A novel crown ether generation containing the pyrilium heteroaromatic cation — Synthesis, characterization, and solid-phase 13C NMR

The synthesis and structural characterization of two members, 5 and 6, of a novel generation of crown ethers containing the pyrilium cation are reported. These two compounds contain two and three benzo-9-crown-3 (B9C3) units, respectively, connected to the 2 and 6 or 2, 4, and 6 positions of a central pyrilium cation. The crown ethers 5 and 6 were obtained from the condensation reaction of appropriate aldehydes and methyl ketones in the presence of POCl3. A variety of physicochemical methods including electrospray mass spectroscopy, solution 1H and 13C NMR, and solid-phase 13C NMR were applied for structural characterization. The 1H and 13C NMR studies indicate rapid rotation of the B9C3 unit about the C–C bond that connects the two units to each other. The solid-phase 13C NMR spectra of these two crown ethers reveal that there are deviations from planarity for the two phenyl rings attached at the 2 and 6 positions of the central pyrilium cation.Key words: crown ether, pyrilium, solid-phase NMR, crownpyrilium, heteroaromatic.

Kinetics of the reduction of the tropylium and xanthylium cations by 1,4-dihydropyridine derivatives

The pH-dependences of the apparent second-order rate constants [Formula: see text] for the reduction of 2,4,6-cycloheptatrien-1-ol and 9-xanthydrol by each of 1-benzyl-1,4-dihydronicotinamide (BNH) and 10-methyl-9,10-dihydroacridine (MAH) have been measured in 20% acetonitrile – 80% water, at 25 °C and ionic strength 1.0. For each of these reactions, the pH-dependence of [Formula: see text] is only consistent with reduction occurring via the aromatic cation (either tropylium or xanthylium) that is present in equilibrium with these alcoholic species. The relative second-order rate constants [Formula: see text] for reductions by these two reducing agents (1700 for tropylium and 770 for xanthylium) are similar for these two cations. These ratios are also similar to those observed for a variety of nitrogen heteroaromatic hydride acceptors, even though the absolute magnitudes of these rate constants vary by 1010-fold. The second-order rate constants for the reductions of the tropylium and xanthylium cations are predicted reasonably well by their [Formula: see text] values, with the latter cation being (7 × 105)-fold more reactive than its π-isoelectronic N-methyl acridinium cation. The xanthylium cation has the greatest [Formula: see text] ratio yet observed for any heteroaromatic cation, and this value further extends the known range of this ratio as a function of reactivity. Keywords: hydride transfer, kinetics of reduction, 1,4-dihydropyridine derivatives, tropylium cation, xanthylium cation.

Specificity of xanthine oxidase for nitrogen heteroaromatic cation substrates

A variety of pyridinium, quinolinium, and benzoquinolinium cations have been investigated as potential substrates for milk xanthine oxidase at pH 9.9 and (or) pH 10.6. Steady-state kinetic parameters (kc, Km and (or) kc/Km) have been evaluated for all substrates which are enzymically oxidized. Simple N-alkyl pyridinium cations are neither substrates nor inhibitors, although N-aryl pyridinium cations are slowly oxidized to the 4-pyridinones. N-Methylpyridinium cations bearing 3-CONH2, 3-CONHCH3, 3-COCH3, 3-CO2− or 3-CN substituents are readily oxidized at C-6 and this suggests an important hydrogen-bonding interaction between an enzyme donor and the C-3 carbonyl substituent. A variety of N-methylquinolinium cations bearing C-6 substituents are enzymically oxidized at C-2. Analogous substituent effects on kc/Km for these 6-substituted 1-methylquinolinium cations and the corresponding 1-(substituted phenyl)-pyridinium cations is suggestive of the relative productive binding orientations of these two classes of substrate in the active site. N-Methyl benzoquinolinium and 1,10-phenanthrolinium cations are the best cationic substrates found to date, and suggest a relatively large active-site region for the reducing substrate, and important hydrophobic interactions between enzyme and substrate. The overall enzymic specificity observed for these cationic substrates allows a mapping of the general features of the reducing substrate binding site of this enzyme