Covalent Chemistry and Conformational Dynamics of Topologically Chiral Amide‐Based Molecular Knots

Abstract

The readily available in gram quantities tris(allyloxy)knot of the amide‐type 5 (knotane) can be completely and partially deprotected with nBu₃SnH in the presence of a palladium catalyst resulting in hydroxyknotanes 7–9. These, in turn, react with diethylchlorophosphate giving rise to knotanes equipped with between one and three phosphoryl groups. Sulfonylation of bis(allyloxy)monohydroxyknotane 8 with p‐toluenesulfonyl chloride and, following removal of one or two allyl groups from the intermediate monosulfonate 13, give rise to sulfonyloxy‐allyloxy‐hydroxy‐ and sulfonyloxy‐dihydroxy‐knotanes 15 and 14, respectively. This provides a convenient method for the preparation of knotanes with any substitution pattern. All new knotanes have been isolated in preparative amounts and as highly pure substances with an exception of allyloxy‐dihydroxyknotane 9. This compound could only be obtained as a mixture with the corresponding monohydroxy‐derivative 8. The structures of all synthesized compounds were established by means of FAB and MALDI TOF mass spectrometry, ¹H and ³¹P NMR spectroscopy. The triphosphorylated knotane 10 exhibits high solubility in alcohols, allowing its complete enantiomeric resolution with a commercially available chiral HPLC column. ¹H,¹H DQF‐COSY correlation spectroscopy along with H/D exchange experiments and ab initio calculations provided the first detailed ¹H NMR signal assignments of knotanes in [D₆]DMSO solution. The combination of variable temperature ¹H and ³¹P NMR spectroscopy and molecular modeling has been applied to study the conformational behavior of the new knotanes in different solvents. It has been shown that in DMSO solution at room temperature knotanes exist in a relatively rigid nonsymmetrical conformation similar to that found in the solid state while faster conformational exchange leading to the average D₃ symmetrical structure was detected in a number of other solvents.