Synthesis of 5-epiaragusterol A

Synthesis and anti-tumor activity of new steroidal nuclear analogues of aragusterol A

3alpha,7alpha-Dihydroxy-5-epiaragusterol A (3) was synthesized from bile acid (cholic acid) as a new steroidal nuclear analogue of antitumor marine steroid aragusterol A. 7alpha-Hydroxyaragusterol A (4) was also derived from xestokerol B. The in vitro anti-proliferative activity of each of these analogues toward KB cells as well as in vivo anti-tumor activity of 5-epiaragusterol A (2) previously synthesized by the authors and 3 were assessed.

[Studies on discovery and synthesis of bioactive marine organic molecules]

This paper describes the discovery and total synthesis of bioactive marine natural products conducted in our laboratory. Clavulone, chlorovulone, bromovulone, and iodovulone are antitumor marine prostanoids isolated from the Okinawan soft coral Clavularia viridis. The synthesis of clavulone and chlorovulone was achieved from chiral 4-hydroxy-2-cyclopentenone. Marine prostanoid punaglandins 3 and 4 were synthesized via similar methodology. The chemical structures of punaglandins 3 and 4 were revised by these syntheses. Dollaberane-type diterpenoid stolonidiol and claenone were isolated from Okinawan soft coral Clavularia sp. Stolonidiol showed potent choline acetyltransferase-inducible activity in cultured basal forebrain cells. The synthesis of stolondiol and claenone was conducted via sequential Michael reaction and retro-aldol reaction. Aragusterols, isolated from the Okinawan marine sponge Xestospongia sp., are structurally unique steroids possessing a rare 26,27-cyclo structure in the side chain. Aragusterols express potent in vivo antitumor activity against L1210 leukemia in mice. The synthesis of aragusterols was carried out via steroselective construction of the side chain and stereocontrolled coupling reaction with the steroid skeleton. Kalihinane-type diterpenoid kalihinol A, isolated by Scheuer, has remarkable in vitro antimalarial activity. The absolute configuration of kalihinol A was determined by applying the CD exciton chiral method. Synthesis of kalihinene X, a kalihinane-type diterpenoid, was achieved. This synthesis involves the regioselective coupling reaction of carbanion of alkyl sulfone with epoxyalcohol and construction of cis-decalin by an intramolecular Diels-Alder reaction.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.