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Figure 11.61

c18, the connection of c6 to c12 completes a six-membered ring: ring E of the triterpene (Fig. 11.62).

Some crosspeaks are ambiguous due to overlap of chemical shifts. For example, a COSY crosspeak is observed at 2.02 ppm (h11a) and 1.80 ppm (Fig. 11.60). Since h5a is in the list at 1.80 ppm, we might assume that c5 is connected to c11. But h11b has a chemical shift of 1.81 ppm, so the crosspeak could also be a geminal (two-bond) correlation between h11a and h11b. Sometimes these ambiguities can be resolved by careful alignment and comparison of the COSY and HSQC spectra, but in this case the ambiguity can be easily resolved since another crosspeak is observed at 2.02 ppm (h11a) and 1.65 ppm (h5b). Since h5b is a unique chemical shift, not overlapped with any other proton, we can safely conclude that c5 is connected to c11. This completes another six-membered ring due to the shared HMBC correlation from h2 (CH3) and h13 (CH3) to c28 (Fig. 11.63). COSY crosspeaks between h14a and h4b, and between h14b and h4a (Fig. 11.60) close the D ring of the triterpene (Fig. 11.64).

Finally, let's examine the olefinic/aromatic fragment starting with the three olefinic protons in the 1D 1H spectrum, identified from the HSQC spectrum as h24, h25 and h26. There is a 6.7 Hz coupling between h24 and h26, confirmed by a COSY crosspeak and consistent with a vicinal coupling in a cis olefin. The long-range coupling between h26 and h25 (1.7 Hz) is confirmed by a weak COSY cross peak and indicates that h25 is located farther away than three bonds from h26 on the h26 side of the olefin. In six-membered ring aromatic systems, we usually see strong three-bond HMBC correlations and weak or missing

10 hx co2ch

10 hx co2ch ch3

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