Lewis Acid Catalyzed Ring‐Opening Reactions of Sugar‐Derived Semicyclic N,O‐Acetals

N‐(Alkoxycarbonyl)‐N‐glycosides (polyoxygenated semicyclic N,O‐acetals) were efficiently synthesized from regular acetyl or methyl glycosides (glucopyranoside, ribofuranoside, arabinofuranoside, and 2‐deoxyribofuranoside) and a carbamate by treatment of trimethylsilyl trifluoromethanesulfonate and 4 Å molecular sieves. It was found that these N‐glycosides underwent Lewis acid catalyzed ring‐opening reactions with silylated nucleophiles to give ring‐opened amino alcohols with good‐to‐high diastereoselectivity. The reactivity order, 2‐deoxyribofuranoside > arabinofuranoside > ribofuranoside > glucopyranoside, was revealed. Ring‐opening reductions were also investigated with silanes or diisobutylaluminium hydride. An appropriate reducing agent was found to be dependent on the N‐glycosides used. A glycosidase inhibitor, (2S,3R,4R)‐2‐hydroxymethylpyrrolidine‐3,4‐diol ( 7 ) was synthesized by means of the reactions.     

Ab initio GB study of prebiotic synthesis of purine precursors from aqueous hydrogen cyanide: dimerization reaction of HCN in aqueous solution

Ab initio MO GB theory which includes the continuum model of solvent effect using generalized Born formula has been applied to the dimerization reaction of HCN in aqueous solution which is the starting step in prebiotic synthesis of purine precursors from aqueous hydrogen cyanide. Three steps considered were: (i) the reaction of HCN and H₂O to produce the CN⁻ anion, (ii) the reaction of CN⁻ with HCN to give the NC–CH=N⁻ anion, and (iii) the addition of a proton to the anion to give iminoacetonitrile. The formation of CN⁻ ion from HCN in aqueous solution requires 15.1 kcal/mol (the experimental value estimated from the dissociation constant of HCN in water is 14.8 kcal/mol). The reaction of CN⁻ with HCN requires the activation energy of 32.2 kcal/mol (MP2/6-31++G^**//HF/6-31++G^**) to give the dimer. This barrier height is reduced to 26.1 kcal/mol when HCN is associated with H₃O⁺. In the presence of NH₃ in aqueous solution, CN⁻ is produced easily by the reaction of HCN and NH₃ with a low activation energy of 4.3 kcal/mol. It was shown that the formation of CN⁻ becomes easier in ammoniacal solution, and the dimerization occurs efficiently in aqueous solutions which contain NH₃.     

Thermo- and pH-responsive gels for application in colon delivery systems

Swelling-controlled drug delivery copolymer gels were synthesized by introducing thermo- and pH-responsive methacryloyl-glycine (MA-Gly) of pH-responsive methacrylic acid (MA-Ac) into thermo-responsive acryloyl- -proline ethyl ester (A-ProOEt). The threshold of swelling for copolymer gels consisting of A-ProOEt/MA-Gly and A-ProOEt/MA-Ac with a composition of 40/60 mol% was found to be pH 3.0 and pH 5.5, respectively, in buffer solutions at 37°C. The diffusion characteristics of 2-(3-benzoylphenyl)propionic acid (ketoprofen) from such copolymer gels were evaluated in buffer solutions at pHs more than 5.5, and it was found that A-ProOEt/MA-Gly gel possessed a Case II transport, while the A-ProOEt/MA-Ac gel exhibited a non-Fickian (or anomalous) diffusion behavior under the same conditions.     

On the D-affinity of the flag variety in type B2

The flag varieties in characteristic 0 are well-known to be D-affine. In positive characteristic, however, only those in type A ₁ and A ₂ have been proved to be so. In this paper we will show in type B ₂ the cohomology vanishing of the first term in the p-filtration of the sheaf of differential operators on the flag variety. This is a necessary condition for the variety to be D-affine. Received: 7 February 2000 / Revised version: 30 June 2000     

Generation of coherent terahertz radiation by photomixing of dual-mode lasers

We have reviewed and discussed the generation of continuous-wave terahertz radiation by the photomixing of dual-mode lasers, which includes a multi-mode laser diode (LD), a dual-mode microchip laser, a monolithic dual-mode DBR LD, and a widely tunable dual-mode external cavity LD. It is demonstrated by several experiments that the beat frequency of a dual-mode laser is more stable than the original laser modes because of the common-mode rejection effect. We also suggest a novel scheme by which the optical beat is stabilized to less than 1 kHz and its frequency can be precisely determined as well.     

