Yohimban

Thus the critical synthetic 1,6-dihydropyridine precursor for the unique isoquinuclidine system of the iboga alkaloids, was generated by reduction of a pyridinium salt with sodium borohydride in base (137-140). Lithium aluminum hydride reduction of phenylisoquinolinium and indole-3-ethylisoquinolinium salts gave enamines, which could be cyclized to the skeletons found in norcoralydine (141) and the yohimbane-type alkaloids (142,143). 

Wender and Aube have independently described the use of the Bischler-Napieralski reaction in the synthesis of Yohimban alkaloids. Aube s approach involved the cyclization of indole 50 followed by reduction of the resulting dihydroisoquinoline... 

Chemical Name 1 -[2-(Diethylamino)ethyl] -11,17-dimethoxy-18-[(3,4,5-trimethoxyben-zoyl)oxy 1 yohimban-16-carboxylic acid methyl ester... 

Chemical Name 11,l7a-dimethoxy-18 3-[[1-oxo-3-(3,4,5-trimethoxyphenyl)-2-propenyl]-oxy]-3/3,20a-yohimban-16 3-carboxylic acid methyl ester... 

Yohimbine (36) is a well-known and reasonably available alkaloid from Corynanthe yohimbe, inter alia. For this reason, and partly because of its intrinsic pharmacological activity (including reputed aphrodisiac activity), chemists have frequently studied its properties. Oppenauer oxidation is usually attended by saponification and decarboxylation in this series, and yohimbone (37) is the product. Wolf-Kischner reduction to yohimbane (38), followed by sodium hydride mediated alkylation, leads to the analgesic agent, mimbane (39). °... 

The structural formula of aspexcine, isolated first from Aspidosperma excel-sum Benth. and identified as a 10-methoxyisositsirikine (4), had to be corrected. Recently, Verpoorte et al. (53) established that, instead of a corynane skeleton, aspexcine has a 10-methoxy, normal yohimbane structure. 

The quaternary alkaloid anhydroalstonatine (70), which has a completely aromatic yohimbane ring system, has been isolated from Alstonia venenata R.Br. (64). 

Epiallo Series. Among alkaloids containing the epiallo yohimbane skeleton, only 3-epi-a-yohimbine (105) was formerly known. In the past decade, four additional members of this alkaloid subgroup have been isolated, namely, quatematine (106) from Alstonia quaternata (84), venenatine (107) and its N-oxide (venoxidine) from Alstonia venenata R.Br. (79, 80, 82a, 85) and 16-epivenenatine (108) also from Alstonia venenata R.Br. (81). 

The previously known reserpine-type alkaloids (3) are shown below. They are reserpine (109), deserpidine (110), rescinnamine (111), veneserpine (112), pseudoreserpine (113), raunescine (114), rescidine (115), raugustine (116), and isoraunescine (117). It should be mentioned that all reserpine-type alkaloids are trisubstituted at ring E and that the substituents at C-16, C-17, and C-18 are all in equatorial positions on the epiallo yohimbane ring system existing predominantly... 

By investigating the biogenetic connection between the yohimbane and cory-... 

The conversion of indole alkaloids to oxindole derivatives has been studied extensively. Since this review does not deal with the literature of oxindole alkaloids, only the most important transformations of the alkaloids having indo-loquinolizine as well as yohimbane skeletons are to be mentioned. 

A series of publications appeared regarding the transformation of corynanthe-type alkaloids to yohimbane derivatives (285-287). The saponification of cory-nantheine (52) in methanol yielded a complex mixture of corynantheic acid (588), two C-16-epimer acetal acids (589), and demethoxycarbonylcory-nantheine (590). All four compounds upon hydrolysis in acid resulted in cory-... 

On the other hand, oxymercuration of demethylcorynantheine (593), followed by sodium borohydride reduction, gave 1 -abeo-( 17- 16)-yohimbane derivative 596 as the major product besides small quantities of ajmalicine (597) and 19-epiajmalicine (598) (287). 

It has been shown that CD measurement is a proper tool to determine the absolute configuration of the C-3 stereo center in corynantheine and yohimbine alkaloids (300). The chiroptical properties of stereoisomeric yohimbanes and 17-ketoyohimbanes also have been studied. Cotton effects due to aromatic and ketone absorptions have been considered in terms of the appropriate sector and... 

Different Na-substituted normal as well as pseudo yohimbane derivatives have antiflammatory and analgesic activity (354, 355). 

Compounds containing condensed yohimbane and pyrazole/or pyrimidine ring systems (616, 617) are potent central nervous system depressants (294, 297). 

Reserpine Reserpine is methyl ester 2a,ll-dimethoxy-3-(3,4,5-trimethoxybenzoyloxy)-yohimban-1-carboxylic acid (12.3.1). Reserpine is one of the alkaloids isolated from a perennial shrub of the Rauwolfia family [67-72]. It can also be synthesized [73-76]. 

The inside yohimbanes also illustrate the importance of interring interactions, and marked changes in conformational equilibria are observed on N-methylation.50 This is illustrated by the changes 204 - 205 and 206 - 207. The benzo analog of 204 prefers the cis conformer 208 because the interring interaction is greater in trans-fused 209 than in 204 224... 

The iron-mediated [2 + 2 + 1]-cycloaddition to cyclopentadienones has been successfully applied to the synthesis of corannulene [24] and the yohimbane alkaloid ( )-demethoxycarbonyldihydrogambirtannine [25]. A [2 + 2 + l]-cydoaddition of an alkene, an alkyne and carbon monoxide mediated by pentacarbonyliron, related to the well-known Pauson-Khand reaction [26], has also been described to afford cyclopentenones [27]. 

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