Syntheses of aza analogues of kainic acid

The kainate receptors are one of the three major groups of ionotropic glutamate receptors in the mammalian central nervous system. They are so named after their most potent agonist, kainic acid (KA), a natural product isolated from the seaweed Diginea simplex. This compound shows both neuroexcitatory and excitotoxic activities, and is an important pharmacological tool for neurophysiological studies. We predict that the more synthetically accessible aza analogues of kainic acid, could act as functional mimics of KA. These could be produced by the 1,3-dipolar cycloaddition of diazoalkanes with trans glutaconate esters. ^ 1,3-Dipolar cycloadditions have been shown to produce 1-pyrazolines that isomerize into 2-pyrazolines. The 1- and 2-pyrazolines can be precursors to aza analogs of kainoids. The regioselectivity, relative stereochemistry and isomerization of the 1-pyrazolines into 2-pyrazolines have been evaluated. Reductions of the 1- and 2-pyrazolines produced aza analogs of kainoids. TMS diazomethane was used as the dipole in 1,3-dipolar cycloaddition reactions leading to aza KA analogs via 2-pyrazolines. A systematic study of cycloaddition-isomerization processes involving TMS-diazomethane and various α, β-unsaturated dipolarophiles has been undertaken. 1H-NMR monitoring of the reaction mixture compositions during the cycloaddition reaction revealed evidence of retro-dipolar cycloaddition processes. Faster formation of 4,5- trans-1-pyrazoline at the beginning of the reaction and subsequent isomerization of this product into 4,5-cis-1-pyrazoline via a retro-dipolar cycloaddition has been observed. Increased reaction time and/or reaction temperature preferentially caused the irreversible isomerization of 4,5-cis-1-pyrazoline into 4,5-cis-2-pyrazoline, which led to high yields of 4,5-cis-2-pyrazolines in the overall process. ^ Two syntheses of the 5-unsubstituted aza-kainic acid have been performed; first, via the reduction of the TMS-eliminated 2-pyrazoline from TMS diazomethane; second by the direct reduction of 1-pyrazoline with Hg/Al-amalgam. 5-Phenyl aza-kainic acid has been produced by direct reduction of 1-pyrazoline, obtained in the reaction of phenyldiazomethane and dibenzyl glutaconate, with Hg/Al-amalgam. ^ Current responses to aza kainate analogs in Aplysia whole cell buccal ganglia indicate potent neuroexcitatory activity. The repetitive exposure of neuronal cells to the 5-unsubstituted aza-kainic acid led to non-desensitizing current responses, showing both binding affinity and neuronal ion-channel activation by the synthesized agonist compound. ^

Identification of Anziaic Acid, a Lichen Depside from Hypotrachyna sp., as a New Topoisomerase Poison Inhibitor

Topoisomerase inhibitors are effective for antibacterial and anticancer therapy because they can lead to the accumulation of the intermediate DNA cleavage complex formed by the topoisomerase enzymes, which trigger cell death. Here we report the application of a novel enzyme-based high-throughput screening assay to identify natural product extracts that can lead to increased accumulation of the DNA cleavage complex formed by recombinant Yersinia pestistopoisomerase I as part of a larger effort to identify new antibacterial compounds. Further characterization and fractionation of the screening positives from the primary assay led to the discovery of a depside, anziaic acid, from the lichen Hypotrachyna sp. as an inhibitor for both Y. pestis and Escherichia colitopoisomerase I. In in vitro assays, anziaic acid exhibits antibacterial activity against Bacillus subtilis and a membrane permeable strain of E. coli. Anziaic acid was also found to act as an inhibitor of human topoisomerase II but had little effect on human topoisomerase I. This is the first report of a depside with activity as a topoisomerase poison inhibitor and demonstrates the potential of this class of natural products as a source for new antibacterial and anticancer compounds.

Developing of Germyldesulonylation and Thiodesulfonylation Reactions for the Synthesis of Novel Nucleoside Analogues. Efficient Synthesis of Novel (α-Fluoro)vinyl Sulfides

S-adenosyl-L-homocysteine (AdoHcy) hydrolase effects hydrolytic cleavage of AdoHcy to produce both adenosine and L-homocysteine and is a feedback inhibitor of S-adenosyl- L-methionine (SAM). Nucleoside analogues bearing an alkenyl or fluoroalkenyl chain between sulfur and C5' utilizing Negishi coupling reactions were synthesized. Palladium-catalyzed cross-coupling between the 5'-deoxy-5'-(iodomethylene) nucleosides and alkylzinc bromides gives analogues with the alkenyl unit. Palladium-catalyzed selective monoalkylation of 5'-(bromofluoromethylene)-5'-deoxy-adenosine with alkylzinc bromide afford adenosylhomocysteine analogues with a 6'-(fluoro)vinyl motif. The vinylic adenine nucleosides produced time-dependent inactivation of the S-adenosyl-L-homocysteine hydrolases. Stannydesulfonylation reaction is a critical step in the synthesis of E-fluorovinyl cytidine (Tezacitabine) a ribonucleoside reductase inhibitor with a potent anticancer activity. The synthesis involves the removal of the sulfonyl group by a radical-mediated stannyldesulfonylation reaction using tributyltin hydride. In order to eliminate the toxicity of tin, I developed a radical-mediated germyldesulonylation utilizing less toxic germane hydrides. Treatment of the protected (E)-5'-deoxy-5'-[(p-toluenesulfonyl)-methylene]uridine and adenosine derivatives with tributyl- or triphenylgermane hydride effected radical-mediated germyldesulfonylations to give 5'-(tributyl- or triphenylgermyl)methylene-5'-deoxynucleoside derivatives as single (E)-isomers. Analogous treatment of 2'-deoxy-2'-[(phenylsulfonyl)methylene]uridine with Ph3GeH afforded the corresponding vinyl triphenylgermane product. Stereoselective halodegermylation of the (E)-5'-(tributylgermyl)-methylene-5'-deoxy nucleosides with NIS or NBS provided the Wittig-type (E)-5'-deoxy-5'-(halomethylene) nucleosides quantitatively. Radical-mediated thiodesulfonylation of the readily available vinyl and (α-fluoro) vinyl sulfones with aryl thiols in organic or aqueous medium to provide a bench and environmentally friendly protocol to access (α-fluoro)vinyl sulfides were developed. Methylation of the vinyl or (α-fluoro)vinyl phenyl sulfide gave access to the corresponding vinyl or (α-fluoro)vinyl sulfonium salts. These sulfonium ions were tested as possible methyl group donors during reactions with thiols, phenols or amino groups which are commonly present in natural amino acids.