Supramolecular Chiral Self-assembly Of Natural Triterpenoids

Triterpenoids,one class of naturally occurring compounds,own rigid skeletons,multiple functional groups,abundant chiral centers,relatively low toxicity and good biocompatibility.These features render them as one of the ideal building blocks for creating various supramolecular structures.However,how to fabricate nano/microscale chiral structures using triterpenoids as the basic unit is still rare,as well as the role of molecular chirality during the assembly process.Thus,it is of great importance to study the chiral assembly behavior of functional natural triterpenoids,which will not only expand the applications of natural products in supramolecular chemistry,but also help us better understand the chiral phenomena in nature and life.In this dissertation,a series of triterpenoid-based amphiphiles have been designed and synthesized by taking glycyrrhetinic acid and oleanolic acid as building blocks,respectively.By exploring their assembly behaviors,the regular chiral nano/microscale structures have been constructed,meaning the molecular chirality has been transferred to the supramolecular level successfully.In addition,the mechanism of these chiral assemblies has been investigated,as well as their applications in preparing chiral silica materials.The major results are as follows:1.By adjusting the solvent polarity and composition,hierarchical self-assembly of a pyridinium-tailored methyl oleanolate amphiphile was achieved.Morphologies underwent a pathway from spherical nanoparticles to microrods passing through nanofiber bundles in nonpolar solvent,while the morphological transition from irregular nanoparticles to nanofibers and flexible ribbons in polar solvent.During the assembly process,opposite molecular orientations and supramolecular chirality were detected as a result of the solvation effect.These preliminary results provided a facile approach for adjusting the supramolecular chirality of triterpenoid-based amphiphiles.2.By modifying various hydrophilic groups on the hydrophobic skeleton,a series of glycyrrhetinic acid-containing amphiphiles were synthesized,and their chiral self-assembly behaviors were investigated.Driven by the combination of ?-? stacking,van der Waals forces,and hydrophobic interactions,a pyridinium-functionalized methyl glycyrrhetate aggregated orderly in chloroform/aromatic solvents by sequestering the hydrophilic pyridinium within the interior and projecting the hydrophobic skeleton on the outside surface.Due to the steric hindrance among stacked molecules,this protrusion generated a staggered angle,which further led to the molecular misalignments and the formation of helical fibrils,strongly revealing the chirality transfer from molecular scale to nano/microscale.Morever,their structure-function relationship in chiral assembly behaviors were revealed by comparing with other amphiphilic glycyrrhetinic acid derivatives.Apparently,such results offered a bottom-up strategy to fabricate supramolecular macroscopic chiral nanostructures.3.By analysing the temperature-dependent CD and UV-Vis spectra,the chiral assembly mechanism of triterpenoid-containng amphiphiles was revealed from the view of thermodynamics,during which molecules assembled into helices in a cooperative fashion(nucleation-elongation mechanism).On this basis,chiral silica materials have been prepared using these organic helices as templates through the electrostatic interactions between tetraethyl orthosilicate and molecules during the sol-gel process.