Stereo- and Regioselective Synthesis of Molecular Baskets

We describe a stereoselective method
for obtaining multigram quantities
of molecular basket 1syn in
overall 11% yield, using inexpensive cyclopentadiene and diethyl fumarate
as starting materials. First, an asymmetric synthesis of enantioenriched
bromo­(trimethylstannyl)­alkene (−)-8 was accomplished
by the stereoselective bromination of dibromonorbornene (+)-4 guided by anchimeric assistance and subsequent syn-exo-elimination of tetrabromonorbornane (−)-5a as the key steps. Subsequent Cu­(I)-catalyzed cyclotrimerization
of (−)-8 was optimized to give 1syn/anti in 85% yield and 1:1 ratio of diastereomers. Importantly, the results
of our mechanistic experiments were in line with the cyclotrimerization
occurring in a chain-type fashion with racemization of a Cu­(I) homochiral
dimeric intermediate, reducing the stereoselectivity of the transformation.
Enabled by more facile access to molecular baskets of type 1syn, a range of recognition studies can
now be completed for producing novel supramolecular catalysts, organophosphorus
scavengers, and nanostructured materials.