It is shown that heteronuclear correlated spectra in which delay times are optimized for geminal and vicinal 13C–1H couplings rather than direct, 13C–1H couplings can be used to assign quaternary carbons in the 13C spectrum of kauradien-9(11),16-oic acid. These spectra in combination with normal heteronuclear correlated spectra plus homonuclear correlated spectra can be used to completely and unambiguously assign the 13C and 1H spectral peaks for the title compound. The main advantage of this approach over the use of 13C–13C connectivity ("INADEQUATE") experiments is sensitivity.

Complete 1 H and 13 C spectral assignments are reported for lupeol (1a) and two derivatives where the C-30 methyl group is replaced by CH 2 OH (1b) and HC O (1c). Compound 1c shows conformationally dependent substituent effects on 1 H chemical shifts. It also shows line broadening of some 13 C signals at 25°C, suggesting hindered rotation of the side-chain group. This is confirmed by low-temperature spectra which show splitting of broadened peaks into pairs in a ca 2 : 1 area ratio. The free energy of activation of hindered rotation is estimated as 13.5 kcal mol 1 . By contrast, 1a shows no evidence of hindered rotation down to 40°C although NOE data suggest the presence of two conformers. Spartan molecular mechanics calculations confirm the presence of two stable conformers for 1a and 1c but overestimate the rotational barrier in 1a. The additional barrier in 1c probably reflects loss of conjugative stabilization during rotation. since these had been determined for much higher sample concentrations and it was anticipated that there would be some dilution shifts. This was considered important since we wished to have chemical shifts measured under comparable conditions to probe the effect of changes in the side-chain group on remote chemical shifts.While carrying out spectral assignments, we noted that the 13 C spectrum of 1c, but not for 1a and 1b, showed clear evidence for some type of slow conformational process involving the side-chain group. Therefore, we decided also to investigate the origins of the phenomenon.

From the stem bark of Mangifera indica, seven cycloartane-type secondary metabolites were isolated. Compound 1 has been isolated for the first time from M. indica, whereas compounds 2 (2a and 2b, as an epimeric mixture), 3, and 4 are new triterpenoid-type cycloartanes. Unambiguous (13) C and (1) H NMR assignments for these compounds and the known compounds mangiferonic acid (compound 5), isomangiferolic acid (compound 6), ambolic acid (compound 7), and friedelin (compound 8) are reported; the latter because full NMR data for these compounds are not available in the literature.

Several modifications of the basic "C-'H shift correlation sequence as optimized for polarization transfer via long-range "C-'H coupling constants are described and evaluated. Modifications involve incorporation of BIRD pulse(s) at the mid-point of the evolution period and/or at the mid-point of the fixed delay prior to polarization transfer, in addition to the previously discussed incorporation of a BIRD pulse during the final fixed delay. These modifications give significantly improved sensitivity for many cross-peaks, excellent suppression of one-bond peaks and, in the case of the sequence incorporating two additional BIRD pulses, some ability to distinguish two-bond and three-bond connectivity peaks. The latter sequence is, overall, comparable in sensitivity to the fixed time sequences XCORFE and COLOC. It is also easier to parameterize and gives better suppression of one-bond peaks and other artifacts.

No abstract