Single-Step Enantioselective Synthesis of Mechanically Planar Chiral [2]Rotaxanes Using a Chiral Leaving Group Strategy

We report a one-step
enantioselective synthesis of mechanically
planar chiral [2]­rotaxanes. Previous studies of such molecules have
generally involved the separation of enantiomers from racemic mixtures
or the preparation and separation of diastereomeric intermediates
followed by post-assembly modification to remove other sources of
chirality. Here, we demonstrate a simple asymmetric metal-free active
template rotaxane synthesis using a primary amine, an activated ester
with a chiral leaving group, and an achiral crown ether lacking rotational
symmetry. Mechanically planar chiral rotaxanes are obtained directly
in up to 50% enantiomeric excess. The rotaxanes were characterized
by NMR spectroscopy, high-resolution mass spectrometry, chiral HPLC,
single crystal X-ray diffraction, and circular dichroism. Either rotaxane
enantiomer could be prepared selectively by incorporating pseudoenantiomeric
cinchona alkaloids into the chiral leaving group.