Steroidal alkaloids dimeric

Fukuzawa, S., Matsunaga, S., and Fusetani, N., Ritterazine A, a highly cytotoxic dimeric steroidal alkaloid, from the tunicate Ritterella tokioka, J. Org. Chem., 59, 6164, 1994. 

Pettit et al., have proposed a biogenetic pathway for the dimeric pyrazine-containing steroidal alkaloids, the cephalostatins isolated from the marine worm Cephalodiscus gilchristi and the tunicate Ritterella tokioka. These alkaloids contain at least 13 fused rings, which constitutes the largest such system known in marine animals. 

The family of dimeric steroid-pyrazine alkaloids isolated from Cephalodiscus and Ritteria now stands at thirty members. There are undoubtedly other examples of this group of steroidal alkaloids that have yet to be discovered, and it is probable that members common to both sources will be found. 

These alkaloids do not contain functional groups such as alkylation sites, Michael acceptors, intercalators, or redox-active qui-nones commonly associated with cytotoxicity, while their scarcity has hindered investigation of the mechanism of action. An early proposal, [16] taking into account the steroidal and dimeric nature of the alkaloids, was that these compounds span the lipid bilayer (cephalostatin 1 is 30 A long) and perturb the eukaryotic cell membrane. 

The ritterazines are highly ox ygenated dimeric steroidal alkaloids isolated by Fusetani and co-workers from 1994 to 1997 from the colonial tunicate Ritterella tokioka (family Policlinidae) [107, 108],... 

Gonopharyngia durissima has afforded iboga bases, two dimeric alkaloids discussed under Voacanga alkaloids, and a trace of a base, alkaloid E, mp 191°-193°, pif a 7.26, UV-maxima at 210 and 305 mp, which differed from the other isolates in having no carbonyl absorption in the IR-spectrum (14). Gonopharyngia pachysiphon, in contrast to C. durissima, has yielded only steroidal bases (45). 

The biogenesis of the eastern hemisphere of the molecule is derivable from a 12-hydroxy-A14-steroid skeleton as shown in Scheme 7. The isolation of closely related dimeric alkaloids from two different phyla (Hemichordata... 

When Two Steroids are Better than One The Dimeric Steroid-Pyrazine Marine Alkaloids... 

Biosynthesis In contrast to steroids and terpenes, there is no equivalent to the isoprene rule for alkaloids as a useful aid for structure determination and biogenetic investigations. It is generally accepted today that A. are formed in cyclization, condensation, and dimerization reactions from amino acids and biogenic amines with biogenic aldehydes and ketones. 

Ganesan, A., 1996. The dimeric steroid-pyrazine marine alkaloids challenges for isolation, synthesis, and biological studies. Angew. Chem. Int. Ed. Engl 35, 611-615. 

Correlations within a stereochemically homogeneous series of compounds, such as monosaccharides and steroids, which are well documented elsewhere in the literature are considered in outline only. Series of compounds which are essentially dimeric or polymeric types built up from chiral monomeric units are not covered in detail, since their configurations follow readily from those of the monomeric unit. Examples of such polymeric-type compounds are di-, oligo- and polysaccharides bis (benzylisoquinoline) and other dimeric alkaloids biflavonoids and polypeptides. Series of natural products having essentially the same carbon skeleton but known in a variety of stereochemical types due to epimerism at one or more chiral centres are frequently covered by general notes rather than by large numbers of examples. This treatment is given, for example, to the labdane-type diterpenes and the yohimbane-type alkaloids. 

Ganesan, A. (1996) When two steroids are better than one the dimeric steroid-pyrazine marine alkaloids, in Studies in Natural Products Chemistry, vol. 18 (ed. Atta-ur-Rahman), Elsevier Science, Amsterdam, Oxford, New York, pp. 875-906. 

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