Thanasis Gimisis

Naturally Occurring Pentacyclic Triterpenes as Inhibitors of Glycogen Phosphorylase : Synthesis, Structure-Activity Relationships, and X-ray Crystallographic Studies

Twenty-five naturally occurring pentacyclic triterpenes, 15 of which were synthesized in this study, were biologically evaluated as inhibitors of rabbit muscle glycogen phosphorylase a (GPa). From SAR studies, the presence of a sugar moiety in triterpene saponins resulted in a markedly decreased activity ( 7, 18- 20) or no activity ( 21, 22). These saponins, however, might find their value as potential natural prodrugs which are much more water-soluble than their corresponding aglycones. To elucidate the mechanism of GP inhibition, we have determined the crystal structures of the GPb-asiatic acid and GPb-maslinic acid complexes. The X-ray analysis indicates that the inhibitors bind at the allosteric activator site, where the physiological activator AMP binds. Pentacyclic triterpenes represent a promising class of multiple-target antidiabetic agents that exert hypoglycemic effects, at least in part, through GP inhibition.

C‐1′ Radical‐Based Approaches for the Synthesis of Anomeric Spironucleosides

A 1,5-radical translocation from a conveniently situated alkoxyl or vinyl radical next to the C-1′ position is the key step for the successful radical-based cascade reactions that allow the construction of two classes of anomeric spironucleosides 1 and 2 in the ribo and 2′-deoxyribo series.

5,10-DIHYDRO-SILANTHRENE AS A REAGENT FOR THE BARTON-MCCOMBIE REACTION

Abstract The deoxygenation of secondary alcohols via thionoesters with the use of 5,10-dihydro-silanthrene as the radical reducing agent has been studied in detail. The ability of hydrogen donation of this silane has been measured using the one-carbon ring expansion of 1-(2-oxocyclopentyl)ethyl radical as a timing device.

Generation of C-1′ radicals through a β-(acyloxy)alkyl rearrangement in modified purine and pyrimidine nucleosides

Abstract The synthesis of protected 1′,2′-didehydro-2′-deoxyadenosines has been optimized by incorporating a phosphoranylidene protection of the adenine amino function. These unsaturated adenosines have served as substrates for the electrophilic iodopivaloyloxylation leading to new nucleosides modified at the anomeric position. Reaction of halopivaloates 10, 11 and 12 with tributyltin hydride generates indirectly C-1′ radicals through a β-(acyloxy)alkyl rearrangement. Rate constants for these rearrangements have been measured by using free-radical clock methodology and comparison of these data with previous reported results provides structural information about the nature of this important class of radicals.

RADICAL TRANSFORMATIONS OF NUCLEOSIDES WITH (ME3SI)3SIH. GENERATION OF A C-1' RADICAL THROUGH 1,2-MIGRATION OF AN ACYLOXY GROUP

Abstract A number of nucleoside substrates has been reduced under free radical conditions with tris(trimethylsilyl)silane. In one case a novel type of a β-(acyloxy)alkyl radical rearrangement has been observed, which leads through the generation of a C-1’ radical species to the stereoselective preparation of an α-ribonucleoside. The rate of the above 1,2-migration has been estimated, and a comparison with previously reported results has been made.

Spectra and structure of the 2′-deoxyuridin-1′-yl radical

Abstract The title C-1′ radical, obtained by photolysis of the corresponding tert -butyl ketone in water, was studied spectroscopically by EPR and laser flash photolysis methods and computationally.

Fate of the C-1′ peroxyl radical in the 2′-deoxyuridine system

The mechanism of 2-deoxyribonolactone formation from the reaction of photogenerated 2′-deoxyuridin-1′-yl radical with molecular oxygen in water has been investigated.

CATALYTIC FERRIER REARRANGEMENT OF UNSATURATED NUCLEOSIDES

Abstract The attempted intermolecular addition of malonyl radicals to 1′,2′-unsaturated nucleosides has led to the unexpected formation of furanones. Thus, only catalytic amounts of ceric(IV) ammonium nitrate (CAN), induce a Ferrier rearrangement. The unsaturated lactone was isolated in good yield and can serve as a precursor for the synthesis of optically active products.

FR258900, a potential anti-hyperglycemic drug, binds at the allosteric site of glycogen phosphorylase

FR258900 has been discovered as a novel inhibitor of human liver glycogen phosphorylase a and proved to suppress hepatic glycogen breakdown and reduce plasma glucose concentrations in diabetic mice models. To elucidate the mechanism of inhibition, we have determined the crystal structure of the cocrystallized rabbit muscle glycogen phosphorylase b–FR258900 complex and refined it to 2.2 Å resolution. The structure demonstrates that the inhibitor binds at the allosteric activator site, where the physiological activator AMP binds. The contacts from FR258900 to glycogen phosphorylase are dominated by nonpolar van der Waals interactions with Gln71, Gln72, Phe196, and Val45′ (from the symmetry‐related subunit), and also by ionic interactions from the carboxylate groups to the three arginine residues (Arg242, Arg309, and Arg310) that form the allosteric phosphate‐recognition subsite. The binding of FR258900 to the protein promotes conformational changes that stabilize an inactive T‐state quaternary conformation of the enzyme. The ligand‐binding mode is different from those of the potent phenoxy‐phthalate and acyl urea inhibitors, previously described, illustrating the broad specificity of the allosteric site.

Ex-Novo and Revisum Procedures for the Preparation of C-1′ Branched Nucleosides

Abstract C-1′ acylated derivatives of 2′-deoxyuridine were obtained either by revising the existing procedures or by introducing a new methodology.