R(4, 5) = 25

The Ramsey number R(4, 5) is defined to be the least positive integer n such that every n-vertex graph contains either a clique of order 4 or an independent set of order 5. With the help of a long computation using novel techniques, we prove that R(4, 5) = 25. © 1995 John Wiley & Sons, Inc.     

2α-Hydroxy-3α-[N-alkyl(aryl)]-pinanoamines. Part II. Synthesis and the nature of some pinanoamines and crystal and molecular structure of 2α-hydroxy-3α-(N-n-butyl)-pinanoamine hydrochloride

Several optically pure representatives of 2-hydroxy-3-[N-alkyl (aryl)] pinanoamines, potential chiral auxiliaries in organic synthesis, were obtained. In addition, a structural examination of 2-hydroxy-3-(N-n-butyl)-pinanoamine hydrochloride was conducted. Results of these studies confirm the structure of the amines obtained and the correctness of the proposed reaction pathway.Crystal data: C₁₂H₂₈NOCl,P2₁/c,a=13.196(1),b=8.562(1),c=14.276(1) Å,=110.41(3)°,R=0.034 for 2046 reflections.Part I,J. Cryst. Spec. Res. (1989)19, 535.     

Interaction of proteins with polymeric latexes

Syntheses, modifications and properties of latexes containing polyacrolein, suitable for the covalent immobilization of proteins, are described. Results of the studies of physical adsorption and covalent attachment of human serum albumin and gamma globulins are discussed. Procedures allowing immobilization of the enzyme, horseradish peroxidase, are described. Changes in protein molecules, owing to their contact with latex particles, are analyzed. Applicability of polyacrolein containing latexes for diagnostic purposes is discussed.     

Analytical applications of triethylenetetraminehexaacetic acid : Part I. the influence of TTHA on the polarographic reduction of some metal ions

The influence of triethylenetetraminehexaacetic acid (TTHA) on the d.c. and square-wave polarographic curves of Cu(II), Pb(II), Cd(II), Ni(II), Co(II), Zn(II), Fe(III), Bi(III), Sb(III), As(III) and In(III) has been investigated in several supporting electrolytes over the pH range 1–13. The E_{$\text{1}\text{2}$} and E_p values with and without a 10-fold excess of TTHA are compared. Some analytical applications are suggested.     

Chemistry of α-(benzotriazol-l-yl)alkylpyridines. Unusual nucleophilic attack at c-3a of the benzotriazolyl ring

Lithiation of 2-, 3- and 4-[α-(benzotriazol-l-yl)methyl]pyridines with butyllithium followed by reactions with electrophiles (alkyl halides, aldehydes, ethyl benzoate and diphenyl disulfide) gave the corresponding α-sub-stituted derivatives in good yields. Repetition of the reaction sequence allowed substitution of the second α-proton by an electrophile. 2-[α-(Benzotriazol-l-yl)-α-(phenylthio)pentyl]pyridine thus obtained gave an unusual attack of the Grignard reagents at C-3a of the benzotriazole system on treatment with arylmagnesium bromides.     

Molecular Interactions and Inclusion Phenomena in Substituted β-Cyclodextrins. Simple Inclusion Probes: H2O, C, CH4, C6H6, NH4 +, HCOO−

Using a simple molecular mechanics approach interaction energy profiles of simple probes (C, CH₄, C₆H₆, H₂O, NH₄ ⁺, and HCOO^-) passing through the center of the -CD ring cavity along the main molecular symmetry axis were first evaluated. Molecular Electrostatic Potential (MEP) values along the same path were also evaluated. The effect of the flexibility of the host -CD molecule together with solute-solvent (H₂O) interactions have been represented by averaging structures of MD calculations for -CD alone and -CD surrounded by 133 H₂O molecules. The effect of various substitutions of -CD has also been evaluated. Small symmetric hydrophobic probes (such as C, CH₄, C₆H₆) are predicted to form stable inclusion complexes with non-substituted and substituted -CDs, the probe position typically being near the cavity center. The stability of the inclusion complexes will increase with increasing size and aliphatic character of the probe. Small polar and charged probes (such as H₂O, NH₄ ⁺, HCOO^-) are predicted to prefer the interaction with the solvent (water) in the bulk phase rather than the formation of inclusion complexes with non-substituted and substituted -CDs. Guest–host interactions in the stable inclusion complexes with hydrophobic probes are almost entirely dominated by dispersion interactions. The MEP reaches magnitudes close to zero in the center of the non-substituted -CD ring cavity and in most of the studied substituted -CDs and shows maximum positive or negative values outside of the cavity, near the ring faces. Substitution of -CD by neutral substituents leads to enhanced binding of hydrophobic probes and significant changes in the MEP profile along the -CD symmetry axis.     

High-pressure [2 + 4]-cycloaddition of dimethylmaleic anhydride to 3,4-dimethoxyfuran; synthesis of dimethoxycantharidin preliminary communication

As an active diene (more active than furan itself), 3,4-dimethoxyfuran ( 1 ) affords with many dienophiles the respective cycloadducts in a high yield [2]. It has recently been found that under thermal conditions 1 easily reacts with maleic anhydride and its monomethyl derivative, but not with dimethylmaleic anhydride ( 2 ) [3]. This is probably due to steric hindrance resulting from the location of two methyl groups on the double bond of the dienophile. Since all Diels-Alder reactions in particular those with steric hindrance are pressure-sensitive [4]. we resolved to perform the title reaction under conditions of static high pressure.     

The Photophysics of β-Tyrosine and Its Simple Derivatives

Synthesis and photophysical studies of (O-methyl)--tyrosine (-tyrosine; an analogue of tyrosine, in which the amino group is moved from the - to the -carbon, closer to the phenol ring) and its derivatives with a blocked amino and/or carboxyl group were performed to explain the nature of the fluorescence of tyrosine derived analogues. All -tyrosine derivatives, except Ac-Tyr(Me), displayed the monoexponential fluorescence decay. The biexponential fluorescence decay observed for Ac-Tyr(Me) is assumed to be the result of the presence of two low-energy conformations (extended and with an intramolecular hydrogen bond). Higher quenching of the fluorescence of -tyrosine derivatives by the N-acetyl group than by the N-methylamide group moved farther was found, contrary to the data found for the respective derivatives of natural tyrosine. The obtained photophysical data are discussed with theoretical calculations (AMBER, AM1) on the basis of the rotamer model.