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Configurational assignment in alkyl‐branched sugars via the geminal C,H coupling constants
Author(s) -
Schulte Jürgen,
Lauterwein Jürgen,
Klessinger Martin,
Thiem Joachim
Publication year - 2003
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1141
Subject(s) - geminal , chemistry , dihedral angle , substituent , coupling constant , epimer , sign (mathematics) , stereochemistry , crystallography , absolute configuration , coupling (piping) , hydrogen , hydrogen bond , molecule , physics , organic chemistry , quantum mechanics , mechanical engineering , mathematical analysis , mathematics , engineering
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 ϕ 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, ϕ = 174° in 1 and −6.6 Hz, ϕ = 47° in 2 , and 2 J (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 2 J 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.