Synthesis and chiroptical properties of a new type of chiral depsipeptide dendrons

A new type of chiral depsipeptide dendrons based on tartaric acid as branching juncture and ω-aminocapronic acid as spacer has been prepared. Natural and unnatural tartaric acid building blocks have been incorporated, providing access to combinatorial libraries. All compounds have been completely characterized by FAB-MS, EA analysis, 1H/13C NMR- and UV/Vis-spectroscopy. 1H NMR relaxation measurements have been used to examine the conformational flexibility of these dendrons in CH3CN and CH3OH and indicate a less flexible structure in CH3CN. Pulse gradient spin echo (PGSE) NMR measurements correlate these findings to molecular dimensions. A reduced size for the dendrons in CH3CN compared to CH3OH leads to the assumption of a more compact structure in this solvent. Additional polarimetric data reveal, that observed changes in optical activity with increasing generation can in CH3OH be explained by constitutional effects of the dendron structure but not in CH3CN. CD measurements are in agreement with these findings and show a linear increase of the Cotton effects with increasing generation for CH3OH but not for CH3CN. It can be concluded that the conformation within the dendrons is very sensitive to environmental conditions and that a chiral secondary structure might be stabilized in CH3CN. Initial studies revealed that chirality transfer to the focal functionality occurs.