Oxygenation pyridine equivalence

The immediate outcome of the Hantzsch synthesis is the dihydropyridine which requires a subsequent oxidation step to generate the pyridine core. Classically, this has been accomplished with nitric acid. Alternative reagents include oxygen, sodium nitrite, ferric nitrate/cupric nitrate, bromine/sodium acetate, chromium trioxide, sulfur, potassium permanganate, chloranil, DDQ, Pd/C and DBU. More recently, ceric ammonium nitrate (CAN) has been found to be an efficient reagent to carry out this transformation. When 100 was treated with 2 equivalents of CAN in aqueous acetone, the reaction to 101 was complete in 10 minutes at room temperature and in excellent yield. 

Reduced nicotinamide-adenine dinucleotide (NADH) plays a vital role in the reduction of oxygen in the respiratory chain [139]. The biological activity of NADH and oxidized nicotinamideadenine dinucleotide (NAD ) is based on the ability of the nicotinamide group to undergo reversible oxidation-reduction reactions, where a hydride equivalent transfers between a pyridine nucleus in the coenzymes and a substrate (Scheme 29a). The prototype of the reaction is formulated by a simple process where a hydride equivalent transfers from an allylic position to an unsaturated bond (Scheme 29b). No bonds form between the n bonds where electrons delocalize or where the frontier orbitals localize. The simplified formula can be compared with the ene reaction of propene (Scheme 29c), where a bond forms between the n bonds. 

The other major group of oxygen heterocycles which can give pyridines are the pyrylium salts. With ammonia or ammonium salts the products are pyridines, while with primary amines the products are pyridinium salts, as shown in equation (91). The simplified mechanism shows the important stages it is notable that a second molar equivalent of base (which... 

Chemical shifts for aromatic azoles are recorded in Tables 17-20. Fast tautomerism renders two of the C-13 chemical shifts equivalent for the NH derivatives just as in the proton spectra (Table 17). However, data for the A-methyl derivatives (Table 18) clearly indicate that the carbon adjacent to a pyridine-like nitrogen shows a chemical shift at lower field than that adjacent to a pyrrole-like N-methyl group (in contrast to the H chemical shift behavior). In azoles containing oxygen (Table 19) and sulfur (Table 20), the chemical shifts are generally at lower field than those for the wholly nitrogenous analogues, but the precise positions vary. 

The roles of NADH and NADPH in the overall strategy of metabolism are shown in Fig. 5.17. Fuel molecules, such as glucose, are oxidised in catabolism they lose electrons and these reducing equivalents are transfered to an environmental acceptor such as oxygen, with concomitant ATP production (see oxidative phosphorylation, Section 5.5.6). However, some reducing equivalents are conserved and re-utilised in the synthesis of cellular components, with the consumption of ATP, as oxidised intermediates are reduced to synthetic precursors with subsequent polymerisation. The pyridine nucleotides thus have roles in both synthetic and energy generation process. 

From 3,4-dibromotetrahydrothiophene 1,1-dioxide in the presence of pyridine and mesitonitrile oxide is obtained 4,4-dioxo-3-(2,4,6-trimethylphenyl)-3a,6a-dihydro-thieno[2,3-d]isoxazoline (172). A second equivalent of nitrile oxide leads to the formation of the 2 1 adduct (173). NMR analysis and crystallographic studies show the formation of the adduct where the regioselectivity corresponds to the oxygen atom of the dipoles bonded to the carbon atom /3 to the sulfone group, the endo nature of the addition, and the anti situation of the two rings in the diadduct (173) (81JOC3502). 

In this paper we report on factors which affect the conversion of fuel nitrogen to TFN in laboratory jet-stirred combustors which serve to simulate the primary zone in a gas turbine. The independent variables in the experiments were fuel type (aliphatic isooctane vs. aromatic toluene), equivalence ratio (fuel-to-oxygen ratio of combustor feed divided by stoichiometric fuel-to-oxygen ratio), average gas residence time in the combustor, and method of fuel injection into the combustor (prevaporized and premixed with air vs. direct liquid spray). Combustion temperature was kept constant at about 1900K in all experiments. Pyridine, C5,H5N, was added to the fuels to provide a fuel-nitrogen concentration of one percent by weight. 

With regard to the preparation of the Zn/Cu couple, several methods have been developed Dibromomethane may be used instead of diiodomethane when a specifically prepared Zn/Cu couple is applied . The use of Zn/Ag couple often gives better results . Ultrasound irradiation of the reaction mixture has been shown to facilitate the reaction . The cyclopropanation of alkenes with diiodomethane and diethylzinc can be carried out in hydrocarbon solvents and is particularly suitable with easily polymerizable olefins such as vinyl ethers . It has been reported that molecular oxygen remarkably promotes the reaction of diiodomethane with diethylzinc and substantially increases the yield of the adducts . A convenient modification which avoids the handling of pyrophoric diethylzinc has been reported . In reaction of olefins which are sensitive to the unavoidably produced zinc iodide (the Lewis acid), the addition of one equivalent of dimethoxyethane (DME) to the solvent has been recommended . Zinc iodide is then precipitated as the 1 1 DME complex as it is formed. Zinc salts, which often complicate the workup of the reaction mixture, can also be removed as precipitates by the addition of pyridine prior to the workup . ... 

---