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A range of 13 C-labeled carbohydrates containing C-O-C-C coupling pathways having different structures and dihedral angles has been prepared and used to identify structural factors affecting 3 J COCC , especially those across the O-glycosidic linkages of oligosaccharides. Model mono-and disaccharides were geometrically optimized using density functional methods, and scalar couplings involving carbon were calculated using a similar approach coupled with finite-field perturbation theory. Experimental and calculated 3 J COCC values were in close agreement, thus allowing use of the latter to better define the effect of carbohydrate structure on 3 J COCC magnitude. In addition to dihedral angle, the disposition of terminal electronegative substituents along the C-O-C-C coupling pathway significantly affects 3 J COCC values, and structural motifs have been identified where these effects may be encountered in oligosaccharides. A simple Karplus equation for trans-O-glycoside 3 J COCC values is proposed and has been applied in the reanalysis of trans-O-glycosidic couplings in 13 Clabeled methyl β-lactoside and sucrose. The behavior of trans-O-glycosidic 2 J COC and 3 J COCH values, which provide structural information complementary to that derived from 3 J COCC values, is also discussed.

13C-13C Spin-spin coupling constants (JCC) have been measured in a group of aldohexopyranoses and methyl aldopyranosides singly labeled with 13C at different sites to confirm and extend prior correlations between JCC magnitude and sign and saccharide structure. Structural correlations for 2JC1,C3, 2JC2,C4, 2JC4,C6, and 2JC1,C5 have been confirmed using density functional theory calculations to test empirical predictions. These geminal couplings depend highly on the orientation of C-O bonds appended to the terminal coupled carbons, but new evidence suggests that 2JCCC values are also affected by intervening carbon structure and C-O bond rotation. 3JC1,C6 and 3JC3,C6 values show Karplus-like dependences but also are affected by in-plane terminal hydroxyl substituents. In both cases, rotation about the C5-C6 bond modulates the coupling due to the alternating in-plane and out-of-plane O6. 3JC3,C6 is also affected by C4 configuration. Both 3JC1,C6 and 3JC3,C6 are subject to remote effects involving the structure at C3 and C1, respectively. New structural correlations have been determined for 2JC3,C5, which, like 3JC3,C6, shows a remote dependence on anomeric configuration. Investigations of dual pathway 13C-13C couplings, 3+3JC1,C4 and 3+3JC2,C5, revealed an important additional internal electronegative substituent effect on 3JCC in saccharides, a structural factor undocumented previously and one of importance to the interpretation of trans-glycoside 3JCOCC in oligosaccharides.

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