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排序方式: 共有112条查询结果,搜索用时 15 毫秒
1.
Garcia Ruano JL Alcudia A del Prado M Barros D Maestro MC Fernandez I 《The Journal of organic chemistry》2000,65(10):2856-2862
The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported. 相似文献
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Garcia Ruano JL Alemparte C Martin Castro AM Adams H Rodriguez Ramos JH 《The Journal of organic chemistry》2000,65(23):7938-7943
The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles. 相似文献
5.
The synthesis of densely functionalized pyrrolo[2,1-a]isoquinolines was accomplished using the organocatalytic [3+2] cycloaddition between enals and azomethine ylides under iminium catalysis developed in our group some years ago as the key step. The cycloaddition proceeds smoothly yielding the corresponding pyrrolidine as a single endo diastereoisomer with enantiopurity in the range of 94–96% and further manipulations of the obtained cycloadducts using high yielding reaction protocols and a final mesylation/intramolecular N-alkylation sequence allowed the synthesis of target compounds in which the integrity of all stereocenters installed during the first step is maintained. 相似文献
6.
The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton. 相似文献
7.
Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results. 相似文献
8.
Antonio J. Meléndez‐Martínez Maria L. Escudero‐Gilete Isabel M. Vicario Francisco J. Heredia 《Journal of separation science》2009,32(11):1838-1848
The efficiency of C30 stationary phases in the separation of carotenes and diverse hydroxycarotenoids has been the subject of several studies. However, little is known concerning their ability to resolve epoxycarotenoids isomers, whose study is of great importance due to the functions they serve and the information they can reveal concerning the processing of foods. We have concluded that C30 columns provide an excellent separation of structural, geometrical and optical isomers of epoxycarotenoids and that the presence of 5,8‐epoxide groups leads to a better shape recognition, to the extent that over 10 geometrical–optical isomers of 5,8‐epoxycarotenoids have been separated. Additionally, it was observed that these carotenoids elute later than their 5,6‐epoxide counterparts, albeit the latter have a longer chromophore. 相似文献
9.
Andrés Reyes José Luis Sanz‐Vicario 《International journal of quantum chemistry》2010,110(13):2331-2331
10.
Fernández N Carrillo L Vicario JL Badía D Reyes E 《Chemical communications (Cambridge, England)》2011,47(45):12313-12315
We have developed a highly efficient procedure for carrying out the catalytic enantioselective (3+2) cycloaddition between enals and stable azomethine ylides such as isoquinolinium and phthalizinium methylides. Under the optimized reaction conditions highly substituted chiral pyrroloisoquinolines and pyrrolophthalazines have been obtained in high yields and excellent diastereo- and enantioselectivities. 相似文献