Methyl groups, neighboring

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

The c arlrnn atoms of the other two methyl groups (on the right-hand side) obtain an EC value of 3 because they arc adjacent to the racthinc carbon atom. The carbon atom of the methylene group obtains an EC value of 4 in the second rcla.xation process, as the sum (1 + 3) of the eonncctivity values of its neighboring atoms in the first iteration. 

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

The features that occur near 5 = 1.0 ppm are associated with the protons of the a-methyl group. The location of this peak depends on the configurations of the nearest neighbors. 

A pyrrole-like nitrogen in a ring iV-methyl group gives a peak around -220 p.p.m. if adjacent to two carbon atoms one or two neighboring nitrogens shift the peak downfield by about 40 and 90 p.p.m., respectively. 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

The irradiation of C-5 proton at 3.73 p.p.m. shows a singlet resonance at 1.27 p.p.m. for three protons of the methyl group and reverse irradiation shows a doublet at 3.73 p.p.m. for one proton at C-5 with coupling constant, 9.6 c.p.s., confirming the neighboring relationship of the methyl... 

The fine structure of the spectrum is the splitting of the resonance into sharp peaks. Note that the methyl resonance in ethanol at 8 = 1 consists of three peaks with intensities in the ratio 1 2 1. The fine structure arises from the presence of other magnetic nuclei close to the protons undergoing resonance. The fine structure of the methyl group in ethanol, for instance, arises from the presence of the protons in the neighboring methylene group. 

In the structure on the left, the charge is somewhat stabilized by the inductive effects of the neighboring chlorine atoms. In contrast, the structure on the right is destabilized by the presence of methyl groups. Therefore, the structure on the left is more stable. 

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