2-BOND C-13-C-13 SPIN-COUPLING CONSTANTS IN CARBOHYDRATES - NEW MEASUREMENTS OF COUPLING SIGNS

D-(1,3,6-C-13(3))Allose (1), (C-13)methyl alpha-D-(1,2-C-13(2))glucopy ranoside (2) and (C-13)methyl beta-D-(1,2-C-13(2))glucopyranoside (3) were synthesized and used to establish the signs of their constituent (2)J(CCC) or (2)J(COC) values ((2)J(C1,C3) in the alpha-pyranose of 1 (15), and (2)J(C1,CH3) in 2 and 3). Compounds 2, 3 and 15 contain thre e mutually coupled labeled carbons, thus creating a three-spin system from which crosspeak displacements in C-13-C-13 COSY-45 spectra were u sed to determine coupling signs. In all compounds, at least one (3)J(C C) value was present as an internal reference: (3)J(C2,CH3) in 2 and 3 , and (3)J(C1,C6) and (3)J(C3,C6) in 15. (2)J(C1,CH3) in 2 and 3, and (2)J(C1,C3) in 15, were found to be negative, thus providing experimen tal confirmation of the sign predictions made via the projection resul tant rule described recently. (C) 1998 Elsevier Science Ltd. All right s reserved.