Syntheses of aza analogs of kainoids

The kainoids are a class of non-proteinogenic pyrrolidine dicarboxylates that exhibit both excitatory and excitotoxic activities. These activities are a result of the ability of the kainoids to act as glutamate receptor agonists by activating ionotropic glutamate receptors. The parent of this group of compounds is α-kainic acid. Kainic acid is isolated from the seaweed Diginea simplex and has been used in Asian countries as a treatment for intestinal worms in children. In addition it is used extensively by neuropharmacologists for the study of glutamate receptors. Several years ago, the world's sole supplier of kainic acid discontinued this product. Since that time, other sources have appeared, however, the price of kainic acid remains significantly higher than it once was. We have thus been working on synthesizing aza analogs of kainoids which would be less costly but potentially potent alternatives to kainic acid via the dipolar cycloadditions of diazoalkanes with trans diethyl glutaconate. These 1, 3-dipolar cycloadditions yielded 2-pyrazolines or pyrazoles. The 2-pyrazolines may be precursors to aza analogs of kainoids. The regioselectivity of these 1, 3-dipolar cycloadditions and isomerization of the 1-pyrazolines to 2-pyrazolines was evaluated. Reductions of the 2-pyrazolines yielded aza analogs of kainoids.^ TMS diazomethane, due to the commercial availability, has been frequently used as a synthetic reagent in 1, 3-dipolar cycloadditions, particularly in the preparation of novel amino acid analogs. A survey of the recent literature indicates that the regioselectivity of the double bond isomerization of TMS substituted 1-pyrazolines is variable and at first glance, unpredictable. In an effort to develop a mechanistic rational for the isomerization which could account for the products obtained, a systematic survey of dipolar cycloadditions between TMS diazomethane and α, β-unsaturated dipolarophiles was undertaken. It was suggested that the steric demand of the dipolarophiles had a profound effect on both the relative stereochemistry of dipolar cycloaddition reactions of TMSCHN2 and the preferred direction of isomerization of the resulting 1-pyrazoline.^

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. ^

Mechanistic studies on the advanced oxidation and photochemical transformation of cyanotoxins (microcystins)

The increased occurrence of cyanobacteria (blue-green algae) blooms and the production of associated cyanotoxins have presented a threat to drinking water sources. Among the most common types of cyanotoxins found in potable water are microcystins (MCs), a family of cyclic heptapeptides containing substrates. MCs are strongly hepatotoxic and known to initiate tumor promoting activity. The presence of sub-lethal doses of MCs in drinking water is implicated as one of the key risk factors for an unusually high occurrence of primary liver cancer. ^ A variety of traditional water treatment methods have been attempted for the removal of cyanotoxins, but with limited success. Advanced Oxidation Technologies (AOTs) are attractive alternatives to traditional water treatments. We have demonstrated ultrasonic irradiation and UV/H2O2 lead to the degradation of cyanotoxins in drinking water. These studies demonstrate AOTs can effectively degrade MCs and their associated toxicity is dramatically reduced. We have conducted detailed studies of different degradation pathways of MCs and conclude that the hydroxyl radical is responsible for a significant fraction of the observed degradation. Results indicate preliminary products of the sonolysis of MCs are due to the hydroxyl radical attack on the benzene ring and substitution and cleavage of the diene of the Adda peptide residue. AOTs are attractive methods for treatment of cyanotoxins in potable water supplies. ^ The photochemical transformation of MCs is important in the environmental degradation of MCs. Previous studies implicated singlet oxygen as a primary oxidant in the photochemical transformation of MCs. Our results indicate that singlet oxygen predominantly leads to degradation of the phycocyanin, pigments of blue green algae, hence reducing the degradation of MCs. The predominant process involves isomerization of the diene (6E to 6Z) in the Adda side chain via photosensitized isomerization involving the photoexcited phycocyanin. Our results indicate that photosensitized processes play a key role in the environmental fate and elimination of MCs in the natural waters. ^