Configurational assignment in alkyl‐branched sugars via the geminal C,H coupling constants

Abstract

The measurement of the magnitude and sign of 2 J(C,H) couplings offers a reliable way to determine the absolute configuration at a carbon center in a fixed cyclic system. A decrease of the dihedral angle f in the O -C A -C B -H fragment always leads to a change of the 2 J(C A ,H B ) coupling to more negative values, independent of the type and position of substituents at the two carbon centers. The orientations of the two substituents at C-3 of the epimeric pair 1 and 2 were determined unambiguously through the measurement of the geminal coupling constants between C-3 and the hydrogen atoms at C-2 and C-4. In particular, 2 J(C-3,H-2ax) with −1.5 Hz, f = 174• in 1 and −6.6 Hz, f = 47