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1.
A combination of pentafluorophenylboronic acid and oxalic acid catalyses the dehydrative substitution of benzylic alcohols with a second alcohol to form new C−O bonds. This method has been applied to the intermolecular substitution of benzylic alcohols to form symmetrical ethers, intramolecular cyclisations of diols to form aryl-substituted tetrahydrofuran and tetrahydropyran derivatives, and intermolecular crossed-etherification reactions between two different alcohols. Mechanistic control experiments have identified a potential catalytic intermediate formed between the aryl boronic acid and oxalic acid.  相似文献   
2.
 Let F be a function field of characteristic over a finite field . Then for any finite set of F-primes S and any , there exists a set of F-primes W of density greater than such that and has a Diophantine definition over . (Here and is defined analogously.)  相似文献   
3.
A nonlinear operator equation F(x)=0, F:HH, in a Hilbert space is considered. Continuous Newton’s-type procedures based on a construction of a dynamical system with the trajectory starting at some initial point x 0 and becoming asymptotically close to a solution of F(x)=0 as t→+∞ are discussed. Well-posed and ill-posed problems are investigated. Received: June 29, 2001; in final form: February 26, 2002?Published online: February 20, 2003 This paper was finished when AGR was visiting Institute for Theoretical Physics, University of Giessen. The author thanks DAAD for support  相似文献   
4.
Let F be a function field of characteristic p > 2, finitely generated over a field C algebraic over a finite field C p and such that it has an extension of degree p. Then Hilbert's Tenth Problem is not decidable over F.  相似文献   
5.
Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.  相似文献   
6.
7.
A number of new 10-oxa-13-aza, 11-oxa-13-aza, and 9-oxa-7-aza prostanoids belonging to the B series were synthesized on the basis of 3-acyl-and 3-(3-arylprop-2-enoyl)furan-2,4(3H,5H)-diones. The scheme of synthesis includes selective hydrogenation of the exocyclic carbonyl group and reduction of the conjugated double bond in the acyl fragment of 3-acyl-and 3-(3-arylprop-2-enoyl)furan-2,4(3H,5H)-diones to obtain 3-alkyl-and 3-(3-arylpropyl)furan-2,4(3H,5H)-diones, transformation of the latter into the corresponding regioisomeric enol ethers via regioselective O-alkylation, and treatment of the enol ethers with primary aliphatic amines.  相似文献   
8.
Summary A preparative method was developed for obtaining L-and D-2-hydroxy-3-methylbutyric acids by the separation of racemic 2-hydroxy-3-methylbutyric acid with the aid of D-and L-threo-2-amino-1-p-nitrophenyl-1,3-propanediols.  相似文献   
9.
The energies of the gauche and anti conformers of 2-fluoroethylamine, 2-fluoroethanol and their protonated analogues are calculated using density functional theory. Unlike the non protonated systems, the protonated systems show a strong gauche effect where the C-F and the C-(+)NH(3) or C-F and C-(+)OH(2) bonds are gauche rather than anti to each other. Single crystal X-ray diffraction studies of 2-fluoroethylammonium compounds identify the same conformational preference.  相似文献   
10.
Summary 1. The solid-phase method of peptide synthesis has been used to obtain the heptapeptide HGly-L-Ala-L-Phe-L-Val-Gly-L-Leu-L-Met which is an analog of eledoisin.2. The possibility has been shown of using o-nitrophenylsulphenyl derivatives of amino acids in the solid-phase method of peptide synthesis.Khimiya Prirodnykh Soedinenii, Vol. 2, No. 3, pp 200–202, 1966  相似文献   
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