Selective tandem cyclization-addition reactions using catalytic quantities of BiBr 3 and π-nucleophiles

ORGN 116

Melissa M. Sprachman, mmspra@wm.edu, Mamio C. Mattern, Yajing Lian, yxlian@wm.edu, and Robert J. Hinkle, rjhink@wm.edu. Department of Chemistry, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187-8795

 

A tandem cyclization-addition route to 2-6 substituted tetrahydropyran systems can be accomplished by using delta-silyloxy carbonyl compounds, catalytic quantities of Bi(III) reagents and silyl ketene acetals as nucleophiles.  These Bi(III) initiated reactions occur via the intermediate formation of a cyclic oxocarbenium ion followed by addition of the silyl ketene acetal nucleophile. Axial approach of the nucleophile to the oxocarbenium ion results in formation of the trans-2,6-disubstituted product by 5-6:1 (trans-/cis-).  When using allyl-TMS as a nucleophile rather than a silyl ketene acetal, very high diastereoselectivity (>99:1) is observed. We will present our results of selectivity studies involving different Bi(III) compounds and varying silyl moieties on the silyl ketene acetal nucleophiles