STABILITY NUMBER IN SUBCLASSES OF P5-FREE GRAPHS

Two new hereditary classes of P5-free graphs where the stability number can be found in polynomial time are proposed. They generalize several known results.     

Toward the development of a structurally novel class of chiral auxiliaries: diastereoselective aldol reactions of a (1R,2S)-ephedrine-based 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one

[reaction: see text] Asymmetric aldol addition reactions have been conducted with (1R,2S)-ephedrine-derived 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one (2). Diastereoselectivities range from 75:25 to 99:1 for the formation of the crude non-Evans syn adducts 8a-h. The facial selectivity of the enolate is directed by the stereogenic N(4)-methyl substituent. Aldol adduct 8a is readily cleaved by acid hydrolysis to afford (2S,3S)-3-hydroxy-2-methyl-3-phenylpropionic acid (9) in >95% ee.     

Gargano,Lewinter and Malerba问题的解

§1　IntroductionLet G be a graph with vertex-set V(G) ={ v1 ,v2 ,...,vn} .A labeling of G is a bijectionL:V(G)→{ 1,2 ,...,n} ,where L (vi) is the label of a vertex vi.A labeled graph is anordered pair (G,L) consisting of a graph G and its labeling L.Definition1.An increasing nonconsecutive path in a labeled graph(G,L) is a path(u1 ,u2 ,...,uk) in G such thatL(ui) + 1相似文献   

Toward the development of a structurally novel class of chiral auxiliaries. Conformational properties of the aldol adducts of oxadiazinones: observation of unusual shielding effects

Asymmetric aldol reactions were conducted with the titanium enolate of N(3)-hydrocinnamoyl-3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one to afford aldol adducts 5a-j. The dominant product of the asymmetric aldol reaction was the non-Evans syn adduct as determined by (1)H NMR spectroscopy and X-ray crystallography. When evaluating the (1)H NMR spectra of adducts 5a-j, a highly shielded signal with an average chemical shift of 0.05 ppm was observed. This signal was readily determined to be the C(5)-methyl group of the oxadiazinone. It is presumed that the overall conformation adopted by the aldol adducts in solution places an aromatic ring of the N(3)-substituent in close proximity to the C(5)-methyl group. An investigation of this conformational preference is conducted employing (1)H NMR spectroscopy, X-ray crystallography, and computational methods.     

翼型模型振动的实验研究与气动特性分析

通过在NF-3低速风洞专门研制的翼型模型及相应的俯仰和沉浮振动机构,选用NACA0012翼型进行大迎角下不同频率的振动实验,研究了模型振动平均状态下对其气动力特性的影响情况,并在N-S方程基础上对振动流场进行了初步分析。实验与计算研究的结果表明:在临近定常失速迎角的大迎角条件下,翼型的振动可以引起旋涡分离,导致翼型升力减小和失速迎角的提前;就所讨论的两种振动模式而言,俯仰振动的影响大于沉浮振动。所以,为了提高飞机模型,尤其是大展弦比飞机模型的风洞实验精准度,在模型设计和加工时要特别注意加强机翼弦向的扭转刚度。     

Temperature and isotope substitution effects on the structure and NMR properties of the pertechnetate ion in water

The uniquely well-resolved (99)Tc NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the (99)Tc chemical shift as a function of temperature has been measured over the range 10-45 degrees C for the three isotopomers Tc((16)O)(4)(-), Tc((16)O)(3)((18)O)(-), and Tc((16)O)(3)((17)O)(-) at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc-O scalar coupling was determined for the Tc((16)O)(3)((17)O)(-) isotopomer. Values for these parameters were computed using relativistic spin-orbit density functional theory with an unsolvated ion approximation and with treatments of the solvated ion based on the COnductor-like Screening MOdel (COSMO) approach. The temperature and isotope dependence of (99)Tc NMR parameters inferred by these methods were in good quantitative agreement with experimental observations. The change in the Tc-O bond length associated with the changes in temperatures considered here was determined to be of the order of 10(-)(4) A. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental studies.     

Conformational preferences of the aldol adducts of oxadiazinones. H NMR spectroscopy and computational studies of N4-methyl and N4-isopropyloxadiazinones

The conformational properties of the aldol adducts of some N₄-isopropyl-oxadiazinones have been investigated by ¹H NMR spectroscopy and computational studies. An earlier study of the syn-aldol adducts of N₄-methyl-oxadiazinone 2 led to the conclusion that the solution and solid state conformation of these compounds involve syn-parallel arrangement of the C₂- and N₃-carbonyls of the oxadiazinones. However, the synthesis and asymmetric aldol reactions of an N₃-hydrocinnamoyl-N₄-isopropyl-oxadiazinone 4 has yielded aldol adducts 5a-e in which the orientation of the C₂- and N₃-carbonyls are most likely in the anti-parallel arrangement. These aldol adducts have been studied by ¹H NMR spectroscopy and the shielding aspect observed clearly suggests the presence of the anti-parallel arrangement. The installment of a N₄-d₆-isopropyl group further confirmed this assertion. Computational studies support the conclusion that solution state conformation of the N₄-methyl and N₄-isopropyl-oxadiazinones involves anti-parallel carbonyls in contrast to the solid state evidence of the X-ray crystallographic data of oxadiazinone 2.