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.

Etude, par spectroscopie infra-rouge, de la conformation de quelques composés peptidiques modèles

Abstract: Some experimental data are given on the infrared spectra between 3300 and 3500 cm−1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH3−CONH-CH(R1)-CONH(R2), (I); CH3-CON(CH3)-CH(R1)-CONH(R2), (II) and CH3-CONH-CH(R1)-CON(R2)2, (III). In studying the N-H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C5 and C7, which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N-H stretching bands which are to be ascribed to the N-H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N-H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm−1. The first two are ascribed to the N-H oscillators included in the Hbonds which lock the C7 and C5 conformations; the last two correspond to free N-H which differ with the substituent on the nitrogen atom.

Cyclopentannulation of 3-Alkylindoles: A Synthesis of a Tetracyclic Subunit of the Kopsane Alkaloids

Abstract: Indoles which bear an alkyl substituent in the 3-position undergo a [3 + 2] annulation reaction when treated with 1,1-cyclopropane diesters in the presence of Yb(OTf)3 resulting in 2,3-cyclopentanoindolines Typically, the reactions are performed at elevated temperatures or at ultrahigh pressures In cases where steric crowding is an issue, ultrahigh pressures are required In reactions involving substituted cyclopropanes, significant regio- and diastereocontrol was observed When the substituent was aromatic or olefinic, the reactions took place at ambient temperature and pressure The applicability of this methodology to the preparation of a key tetracyclic subunit of the kopsane alkaloids was demonstrated

Electron-transfer oxidation of 9-substituted 10-methyl-9,10-dihydroacridines. Cleavage of the carbon-hydrogen vs. carbon-carbon bond of the radical cations

Abstract: Electron-transfer oxidation of various 9-substituted 10-methyl-9,10-dihydroacridines (AcrHR) by Fe(ClO 4 ) 3 and [Fe(phen) 3 ](PF 6 ) 3 (phen=1,10-phenanthroline) results in cleavage of the C(9)-H or C(9)-C bond of AcrHR .+ depending on the substituent R. Transient electronic absorption spectra as well as electron spin resonance (ESR) spectra of AcrHR .+ have been detected by using a stopped-flow spectrophotometer and a rapid mixing flow ESR technique, respectively

Origin of enantioselectivity in the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones.

Abstract: The origin of the enantioselectivity in the ruthenium-catalyzed transfer hydrogenation has been studied by means of experiment and density functional theory calculations. The results clearly show that electrostatic effects are of importance, not only in the T-shaped arene-aryl interaction in the favored transition state but also between the aryl of the substrate and the amine ligand in the disfavored TS. In addition, the electrostatic interaction between the alkyl substituent of the substrate and the catalyst is of importance to the enantioselectivity. The major cause of enantioselection is found to be of nonelectrostatic origin. This inherent property of the catalytic system is discussed in terms of dispersion forces and solvent effects. Finally, a minor but well-characterized steric effect was identified. The success of this class of catalysts in the reduction of alkyl aryl ketones is based on the fact that all factors work in the same direction.