Synthesis of novel pentacyclo-undecane chiral ligands for application in asymmetric catalysis

Abstract

There is enormous interest in the design and development of efficient chiral ligands for asymmetric

catalysis, as a result, this field has become one of the most popular areas of research in organic

chemistry. This project involved the investigation of the novel chiral pentacyclo-undecane (PCU)

diol 54a, PCU bisimine 87 and PCU bis(oxazoline) 100 type ligands. The PCU diol ligand was

synthesized, but proved to be difficult to obtain enantiomerically pure which hindered further

investigation into this type of ligand. The PCU bisimine ligand 87 was synthesized. However due

to its instability it was not further pursued. Synthesis of the PCU bis(oxazoline) ligand 100 was

successful. This ligand was complexed to various metal salts and its efficiency as a chiral Lewis

acid catalyst was evaluated on the asymmetric Diels-Alder reaction between 3-acryloyloxazolidin-

2-one 52 and cyclopentadiene 33. The anhydrous magnesium perchlorate ligand complex emerged

as the best catalyst providing the endo-cycloadduct product 53 in 81 % enantiomeric excess at -40

oC. Optimizations of the possible conformations of the magnesium complex of ligand 100 with the

substrate 52 were performed using Density Functional Theory (DFT) calculations. The more

energetically favoured complex conformation was established. The Re-face of the dienophile which

was less hindered produced the product consistent with the experimentally observed product 16.

Based on the calculated bond lengths from the computational model binding of the ether oxygen on

the PCU moiety to magnesium was observed. All the novel compounds were fully characterized

using NMR, IR and mass spectroscopy as the main tools.