Aldol reactions of 2-thioxotetrahydropyrimidin-4(1H)-ones: stereoregulations from endo- and exocyclic chiral centres

The steric regulations imparted by the substituent at N1 in lithium mediated asymmetric aldol reactions of conformationally restricted 3-aryl-1-((S)-1-phenylethyl)-2-thioxotetrahydropyrimidin-4(1H)-ones governed the formation of anti aldol adducts, by a kinetic reaction pathway. The preferential formation of the anti aldol diastereomers was also assisted by the steric effects of the electrophile through diastereofacial selection while the electronic effects of the aryl group at N3 remained subtle. Incorporation of an endocyclic methyl group at C6 witnessed the diastereoselective formation of an anti aldol adduct by regulation of π-facial selectivity. The absolute configurations of the aldol adducts were determined by computational calculations and NMR experiments, and confirmed by single crystal X-ray analysis.     

LC Enantioseparation of 30-Component Diastereomeric Mixture of Amino Acids and Detection of d-Isomers Using New Reagents with Amines as Chiral Auxiliaries in Cyanuric Chloride

Two enantiomerically pure amines, viz., (R)-(+)-naphthylethyl amine and (S)-(+)-1-benzyl-3-aminopyrrolidine, were used as chiral auxiliaries for nucleophilic substitution of chlorine atoms in cyanuric chloride or its 6-butoxy derivative. The chiral derivatizing reagents so obtained were characterized and their chiral purity was ascertained. Diastereomers of 15 dl-proteinogenic amino acids were synthesized under microwave irradiation using these reagents. Separation of diastereomeric pairs along with separation of a mixture of 30 diastereomers in a single chromatographic run was carried out on a reversed-phase C₁₈ column. Mixtures of acetonitrile with aqueous trifluoroacetic acid were used as mobile phase. The detection was made at 230 nm using photo diode array detector. The separation behavior in terms of retention times and resolutions was compared on the basis of effect of chiral auxiliaries (i.e. amines) and achiral substituents (i.e. chlorine or butoxy group) in the chiral derivatizing reagents and the hydrophobic side chains of amino acids. The separation method was validated in terms of accuracy, precision, linearity, recovery, limit of detection and limit of quantitation. The method was successful for determination of d-amino acids in the absence of pure d-enantiomers and for separation of 19 diastereomers from a mixture of 30.     

New three-component condensation reaction: synthesis of 1-[(alkylthio)(phenyl)methyl]-naphthalene-2-ol catalyzed by bromodimethylsulfonium bromide (BDMS)

Bromodimethylsulfonium bromide acts as an efficient catalyst for one pot three component condensation reactions of aldehydes, 2-naphthol, and thiols in acetonitrile at room temperature. Various aliphatic and aromatic thiols undergo conjugate addition with in situ generated enone in acetonitrile and provide good yields. The main features of this procedure are mild reaction conditions, good yields, and operational simplicity.     

Direct sulfonylation of Baylis-Hillman alcohols and diarylmethanols with TosMIC in ionic liquid-[Hmim]HSO4: an unexpected reaction

A Brønsted acidic ionic liquid-[Hmim]HSO₄ promoted unexpected reaction of Baylis-Hillman alcohols and diarylmethanols with p-toluenesulfonylmethyl isocyanide (TosMIC) affording the corresponding sulfone derivatives instead of N-tosylmethyl amides is reported. After isolation of the product, the ionic liquid [Hmim]HSO₄ was easily recycled for further use.     

CuBr(PPh3)3] for azide-alkyne cycloaddition reactions under strict Click conditions

A careful methodological study revealed a true Click catalytic system based on commercially available [CuBr(PPh(3))(3)]. This system is active at room temperature, with 0.5 mol % [Cu] (or less), in the absence of any additive, and it does not require any purification step to isolate pure triazoles.     

Thermal—fluid modelling of an air—cooled condenser for refrigerants

A steady state thermodynamic model for an air—cooled finned—tube condenser, used in a typical deep freezer vapour compression system, operating with pure and refrigerant mixtures has been developed using finite difference method. The heat transfer aspects in the condenser are treated uniquely for superheated, two—phase and subcooled regimes and studied for various heat load, pressure, temperature, and mass flow rate. The condensation heat transfer coefficient, the tube length required for condensation, the degree of subcooling, and the temperature glide for different ambient temperatures are estimated. The simulation and the experimental results are in good agreement.     

A new method for estimating the critical current density of a superconductor from its hysteresis loop

The critical current density J_c of some of the superconducting samples, calculated on the basis of the Bean’s model, shows negative curvature for low magnetic field with a downward bending near H = 0. To avoid this problem Kim’s expression of the critical current density, J_c = k/(H₀ + H), where J_c has positive curvature for all H, has been employed by connecting the positive constants k and H₀ with the features of the hysteresis loop of a superconductor. A relation between the full penetration field H_p and the magnetic field H_min, at which the magnetization is minimum, is obtained from the Kim’s theory. Taking the value of J_c at H = H_p according to the actual loop width, as in the Bean’s theory, and at H = 0 according to an enhanced loop width due to the local internal field, values of k and H₀ are obtained in terms of the magnetization values M⁺(?H_min), M^?(H_min), M⁺(H_p) and M^?(H_p). The resulting method of estimating J_c from the hysteresis loop turns out to be as simple as the Bean’s method.     

Synthesis and characterization of 4‐vinyl pyridine‐grafted Teflon‐PFA film for water technologies

To get highly reactive polymeric materials for selective studies of enzyme immobilization, water purification, separation, and enrichment technologies, we attempted to graft 4‐vinyl pyridine (4‐VP) onto Teflon‐PFA by simultaneous γ‐ray initiation. The resulting graft copolymers were quaternized by treatment with some alkylating agents. Optimum conditions for grafting were evaluated through the variation of the total dose of radiation, the amount of water, and the monomer concentration. The effect of the solvent composition (H₂O/MeOH) was also studied. In the presence of MeOH, grafting occurred less often and was nonselective as 4‐VP was incorporated on both sides in comparison with highly selective grafting in an aqueous medium. The percentages of the grafting, total conversion, and grafting efficiency and the rates of the polymerization, grafting, and homopolymerization were also evaluated. Some other monomers such as methacrylic acid, methyl methacrylate, maleic acid, acrylonitrile, and vinyl imidazole were not incorporated onto the backbone film under the optimum grafting reaction conditions evaluated for the grafting of 4‐VP. Although some grafting occurred, the graft yield was too low to be considered of any significance. The grafted films were quaternized with benzyl chloride, and quaternized and unquaternized films were used for the immobilization of lipase. The former showed high activity with lipase and has potential for development as a bioreactor. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4506–4518, 2000