Pleiocarpa

According to a recent report on some unpublished work by Joshi and Taylor (92a), the acid hydrolysis of villalstonine affords pleiocarpamine (LXXIIa) (92b), one of the constituent bases of Pleiocarpa mutica Benth. (92c) and Hunteria ebumea Pichon (92d). 

By the use of mass spectroscopy coupled with deuterium labeling, eburnamonine, (iso (eburnamine, and ebumamenine have been shown to exist (stereochemistry not specified) in Rhazya stricta Decaisine, and ebumamenine itself in Aspidosperma quebracho bianco Schlecht (19). The obtention of ebumamenine from Pleiocarpa species is described in Section III. 

The alkaloids of Hunteria and Pleiocarpa are included here because they were early sources of the ebumamine-type alkaloids. 

A continuing examination of Pleiocarpa mutica Benth. and P. tubicina Stapf has revealed the alkaloids summarized in Table II. Although extracts of these plants have shown prolonged blood pressure-lowering properties (41), the active component(s) has not yet been identified. 

DIPLORRHYNC US, KOPSIA, OCHROSIA, PLEIOCARPA, AND RELATED GENERA... 

A number of alkaloids with the aspidospermine skeleton occur in the genus Vinca and are dealt with in detail in Chapter 12. They include the very important base vindoline (CIII) which not only occurs as the free base in Vinca rosea (= Lochnera rosea = Catharanthus roseus) but also as part of the dimeric alkaloids vinblastine (vincaleucoblastine) and leurosine, whereas the Aa-formyl analog, CIV, forms part of the dimeric alkaloid leurocristine (5, 72a). These dimeric alkaloids have been used successfully for the treatment of certain forms of cancer in man (5). Vindolinine (CVI) has also been isolated from V. rosea and its dihydro-decarbomethoxy derivative, tuboxenin (CVI-A), which is the parent member of the series, occurs in a Pleiocarpa species (53). ( + )-Vincadif-formine (XCIII, 6, 74) has already been mentioned as the racemic form of (— )-6,7-dihydrotabersonine (Section II, O). It has been found in V. difformis and in Rhazya stricta (51b) where the (+) form also occurs. 

From the leaves of Pleiocarpa tubicina, in addition to bases described in Sections III, G, and VI, C, there has been isolated an alkaloid, tubotaiwine, of structure CCXXIV (119). The same substance also occurs in the root bark of Aspidosperma limae (120). 

Three related alkaloids have been found in the leaves of Pleiocarpa tubicina. One is the known compound 19,20-dihydroakuammicine (CCXXXVII, 70, 69, 137a), while the other two, tubifoline and tubi-folidine, are respectively its decarbomethoxy derivative, the indolenine, CCXXXIX, and the corresponding dihydroindole, CCIII (119). A compound strychene of structure CCXXXIX has been prepared from dihydrodeoxyisostrychnine (67, 125). 

To conclude this Section, the structure of the Ci9-skeleton will be briefly discussed the isomeric mixture obtained from cleavage of 6, 7 -dihydropycnan-thine (194) shows a mass spectrum identical to that of (-l-)-tuboxenine, isolated from Pleiocarpa pycnantha. Isotuboxenine could be isolated from the same plant and has been shown to be identical by mass spectroscopy and thin-layer chromatography with the isotuboxenine obtained by reductive cleavage of 6, 7 -dihydropycnanthine (194). Under the conditions employed, tuboxenine is the thermodynamically more stable isomer. Concerning possible mechanisms of the tuboxenine-isotuboxenine interconversion see ref 113. 

These two bases, pleiomutine from Pleiocarpa mutica Benth. and umbellamine from Hunteria umbellata (K. Schum.) Hall. F. Carpodinus umbellatus K. Schum., Polyodoa umbellata Stapf., Picralima umbellata Stapf.), possess a common building block, namely 14,15-dihydroeburnamenine (229). This is bonded through C(14) to the aromatic ring of the second monomeric component, an indoline. Umbellamine is probably identical with the alkaloid hunterine, of unknown constitution, from Hunteria eburnea Pichon. ... 

Once known, the characteristic spectra of alkaloids of the second category are of great value in the classification of new alkaloids. For instance, pycnan-thinine (195), a trace alkaloid of Pleiocarpa pycnantha, exhibits a u.v. spectrum which is virtually identical to that of pycnanthine (193). The nature of its monomeric chromophores and the mode of linkage were thus immediately clear. Two fission reactions then yielded the complete structure. 

Talbotine24 (15) from Pleiocarpa talbotii Wernham is a tetrahydro-version of this type it has the C(16)—N(l) linkage which is also present in mavacurine, and indeed the present base was degraded (Scheme 4) to -mavacurine (16), the... 

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