Facial Selectivity in the Nucleophilic Additions of Vinylmagnesium Bromide to Bicyclo[2.2.2]oct-5-en-2-one Derivatives

Abstract

The levels of diastereoselection attainable by addition of vinylmagnesium bromide to a selection of bicyclo[2.2.2]octenone derivatives 1-6 in the presence of various Lewis acids such as LiBr, CeCl3, TiCl4, ZnBr2, MgBr2, and Et2AlCl have been determined. The 1,2-addition of ketone 1 with vinylmagnesium bromide in THF provided a mixture of anti- and syn-isomers. The reactions of 2 with vinylmagnesium bromide at room temperature afforded anti- and syn-isomers with preference to anti-isomers in most cases. These reactions in the presence of Lewis acids afforded anti-isomers as the major product with an excellent stereoselectivity or as single isomers in some cases. The ketones 3 gave surprisingly different results providing anti-isomers predominantly even in the presence of Lewis acids. The bicyclic ketones 4 and 5 and all-carbon tricyclic ketone 6 furnished the syn-isomer as the main product. There is no significant effect of Lewis acid catalysis in the nucleophilic addition reactions of 1, 4, 5, and 6. The use of a preformed vinylmagnesium bromide-CeCl3 reagent for the addition reactions of 2d-f and 3d-f provided almost exclusively syn-isomers. The substituents and reaction conditions can influence facial selectivity in the nucleophilic additions to the bicyclo[2.2.2]oct-5-en-2-one derivatives.