Microwave-promoted catalyst- and solvent-free aza-Diels-Alder reaction of aldimines with 6-[2-(dimethylamino)vinyl]-1,3-dimethyluracil

A microwave-promoted aza-Diels-Alder reaction between 6-[2-(dimethylamino)vinyl]-1,3-dimethyluracil and aldimines has been developed for the construction of dihydropyrido[4,3-d]pyrimidines. Urea is effectively employed as an environmentally benign source of ammonia in the absence of any catalyst or solvent. The key step in the reaction is in situ generation and trapping of the reactive aldimine formed from urea and aldehyde by the diene system of the uracil. The reaction is clean, and excellent yields are obtained in a matter of a few minutes.     

Facile and convenient strategy towards synthesis of 4-substituted 2-aminothiazolo[4,5-d]pyridazinones

Convenient synthesis of 4-substituted 2-aminothiazolo[4,5-d]pyridazinones has been achieved in 12 steps with overall yield of 19% by employing Grignard reaction as the key step. The route utilizes well established thiazole ring formation followed by Grignard reaction to introduce substitution at 4-position effectively. In addition to the use of inexpensive chemicals, the present route first time gave access to the 4-substituted 2-aminothiazolo [4,5-d]pyridazinones with free amino group at C-2 position.     

Synthesis and evaluation of silica hydride‐based fluorinated stationary phases

Two novel silica hydride‐based fluorinated bonded phases have been synthesized using a hydrosilation procedure to test combined fluorine and hydride selectivity. The bonded moieties were characterized by elemental and spectral analysis. Chromatographic testing was done using hydrophilic analytes in the aqueous normal phase mode. At higher amounts of the nonpolar solvent in the mobile phase, there should be increased retention for solutes such as acids, bases and other polar compounds, whereas nonpolar solutes can be retained when water is increased as in RP chromatography. The synergistic effects of the fluorinated phase selectivity and aqueous normal phase retention on a hydride surface have been explored for small polar molecules. The stability and repeatability of the hydride‐based fluorinated stationary phases were evaluated. The use of acetone as the organic component in the mobile phase was also tested.     

Mesogenic benzothiazole derivatives with methoxy substituents

Two new mesogenic homologous series containing 6-methoxybenzothiazole ring at the terminus of the molecule, viz. 2-(4′-n-alkoxyphenylazo)-6-methoxybenzothiazoles and 2-[4′-(4″-n-alkoxy-benzoyloxy) phenylazo]-6-methoxybenzothiazoles were synthesized. In series I,n-butyloxy to n-tetradecyloxy derivatives exhibit nematic mesophase. Smectic A mesophase commences from then-dodecyl-oxy derivative as a monotropic phase and is retained up to the last member synthesized. All the members of series II exhibit enantiotropic nematic mesophase. The mesomorphic properties of the present series are compared with other structurally related series to evaluate the effect of the benzothiazole ring and the methoxy substituent on mesomorphism. This paper was presented at the 10th National Conference on Liquid Crystals held at Bangalore during 9–11 October 2003     

Azomesogens containing an ethoxyethyl terminal chain: synthesis and characterization

Two homologous mesogenic series, the ethoxyethyl 4-(4'-n-alkoxyphenylazo)benzoates (I) and the ethoxyethyl [4-(4'-n-alkoxybenzoyloxy)phenylazo]-4'-benzoates (II), both containing a terminal ethoxyethyl chain, have been synthesized. In series I, all nine members synthesized exhibit only an enantiotropic smectic A mesophase. In series II, all 12 homologues exhibit an enantiotropic nematic mesophase; a smectic A phase appears in the n-butyloxy derivative as a enantiotropic phase and persists through to the n-hexadecyloxy member. The mesomorphic properties of both series are compared with each other and also with the properties of other structurally related series to evaluate the effects of the ethoxyethyl chain on mesomorphism.     

