Of quassinoids

Spino and colleagues134 studied the Diels-Alder reactions of vinylallenes aiming to synthesize six-membered rings with a tetrasubstituted exocyclic double bond, which were to be employed as precursors of quassinoids. Some representative results of their investigations have been summarized in Table 5 (equation 56). Due to the presence of two different substituents at the allene terminus of 200, facial differentiation occurred, which resulted in non-equivalent amounts of geometrical isomers 201 and 202. The major isomers obtained in each case were formed by endo attack of maleic anhydride 144 at the less hindered face of the diene. 

In approaches to a synthesis of quassinoids, the ketone (124), derived from cholic acid, has been converted into the lactone (125), with the C-7 stereochemistry of quassinoids. ... 

J. Herscovici, S. Delatre, L. Boumaiza, and K. Antonakis, Stereocontrolled routes to functionalized [1,8-bcJ-naphthopyran—a study on the total synthesis of quassinoids and tetra-hydronaphthalene antibiotics, J. Org. Chem. 58 3928 (1993). 

For a recent structure-activity relationship correlation of quassinoids as antitumor agents see M. Okano, F. Narihiko, K. Tagahara, H. Tokuda, A. Iwashima, H. Nishino and K-H. Lee, Cancer Lett. 1995, 94, 139. 

Lactonization of a b,i-unsaturaled ester/ In a projected synthesis of quassinoid diterpenes, the final step involves lactonization of the unsaturated ester 1. Acid-catalyzed solvolysis (HCl in THE) or treatment with an arvlsulfonic acid in toluene results mainly... 

Manganese triacetate has been specifically reported as a reagent for a -oxidation.Mechanistic dualism analogous to LTA (vide supra) is observed, although the radical process may be more dominant. Watt and coworkers used this technique during the synthesis of quassinoids. Enone (113) was con-... 

Table 8 Aqueous, Aggregation-promoted Diels-Alder Reactions of Quassinoid Intermediates at Room Temperature (Scheme 57)...

A final method of alkene formation from a-derivatized ketones entails formation of unsaturated tosyl-hydrazones from a,P-unsaturated ketones followed by conjugate reduction with sodium cyanoborohy-diide in acid media. This transformation, which proceeds with regioselective migration of the original double bond, has been applied to the synthesis of quassinoids by Ganem et al. (Scheme 22). A similar... 

Melianodiol (66) and its diacetate have been foundin Samadara madagas-caneois (Simaroubaceae), a source of quassinoids. In model experiments directed towards the synthesis of simarolide, melianodiol was converted into the... 

Various workers have attempted the chemical 77-80 or microbiological81 modification of quassinoids in the search for compounds with potent antileukaemic activity. 

The alkylating-cyclization method may be extended to the enantioselective introduction of an alkyl chain, if a chiral alcohol is used as starting material. The tributyltin hydride mediated radical cyclization of the bromoacetal of 18,19-bisnor-9/i-podocarp-l 3-en-12-one gives a tetracyclic d-lactone, the characteristic structural feature of quassinoids, a complex family of degraded triterpenes82. The hydrogen abstraction in the last step of the reaction has to occur stereoselectively, because only two C-16 diastereomers are formed during the reaction. 

The highly stereoselective 1,2 rearrangement of allyl vinyl ether 14 gives rise to (3-oxo ester 15 with the correct relative stereochemistry at C-8, C-9, and C-14 of quassinoids (a group of natural products, which exhibit activity against a number of cancer screens e.g., bruceantin), and allows the assembly of rings C, D, and E531. 

The review containing structural data, biological activities and chemical modifications of quassinoids was published last decade with references up to September 1984 by Polonsky [1]. According to Simao et al. [2] more than two hundred quassinoids were isolated and identified until December 1984. 

Eurycoma longifolia Jack is one of the most well known folk medicines for intermittent fever (malaria) in Southeast Asia [11]. This plant possesses the largest number of quassinoids with C19 skeleton identified so far. 7a-hydroxyeurycomalactone (20) [12], 6a-hydroxyeurycomalactone (21) [13], eurycolactone E (22) [10], eurycomalide B (23) [14], and quassinoids (24) and (25) [15] isolated from the E. longifolia Jack and cymosanine (26) [16] isolated from... 

Chemical examination of Ailanthus excelsa has been carried out by several workers resulting in the isolation of quassinoids, alkaloids and... 

In recent years, a number of studies on the synthesis of quassinoids have been reported [126], Among them, total synthesis of quassin (possessing 7 chiral centers) and of castelanolide (possessing 9 chiral centers) both carried out Grieco s group constitute two successful works to synthesize natural quassinoids [1]. Below the main total synthesis of some quassinoids are described. 

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