Diastereoselective Synthesis of and Mechanistic Understanding for the Formation of 2‐Piperidinones from Imines and Cyano‐Substituted Anhydrides

2‐Piperidinones are synthesized in a single step from imines and 2‐cyano glutaric anhydrides. The reaction provides the products in good diastereoselectivity and generates a quaternary stereogenic center. Substitutions on the anhydride skeleton are well tolerated to provide 2‐piperidinones with three stereogenic centers from a single transformation. The pertinent transition structures have also been computed using quantum mechanics and reveal the key interactions controlling the stereochemical outcome of the reaction.     

Natural antifungal compounds from the peels of Ipomoea batatas Lam

Three antifungal compounds have been isolated for the first time from the peels of Ipomoea batatas Lam. Their structures were established on the basis of 1D and 2D NMR spectra data as well as ESI-MS and IR analysis. Urs-13(18)-ene-3β-yl acetate was found to possess a weak activity against Sporothrix schenckii and Trichophyton metagrophytes fungi with an MIC value of 50 μg/mL each. Stigmasterol and 3-friedelanol were equally active against T. metagrophytes.     

The Substituent Effect of 1‐Arylazonaphthen‐2‐ols on Azo‐hydrazone Tautomerization According to NMR Analysis

The substituent effect on azo‐hydrazone tautomerization of 1‐arylazonaphthen‐ols is studied by means of NMR analysis. Among the ¹³C chemical shifts, the C(2) of this series compound is the most sensitive to the variation in the nature of substituent on the phenyl ring. Therefore, the variation in the chemical shifts of C(2) is used to probe the substituent effect by using the substituent chemical shifts and free energy vs. Hammett’s constant (χρ⁺). Both methods give a negative correlation slope, indicating the electron‐with‐ drawing groups favor the hydrazone tautomer form. The effect on the chemical shifts of C(2) of compound 8 in ten solvents can be classified as the solvent with a proton‐donor, proton‐acceptor and arenes system. The substituent with electron‐donating character is more sensitive to the nature of solvent and it favors the hydrazone form. Free energy obtained from the dynamic NMR technique indicates the tautomerization favors the hydrazone‐form for the substituent with electron‐withdrawing character.     

The Isothiocyanato Moiety: An Ideal Protecting Group for the Stereoselective Synthesis of Sialic Acid Glycosides and Subsequent Diversification

The preparation of a crystalline, peracetyl adamantanyl thiosialoside donor protected by an isothiocyanate group is described. On activation at ?78 °C in the presence of typical carbohydrate acceptors, this donor gives high yields of the corresponding sialosides with exquisite α‐selectivity. The high selectivity extends to the 4‐O‐benzyl‐protected 3‐OH acceptors, which are typically less reactive and selective than galactose 3,4‐diols. Treatment of the α‐sialosides with tris(trimethylsilyl)silane or allyltris(trimethylsilyl)silane results in replacement of the C5? N5 bond by a C? H or a C? C bond. The reaction of the isothiocyanate‐protected sialosides with thioacids generates amides, while reaction with an amine gives a thiourea, which can be converted into a guanidine. The very high α‐selectivities observed with the new donor and the rich chemistry of the isothiocyante function considerably extend the scope for optimization at the sialoside 5‐position.     

Visual observation of development of sharkskin melt fracture in polybutadiene extrusion

We studied the sharkskin melt fracture phenomena of polybutadiene (PBD), which exhibits similar flow properties and instabilities to linear low density polyethylene (LLDPE). The advantages of using PBD are that it exhibits larger distortions and slower development due to its high viscosity. By using a video camera with close-up lenses focused at the die exit, we observed the development of sharkskin in profile. It was shown that the sharkskin melt fracture develops with the cohesive failure at the die exit due to a peeling of surface layers as described by Howell and Benbow (1962). Oddly enough, this mechanism is similar to that proposed by Ovaici et al. (1998) for the extrusion of chocolate. A soap solution coating around the die exit to induce a slippery interface eliminated the sharkskin fracture, similar in the effect of fluoro-elastomer coating with LLDPE. Based on our visual observations, we qualitatively modeled the sharkskin fracture by modifying Cogswell‘s (1977) idea on exit stretching at the die exit and the force balance of Ovaici et al. on the ring formation of the chocolate extrusion. Received: 22 May 1998 Accepted: 31 August 1998     

Preparation,characterization, and structural analysis of d8 palladium and platinum compounds containing amino acid ester derivatized diimine ligands. Observation of liquid crystal behavior

Eight new compounds, M(pyca(CH₂)_xCOOR)Cl₂, M = Pd, Pt; R = Me, Et; x = 2, 3, 5, 11, were prepared. The resulting new complexes were characterized by ¹H and ¹³C NMR spectroscopy and each was also structurally elucidated by X-ray crystallography. The compounds share general structural features, but there are differences in the alignment of the alkyl chain; as the chain lengthens, the chain straightens relative to the plane of the metal complex. For the dodecanoic ester derivatives, a nearly linear alkyl chain was observed. These longer alkyl derivatives show mesogenic behavior.     

Synthesis of hollow silicon nanospheres encapsulated with a carbon shell through sol–gel coating of polystyrene nanoparticles

Journal of Sol-Gel Science and Technology - In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell (HSi@C) has been investigated. The key synthesis...     

