Yohimbine-type alkaloids structures

Two important books on the biogenesis of indole alkaloids have been published (5, 6), therefore we do not intend to cover the literature of the biogenesis of corynantheine- and yohimbine-type alkaloids. Our chapter begins with the structure elucidation of the alkaloids isolated during the past two decades and proceeds with the synthesis, transformation, as well as spectroscopy of the alkaloids in question. 

The stem bark of P. yohimbe (= P. johimbe Corynanthe yohimbe), "yohimbe as it is known in Cameroon, Gabon and Congo is used traditionally as an aphrodisiac and stimulant to prevent sleep (41). The bark contains 1-6% of indole alkaloids, most of which are yohimbane-type alkaloids, the main one being yohimbine [7], which is structurally related to reserpine [8] (89). Yohimbine is a selective inhibitor of a-2- adrenergic receptors and, while at low dose it has hypertensive activity, at high dose it is hypotensive (vasodilation of peripheral vessels). It is the vasodilation of peripheral vessels, and especially vasodilation of the corpus cavemosum, which is the cause of the reputation of yohimbine as an aphrodisiac (90). Tests have shown, indeed, that increased... 

Quebracho (cortex). The bark of the quebracho tree, Aspidosperma quebracho-bianco (Apocynaceae), a large tree (up to 20 m high), indigenous to the west of South America. The bark contains ca. 1% monoter-penoid indole alkaloids such as yohimbine, aspido-spermine, quebrachamine. The Aspidosperma alkaloids are structurally related to the Catharanthus alkaloid vindoline. Quebracho bark contains over 25 different alkaloids of widely differing types. ... 

Rauwolfia alkaloids a group of about 50 structurally related indole alkaloids firom the roots and rhizomes of various species of Rauwolfa, Aspidosperma and Corynanihe. All R. a. contain a -carbolene skeleton they are classified into 3 types 1. yohimbine (cory-nanthine), 2. ajmaline, 3. serpentine. The large number of R. a. is due to the existence of stereoisomers. Thus Rauwolfia contains seven stereoisomers of yohimbine. 

The alkaloids imder discussion share a common Corynanthe-type nucleus derived from secologanin. Yohimbine is a carbocyclic variant of ajmalicine, and the enzymes that convert deglycosylated strictosidine to yohimbine have not been identified [124, 125]. Strychnine and bmcine are synthesized from the preakuammicine structure by hydrolysis, decarboxylation, and condensation reactions to aldehyde (Wieland-Gumlich), and subsequently reaction with acetyl-CoA to make a hemiacetal form of aldehyde (Wieland-Gumlich) and strychnine and brucine. Brucine is a dimethoxy form of strychnine. Ellipticine is a representative member of pyrido[4,3-b] carbozole alkaloid, and the formation of ellipticine is from ajmalicine (corynanthean skeleton) [126] (Fig. 20.8). 

Today the molecular formula and the obvious chromophoric moieties along with the type of double bond can be established rapidly by techniques which allow by and large for complete recovery of the alkaloid in question, e.g. for strychnine (Chart 3.1) the units shown by the more heavily shaded positions. This sort of information was available in the past via pyrolytic methods and two examples are given for strychnine and yohimbine in Chart 3.1. It can be seen that despite the so-called brutality of the method it did provide accurate structural information the problem being, of course, that in hindsight it is more obvious. 

AspMospemiine. The first inkling that the structure of aspido spermine was different firom the yohimbine or strychnine types was the formation on zinc dust distillation of 3,5-diethyIpyridine and since this carbon nitrogen pattern turned out not to be present in the original alkaloid it could only lead to erroneous formulae, which it did. The nature of rings A and B became clear... 

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