13C-
13C Spin-spin coupling constants (
JCC) have been measured in a group of aldohexopyranoses andmethyl aldopyranosides singly labeled with
13C at different sites to confirm and extend prior correlationsbetween
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 coupledcarbons, but new evidence suggests that
2JCCC values are also affected by intervening carbon structureand C-O bond rotation.
3JC1,C6 and
3JC3,C6 values show Karplus-like dependences but also are affectedby in-plane terminal hydroxyl substituents. In both cases, rotation about the C5-C6 bond modulates thecoupling 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 dependenceon 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, astructural factor undocumented previously and one of importance to the interpretation of trans-glycoside
3JCOCC in oligosaccharides.
