Predictive Analysis of the Side Chain Conformation of the Higher Carbon Sugars: Application to the Preorganization of the Aminoglycoside Ring 1 Side Chain for Binding to the Bacterial Ribosomal Decoding A Site.

With a view to facilitating prediction of the exocyclic bond to the pyranoside ring in higher carbon sugars, a model is advanced that relates the relative configuration of the three stereogenic centers comprised of the branchpoint and of the two flanking centers (C4-C5-C6 in aldoheptoses and higher and C5-C6-C7 in sialic and ulosonic acids) to that of the simple ring-opened pentoses. Assignment of a given stereotriad as arabino, lxyo, ribo, or xylo by inspection of the Fischer projection formulas permits prediction of conformation of the exocyclic bond by comparison with the known solution (= crystal in all cases) conformations of the simple pentitols. More remote stereogenic centers in the side chain, as in the 8-position of N-acetylneuraminic acid, have little impact on the conformation of the exocyclic bond. On the basis of this model the conformation of the exocyclic bond in ring I of 6'-homologated 4,5-disubstituted 2-deoxystreptamine class aminoglycoside antibiotics was predicted and was borne out by NMR analysis of newly synthesized derivatives in D2O at pD5. The antiribosomal and antibacterial activity of these derivatives is briefly presented and discussed in terms of preorganization of the side chain for binding to the ribosomal decoding A site. It is anticipated that this predictive analysis will also find use in the prediction of the conformation of the exocyclic bonds in other 2-(1-hydroxyalkyl)-3-hydroxytetrahydropyrans and tetrahydrofurans.