| |
[摘要]：The synthesis and conformational anal. in aq. soln. of different a-methyl-a-amino acid diamides, derived from serine, threonine, b-hydroxycyclobutane-a-amino acids, and their corresponding model b-O-glucopeptides, are reported. The study reveals that the presence of an a-Me group forces the model peptides to adopt helix-like conformations. These folded conformations are esp. significant for cyclobutane derivs. Interestingly, this feature was also obsd. in the corresponding model glucopeptides, thus indicating that the a-Me group and not the b-O-glucosylation process largely dets. the conformational preference of the backbone in these structures. On the other hand, atypical conformations of the glycosidic linkage were exptl. detd. Therefore, when a Me group was located at the Cb atom with an R configuration, the glycosidic linkage was rather rigid. Nevertheless, when the S configuration was displayed, a significant degree of flexibility was obsd. for the glycosidic linkage, thus showing both alternate and eclipsed conformations of the ys dihedral angle. In addn., some derivs. exhibited an unusual value for the fs, angle, which was far from a value of -60?expected for a conventional b-O-glycosidic linkage. In this sense, the different conformations exhibited by these mols. could be a useful tool in obtaining systems with conformational preferences "a la carte". |
|
