Pentacyclic indole

Among the alkaloids of Hunteria ebumea Pichon (Table I) were four interrelated ones (11). Two of them, eburnamine and isoeburnamine, were diastereoisomeric pentacyclic indoles (XVII) convertible by acids into eburnamenine, an A-vinylindole (XVIII), on the one hand, and by chromic acid into eburnamonine, an V-acylindole (XIX), on the other. Reduction of eburnamonine with lithium aluminum hydride regenerated the alcohols, XVII (7,11). When eburnamonine was heated with selenium... 

The key step of this synthesis is a sequential asymmetric Heck-N-acyliminium ion cyclization of dienyl carbamate triflate 13 to provide enantiopure 3,4-dihydro-9a,4a-(iminoethano)-9/f-carbazole 16. This intermediate and related structures containing a 2,3-double bond should represent versatile precursors for constructing a variety of pentacyclic indole alkaloids containing the (hydroiminoethano)-carba-zole fragment. 

Fischer indolization of the arylhydrazono derivatives 412 in polyphos-phoric acid or in the presence of anhydrous zinc chloride at 160-180°C afforded pentacyclic indole derivatives. - - " Kdkdsi and coworkers started from tricyclic arylhydrazones of type 412 and synthesized various products in high yields, including the alkaloid rutecarpine (413, R = R = = H, = 1, ring E = aromatic), its homologs in ring C (413,... 

The usefulness of aminosiloxy diene Diels-Alder chemistry to the preparation of different substituted cyclohexenones is demonstrated in Table 11.3 The functionality at the 4 and 5-positions of the cyclohexenones can be easily controlled by the substitution pattern in the dienophile. The differing endo-exo selectivity found in the initial cycloadducts does not impact the usefulness of this route to cyclohexenones, since the amino group is eliminated in the last step. Chiral versions of aminosiloxy dienes provide the opportunity for asymmetric synthesis. Indeed, the diphenylpyrrolidine-substituted diene allows the synthesis of a variety of cyclohexenones, with good to excellent ee s 2b The usefulness of aminosiloxy diene Diels-Alder reactions to natural product synthesis is exemplified through the stereocontrolled synthesis of the pentacyclic indole alkaloid tabersonine 2c... 

A new monoterpene alkaloid, kinabalurine G (330) has been obtained from the leaf extract of Kopsia dasyrachis in addition to 11 known alkaloids and the novel pentacyclic indole, danuphylline vide infra) [223]. Analysis of the spectral data allowed assignment of the gross structure as well as determination of the stereochemistry at the various centers. Kinabalurine G (330) is the AZ-oxide of 9-hydroxy-S-skytanthine, which is unknown, although a 9-hydroxy-skytanthine of unknown stereochemistry has been previously reported from Tecoma stans [224]. 

Yohimbine alkaloids possess a characteristic pentacyclic indole skeleton. Representative members of the family include the rauwolfia (reserpine and deserpidine) and the yohimbines. A wide range of medicinal properties has been associated with these compounds and extensive studies have been carried out on the synthesis of the yohimbine alkaloids, including enantioselective syntheses [13,14]. In our approach, we view the acetylenic sulfoxide as a two-carbon synthon for the C3-C14 segment of the pentacyclic system (see 27). The chirali-... 

A pentacyclic indole alkaloid isolated from the leaves of Hunteria zeylanica was identified as pleiocarpamine (167) by Arambewela and Khuong-Huu (78). 

In a study on biomimetic alkaloid synthesis the pentacyclic indole alkaloid vincadifformin (1) was synthesized in two steps starting from tryptamine hydrochloride (X) ... 

Both 2- and 3-vinylindoles take part quite readily as 4 components in Diels-Alder cycloadditions often, but not always, these employ A-acyl- or A-arylsulfony-lindoles, in which the interaction between nitrogen lone pair and 7r-system has been reduced. The example below shows how this process can be utilised in the rapid construction of a complex pentacyclic indole. ... 

More recently, Kalaus and Szantay have made use of a similar approach to synthesize a variety of pentacyclic indole alkaloids containing the aspidospermane, ibophyUidine, and iboxyphylline core structures. In one approach, featured in the formal total synthesis of ( )-12-demethoxy-N(l)-acetylcylindrocarine (188) [73], a benzyl-protected tryptamine derivative 183 was treated with an appropriately substituted aldehyde (184) to form the enamine species 185a-b, which then underwent... 

The synthesis of pentacyclic indole derivatives, 9-phenyl-9,13c-dihydro-6//-chromeno[4, 3 4,5]-thiopyrano[2,3-( ]indoles (Scheme 2.3), has been achieved via domino Knoevenagel-hetero-Diels-Alder reactions of indolin-2-thiones and O-propargylated salicylaldehyde derivatives in the presence of 10 mol% of ZnO as a heterogeneous catalyst in CH3CN (Kiamehr and Moghaddam 2009). When the... 

Novel tetracyclic and pentacyclic indole derivatives can be prepared from readily available gem-dibromovinyl substrates in a single step by means of an efQdent Pd-catalyzed domino Buchwald-Hartwig amination/direct arylation reaction. Enhanced reactivity and selectivity are obtained by the addition of silver salts (Bryan and Lautens 2008). 

The next stage of the synthesis required introduction of the indolylethyl group and oxidative cyclization to afford the pentacyclic skeleton of reser-pine. First the carbamate was removed from decahydroisoquinoline 133 to give 134. Alkylation then gave 135. The next task was oxidative cyclization to provide the pentacyclic indole skeleton. At this point it becomes clear why loss of the enamide was unfortunate. 

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