, be an artifact and not a genuine plant alkaloid (55). 

A stereoselective synthesis of (+)-pilocarpine (7) starting from L-histidine (2) has been worked out by Noordam et al. (88 - 90). Use was made of the S configuration of the amino acid, which is the same as that of C-3 of the lactone ring in both (+)-pilocarpine and (+)-isopilocarpine. Furthermore, regioselective N-alkylation reactions of the imidazole nucleus of histidine had been developed by Beyerman et al. (29,91). Schemes 3 and 4 depict the different ways of the regioselective alkylations. For the synthesis of pilocarpine, the N7I-methylation has been performed via Nb-protection with the 4-nitrobenzenesulfonyl group, instead of the benzoyl group (29). 

Compound 44 was decarboxylated at 140°C, yielding a mixture of diastereoisomers 45. The ester group of 45 was reduced selectively to the alcohol function, affording 46. Lactonization of 46 under acidic conditions yielded a 1 1 mixture of (+)-pilocarpine (7) and (-l-)-isopilocarpine (8) (88-90). The separation of the isomers is well established and the conversion of (-f)-isopilocarpine to (-l-)-pilocarpine has been described (92). 

Metapilocarpine was first reported by Pinner he obtained it by heating pilocarpine hydrochloride at 225-235°C (118). Polonovski proposed a betaine structure (58) for the compound (119). On reinvestigation, metapilocarpine was shown to be a racemic mixture of isopilocarpine (120). The structure was proved by spectral analysis and by GLC comparison with authentic isopilocarpine. The pharmacological activity of the racemic product was compared to that of (+)-pilocarpine and (+)-isopilocarpine (120). 

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