Marine terpenes

The diverse, widespread and exceedingly numerous class of natural products that are derived from a common biosynthetic pathway based on mevalonate as parent, are synonymously named terpenoids, terpenes or isoprenoids, with the important subgroup of steroids, sometimes singled out as a class in its own right. Monoterpenes, sesquiterpenes, diterpenes and triterpenes are ubiquitous in terrestrial organisms and play an essential role in life, as we know it. Although the study of terrestrial terpenes dates back to the last century, marine terpenes were not discovered until 1955. 

Participation of Br in marine terpene biosynthesis (Section II.C) is reflected in the impressive number (several hundred) of halogenated terpenes, some with useful biological activities, that have been identified. For example, the poly halogenated sesquiterpenes isoobtusol (14) and isoobtusol acetate (15), isolated from the red alga Laurencia obtussa, showed potent activity against HeLa 229 human carcinoma cells1011. 

Compared to isocyanides and isothiocyanates, there are far fewer examples of thiocyanate (135-139), dichloroimine (= carbonimidic dichlorides 140-146), isocyanate (36, 60, 70, 71) or amine (61,63, 65-67) groups in the marine terpene literature. The isocyanates which have been isolated [18-21] are shown in Table 1 or 2 respectively because these unusually-functionalised compounds are generally isolated together with the corresponding isocyanides or isothiocyanates, however most sesquiterpene thiocyanates (e.g. (135) [8]) represent a unique structural classes for which no isocyanide or isothiocyanate representative has yet been isolated, and these are therefore listed separately in Table 3. To date, 2-thiocyanatopupukeanane (139) [10] is the only example of a thiocyanate whose carbon skeleton has also been isolated with an isocyanide substituent 2-isothiocyanatopupukeanane is a known metabolite but has not been formally characterised in the literature. Dichloroimines such as stylotellanes A and B (141) and (140) have been found in sponges together with farnesyl isothiocyanate (147), but as yet not with any isocyanides [14-17], while very few ureas and thioureas have been isolated [23, 25, 26]. For convenience, we have therefore shown separately a list of all reported isolations of thiocyanates, dichloroimines, ureas and thioureas by structural class (Table 3). 

Our results suggest thiocyanate acts as a precursor to the isocyanide moiety of marine terpenes, as well as acting as a precursor to the isothiocyanate group. This strongly suggests either (i) an interconversion between inorganic cyanide and thiocyanate and/or (ii) an equivalent interconversion at the secondary metabolite level, i.e. between (20) and... 

Catalytic cycloadditions of carbenes to alkenes is a straightforward method for synthesizing cyclopropanes [40]. In fact, cyclopropanes are present in a variety of natural products [41-43]. For example, they occur in some unusual amino acids, in natural phytotoxins such as coronatine, as well as in marine terpenes [44], sesquiterpenes [45], cyclosteroids [46-47] (as part of the A-cycle) or in the side chain of steroids [48]. 

Nakagawa, T., M. Kobayashi, K. Hayashi, and H. Mitsuhashi Marine Terpenes and Terpenoids. II. Structures of Three Cembrane-type Diterpenes, Sarcophytol-C, Sarcophytol-D, and Sarcophytol-E, from the Soft Coral, Sarcophyton glaucum Q. et G. Chem. Pharm. Bull. (Japan) 29, 82 (1981). 

Kobayashi, M., Chavakula, R., Murata, O., and Sarma, N.S. (1992c) Marine terpenes and terpenoids. Part 16. Revised structure of the marine furanoterpene (+)-furospongin-l. J. Chem. Res. (S), 366— 367. 

Yong, K.W.L., Jankam, A., Hooper, J.N.A., Suksamram, A., and Garson, M.J. (2008) Stereochemical evaluation of sesquiterpene quinones from two sponges of the genus Dactylospongia and the implication for enantioselective processes in marine terpene biosynthesis. Tetrahedron, 64, 6341-6348. 

Kobayashi, M., Nakagawa, T, and Mitsuhashi, H. (1979b) Marine terpenes and terpenoids I. Structure of four cembrane-type diterpenes from the soft coral Sarcophyton aucum. Chem. Phann. Bull, 27, 2382-2387. 

Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenicaHy derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the perhydro-l,2-cyclopentenophenanthrene ring system (1) and are found in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primarily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring junctions. 

Tetraterpenes. Carotenoids make up the most important group of C q terpenes and terpenoids, although not all carotenoids contain 40 carbon atoms. They are widely distributed in plant, marine, and animal life. It has been estimated that nature produces about 100 million t/yr of carotenoids synthetic production amounts to several hundred tons per year (207,208). 

Phosphonate-based cyclization has been increasingly used in macrocyclic terpene synthesis. In a synthesis of the marine cembranoid (+)-desepoxyasperidiol the cyclization of the phosphonate (192) was attempted under a variety of conditions without success. 

In order for allelochemicals to enter the body of a herbivore, absorption must occur across the gut lining. Curtailing the initial absorption of dietary allelochemicals may be a herbivore s first line of defense against plant toxins. Studies have citied the lack of absorption or metabolism of lipophilic plant secondary metabolites (i.e., terpenes), conducive to phase I or II detoxification, in the gut of terrestrial herbivores rather these compounds are excreted unchanged in the feces (Marsh et al. 2006b). While physical barriers or surfactants have been used to explain this limited adsorption in both marine and terrestrial herbivores (Lehane 1997 Barbehenn and Martin 1998 Barbehenn 2001 for review of marine herbivores, see Targett and Arnold 2001), active efflux of plant allelochemicals out of enterocytes into the gut lumen has received limited attention until now. 

Phytene is an example of a diterpene. It is found as the phytyl side chain in chlorophyll a and vitamin K. Haslene is an example of a sesterpene. It is an imsaturated and branched simple lipid synthesized by marine pennate diatoms. One of the largest femilies of terpenes... 

Butlo", M.S. Capon, R.J. (1992B) The luffarins (A-Z), novel terpenes from an Australian marine sponge, Luffca-iella geometrica. Aust. J. Chan., 45, 1705-43. 

The marine isothiocyanates, with more than 80 compounds isolated so far, form the largest group of naturally occurring isothiocyanates. This well-established group of marine natural products is constituted mainly by terpene metabolites present as sesquiterpene and diterpene derivatives. The non-terpene isothiocyanate compounds include two cylindricine alkaloids and a series of long-chain aliphatic metabolites. Marine sponges constitute the main source of these compounds, although they are also found in nudibranches and tunicates. 

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