Search results for "steric"

ChemInform Abstract: Gas-Phase and Solution Basicities of Some Alkyl 2,6-Dialkylphenyl Ketones: A Comparative Analysis.

2000

Abstract The gas-phase basicity of a number of alkyl 2,6-dialkylphenyl ketones (2,6-R2C6H3COR′) has been found to be almost insensitive to structural variations, as a result of a compensation of steric and electronic effects associated with the bulkiness and to the polarizability, respectively, of R and/or R′. On the contrary, the basicity in concentrated sulfuric acid undergoes, along the same series of compounds, a variation of nearly 8 pK units, as a consequence of steric inhibition of solvation of the protonated carbonyl as the main effect played by R and/or R′. The results in the condensed phase agree very nicely with recent findings relevant to some 4-substituted 2,6-dimethylacetophen…

ChemInform Abstract: Stereoselective Synthesis of Substituted Tetrahydropyran Rings via 6-exo and 6-endo Selenoetherification.

2010

Eight unsaturated alcohols were cyclized by selenoetherification in 6-exo or 6-endo manner to give substituted tetrahydropyran rings. Yields, regio- and stereoselectivities were discussed in terms of steric and electroniceffects such as Se-O interaction. For the first time examples of the use of silica gel in selenoetherification and the effect of the X - counter ion of PhSe + on the reaction course are discussed. These effects are related to the occurrence of Se-O interaction.

Crystal structure of ethyl 2-(di-eth-oxy-phosphor-yl)-2-(2,3,4-tri-meth-oxy-phen-yl)acetate.

2014

The title compound, C17H27O8P, was prepared by Michaelis–Arbuzov reaction of ethyl 2-bromo-2-(2,3,4-trimethoxyphenyl)acetate and triethyl phosphite. Such compounds rarely crystallize, but single crystals were recovered after the initial oil was left for approximately 10 years. The bond angle of thesp3-hybridized C atom connecting the benzene derivative with the phospho unit is widened marginally [112.5 (2)°]. The terminal P—O bond length of 1.464 (2) Å clearly indicates a double bond, whereas the two O atoms of the ethoxy groups connected to the phosphorous atom have bond lengths of 1.580 (2) Å and 1.581 (3) Å. The three methoxy groups emerge out of the benzene-ring plane due to steric hind…

ChemInform Abstract: Ortho-Substituent Effects in N-Arylacetamides. NMR and Molecular Mechanics Investigation.

2010

1H, 13C, 15N and 17O NMR spectra of N-phenylacetamide (acetanilide) and 21 ortho-substituted acetanilides were measured and assigned. The observed NMR parameters are related to the Hammett substituent parameters and conformational characteristics of the acetamido moiety estimated by molecular mechanics calculations. Significant relationships were found for the 13C NMR chemical shifts of C-5 (para to substituent) and the direct spin–spin coupling constant, 1J(C, H), of C-3 (ortho to substituent) with Hammett substituent parameters. For 15N NMR chemical shifts of the amido nitrogen, no general correlation with the Hammett substituent parameters was found. The interactions between functionalit…

Durch sterische effekte stabilisierte ß-ketocarbonsäuren

1989

Abstract Increasing steric hindrance in β-keto carboxylic acids leads to an increasing kinetic stability towards decarboxylation, till systems are reached which are completely stable at room temperature. Simultaneously the tautomeric equilibrium is changed in favour of the (Z)-enol, and finally in favour of the (E)-configurated enol.

Complexation of Small Molecules by Open-Ended Resorcarene Hosts

2002

[structure: see text] Sterically hindered tetraaminomethylated resorcarenes form inclusion complexes in CDCl(3) with acetonitrile and acetaldehyde, which are kinetically stable on the NMR time scale at 233 K.

Stereoselektive Glycosylierung von Steroidalkoholen mit 2,3,4,6-Tetra-O-privaloyl-α-D-glucopyranosylbromid (Pivalobromglucose) und 2,3,4,6-Tetra-O-(o…

1986

Steroidalkohole verschiedener Struktur, deren Hydroxyfunktionen aus elektronischen und sterischen Grunden in ihrer Reaktivitat differieren und die daruber hinaus empfindliche Gruppierungen enthalten, werden mit 2,3,4,6-Tetra-O-pivaloyl-α-D-glucopyranosylbromid (1) selektiv und effektiv in β-Glucoside ubergefuhrt. Dank des lenkenden Einflusses des 2-O-Pivaloyl-Substituenten wird eine Orthoesterbildung bei den Koenigs-Knorr-Reaktionen stark unterdruckt. Mit dem o-Toluoylrest als Hydroxyschutzgruppe wird diese Lenkung nur in geringem Mas erreicht. Stereoselective Glycosylation of Steroid Alcohols Using 2,3,4,6-Tetra-O-pivaloyl-α-D-glucopyranosyl Bromide (Pivalobromoglucose) and 2,3,4,6-Tetra-O…

A sterically crowded triazene: 1,3-bis(3,5,3′′,5′′-tetramethyl-1,1′:3′;1′′-terphenyl-2′-yl)triazene

2007

ChemInform Abstract: Two Strained Hexahelicenophanes.

2000

The crystal structures of the [6]­helicenes 4,13-(1,10-deca­methyl­ene­dioxy)­hexahelicene, C36H34O2, (I), and 4,13-(1,8-octa­methyl­ene­dioxy)­hexahelicene, C34H30O2, (II), show strong steric interactions between the terminal benzene rings and the poly­methyl­ene­dioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) A, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]­helicene.

Tropylium cation capsule of hydrogen-bonded tetraurea calix[4]arene dimers

2001

The interaction between tropylium salts and tetraurea calix[4]arene derivatives (such as 1 and 2) was studied in solution using 1D, 2D, diffusion, VT NMR and UV–visible spectroscopy. It was found that tropylium salts form charge transfer complexes with both the monomers and dimers of the tetraurea calix[4]arene derivatives depending on the experimental conditions. Compound 1 increases dramatically the solubility of tropylium salts in apolar solvents such as C2D4Cl2, CDCl3 and CD2Cl2 by forming the molecular capsule 1·C7H7+·1. In contrast to the benzene capsule of 1, in 1·C7H7+·1 the hydrogen bonds in the equatorial region that hold together the two parts of the dimer change their directiona…