Nor-triterpenes

An impressive cationic domino polycyclization has been developed by Corey and coworkers in their short and efficient enantioselective total synthesis of aegicer-adienol (1-150), a naturally occurring pentacyclic nor-triterpene belonging to the 3-amyrin family [42]. Thus, the treatment of the enantiopure monocyclic epoxy tet-raene 1-147 with catalytic amounts of methylaluminum dichloride induces a ca-tion-JT-tricyclation by initial opening of the epoxide to form the tetracyclic ketone 1-148 in 52 % yield, and its C-14 epimer 1-149 in 23 % yield, after silylation and chromatographic separation (Scheme 1.37). Further transformations led to aegicer-adienol (1-150) and its epimer 1-151. 

A wide range of biological activities has been described for triterpenes and nor-triterpene quinonemethides. These include anti-inflammatory, anti-hyperlipidemic, antitumour-promotion, hepatoprotective, anti-ulcer, anti-microbial and hypoglycemic activitiy, among others [16,35]. Among the compound mentioned previously in this review, antitumour-promotion, anti-inflammatory, inmunosuppressive, anti-tumour, insecticide and anti-AIDS activities were found and are described below. 

Compounds isolated from Celastraceae had showed cytotoxic activity [3,4]. This property has been related mainly to nor-triterpene quinonemethides, but cytotoxic triterpenes had also been reported [96]. As an example, both, tumor initiation and promotion are inhibited by two triterpenes commonly isolated from the familiy, ursolic and oleanolic acids [16]. 

S. krausii is used in Mozambican traditional medicine against bilharziasis and dysentery. However, Celastraceae plants were also reported to possess antimalarial activity [35]. Figueiredo et al. isolated three nor-triterpene quinone methides [77], compounds (60), (61) and (62). The in vitro activity (expressed as IC50 values) of the isolated quinone methides in comparison to the reference compound (chloroquine) is shown in Table 6. The isolated compounds, together with the known celastroloids celastrol (52), pristimerin (53) and isoiguesterol (101), Fig. 

Almost all publications concerning the terpenes of brown algae refer to the above-mentioned orders Dictyo-tales and Fucales, in which numerous diterpenes and some sesquiterpenes, a single linear tri-nor-triterpene and a linear tetraterpene have been identified, but no monoterpene. Because, with very few exceptions, the terpenes of brown algae do not contain any halogens, it is necessary to distinguish clearly between the terpenes of Phaeophyceae and those of Chlorophyceae and Rhodo-phyceae. However, similar to what has been observed in... 

Turbinaric acid is a tris-nor-triterpene (C27), a derivative of squalene, which possesses cytotoxic properties. 

The two previous compoimds correspond respectively to the skeletons of malabaricane (81 , 9R) and isomalabaricane (8S,9S), most frequently encountered, faspiferals and geoditins represent a series of nor-triterpenes from C29 to 22-... 

At the time of this writing neither monoterpenes nor triterpenes have been reported from sponge sources, apart from squalene which was found in Ircinia spinosula 47) and I. muscarum 48). 

---