Two-bond 13C–13C spin-coupling constants in carbohydrates: New measurements of coupling signs

d-(1,3,6-13C3)Allose (1), (13C)methyl α-d-(1,2-13C2)glucopyranoside (2) and (13C)methyl β-d-(1,2-13C2)glucopyranoside (3) were synthesized and used to establish the signs of their constituent 2JCCC or 2JCOC values (2JC1,C3 in the α-pyranose of 1 (15), and 2JC1,CH3 in 2 and 3). Compounds 2, 3 and 15 contain three mutually coupled labeled carbons, thus creating a three-spin system from which crosspeak displacements in 13C–13C COSY-45 spectra were used to determine coupling signs. In all compounds, at least one 3JCC value was present as an internal reference: 3JC2,CH3 in 2 and 3, and 3JC1,C6 and 3JC3,C6 in 15. 2JC1,CH3 in 2 and 3, and 2JC1,C3 in 15, were found to be negative, thus providing experimental confirmation of the sign predictions made via the projection resultant rule described recently.