High-resolution gamma-ray spectroscopy of ΛLi

We report on the first observation of hypernuclear γ transitions using germanium detectors. Using a large-acceptance Ge detector system, we observed two γ transitions in _Λ⁷Li, the spin-flip transition at 689±4keV and the transition at 2050±2 keV (preliminary). The strength of the ΛN spin-spin interaction is derived from the energy of the former transition. As for the latter transition, B(E2) was measured to be 3.9±0.6±0.4 e²fm⁴ (preliminary), which indicates a shrinkage of the nuclear size of _Λ⁷Li from ⁶Li and confirms “glue-like role” of Λ.     

On non-orientable surfaces in 4-space which are projected with at most one triple point

We show that if a non-orientable surface embedded in 4-space has a projection into 3-space with at most one triple point, then it is ambient isotopic to a connected sum of some unknotted projective planes and an embedded surface in 4-space with vanishing normal Euler number.

     

Solvent Effects on Ion-Pair Distribution and Dimerization of Tetraalkylammonium Salts

Summary.  The distribution of tetraalkylammonium ions (C _n H _2n+1 )₄N⁺ (R ⁺, TAAn ⁺, n = 4–7) with picrate ion (pic ⁻) and inorganic anions X ⁻, (Cl⁻, Br⁻, ClO⁻ ₄), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R ⁺ · X ⁻, and the dimerization of the ion pairs, (R ⁺ · X ⁻)₂, in the organic phase. The ion-pair, distribution constant, K _dist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of K _dist show a good correlation with the E _T value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing E _T. The effect of the solvent on the dimerization constants, K _dim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, K _dim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of K _dim decreases with decreasing E _T and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs. Received May 15, 2001. Accepted (revised) July 18, 2001     

X-Ray Diffraction and Raman Spectroscopic Study of Bis-(Pyridine Base) Complexes of Cadmium(II) Halogenides

Summary.  The molecular structures of bis-(pyridine base) complexes of cadmium(II) chloride and bromide, where the pyridine base is pyridine ( py), 3-methylpyridine (3-Me-py), 4-methylpyridine (4-Me-py), and 4-ethylpyridine (4-Et-py), were investigated by means of single-crystal X-ray diffraction and Raman spectroscopy. The crystal structures of CdCl₂ py ₂ (1), CdCl₂(3-Me-py)₂ (2), and CdCl₂(4-Me-py)₂ (3) were determined. All crystals are monoclinic; 1: a = 17.784(2), b = 8.666(1), c = 3.8252(7) ?, β = 91.54(1)°, space group: P2₁/n; 2: a = 11.89(1), b = 14.41(1), c = 3.874(6) ?, β = 92.3(1)°, space group: P2₁/a; 3: a = 21.091(2), b = 3.8884(5), c = 18.2317(3) ?, β = 113.64(1)°, space group: C2/c. The structures were refined to R/R _w values (%) of 3.2/5.5, 3.0/5.0, and 3.4/5.1 for 1–3. All cadmium atoms are octahedrally coordinated with the chloride ions forming infinite di-μ-chloro polymeric linear chains and the nitrogen atoms of the pyridine base in trans configuration. The Cd chains are oriented along the c-axis in 1 and 2 and along the b-axis in 3. The crystal structures indicate the absence of a peculiar interaction between the polymeric chains. The Raman spectra of eight complexes were measured in the range of 550–50 cm⁻¹, and the Raman peaks originating from cadmium-halogen vibrations were assigned. The Raman spectra of 1 and 2 are quite alike in the lattice mode vibration region. The resemblance of the cadmium-halogen vibration peaks indicates the same halogenide ion bridged octahedral structure for all complexes. Received March 27, 2001. Accepted (revised) June 19, 2001