Examples NMR Spectroscopy of Oligosaccharides and Terpenoids

The third category is rare in carbohydrates because the bulky OH groups prefer the equatorial position, pushing the H into the axial position. 

NMR chemical shifts give us information about the proximity of electronegative atoms (e.g., oxygen) and unsaturated groups (double bonds and aromatic rings). In this discussion we will ignore the protons attached directly to oxygen (OH) because they provide little 

Menthol (Fig. 1.19) is a monoterpene natural product obtained from peppermint oil. Typical of terpenoids, menthol is only slightly soluble in water and is soluble in most organic solvents. The trans arrangement of the methyl and isopropyl substituents on the cyclohexane 

Cholesterol (Fig. 1.20) is a steroid, the same rigid five-ring backbone used for the mammalian sex hormones. There are only two functional groups an olefin (C-5, C-6) and an alcohol (C-3). The bulk of the molecule can be described as saturated hydrocarbon. There are five methyl groups two are attached to quaternary carbons so they should appear as singlets and three are attached to CH carbons so they should appear as doublets. Most of the protons in the A, B, and C rings can be described as axial or equatorial due to the rigid, 

In addition to these solvent peaks, we can count 26 peaks in the spectrum. Because there are 27 carbons in the cholesterol molecule (three are lost in the biosythesis from a triterpene precursor), there must be one peak that accounts for two carbons. The tallest peak (labeled 1, m ) in fact corresponds to two different carbons with nearly identical chemical shifts. The most downfield peaks ( aa and z ) are in the olefin/aromatic region of the 13 C spectrum (120-140 ppm), so they must correspond to C-5 andC-6. Peak aa is less intense ( shorter ) than all of the other peaks because of slow relaxation it must be a quaternary carbon. We will see that the proximity of protons is the primary means of relaxation of 13 C nuclei, so carbons lacking a proton relax much more slowly and give less intense peaks, especially if 

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