Terpenoids sesterterpenoids

Tables 2-5 list monoterpenoids (Mo), sesquiterpenoids (Sq), diterpenoids (Di), and retinoids (Re), sesterterpenoids (St), meroterpenoids and miscellaneous terpenoids (Me), respectively, discussed in this review. The bioassay systems in which the compounds exhibited inhibitory effects, together with the major sources of the compounds, are included in the Tables. Most of these terpenoids are isolated from natural sources, and their structures are shown in Figs. (2)-(5). Some terpenoids which exhibit significant and/or a broad range of chemopreventive and anti-inflammatory activities are discussed below.

Table 5. Retinoids (Re), Sesterterpenoids (St), and Miscellaneous Terpenoids from Natural Sources and the Bioassay Systems in which the Compounds Exhibited Inhibitory Activities Code Compound Source Occurrence Assay System References...

Fig. (5). Structures of retinoids, sesterterpenoids, meroterpenoids, and miscellaneous terpenoids described in this review...

Several reviews have appeared, on such topics as steroid and terpenoid biogenesis, hemiterpenoid alkaloids,indole alkaloids (monoterpenoids), ophiobolins (sesterterpenoids), sterols in Digitalis, oestrogens, plant tri-terpenoids, and carotenoids. ... 

Pentanortriterpenoids.—Recently a fascinating group of highly cleaved C25 terpenoids has been isolated from the Cneoraceae (for a review see ref. 55). Initially these compounds, e.g. cneorin C (76), were considered to be sesterterpenoids, but with the structural elucidation of cneorin B the biogenetic relationship with tetranortriterpenoids became more apparent. The carbon framework (78) helps to emphasize this relationship. It has now been proposed that the cneorins and related compounds are pentanortriterpenoids with the same biogenetic origins as tetranortriterpenoids with which they co-occur. 

Terpenoids are widespread natural products that are formed from C5 isoprene units leading to their characteristic branched chain structure. Terpenoids are divided into families on the basis of the number of isoprene units from which they are formed. Thus there are monoterpenoids (Cio), sesquiterpenoids (C15) diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30) and carotenoids (C40). The isoprene units are normally linked together in a head-to-tail manner. However, the C30 triterpenoids and C40 carotenoids are formed by the dimerization of two Ci5 and C20 units, respectively. Hence, in these cases the central isoprene units are linked in a head-to-head manner. The presence of tertiary centres in the isoprenoid backbone of the terpenoids facilitates skeletal rearrangements in the biosynthesis of these natural products. As a consequence, on first inspection some structures appear not to obey the isoprene rule. 

Sesterterpenoids are not well represented among the geochemically important terpenoids, other than the highly branched alkenes found in planktonic (Belt et al. 

Sesterterpenoids refers to those terpenoids having 25C skeleton compounds. Up to now, only about 40 species are isolated and indentihed from several species of plants, marine organisms, and insects. Phytotoxin ophiobolin A, a natural product inhibitor of calmodulin, is an example of sesterterpenoids. 

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