Nuclear Overhauser effect carbonyl

Nuclear Overhauser effect (NOE) experiments clarified the preference of the cis-trans-geometry in solution for cyclic lactams 19. For 19a-c, X-ray geometries (Section 14.10.3.1) retain in solution, and NOEs were observed between the methylene protons next to the ring carbonyl and the NCH2 protons, whereas no such NOE was observed in 19d <2002CC2656>. 

Two new chiral carbon atoms are formed in the condensation and four diastereoisomeric -sulfinyl y-lactones can therefore in principle be obtained. However, only two diastereoisomers, 3S,4P,Ps) and (3P,4S,Ps), are isolated when the carbanion is condensed with pivalic aldehyde, benzaldehyde, or pinacolone (yield 65-70% for aldehydes, ratio 53 47 yield 47% for pinacolone, ratio 81 19). The diastereoselectivity decreases when the two substituents of the carbonyl group are sterically similar. However, single diastereoisomers can easily be separated through chromatography and transformed in high yield into both enantiomers of optically pure saturated (by desulfurization) and a,(i-unsaturated p-lactones (by pyrolytic sulfoxide elimination) (eq 3). The relative and absolute stereochemistry of all the products have been determined by circular dichroism, nuclear Overhauser effects, and X-ray analyses. 

The stereochemistry of the double bond in 4-(a-arylethylidene)-2-phenyl-5(4//)-oxazolones can be determined by measurements of long-range heteronuclear selective carbon-13 proton nuclear Overhauser enhancements. In the (Z)-isomers 774, large nuclear Overhauser enhancements are observed for the carbonyl carbon atom upon presaturation of the methyl group (Fig. 7.65). These effects are much smaller for the ( ) isomers. ... 

C Spin lattice relaxation times, Tj, spin-spin relaxation times, T2, and nuclear Overhauser enhancements, NOE, for the a-carbons of PBLG of various molecular weights have been used to study transitions from rigid to flexible forms of this polymer (Allerhand and Oldfield, 1973). Effective rotational correlation times, reff, calculated from 7 - and NOE-values, for the a-carbons were 24-32 nanoseconds for the helical form and approximately 0-8 nanoseconds for the random coil (Allerhand and Oldfield, 1973). The transition from the a-helix to the random-coil of PLM causes the resonances of the a- and carbonyl carbons to move upfield 2-3 and 3-4 ppm respectively (Tadokoro et al., 1973), which is consistent with results obtained for PBLG and PCBO. Further work is required before the reasons for the chemical shift differences between the corresponding carbons in the helical and random-coil forms in deuterochloroform-TFA systems can be elucidated. Plots of chemical shifts and relaxation times vs. pH have been used to study the helix-coil transition of poly-L-lysine hydrochloride in aqueous solution (Saito and Smith,... 

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