1,348 research outputs found
Diels-alder cycloaddition in the synthesis of 1-azafagomine, analogs, and derivatives as glycosidase inhibitors
This comprehensive review deals with the synthesis of 1-azafagomine, analogs, and derivatives having the Diels-Alder cycloaddition as the key step. Most of the compounds referred are racemic or have been resolved by lipase transesterification. There are two asymmetric cycloadditions leading to 1-azafagomine or to an analog. In one case both enantiomers of 1-azafagomine were prepared together with a pair of derivatives. The study comprises glycosidase inhibition studies of the target compounds to a set of glycosidic enzymes, and evidenced molecular features that enhance or diminish their activity as glycosidase inhibitors
Statistical Mechanics of Two-dimensional Foams
The methods of statistical mechanics are applied to two-dimensional foams
under macroscopic agitation. A new variable -- the total cell curvature -- is
introduced, which plays the role of energy in conventional statistical
thermodynamics. The probability distribution of the number of sides for a cell
of given area is derived. This expression allows to correlate the distribution
of sides ("topological disorder") to the distribution of sizes ("geometrical
disorder") in a foam. The model predictions agree well with available
experimental data
Oxide thermoelectrics prepared by laser melting: effects of processing atmosphere
Traditional thermoelectric materials suffer from two main problems, the toxicity/scarcity of
the elements used and their stability at high temperatures or in non-inert atmospheres.
Thermoelectric oxides appear to be a promising alternative to traditional materials due to
natural abundance of the constituents and high thermal stability. This work focuses on the
processing of these materials using the Laser Floating Zone (LFZ) technique. Particular
emphasis is given to LFZ processing under various redox atmospheres, allowing unique
opportunities for tuning the structural, microstructural and thermoelectric properties. This
technique allows the growth of fully dense fibres, as well as the formation of metastable
phases and/or promoting different oxidation states by adjusting the growth conditions. Here
we report the processing of model manganite- and titanate-based materials including donorsubstituted Ca(Pr)MnO3 and Ti(Ta)O2 systems. XRD/SEM/EDS studies demonstrate some
guidelines for tuning the phase composition and microstructure by adjusting the growth rate
under different redox conditions. We report high power factor values of 303 μWm-1K
-2 at
1120 K for the Ca(Pr)MnO3 system and 317 μWm-1K
-2 for the Ti(Ta)O2 system. The obtained
results suggest that LFZ is a suitable technique for processing thermoelectric oxides if
optimized control over growth parameters and re-equilibration conditions is imposed.publishe
Diastereo-selectivity in diels–alder cycloadditions of erythrose benzylidene-acetal 1,3-Butadienes with maleimides
Maleimides were combined with D-erythrose benzylidene-acetal 1,3-butadienes 1 and 2 to study the facial selectivity of the Diels-Alder cycloadditions. The selectivity was found to be from moderate to good. The reaction diastereotopicity can be reversed with the temperature. Simultaneous coordination of diene 1, having a free hydroxyl group, and maleimide 3 to a chiral bimetallic Lewis acid catalyst (LACASA-DA reaction) occurs with complete diastereo-control to give a single adduct, using an extra chiral inductor either R- or S-BINOL.Thanks are due to FCT for project funding PTDC/QUI/67407/2006 and FCT and FEDER for funding the NMR spectrometer Bruker Avance III 400 as part of the National NMR Network. V. C. M. D. also thanks for PhD grant (SFRH/BD/61290/2009)
The synthesis of an homochiral aziridine as a useful synton to iminosugars and amino acid compounds
Fundação para a Ciência e a Tecnologia (FCT) - PTDC/QUI/67407/200
Dimensional crossover of a boson gas in multilayers
We obtain the thermodynamic properties for a non-interacting Bose gas
constrained on multilayers modeled by a periodic Kronig-Penney delta potential
in one direction and allowed to be free in the other two directions. We report
Bose-Einstein condensation (BEC) critical temperatures, chemical potential,
internal energy, specific heat, and entropy for different values of a
dimensionless impenetrability between layers. The BEC critical
temperature coincides with the ideal gas BEC critical temperature
when and rapidly goes to zero as increases to infinity for
any finite interlayer separation. The specific heat \textit{vs} for
finite and plane separation exhibits one minimum and one or two maxima
in addition to the BEC, for temperatures larger than which highlights
the effects due to particle confinement. Then we discuss a distinctive
dimensional crossover of the system through the specific heat behavior driven
by the magnitude of . For the crossover is revealed by the change
in the slope of and when , it is evidenced by a broad
minimum in .Comment: Ten pages, nine figure
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