Figure 11.28

The assignments for cholesterol obtained just from these most intense HMBC crosspeaks are summarized in Figure 11.27. Seventeen of the 27 carbons of cholesterol were assigned from methyl group HMBC correlations alone. The one remaining "uncrowded" aliphatic CH group, l, can be assigned to position 8 by the process of elimination, and z = 6, y = 3, and t = 4 have already been assigned from chemical-shift arguments alone. From the olefin and alcohol functional groups, we assigned k = 2 and m = 7. With the HSQC data, including the "doublet" and "triplet" method for stereospecific assignment, we can precisely assign most of the proton resonances, even those that fall in heavily overlapped regions of the 1H spectrum. This leaves only four unassigned positions: methylene groups d, g, h, and j for positions 11, 15, 16, and 23. COSY data would be the best way to finish the assignments and to confirm those already made. You might also look for protons that are "in the clear" (resolved) in the HSQC and look for correlations from these F2 positions: j' and h have unique 1H chemical shifts to search the HMBC and COSY spectra.

11.6 STRUCTURE DETERMINATION USING HSQC AND HMBC 11.6.1 Testosterone Metabolites

HMBC is a powerful tool for locating the position of a functional group within a known carbon skeleton. Oxidation of testosterone with the enzyme cytochrome P-450 (Fig. 11.29) leads to a number of hydroxylation (C-H ^ C-OH) and di-hydroxylation products. One


0 0

Post a comment