New bioactive annonaceous acetogenins from the bark of Annona squamosa, and the use of countercurrent chromatography for their isolation
Abstract
This project had as its objective the isolation of new bioactive principles from the bark of Annona squamosa Linn. (Annonaceae). Folkloric reports suggested its effectiveness as an insecticide and as a treatment for ticks and headlice. The bioactivity-directed fractionation of this plant resulted in the isolation of twenty Annonaceous acetogenins; twelve are new to the literature while eight are reported for the first time from this species. The structures of these new compounds were characterized through careful examination of the $\sp1$H NMR, $\sp{13}$C NMR, $\sp1$H-$\sp1$H COSY, HMQC, HMBC, and mass spectral data. The relative stereochemistries of the methine protons around the tetrahydrofuran ring were determined by comparison with compounds of known stereochemistry and with synthetic models. Where possible, the absolute stereochemistry of the carbinol centers was established using advanced Mosher methodology. All of the compounds showed significant, and in some instances selective, bioactivities against a panel of six human tumor cell lines (A-549, MCF-7, HT-29, A-498, PC-3, and PACA-2). Squamotacin was found to be selectively cytotoxic against the human prostate tumor cell line, PC-3, with activity one million times that of the positive control, adriamycin. This compound differs from the more generally cytotoxic acetogenin, bullatacin, only in the position of the THF ring system. In bullatacin, the rings are located between C-15 and C-24, in squamotacin, they are shifted two methylene units toward the lactone and are found between C-13 and C-22. The purification and isolation of bioactive natural products from their plant sources is traditionally achieved by running the active fractions through a series of open columns followed by HPLC purification. With the development of powerful spectroscopic methods, the structure elucidation of new compounds can be completed fairly quickly. Therefore, the isolation of the pure compound is the rate-limiting step in the discovery of new natural products. As a result, a countercurrent chromatography method was developed to facilitate the rapid purification of bioactive compounds from crude extracts and column fractions.
Degree
Ph.D.
Advisors
McLaughlin, Purdue University.
Subject Area
Pharmacology
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