Hydroxyl groups, substituent effect carbon chemical shifts

Carbon-13 NMR spectra. A carbon-13 NMR spectrum of HTE polymer (Rn S 500) is shown in Figure 3. The carbon-13 NMR spectra of HTE polymers show the carbon chemical shifts at 79.3 and 72.7 ppm for the backbone and the terminal methine carbons respectively, at 43.9 and 46.2 ppm for the backbone and the terminal chloromethyl carbons, respectively, and in the range of 69.7-72.0 ppm as a multiplet for the methylene carbon. It is a characteristic feature of hydroxyl-terminated polyethers that the terminal carbon exhibits a up-field shift due to the substituent effect of the hydroxyl group, whereas the (0 carbon(s) to the terminal hydroxyl group exhibits a down-field shift (Table III). The terminal methine carbon also suggests that the hydroxyl groups are predominantly secondary. 

Table III. Substituent Effect of Hydroxyl Groups on Carbon Chemical Shifts of Polyethers...

The electronic effects of many substituents have been examined by studies of PMR118,119 sulfinyl and sulfonyl groups have been included in some of these. For example, Socrates120 measured the hydroxyl chemical shifts for 55 substituted phenols in carbon tetrachloride and in dimethyl sulfoxide at infinite dilution, and endeavored to... 

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

The P-configuration assigned to this tertiary hydroxyl group in teupestalin A (108) was supported by the very deshielded resonance of one of the C-11 methylene protons [H-1 la (pro-S), 8 3.47 dd, J= 1.5 and 3.7 Hz] which is close to the lOP-hydroxyl group, and by comparison of the NMR spectral data of 113 and 114 (Table 13). Since the observed differences in the chemical shifts of the C-8 [A8 = 8(1)- 8(3), -6.2 ppm] and C-11 (A8-7.7) y-carbons were only compatible with a y-gauche arrangement [66], whereas for a 10a-hydroxyl substituents, smaller shielding effects are expected on these carbon atoms, at least on C-11 (y-trans carbon) [67]. 

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