Sesquiterpene polycyclic

A quite detailed review of transannular cyclizations was published201 wherein their important role in biomimetic syntheses of sesquiterpenes as well as explanation of the biogenetic formation of the polycyclic natural compounds from their monocyclic precursors is discussed. The great significance of these transformations for the synthesis of natural products is also emphasized in a series of reviews which describe the cyclizations to form terpene derivatives, e.g., of the germacrane and humulene systems202-206. 

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

The isoprenoids contribute most to the list of structural similarities in the sea and on land. They range from common classes in both ecosystems, such as drimane sesquiterpenes, to rare classes in the sea, such as the trichothecenes (Chart 8.3.11). The similarity in marine and terrestrial polyether triterpenes (Chart 8.3.12) may be seen as convergence toward chemically favored structures, starting from squalene as a biosynthetic precursor. Similar conclusions may apply to polycyclic triterpenes. 

For instance, terpene cyclases are known to catalyze the conversion of oligomeric isoprenoid pyrophosphate substrates to polycyclic hydrocarbon products. Sequence comparisons of terpene cyclases with different known specificity can allow them to be classified into monoterpene, sesquiterpene, and diterpene synthases, which utilize the 10-carbon substrate geranyl pyrophosphate, the 15-carbon substrate famesyl pyrophosphate, and the 20-carbon substrate geranyl-geranyl pyrophosphate, respectively, on the basis of sequence criteria. 

Cycloaddition using an enol derivative of a cyclic 1,3-dicarbonyl compound, followed by retro-aldol reaction, results in ring expansion by two carbon atoms. Reactions of this kind have been applied to the synthesis of a number of polycyclic natural products. For example, irradiation of the enol benzoate 160 gave the tricyclic product 161 in almost quantitative yield (3.112). Dimethylation followed by hydrolysis and retro-aldol reaction gave the eight-membered ring diketone 162, used in a synthesis of the sesquiterpene ep/-precapnelladiene. 

Mehta, G., D. N. Dhar, and S. C. Suri Addition of Chlorosulphonyl Isocyanate to Some Polycyclic Sesquiterpenes, Longifolene, Isolongifolene, a-Cedrene, Caryo-phyllene and Thujopsene. Indian. J. Chem. 16B, 87 (1978). 

M. Vandewalle and P. De Clercq, Total Synthesis of Polycyclic Sesquiterpenes. A Survey of Novel Methods and Reactions, Tetrahedron, 1985, 1, 1767. 

The acid-catalysed rearrangement of two enantiomerically pure epoxides derived from sesquiterpenic himachalenes with Lewis and Brpnsted acids results in polycyclic compounds and mechanisms to explain the products have been proposed. Treatment of acyclic a-hydroxy and a-tosylamino sulflnyl dienes with amines gives enantiopure 1,4-diol or 1,4-hydroxysulfonamide derivatives.This one-pot proce- (g) dure entails a conjugate addition that triggers a diastereoselective sulfoxide-sulfenate [2,3]-sigmatropic rearrangement (Scheme 61). 

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