Methyl configurational sensitivity

There are only three unique triad combinations, mm, mr and rr thus a methyl configurational sensitivity to just nearest neighbor configurations would produce only three resonance in the methyl region of the C-13 spectrum. From an earlier spectrum of the amorphous polymer, we noted at least ten methyl resonances. We must therefore consider the situation where the next-nearest as well as nearest neighbor configurations are affecting the chemical shift, that is. 

The ester methyl resonance could in principle give the same information but in most solvents, as in chloroform, it is not configurationally sensitive). In Fig. 2, we see three a-methyl proton resonances having the same chemical shifts but very different intensities in each spectrum. From their measured areas, the triad probabilities are readily determined. 

The configurational sensitivity falls within a range from triad to pentad for most vinyl polymers. In non-crystalline polypropylenes, three distinct regions corresponding to methylene (46 ppm), methine (28 ppm) and methyl (20 ppm) carbons are observed in the C-NMR spectrum. (The chemical shifts are reported with respect to an internal tetramethylsilane (TMS) standard.) The spectrum of a 1,2,4-trichlorobenzene solution at 125 "C of a typical amorphous PP is shown in... 

Simple diastereoselection in the reactions of 2-butenylboron compounds and aldehydes is critically dependent on the configurational stability of the reagentslb. As a general rule, most 2-bulenylorganometallics arc sensitive to sequential 1,3-metal shifts (1,3-metallotropic rearrangements) that result in E- to Z-olefin isomerization via the l-methyl-2-propenylmetal isomer. 

WIN 64821 (10) and (—)-ditryptophenaline (11) syntheses [7], not only effectively differentiated the two amide moieties but also most importantly marked the first in a series of stereochemical transfer steps in which the stereochemistry of the constituent L-amino acids was relayed to ultimately define each of the relative and absolute stereochemical configurations at all eight stereogenic centers found in the target compound. Finally, /V-methylation of the base-sensitive amide in 77 % yield using methyl iodide and potassium carbonate in acetone completed the 5-step synthesis of our key tetracyclic bromide monomer starting from commercially available amino acid derivatives. 

Methyl sorbate gives three dimeric products from sensitized irradiation, Eq. 52. The trans configuration of the reactant double bonds is retained in all of the products. 134> Over 80% of the product arises from... 

Direct irradiation of BDAF in benzene or acetonitrile containing methyl alcohol gives the expected ether in high yield. The triplet-sensitized reactions give the same result. Similarly, BFL reacts with styrene and with a-methyl-styrene to give cyclopropanes. In the latter case, 80% retention of configuration is observed for both the direct and triplet-sensitized irradiations. 

The indifference of /3-fructofuranosidase towards substitution in the afructon part of sucrose is contrasted by its extreme sensitivity towards any change in the structure and configuration of the fructon. Change from the furanose to the pyranose ring structure in methyl /3-D-fructoside is incompatible with the action of /3-fructofuranosidase.60... 

Oxo esters undergo an intramolecular dicarbonyl coupling reaction on treatment with a low-valent titanium reagent to form a ketone. This was used in the synthesis of (Z)-7,l0,10-trimethyl-bicyclo[7.2.0]undec-6-en-2-one (4) from ethyl ( )-6-methyl-7-(4,4-dimethyl-2-oxocyclobutyl)-hept-5-enoate,108 Interestingly, the configuration of the isolated C -C double bond in the isolated product is sensitive to the reaction conditions. 

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