Efficient catalytic effects of Lewis acids in the 1,3-dipolar cycloaddition reactions of carbonyl ylides with imines

[reactions: see text] 1,3-Dipolar cycloaddition reactions between imines and carbonyl ylides generated by tandem intramolecular carbenoid-carbonyl cyclizations were found to be effectively catalyzed by Lewis acids (10 mol %). The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)-alpha-diazoacetophenone with imines such as N-[2-(benzyloxy)benzylidene]aniline in the absence of Lewis acid gave no 1,3-dipolar cycloaddition products, but rather the dimeric product of the corresponding carbonyl ylide. In contrast, in the presence of Lewis acids such as Yb(OTf)3, the 1,3-dipolar cycloaddition reactions of the corresponding 1-methoxy-2-benzopyrylium-4-olate proceeded smoothly with several imines, giving in most cases exo-selectivity and no formation of the dimeric product. When Yb(OTf)3 was used as a Lewis acid catalyst, a fundamental catalytic effect was also observed in the cycloaddition reactions of imines with carbonyl ylides generated from 1-diazo-5-phenyl-2,5-pentanedione, 1-diazo-2,5-hexanedione and diazomethyl 2,3,4,5-tetrachloro-6-methoxycarbonylphenly ketone. This efficient catalytic effect can be satisfactorily explained in terms of energetics of the cycloaddition in the absence and the presence of Lewis acid by calculations using the ONIOM (B3LYP/6-31G(d):PM3) method.     

A fully automated amino acid analyzer using NBD-F as a fluorescent derivatization reagent

A fully automated amino acid analyzer using NBD-F (4- fluoro-7-nitro-2,1,3-benzoxadiazole) as a fluorescent derivatization reagent was developed. The whole analytical process was fully automated from derivatization, injection to HPLC separation and quantitation. The derivatization reaction conditions were re-evaluated and optimized. Amino acids were derivatized by NBD-F for 40 min at room temperature in the borate buffer (pH 9.5). The derivatives were separated within 100 min and fluorometrically detected at 540 nm with excitation at 470 nm. The detection limits for amino acids were in the range of 2.8-20 fmol. The calibration curves were linear over the range of 20 fmol to 20 pmol on column with the correlation coefficients of 0.999. The coefficients of variation were less than 5% at 3 pmol injection for all amino acids. Amino acids in rat plasma were determined by the proposed HPLC method.     

Coumarin-4-ylmethoxycarbonyls as phototriggers for alcohols and phenols

Caged compounds can be used to regulate the spatial and temporal dynamics of signaling molecules in live cells. Photochemical properties of coumarin-4-ylmethoxy carbonates (1a-d) are investigated to construct caged compounds of hydroxy-containing molecules. All the compounds possess desired properties as phototriggers for alcohols and phenols. The 6-bromo-7-hydroxycoumarin-4-ylmethoxycarbonyl (Bhcmoc) group has the highest photochemical efficiency and is applied to make caged compounds of 1,2-dioctanoylglycerol (diC(8)), Tyr-OMe, and adenosine. [reaction: see text]     

Acid-base characterization of molecular weight fractionated humic acid

Acid-base properties of molecular weight fractionated humic acids (HAs) were investigated by the acid-base potentiometric titration. The acidic group contents (C(A(t))) and the average values of apparent pK (pK(app)) were evaluated by applying a modified Henderson-Hasselbalch equation to the experimental titration curves. The average values of pK(app) of the fractionated and unfractionated HAs were about 4.1-4.4, and the distribution of pK(app) values could be represented by the relationships between alpha and pK(app) plots in the range 2-8. The C(A(t)) values increased with a decrease in molecular size, as did the aromaticity. This suggests that the acidic group contents are related to the aromaticity of the HA.     

Characterization of silica supported Rh-Fe catalysts by Mössbauer technique

Silica supported Rh–Fe catalysts were characterized by means of in situ⁵⁷Fe Mössbauer spectroscopy. The Mössbauer spectra indicated that iron on the silica support existed either as Fe/O/ in the Rh–Fe alloy or as Fe³⁺ in Rh–Fe metal cluster compounds. The (Fe³⁺/Fe/O/) ratio and Mössbauer parameters were found to depend on the (Fe/Rh) atomic ratio in the catalysts. Such dependence corresponded to the change of catalytic properties of the supported Rh–Fe catalyst with the varying (Fe/Rh) ratio.     

Synthesis of δ-thiolactams by the aza-Diels-Alder reaction of in situ generated allenyltrimethylsilylthioketenes with imines

Allenyltrimethylsilylthioketenes, generated in situ through [3,3] sigmatropic rearrangement of trimethylsilylethynyl propargyl sulfides, underwent facile [4+2] cycloaddition with imines to afford the corresponding δ-thiolactams. The resulting 2-trimethylsilyl-4-methylenetetrahydroquinolidine-2-thione, obtained by the [4+2] cycloaddition using piperideine as a dienophile, was transformed into (±)-lupinine in six steps.     

A successive quadratic programming method for a class of constrained nonsmooth optimization problems

In this paper we present an algorithm for solving nonlinear programming problems where the objective function contains a possibly nonsmooth convex term. The algorithm successively solves direction finding subproblems which are quadratic programming problems constructed by exploiting the special feature of the objective function. An exact penalty function is used to determine a step-size, once a search direction thus obtained is judged to yield a sufficient reduction in the penalty function value. The penalty parameter is adjusted to a suitable value automatically. Under appropriate assumptions, the algorithm is shown to produce an approximate optimal solution to the problem with any desirable accuracy in a finite number of iterations.     

The mechanism and rate of dtμ formation in solid hydrogen

The partial spectral distribution function for muonic molecule formation turns out to play an important role in understanding the effect of interactions with surrounding spectator molecules. We formulate and numerically calculate it for a solid hydrogen. First of all, in addition to a conventional Lorentzian peak centered at a resonance energy , there appears another very broad peak in the spectral distribution, which extends from the resonance energy to energies higher by the Debye energy _D of the solid. It is shown that the latter corresponds to the spectral distribution of phonon excitations caused by a sudden dt formation. Secondly, a strong intensity borrowing from the Lorentzian peak to the broad one occurs. Thanks to this fact, a large formation rate is yielded for subthreshold transitions. The comparison with Vesman's spectral distribution function, namely, of a dt formation for an isolated D₂, is made.     

Transformation of microcrystalline state of hydrogenated silicon to amorphous one due to presence of more electronegative impurities

When a slight fraction (～ 2 mol %) of N₂ is added into H₂, sputtering atmosphere for microcrystalline hydrogenated Si films, without changing other fabrication parameters, the amorphous film rather than microcrystalline one forms. The stabilization mechanism of the amorphous film of Si is discussed through TEM and IR observations of this kind of transformation from microcrystal to amorphous state.