Restricted Flexibility of the Glycosidic Linkage in α(1→3)‐L‐Digitoxosides as Determined by NMR Spectroscopy and MMC Calculations

Abstract

AbstractThe preferred conformations of nine protected α(1→3)‐linked digitoxoside trisaccharides 1–9, α‐L‐Dig‐(1–3)‐α‐L‐Dig‐(1–3)‐α‐L‐Dig (C–B–A), constituents of tetronolide antibiotics, were determined by a combination of 300‐MHz NMR spectroscopy and potential energy calculations using the GEGOP programme. Chemical shift differences and nuclear Overhauser enhancements indicated some unusual short distances between not directly bound digitoxose residues A and C. Thus, the glycosidic linkages of the nine α(1→3)‐linked digitoxosides have a fairly high rigidity as indicated by the relatively large absolute NOE (≈︁3%) from 1C‐H to 5A‐H. MMC simulations of the conformational flexibility resulted in preferred conformations of the glycosidic linkages of ΦC,B/ΨC,B ≈︁ 70°/40° and ΦB,A/ΨB,A ≈︁ 65°/50° with ranges of 40°/50° and 50°/60°. By energy minimisations a second minimum was identified for the B–A glycosidic linkage at ΦB,A/ΨB,A ≈︁ 0°/40° with an energy higher than 2.0 kcal/mol to the main minimum. The MMC‐derived ensemble‐averaged NOEs that were obtained by sampling the main minimum agree well with the experimental values, whereas NOEs derived from MMC runs that stayed at the local minima are not in accord with experimental values. The MMC simulations failed to give transitions between the two minima presumably because of a high energy barrier between the minima. All nine trisaccharides seem to adopt a well‐defined conformation characterised by a narrow distribution in the conformational space that is located in the region of the global minimum.