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51.
Teresa M.V.D. Pinho e Melo Catarina I.A. SantosAntónio M.d'A. Rocha Gonsalves José A. PaixãoAna M. Beja Manuela Ramos Silva 《Tetrahedron letters》2003,44(45):8285-8287
Novel 3-methylene-2,5-dioxo-3H,9bH-oxazolo[2,3-a]isoindoles were prepared from the thermolysis of (3R,9bS)-5-oxo-2,3,5,9b-tetrahydrothiazolo[2,3-a]isoindole-3-carboxylic acids in acetic anhydride. The structure of 3-methylene-2,5-dioxo-3H,9bH-oxazolo[2,3-a]isoindole 6a was determined by X-ray crystallography. 相似文献
52.
Pinho e Melo TM Soares MI Rocha Gonsalves AM Paixão JA Beja AM Silva MR Alte da Veiga L Pessoa JC 《The Journal of organic chemistry》2002,67(12):4045-4054
Intramolecular dipolar cycloaddition of bicyclic münchnones, 5H,7H-thiazolo[3,4-c]oxazol-4-ium-1-olates, derived from cyclodehydration of 2-substituted-N-acylthiazolidine-4-carboxylic acids are reported. A range of new pyrrolo[1,2-c]thiazole derivatives (7, 14, 15, 20, 23, and 26) were obtained as single enantiomers from 2-phenylthiazolidines, 2-benzoylthiazolidines, and 2-methylthiazolidine-4-carboxylates. Pyrrolo[1,2-c][1,4]thiazine derivative 27 was also obtained from pyrrolo[1,2-c]thiazole derivative 26. The structures of methyl (2R,4R)-2-(p-methoxybenzoyl)thiazolidine-4-carboxylate (17a), methyl (2R,4R)-2-(p-methoxybenzoyl)-N-(prop-2-ynyloxyacetyl)thiazolidine- 4-carboxylate (18), and 3-oxo-4-phenyl-3,4,6,8-tetrahydro-1H-furo[3',4':2,3]pyrrolo[1,2-c][1,4]thiazine (27) were determined by X-ray crystallography. Chirooptical studies of the pyrrolo[1,2-c]thiazoles were done by confirming the absolute configuration at the chiral center C-3. 相似文献
53.
Teresa M.V.D. Pinho e Melo Ana Lúcia Cardoso Jesús M. de los Santos Paulo E. Abreu Ana Matos Beja 《Tetrahedron》2008,64(35):8141-8148
A highly selective two-step approach to chiral β-amino esters via the hydride reductive amination of chiral allenes is reported. β-Enamino esters were obtained from the nucleophilic addition of amines to 2,3-allenoates bearing a chiral auxiliary. The reduction of the (1R)-(−)-10-phenylsulfonylisobornyl β-enamino esters gave the corresponding β-amino esters with S configuration whereas the reduction of the (1S)-(+)-10-phenylsulfonylisobornyl β-enamino esters led to β-amino esters with R configuration. The rationalization of the observed selectivity was supported by semi-empirical molecular orbital calculations (PM3). 相似文献
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This paper discusses the estimation of discretization errors on the basis of power series expansions for grid sets that are not geometrically similar, that is, grids not exhibiting a constant grid refinement ratio for the entire computational domain. Simple test cases with structured and unstructured grids are used to demonstrate that reliable error estimates on the basis of power series expansions can be made if the grids are refined systematically. However, if the grid refinement ratio is not constant in the complete domain, the definition of the typical cell size is not obvious, and the observed order of accuracy may not be equal to the expected theoretical order of the discretization. Some alternatives for the definition of the typical cell size are tested. In these tests, the error estimation does not show a significant effect of the definition of the typical cell size even for some cases with data sets clearly outside the ‘asymptotic range’. For non‐geometrically similar grids, the best estimates of the observed order of accuracy are obtained with the typical cell size on the basis of the mode of the cell size (the cell size that occurs more often in a given grid). Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
58.
M. Ramos Silva J. A. Paixo A. Matos Beja L. Alte da Veiga 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(4):421-422
The title compound, C6H13NO5, adopts a zwitterionic form where the carboxylic acid H atom is transferred to the amino group. The methyl–glycine backbone is planar. The tris(hydroxymethyl)methyl group is rotated as a rigid group around the amino–methyl bond by 22 (1)° and the carboxylic acid plane is rotated by 19.76 (12)° from the plane of the main skeleton. Apart from their H atoms, the three hydroxymethyl groups adopt a propeller‐like conformation around the amino–methyl bond, close to C3 symmetry. 相似文献
59.
Manuela Ramos Silva Jose A. Paixo Ana Matos Beja Luiz Alte da Veiga 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(7):838-840
The title compound, C3H7NO2·C3H8NO2+·NO3?, contains l ‐alanine–alaninium dimers bonded via the carboxyl groups by a strong asymmetric hydrogen bond with an O?O distance of 2.4547 (19) Å. The neutral alanine molecule exists as a zwitterion, where the carboxyl group is dissociated and the amino group is protonated. The alaninium cation has both groups in their acidic form. The alanine molecule and the alaninium cation differ only slightly in their conformation, having an N—Cα—C=O torsion angle close to ?25°. The dimers and the nitrate anion are joined through a three‐dimensional hydrogen‐bond network, in which the full hydrogen‐bonding capabilities of the amino groups of the two alanine moieties are realised. 相似文献
60.
Ochoa Marta Durães Luísa Beja Ana Matos Portugal António 《Journal of Sol-Gel Science and Technology》2012,61(1):151-160
Silica based materials were synthesized using ethyltrimethoxysilane (ETMS) and/or methyltrimethoxysilane (MTMS) precursors
by sol–gel technology, in order to ascertain if their properties are suitable for aerospace applications. When ETMS was used
alone and in equimolar ETMS/MTMS mixtures, no gel formation took place and a resin-like precipitate was observed. After drying,
a compact tablet was formed. When mixtures of 25% ETMS/75% MTMS and MTMS alone were used, gel formation occurred and xerogels
were produced upon drying. Chemical and structural characterization of the obtained materials was performed using Elemental
Analysis, FTIR, XRD and SEM. Bulk density, specific surface area, contact angle and the thermal behavior were also evaluated.
For materials from ETMS, the chemical structure grows preferentially in one direction and, in the case of MTMS the growth
follows a 3D pattern. The use of ETMS precursor leads to a significant increase in the product density, accompanied by a decrease
in the specific surface area. It also leads to a decrease in the thermal stability limit of the synthesized materials. Then,
ETMS precursor is less appropriate than MTMS precursor for space applications. However, ETMS co-precursor in mixtures with
MTMS contributes to the increase in the hydrophobic character of the synthesized materials. 相似文献