[3.3] SIGMATROPY WITHIN 1-VINYL-2-ALKENYL-7,7-DIMETHYL-EXO-NORBORNAN-2-OLS - THE 1ST ATROPSELECTIVE OXYANIONIC COPE REARRANGEMENT

The anionic oxy-Cope rearrangement of several title compounds is shown to occur rapidly at room temperature exclusively via the respective endo-chair transition states. These reactions occur with complete stereoinduction to generate stereochemically homogeneous bridgehead olefinic ketones and therefore offer especially stringent probes of transition-state topographical stereoselection. Evidence is provided to show that these conversions are remarkably atropselective as well. The illustrative example selected for study was 17, its α-methyl group serving as a utilitarian1H NMR probe of structural homogeneity and conformation. This ketone is the product of a tandem [3.3] sigmatropic shift-methylation sequence. On being heated in tetrahydrofuran for several days, 17 is completely transformed into its more thermodynamically favored conformational isomer 18. These results are nicely accommodated by molecular mechanics calculations. The stereochemical course of the oxy-Cope rearrangements is compared to the pathways followed by structurally related exonorboman-2-ols and allied molecules. © 1990, American Chemical Society. All rights reserved.