Synthesis of chiral mesoporous silicas with oligo(saccharide) surfaces and their use in separation of stereoisomers

Novel chiral mesoporous silicas (SBA-15 motif) with chemically bonded oligo(saccharides) (1, 3, and 7 glucose units) were obtained through the cocondensation of organosilicon derivatives of the oligo(saccharides) and silica precursors in the presence of polymer surfactant template under mild acidic conditions. The pore order and structure of the materials prepared were characterized by transmission electron microscopy and nitrogen adsorption. The direct application of the oligo(saccharide)-grafted SBA-15 stationary phases in the HPLC separations of stereoisomers was demonstrated for the first time.     

Povarov reactions of exo-glycals: preparation of C-linked, quinoline analogues

A one-pot approach based upon the Povarov reaction has been efficiently employed with a number of exo-glycals and para-substituted benzanilines to synthesize novel open-ring, carbohydrate-derived quinolines. The mechanism of this reaction was studied and an explanation for the observed stereoselectivity is proposed. Treatment of the compounds with the Lewis acid, boron trichloride, successfully removes the benzyl ether protecting groups in good yields. Several of the prepared compounds have been screened in the National Cancer Institute’s (NCI’s) 60 cell line model. Moderate activity was observed for several leukemia cell lines.     

Silica Adsorbents with Well-Defined Peracetylated Thiosaccharide Surfaces: Synthesis and Use for Stereoselective Separations in RP and NP HPLC

New chiral silica adsorbents with well-defined peracetylated thiosaccharide (a.k.a. thiosaccharide) surfaces containing 1, 3, 7, and 13 glucose units have been synthesized through the reaction of trimethoxysilyl-derivatives of thiosaccharides with bare silica gel. As determined by FTIR, nitrogen adsorption, thermo-gravimetric analysis, and chemical analysis, the adsorbents prepared contained closely packed uniform monolayer surfaces of thiosaccharide groups grafted to silica. The chromatographic behavior of the thiosaccharide-silicas prepared has been evaluated in the HPLC separations of stereoisomers in both normal and reversed phase modes. With the exception of low grafted thiomaltodecatriose-silica, there was no effect of the size of thiosaccharide on the selectivity of the separations.     

Silica Adsorbents with Well-Defined Peracetylated Thiosaccharide Surfaces: Synthesis and Use for Stereoselective Separations in RP and NP HPLC

New chiral silica adsorbents with well-defined peracetylated thiosaccharide (a.k.a. thiosaccharide) surfaces containing 1, 3, 7, and 13 glucose units have been synthesized through the reaction of trimethoxysilyl-derivatives of thiosaccharides with bare silica gel. As determined by FTIR, nitrogen adsorption, thermo-gravimetric analysis, and chemical analysis, the adsorbents prepared contained closely packed uniform monolayer surfaces of thiosaccharide groups grafted to silica. The chromatographic behavior of the thiosaccharide-silicas prepared has been evaluated in the HPLC separations of stereoisomers in both normal and reversed phase modes. With the exception of low grafted thiomaltodecatriose-silica, there was no effect of the size of thiosaccharide on the selectivity of the separations.

     

Experimental study of the boundary layer over an airfoil in plunging motion

This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions.It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer.The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake.The experiments were conducted at Reynolds numbers of 0.42×10 6 to 0.84 × 10 6 and the reduced frequency was varied from 0.01 to 0.11.The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases.For the static tests,boundary layer transition occurred through a laminar separation bubble.By increasing the angle of attack,disturbances and the transition location moved toward the leading edge.For the dynamic tests,earlier transition occurred with increasing rather than decreasing effective angle of attack.The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer.By increasing the reduced frequency,the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack,but the quasi skin friction coefficient was decreased.     

Why are silyl ethers conformationally different from alkyl ethers? Chair-chair conformational equilibria in silyloxycyclohexanes and their dependence on the substituents on silicon. The wider roles of eclipsing, of 1,3-repulsive steric interactions, and of attractive steric interactions

An NMR study of the diaxial/diequatorial chair equilibrium in a range of silylated derivatives of trans-1,4- and trans-1,2-dihydroxycyclohexane is reported and discussed with a view to explaining unusually large populations of chair conformations with axial substituents, noted previously for some monosilyloxycyclohexanes and in some silylated sugars. X-ray diffraction studies of three bis-triphenylsilyloxycyclohexanes are reported and show both axial and equatorial silyloxy groups with the exocyclic bonds eclipsed. Eclipsing is also suggested by molecular mechanics (MM3) calculations on such derivatives. Both axial and equatorial tertiary silyl groups have 1,3-repulsive interactions with whatever substituents or hydrogen atoms are at the two adjacent equatorial positions, and these are relieved by rotation toward the eclipsed conformation of the exocyclic C-O bond. The three substituents on silicon interact attractively with the nine atoms at the 3, 4, and 5-positions of the cyclohexane ring and calculations suggest that these stabilizing interactions are significantly greater in the axial than in the equatorial conformation. An equatorial C-OSiR(3) bond with one or two equatorial neighbors has a restricted potential energy well that becomes much broader when the bond is axial without any equatorial neighbors in the alternative chair. Adjacent silyl groups in the 1,2-disubstituted series interact in a stabilizing way overall in all conformations, this being particularly marked in the diaxial conformation of the more complex ethers. These factors lead to unusually large axial populations.     

On-resin generation and reactions of orthogonal glycosyl donors

An orthogonal series of glycosyl donors has been generated in situ from thiophenylglycosides appended to a hydroxymethylpolystyrene resin through a succinate linker. The conditions used to generate these donors and their subsequent reactions with sugar acceptors will be described.