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.
New Reactions and Methodology, Materials, Total Synthesis, Process R&D
8:00 PM-10:00 PM, Sunday, 28 August 2005 Washington DC Convention Center -- Hall A, Poster