New Developments in the Chemistry of Pyrans

The second chapter of this book provides information about all category of six membered ring system with N and/or O and /or sulfur atom, and their benzo derivatives. These six membered heterocycles and their benzo derivatives are not only present in drugs, vitamins, natural products, biomolecules, exhibiting antitumor, anti-inflammatory, anti-fungal, anti-bacterial, anti-depressant, anti-HIV anti-microbial, anti-malarial and anti-diabetic activities but they are also the key structural unit of pharmaceutical drugs and agrochemicals available in the market. Thus, each unit of this chapter gives due emphasis on the importance of a particular six membered heterocycle or its derivative under study in natural product, medicine and material. Each unit of the chapter also provides information about the structure of the individual heterocycle especially in terms of X-ray and spectroscopic data. After that synthesis, chemical reactivity and application of these heterocycles have been discussed. It is worth mentioning that wherever necessary, possible mechanism of the some typical reactions as well as name reactions have been discussed.

Six-Membered Heterocycles

First and second wave E½redn potentials have been measured by cyclic voltammetry for a set of fifteen 2,6-diphenyl-4-(X-phenyl)-pyrylium, -thiopyrylium and -N-methylpyridinium tetrafluoroborates, with X =p-CH3, -H, m-Cl, p-CF3 and m,m˙-(CF3)2. There are linear correlations between Hammett substituent constants and scaled potentials (E/0.059 15) giving slopes of ρ= 2.59 ± 0.06 (r2= 0.9984), 2.38 ± 0.07 (r2= 0.9973) and 3.10 ± 0.16 (r2= 0.9918), respectively, for the first wave potentials.Products, rates and kinetic hydrogen isotope effects for reductions of the cations with sodium and tetrabutylammonium cyanoborohydride in acetonitrile solution have been determined. Reaction of the thiopyrylium 2 (X = H) yields kinetically controlled 54 : 46 mixture of the 4H- and 2H-thiopyrans. With the pyrylium ion 1 (X = H) the 4H- to 2H- ratio is 24 : 76 with the 2H-pyran suffering electrocyclic ring opening to 1,3,5-triphenylpentadienal. Product ratios are weakly sensitive to substituents on the 4-phenyl, with electron withdrawing groups increasing the amount of 2H-product for both thiopyryliums and pyrylium cations. Reductions are first order in cation and cyanoborohydride. For sodium cyanoborohydride and 2,4,6-triphenylpyrylium, k(25 °C)= 65.6 dm3 mol–1 s–1 and for 2,4,6-triphenylthiopyrylium, k(25 °C)= 16.7 dm3 mol–1 s–1, with kinetic isotope effects (kBH3CN/kBD3CN) of 1.17 and 1.68, respectively. Logarithms of rate constants correlate with Hammett constants giving ρ= 1.46 ± 0.06 (r2= 0.9951) for the pyrylium series and ρ= 1.12 ± 0.08 (r2= 0.9831) for the thiopyryliums. The N-methylpyridinium ions do not react with cyanoborohydride.The X-ray crystal structure of 2,4,6-triphenyl-N-methylpyridinium tetrafluoroborate has been determined. Dihedral angles about bonds from the pyridinium ring to the 2- and 6-phenyls are larger than corresponding angles in 2,4,6-triphenylpyrylium and 2,4,6-triphenylthiopyryliums and are ascribed to relief of non-bonded interactions between the 2- and 6-phenyls and the N-methyl group.

Substituent effects in cyanoborohydride reductions of heterocyclic aromatic cations

In methanol, the reactions of aniline with 2-phenoxy-3,5-dinitropyridine, phenyl 2,4,6-trinitrophenyl ether, and 4-nitrophenyl 2,4,6-trinitrophenyl ether and the last-named substrate with N-methylaniline are general-base catalysed. The reaction of 2,6-dinitrophenyl phenyl ether with n-butylamine is not base catalysed, but its reaction with piperidine is specific-base catalysed. The results are discussed in terms of the mechanism of aromatic nucleophilic substitution reactions in protic solvents.

The mechanism of aromatic nucleophilic substitution reactions in protic solvents. The reations of aniline, N-methylaniline, n-butylamine, and piperidine with some nitroaryl phenyl ethers in methanol

Pyrylium Compounds. 43. Arylsubstituted 5-(2-Dialkylamino-thiazol-5-yl)-pentadienones from 2,4,6-Triarylpyrylium Salts and 2-Dialkylamino-4-aryl-thiazoles



2,4,6-Triarylpyrylium salts 1 react with 2-dialkylamino-4-aryl-thiazoles 7 (used in substance or prepared in situ from α-thiocyanato-acetophenones 9) in the presence of an appropriate acid-binding agent (e.g. piperidine acetate or sodium acetate) to give 5-(2-dialkylamino-4-aryl-thiazol-5-yl)-1,3,5-triaryl-penta-2,4-dien-1-ones 8. As reaction medium aliphatic alcohols (ethanol, n-propanol), dipolar aprotic solvents (acetonitrile) or chlorinated hydrocarbons (methylene chloride, chloroform) can be used. On the other hand, under the same conditions 2-amino-4-phenyl-thiazole (10) reacts with salts of type 1 via pyrylium ring transformation yielding 2,4,6-triaryl-1-(4-phenyl-thiazol-2-yl)-pyridinium perchlorates 11.

Pyryliumverbindungen. 43 [1]. Arylsubstituierte 5-(2-Dialkylamino-thiazol-5-yl)-pentadienone aus 2,4,6-Triaryl-pyryliumsalzen und 2-Dialkylamino-4-arylthiazolen

Cinetique des reactions de l'ion triphenyl-2,4,6 pyrylium avec butylamine, cyclohexylamine, pyrrolidine piperidine et morpholine conduisant aux amides vinyliques a cycle ouvert correspondants. Influence de la nature, primaire ou secondaire, des amines utilisees sur l'etape determinant la vitesse

Mauro Mecozzi

Papers

Kinetic study of the reaction of 2,4,6-triphenylpyrylium ion with amines. Base-catalyzed ring-opening reaction of 2H-pyran intermediates