13C-chemical shifts

Jch coupling constants 13c chemical shifts (y effects)... 

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the 13C chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the 170 chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and tram sulfoxide isomers of trans- 1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upfield from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upfield from those of the supposed equatorial conformers. For tram-3, (rans-5-dimethylthiane-1 -oxide (177) with the oxygen axial, the 170 signal was 21 ppm upfield from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

H and 13C chemical shifts and coupling constants of some selected representative four-membered ring sulfoxides and sulfones are given in Table 7. ... 

Carbon-13 chemical shifts of the a- and (8-carbon atoms of various unsubstituted and 3-substituted thietane oxides and dioxides have been recorded and correlated by the equations S = ay + bx and Sf = ax + by where a and b are parameters characteristic of the sulfoxide or sulfone (y) and the substituent (x)216. The values of the substituent parameters were found to parallel those which determine the effect on the 13C chemical shifts when hydrogen is replaced by a substituent224. 

Scheme 7.11 shows the product structures resulting from the dithionite reduction of a simplified version of WV-15. The symmetric sulfite diester was extracted from the reaction mixture with methylene chloride. The isolation and characterization of the sulfite diester confirmed that this species can form in dithionite reductive activation reactions and provided the chemical shift for the 10a-13C center of a mitosene sulfite ester (49.37 ppm). The aqueous fraction of the reaction contained the mitosene sulfonate and trace amounts of Bunte salt, based on their 13C chemical shifts. 

SCHEME 7.23 Mechanism of quinone methide trapping of water with calculated 13C chemical shifts of the five-carbon center of methide enolates. 

NMR spectra were recorded on a JEOL FX90Q (90 MHz) spectrometer. chemical shifts are reported relative to an internal standard of CHCI3 in CDCI3, while 13c chemical shifts are reported relative to the CDCI3 used as solvent. Infrared spectra were recorded on an IBM IR32 FTIR or a Perkin-Elmer 1330 IR spectrophotometer. Uv-visible spectra were obtained using a Varian DMS90 spectrophotometer. 

C-2-0-2 bonds.7 45 The assignments of the carbohydrate-carbon resonances of Arap model compounds (61, 62) were based on comparisons with the 13C chemical-shift data for methyl a- and / -arabinopyrano-side.89-110... 

As may be seen from the comparisons of the chemical-shift data for compounds 59 and 60 with those for compounds 61 and 62, two carbon atoms, C-l and C-4, could be monitored in order to distinguish between the two tautomeric and anomeric forms of arabinose. There is an 6-p.p.m. difference between the C-4 signal of arabinose in the pyra-nose and the furanose form. Determination of the chemical shift of the anomeric-carbon atom is, however, not that straightforward, as there is an overlap between the C-l chemical-shifts of arabino-furanosyl and -pyranosyl derivatives (compare the data for compounds 60,61, and 62). The 13C chemical-shift data for C-l, in conjunction with the 13C chemical-shift data for C-4, allow the detection and assignment of arabinose in either tautomeric and either anomeric form. 

C Chemical-shift Data" for the Disaccharide Moiety of Bleomycin Af, and Some Related Carbohydrate Moieties... 

Disilenes exhibit the relatively low-field (8 = 49-155) 29Si chemical shifts characteristic of low-coordinate silicon compounds (Table I) thus 29Si NMR spectra are very important in their characterization. This deshielding is similar to that observed in the 13C chemical shifts of doubly bonded carbons relative to those of their saturated counterparts. 

FIGURE 2.12. CH2F/CD2F region of the 13C NMR spectrum of 1,6-difluorohexane-1,1 -d2 demonstrating deuterium isotope effect on 13C chemical shifts... 

Both the 13C chemical shifts and the F—C coupling constants for CF2 carbons are quite characteristic in value, as can be seen from the examples in Scheme 4.9. A review article on 13C NMR spectra of fluorinated cyclopropanes has recently appeared.3... 

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