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

The measurement of the magnitude and sign of 2J(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 ϕ in the O—CA—CB—H fragment always leads to a change of the 2J(CA,HB) 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, 2J(C‐3,H‐2ax) with −1.5 Hz, ϕ = 174° in 1 and −6.6 Hz, ϕ = 47° in 2, and 2J(C‐3,H‐4) with +1.5 Hz, ϕ = 175° in 1 and −4.7 Hz, ϕ = 49° in 2 showed the greatest differences between the two epimers. Both couplings therefore allow the determination of the absolute configuration at C‐3. It should be noted, however, that the size of the coupling constants can be different for dihedral angles of nearly identical size, when there are different numbers of electronegative substituents on the two coupling pathways, i.e. no O‐substituent at C‐2, but one axial O‐substituent at C‐4. It becomes clear that it is not sufficient to measure the magnitude of 2J coupling constants only, but that the sign of the geminal coupling is needed to identify the absolute configuration at a chiral center. The coupling of C‐3 with H‐2eq is not useful for the determination of the configuration at C‐3, as the similarity of the dihedral angles ϕ (O—C‐3—C‐2—H‐2eq) (57° in 1 and 70° in 2) leads to identical coupling constants (−6.1 Hz) for both epimers. Copyright © 2003 John Wiley & Sons, Ltd.