Stereo- and Regioselective Synthesis of Molecular Baskets.

Abstract

We describe a stereoselective method for obtaining multigram quantities of molecular basket 1 syn 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 1 syn/ 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 1 syn, a range of recognition studies can now be completed for producing novel supramolecular catalysts, organophosphorus scavengers, and nanostructured materials.