Elastic moduli of nanocrystalline binary Al alloys with Fe,Co, Ti,Mg and Pb alloying elements

The paper studies the elastic moduli of nanocrystalline (NC) Al and NC binary Al–X alloys (X is Fe, Co, Ti, Mg or Pb) by using molecular dynamics simulations. X atoms in the alloys are either segregated to grain boundaries (GBs) or distributed randomly as in disordered solid solution. At 0 K, the rigidity of the alloys increases with decrease in atomic radii of the alloying elements. An addition of Fe, Co or Ti to the NC Al leads to increase in the Young’s E and shear μ moduli, while an alloying with Pb decreases them. The elastic moduli of the alloys depend on a distribution of the alloying elements. The alloys with the random distribution of Fe or Ti demonstrate larger E and μ than those for the corresponding alloys with GB segregations, while the rigidity of the Al–Co alloy is higher for the case of the GB segregations. The moduli E and μ for polycrystalline aggregates of Al and Al–X alloys with randomly distributed X atoms are estimated based on the elastic constants of corresponding single-crystals according to the Voigt-Reuss-Hill approximation, which neglects the contribution of GBs to the rigidity. The results show that GBs in NC materials noticeably reduce their rigidity. Furthermore, the temperature dependence of μ for the NC Al–X alloys is analyzed. Only the Al–Co alloy with GB segregations shows the decrease in μ to the lowest extent in the temperature range of 0–600 K in comparison with the NC pure Al.     

Natural convection in partitioned enclosed spaces of solar collector

The two-dimensional, steady, laminar natural convection phenomena in partitioned enclosure of solar collector has been studied numerically. Heat conduction along the partition is considered. An iterative finite-difference scheme is employed to solve the governing equations in the flow field. The effects of Rayleigh number, thermal conductivity ratio, partition angle, tilt angle, and aspect ratio on both the local and average heat transfer coefficients of the solar collector have been discussed. The range of Rayleigh number tested was up to 5 × 10⁴, the thermal conductivity ratio of 4.5 and 30, partition angle from 10 deg to 170 deg, tilt angle from 10 deg to 90 deg, and aspect ratio varied between 0.2 and 10. The results indicate that the convective heat transfer is strongly affected with the aspect ratio of the enclosures.

---

Freie Konvektion in unterteilten Kammern von Solarkollektoren

Zusammenfassung Die zweidimensionale, stetige, laminare freie Konvektion in unterteilten Kammern von Solarkollektoren wurde numerisch untersucht. Die Wärmeübertragung entlang dieser Kammern wurde betrachtet. Ein iteratives Finite-Differenzen-Schema wurde angewandt um die Gleichungen, welche das Strömungsfeld beschreiben, zu lösen. Der Einfluß der Rayleigh-Zahl, der thermische Leitfähigkeit, des Kammerwinkels, des Neigungswinkels und der Längenverhältnisse auf die örtlichen und durchschnittlichen Wärmeübertragungskoeffizienten des Solarkollektors wurde diskutiert. Der Bereich der Rayleigh-Zahl erstreckte sich bis zu 5 × 10⁴, das Verhältnis der thermischen Leitfähigkeit betrug 4.5 und 30, der Kammerwinkel lag zwischen 10° und 170°, der Neigungswinkel zwischen 10° und 90° und das Längenverhältnis variierte zwischen 0.2 und 10. Die Ergebnisse beinhalten, daß die konvektive Wärmeübertragung sehr stark durch das Längenverhältnis der Kammern beeinflußt wird.

---

Nomenclature a slope of the partition with respect to the horizontal - A H/L=cell aspect ratio - A _w t/L=wall aspect ratio - g acceleration due to gravity - h local heat transfer coefficient - average heat transfer coefficient - H cell length - k thermal conductivity of fluid within the cell - k _w thermal conductivity of the cell wall - L plate spacing - Nu _f h L/k=local cell Nusselt number - L/k=average cell Nusselt number - overall average Nusselt number - qL/k _w t(T _h–T _c)=average wall Nusselt number - Pr /=Prandtl number - q heat transfer in the cell wall from the hot to cold plate per unit depth - Ra g L ³ T/=Rayleigh number - R _k k _w/k=ratio of wall thermal conductivity to that of the fluid - t thickness of cell wall - T _c cold plate temperature - T _f temperature in cell - T _h hot plate temperature - T _w temperature in cell wall - u, U dimension and dimensionless velocities inx-direction - v, V dimension and dimensionless velocities iny-direction - x distance measured from the bottom of the cell (Fig. 1) - X x/L=normalized distance ofx - y distance measured from hot plate (Fig. 1) - Y y/L=normalized distance ofy - x ₁ distance measured in wall (Fig. 1) - X ₁ x/L=normalized distance ofx ₁ Greek symbols thermal diffusivity of fluid - coefficient of volumetric expansion of fluid - partition angle with respect to the hot plate - _f T _f–T _c/T _h–T _c=dimensionless temperature in cell - _w T _w–T _c/T _h–T _c=dimensionless temperature in cell wall - kinematic viscosity of fluid - enclosure tilt angle from horizontal - dimensional vorticity - L ²/=dimensionless vorticity - dimensionless streamline