tone ring configuration.31,23 The method of countercurrent chromatography is now being used to separate known and new acetogenins.41,42
The emergence of the Annonaceous acetogenins as potential agents to thwart biological resistance was first manifested in the work of Oberlies et al.19-21 in tumor cells and in the work of Alali et al.22 in insects; it was found that the acetogenins could effectively combat multidrug resistant cancer cells and pesticide-resistant cockroaches, respectively. The work of Shimada et al.43,44 on the placement of acetogenins, representing several structural classes, within liposomal membranes, now provides a new hypothesis as to how these compounds interact with lipid bilayers to exert their activity on membrane-bound enzymes. Additionally, in cooperation with a group in Japan, our recent work on the structure–activity relationships (SAR) in the isolated submitochondrial particles and work with computer models drastically challenge our understanding of the relevance of the relative stereochemistry across the THF rings.45
In our first review in 1990, 28 Annonaceous acetogenins isolated from 11 species were described; in our second review in 1993, 61 acetogenins isolated from 16 species were summarized; in our third review in 1995, another 80 acetogenins from 20 species of Annonaceous acetogenins were reported; and in our fourth review in 1996, 76 new acetogenins from 26 species were tabulated.3-6 In this review, which covers scientific progress made in the biology, chemistry, and development of Annonaceous acetogenins from December 1995 to July 1998, data have been compiled on 137 new acetogenins from 24 species (Appendix 1). At the time of preparation (August 1998) of this current review, over 350 Annonaceous acetogenins have been isolated from 37 species. Our preliminary efforts show that about 50%, of over 80 Annonaceous species screened, are significantly bioactive and are worthy of fractionation; thus, this class of compounds can be expected to continue to grow, at an exponential rate in the future, provided that financial support for such research efforts can be found. With the demise of the world's tropical rain forests, such work is compelling before the great chemical diversity, contained within these endangered species, is lost.
In our last review,3 the Annonaceous acetogenins were classified according to their relative stereostructures across the THF rings; while that classification is more informative than any other method, it leads to a plethora of subclasses, and we will use herein our earlier system of classification with some additions and modifications. The Annonaceous acetogenins seem to be best classified into mono-THF, adjacent bis-THF, nonadjacent bis-THF, non-THF ring, tri-THF, and nonclassical acetogenins (THP and ring-hydroxylated THF compounds), followed by subclassification of the γ-lactone, substituted γ-lactone, or ketolactone variations (Figure 1).
The names, classifications, chemical and biological data, and references to the new compounds, reported December 1995–July 1998, are tabulated in Appendix 1.
Biogenetically, the Annonaceous acetogenins seem to be derived from the polyketide pathway. The THF, THP, and epoxide rings are suggested to arise from isolated double bonds through epoxidation and cyclization.34 The discovery of earlier precursors (nonring compounds, epoxides, ketones, diols, and double bonds), the location of double bonds Mono-THF acetogenins
505
Mono-THF acetogenins
Adjacent bis-THF acetogenins
Non-adjacent bis-THF acetogenins
Non THF-ring acetogenins
R=-(CH_{2})-, epoxide, ketone, -CH-OH
Non-Classical (THP) acetogenins
Sub-types of the terminal lactone ring
R1 = H, OH cis or trans ring hydroxylated R2 = H, OH ketolactone and reduced
Figure 1. Core units for classification of Annonaceous acetogenins.
in the appropriate positions, and the semisynthesis of additional THF rings from double bond-containing acetogenins strongly support this hypothesis. The discoveries of muridienins, proposed precursors of the mono-THF acetogenins, and chatenaytrienins, proposed precursors of the bis-THF acetogenins, add new evidence that acetogenins are more likely to be derived from laceroic (C-32) and ghedoic (C-34) fatty acids after enzymatic combination with a three-carbon unit.[29] We could not find these free acids or their esterified products in the oily extracts of the paw paw, Asimina triloba; this might suggest that these free acids actually serve as short-lived intermediates in the biosynthetic process and are shuffled along complex enzyme subunits rather than being released as end products that can later be metabolized by γ-lactone formation, dehydrogenation, epoxidation, and cyclization. The sequence of these events is still unknown. The time is at hand to conduct feeding experiments with isotopically labeled precursors to verify the biogenetic pathways and the sequence of events leading to these compounds. Plant tissue culture methods have not yet produced sufficient cell growth to permit such studies.
One wonders why the plants in this family choose to biosynthesize more than 350 different acetogenins, with all of them being either C-32 or C-34 fatty acid derivatives, and not produce C-36 or C-38 or even C-30 or C-28 compounds. The dimensions of cell and mitochondrial membranes43 (the sites of the target enzymes) may dictate this; this particular chain length has likely evolved because it provides the optimum activity and protection of the plant against herbivores and pests. Shorter acetogenins lose activity, and it may be logical to assume that longer ones will also be less active.
The biogenetic story of goniocin is particularly interesting. This tri-THF ring acetogenin was first isolated in our laboratory by Gu et al. in 1994 from Goniothalamus giganteus.[46] All previously isolated acetogenins from this plant, representing over 30 different compounds,[47] had an R configuration at C-10 including cyclogoniodenin T,[48] a semisynthetic tri-THF enantiomer of goniocin; however,