Wistarin

A NiCl2/CrCl2-mediated domino process of the chiral aldehyde 6/4-81 iodotriene 6/4-82 was used for the synthesis of (-)-ircinianin (6/4-79) and (+)-wistarin (6/4-80), as described by Uenishi and colleagues (Scheme 6/4.19) [299]. After the coupling to give 6/4-83, an intramolecular Diels-Alder reaction occurred at room temperature, leading to the desired cyclic product 6/4-84 in 60% yield. 

The chemistry of fiiians and benzofurans was a Held of lively research in the last year. There are a number of reasons for this activity. The furan ring system - both in its native as well as in its reduced form - occurs in a great number of natural products and a wide variety of these compounds has been isolated from natural sources. This subject is treated regularly with care and accuracy in Heterocycles and will not be repeated here. Therefore only a few examples will be given in this chapter. Several new furan derivatives were isolated from natural sources (-)-wistarin 1 from the marine sponge Ircinia sp. <99TA3869>,... 

Although toxins are identified (wistarine and lectin), clear information about their behavior does not exist. As a saponin-containing compound, it is classified as a gastrointestinal irritant. 

Diels-Alder cycloadditions in which heterocycles are the part of dienophile component and not directly involved in reaction (present in the side rings) could be illustrated by synthesis of Wistarin precursor. Lactones and butenolides are frequently found as substructures in the framework of natural products and employed in reaction published by Winterfeldt... 

C. Knappwost-Gieseke, F. Nerenz, R. Wartchow, E. V finterfeldt, High-pressure selectivity studies - A simple route to a homochiral Wistarin precursor, Chem. Eur. J. 9 (2003) 3849-3858. 

A unique tetracyclic furanosesterterpene, wistarin (195), has been discovered in Ircinia wistarii in addition to the previously known isomer ircinianin (196) 131). 196 decomposed readily, whereas 195 was relatively stable. As wistarin (195) was not detected among the decomposition products of 196 and 196 did not undergo cyclization to 195 on treatment with dilute acid, base or Lewis acids, it was proposed that 195 was a natural product and not an artifact. The biosynthesis of these sesterterpenes must involve a different folding of the acyclic 25-precursor. 

Gregson, R.P., and D. Ouvrier Wistarin, a Tetracyclic Furanosesterterpene from the Marine Sponge Ircinia wistarii. J. Nat. Prod. 45, 412 (1982). 

In addition to the classical structures, certain genera of dictyoceratid sponges produce atypical terpenes, most often sesterterpenes, the carbon skeletons of which do not conform to the usual schemes of terpene biosynthesis. This is the case for the next to last carbon skeleton shown in Figure 19.118, which occurs in only a few molecules. These include the wistarins, the first known examples of natural enantiomers whose antipodes were isolated from two different species of the genus Ircinia, and ircinianin, whose sulfated derivative was not discovered until 20 years... 

Fontana, A., Fakhr, I., Mollo, E., and Cimino, G. (1999) (—)-Wistarin from the marine sponge Ircinia sp. the first case of enantiomeric sesterterpenes. Tetrahedron Asymmetry, 10, 3869-3872. 

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