54(P-23) Detection of long-range J_<CC> by 1D and 2D DEPT Long-range C-C relay
Details
Determination of molecular structure by heteronuclear long-range C-H correlation methods such as HMBC and COLOC has a substantial problem in which a carbon bearing no proton within three bond cannot be detected. The structure elucidation of highly hydrogen deficient compounds might thus be problematic since the carbon network of those molecules could not be completely followed by information from indirect ^<2,3>J_<CH>. For the structure elucidation of such molecules, utilization of long-range ^<13>C-^<13>C coupling is necessarry to detect the super long-range H/C correlations via four bonds, ^1H-^<13>C-X-X-^<13>C. Large unwanted signals arising from single ^<13>C isotopomers were supressed by utilization of BIRD pulse or zz gradient-enhanced X-filter. The X-filter dose not contain any parameter to be optimized such as BIRD delay and was more efficiently supress those unwanted signals than the BIRD-included sequence. Accurate values of long-range ^<13>C-^<13>C coupling constants obtained from 1D X-filtered DEPT C-C relay were applied to determine discrimination of stereochemistry in sp^3 carbon systems. A two-dimensional version of the above method was constructed by an insertion of t_1 in place of the DEPT or long-range C-C evolution. The former gave the spectrum in F_1=^1H, F_2=^<13>C and the latter, F_1=F_2=^<13>C. Although the F_1=^1H spectrum showed direct connectivity between ^1H-^<13>C-X-X-^<13>C, its sensitivity was lower than that of the F_1=F_2=^<13>C spectrum probably due to long t_1 evolution time. The F_1=F_2=^<13>C spectrum gave correlation peaks between ^1H-^<13>C-X-X-^<13>C. This spectrum successfully assigned the acyl residues of phenol esters at natural ^<13>C abundance, which could not be determined by conventional HMBC. Spectral editing was also applied for the 2D version to obtain each CH-, CH_2-, and CH_3- subspectrum, in which the correlation peaks close to each other could be discriminated.
