Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

The chemical structure of lipid A. Demonstration of amide-linked 3-acyloxyacyl residues in Salmonella minnesota Re lipopolysaccharide.

Abstract: In Salmonella minnesota lipopolysaccharide the lipid A backbone, a substituted diphosphorylated beta 1,6-linked D-glucosamine disaccharide molecule, carries approximately seven residues of fatty acids: one each of dodecanoic, hexadecanoic, D-3-hydroxytetradecanoic and D-3-O-(tetradecanoyl)-tetradecanoic acid in ester linkage and two of D-3-hydroxytetradecanoic acid in amide linkage. In the present study it is shown that treatment of the lipopolysaccharide with alkali at elevated temperature leads, through a beta-elimination reaction, to the generation of amide-bound delta 2-tetradecanoic acid. This suggested that the 3-hydroxyl group of amide-bound hydroxy fatty acids carried a substituent. To elucidate the nature of the substituent, free Salmonella lipid A was methylated with methyl iodine in the presence of silver salts followed by mild acid hydrolysis, a procedure which is known to cleave amide (and not ester) bonds selectively. In the hydrolysate, by means of combined gas-liquid chromatography/mass spectrometry the methyl esters of 3-O-(dodecanoyl)-tetradecanoic and 3-O-(hexadecanoyl)-tetradecanoic acid were identified. This shows that in lipid A amide-linked 3-hydroxytetradecanoic acid residues are 3-O-acylated by dodecanoic and hexadecanoic acid, respectively. Quantitative analyses suggest that the Salmonella lipid A backbone is substituted by four D-3-hydroxytetradecanoyl residues, two being present as esters and two as amides. The nonhydroxylated fatty acids are not bound directly to the backbone. Rather, they are attached to hydroxyl groups of 3-hydroxytetradecanoyl residues: specifically, tetradecanoic acid substitutes ester-bound and dodecanoic and hexadecanoic acid amide-bound 3-hydroxytetradecanoic acid.

.pi.-Conjugated Soluble Poly(aryleneethynylene) Type Polymers. Preparation by Palladium-Catalyzed Coupling Reaction, Nonlinear Optical Properties, Doping, and Chemical Reactivity

Abstract: Palladium-catalyzed polycondensation between dihalo aromatic compounds X-Ar-X (3-hexyl-2,5-diiodothiophene, 2,5-dibromoselenophene, and 3,4-dinitro-2,5-dibromothiophene) and diethynyl aromatic compounds HC≡C-Ar'-C≡CH (2,5-diethynylpyridine, 3-hexyl-2,5-diethynylthiophene, and p-diethynylbenzene) in the presence of triethylamine gives soluble π-conjugated poly(aryleneethynylene) (PAE) type polymers (-Ar=C≡C-Ar'-C≡C-) n when Ar and/or Ar' contains the long alkyl substituent and/or pyridine ring

Heterocycles by Ring Closure of Ortho-Substituted t-Anilines (The t-Amino Effect)

Abstract: Publisher Summary It is the purpose of this chapter to collate all the available material relating to the “t-amino effect” and to attempt a rationalization of its operative influence in a mechanistic manner. The cyclizations of ortho-substituted t-anilines are described in two main sections. The first discusses ring closures between the ortho substituent and the t-nitrogen. Varieties of seemingly unrelated reactions have been reported in which a t-aniline with an ortho substituent of the type N-C-Y is converted into a benzimidazole with loss of a group from the t-nitrogen. All these cyclizations can be rationalized by the intermediacy of an imine bearing a good leaving group. While the second deals with those ring formations which involve the a-methylene groups attached to the nitrogen. The reactions which fall into this category conform to a basic mechanism for an unsaturated ortho substituent (A=B) capable of mesomerism. The polarized mesomer is set up for further reaction since the hydrogens of the a-methylene group are rendered labile by the charged nitrogen, and are consequently abstracted by the nucleophilic center (B-). Here, the discussion will be divided into sections according to the functiona1 group (o-nitroso and o-nitro groups, o-amino and o-acylamino groups, o-azo group, o-azomethine group (=n=c), and o-carbonyl (c=o) and o-imine (c=n) groups) situated at the ortho position.

Face-to-face arene-arene binding energies: dominated by dispersion but predicted by electrostatic and dispersion/polarizability substituent constants.

Abstract: Parallel face-to-face arene−arene complexes between benzene and substituted benzenes have been investigated at the MP2(full)/6-311G** and M05-2X/6-311G** levels of theory. A reasonably good correlation was found between the binding energies and the ∑|σm| values of the substituted aromatics. It is proposed that a substituent |σm| value informs on both the aromatic substituent dispersion/polarizability and the effect the substituent has on the aromatic electrostatics. Supporting this hypothesis, a combination of electrostatic (∑σm) and dispersion/polarizability (∑Mr) substituent constant terms gives an excellent, and statistically significant, correlation with the benzene-substituted benzene binding energy. Symmetry adapted perturbation theory energy decomposition calculations show the dominant attractive force is dispersion; however, the sum of all nonelectrostatic forces is essentially a constant, while the electrostatic component varies significantly. This explains the importance of including an electros...