Trapping Reactive Intermediate Carbanions Generated by Lithium Tetramethylpiperidide Treatment of 7-Oxabicycloheptenes in the Presence of Trimethylsilyl Chloride

Article abstract of DOI:10.1021/jo00357a006

Standard strong base induced methods for preparing enol ether derivatives fail with oxabicyclic ketone 4, giving instead an aldol product.The trimethylsilyl enol ether 5 can be prepared by addition of lithium tetra-methylpiperidide (LTMP) to a mixture of 4 and trimethylsilyl chloride (Me3SiCl) in THF solvent.Further reactions of 5 are observed under these conditions, leading to proximal bridgehead and vinyl trimethylsilylated products.These reactions appear to be general for benzannulated 7-oxabicyclo<2.2.1>heptenes: i.e., the enol ether function, while exerting a directing influence, is not needed for the reaction.Bridgeheads are somewhat more reactive than vinyl sites.Silylation of 9,10-dihydro-9,10-epoxyanthracene (15), which has pK_a >= 40, occurs readily, demonstrating the utility of this in situ LTMP/Me3SiCl approach for the trapping of very small equilibrium amounts of carbanions (reactive intermediates.) The benzanthracene analogue 18 is similarly mono- and bis((trimethylsilylated) with a modest level of regioselection for the 7-position.