Iodine-Catalyzed Highly Effective Pictet–Spengler Condensation: An Efficient Synthesis of Tetrahydro-β-carbolines

Molecular iodine was found to be an efficient catalyst for the two-component Pictet–Spengler condensation of tryptamine with aldehydes, which leads to the formation of tetrahydro-β-carbolines in high yields.     

Effect of polarization force on the Jeans instability of self-gravitating dusty plasma

The effect of polarization force acting on massive charged dust grains is investigated analytically on the Jeans instability of self-gravitating dusty plasma. The gravitational force acting on the massive negatively charged interstellar dust grains are considered in presence of both electrical and polarization forces. The basic equations of the problem are formulated and a general dispersion relation is obtained using plane wave approximation in low frequency wave mode. The effect of polarization force in the dispersion relation of the problem, condition of the Jeans instability and expression of the critical Jeans wave number is examined. The unstable growing modes due to self-gravitational force are studied in the situation when polarization force on the dust grain exceeds over the electrical force in magnitude. It is observed that the polarization force increases the growth rate of the system.     

Bent-shaped mesogenic oxadiazoles and thiadiazoles with terminal methyl group

Rod-shaped 4-methylbenzoic acid-N′-(4′-n-alkoxybenzoyl) hydrazide (series I) have been synthesized by the Schotten–Baumann reaction of 4-methylbenzhydrazide with 4-n-alkoxy benzoyl chloride using dry pyridine, as a solvent. The series I compounds have been cyclized to bent-shaped mesogenic 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-oxadiazoles (series II) and 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-thiadiazole (series III) using POCl₃ and Lawesson's reagent, respectively. The synthesized compounds are characterized by the combination of elemental analysis and standard spectroscopic methods. In series II, lower and middle members are non-mesogenic. n-Dodecyloxy to n-hexadecyloxy derivatives exhibit enantiotropic nematic mesophase. In series III, all the compounds synthesized exhibit enantiotropic nematic mesophase. n-Tetradecyloxy and n-hexadecyloxy derivatives also exhibit enantiotropic SmA mesophase. The mesomorphic properties of the series II and III used in this study are compared with each other and with other structurally related compound to evaluate the effect of different heterocyclic moieties as well as terminal substituents on mesomorphism.     

Optoelectronics and formaldehyde sensing properties of tin-doped ZnO thin films

Sn-doped ZnO thin films were deposited on clean glass substrates using the chemical spray pyrolysis technique. XRD analyses confirm stable ZnO hexagonal wurtzite structure of the films with crystallite size in the range of 20–28 nm. The surface roughness of the films increases on Sn doping, which favors to higher adsorption of oxygen species on the film surface, resulting in higher gas response. Optical studies reveal that the band gap decreases on Sn doping. All the films show near band edge emission, and on Sn doping the luminescence peak intensity has been found to increase. Photocurrent in the 1.5 at.% doped film enhances about three times to that observed in the undoped ZnO film. Among all the films examined, the 1.5 at.% Sn-doped film exhibits the maximum response (～94.5 %) at the operating temperature of 275?°C for 100 ppm concentration of formaldehyde, which is much higher than the response (～35 %) in the undoped film. The gas response of the film is attributed to the chemisorption of oxygen on the film surface and the subsequent reaction between the adsorbed oxygen species and the formaldehyde molecules.     

Computing Eigen value equation and modal dispersion characteristics of an elliptical Bragg waveguide

Using the method of separation of variables in the elliptical coordinate system, a recursive formula for the electromagnetic fields in a new type of Bragg waveguide having elliptical core cross section with multilayered cladding is derived. The eigen equation is written in the form of Mathieu and the modified Mathieu functions and a dispersion relation is obtained for various modes supported by the proposed Bragg waveguide. The cutoff frequencies for several lower order even-odd modes have been calculated and their propagation characteristics are plotted. The results show that the dispersion curves are discontinuous and modes can exist only in particular wavelength bands. The effects of elliptical eccentricity on the mode cutoff values and mode transmission are addressed. Finally, the modal birefringence in the said waveguide is also estimated.