BOAT CHAIR TOPOGRAPHIC STEREOSELECTION DURING ANIONIC OXY-COPE REARRANGEMENT OF 1-ALKENYL-2-CYCLOPENTENYL-ENDO-NORBORNAN-2-OLS

In order to examine which of four possible [3.3] sigmatropic transition states l-alkenyl-2-cyclopentenyl-endo-norbornan-2-ols would adopt during oxy-Cope rearrangement of their potassium salts at room temperature, the functionalized norbornanones 15,19, 20, and 24-26 were prepared. Since those in the 3,3-dimethyl series are derived from (1R)-(-)-fenchone, they were available in optically pure form. Condensation of these ketones with cyclopentenyllithium (for the nonenolizable series) or cyclopentenyldichlorocerium(III) proceeded exclusively with C-C bond formation from the exo direction. In each example, oxy-Cope rearrangement took place efficiently, and structural assignments to the products were arrived at on the strength of extensive X-ray crystallographic data. In all but one instance, bridgehead olefinic ketones were formed. The major exception materialized because of unusual SN’ displacement of methoxide ion by the transannularly positioned enolate ion. The stereochemistries of these products are inextricably linked to the particular operational transition state. The results show clearly that exo-boat transition states are utilized as long as the sigmatropic change occurs readily, as it does except in one example. A slower reaction rate favors adoption of an exo-chair geometry. From the synthetic viewpoint, the stereochemical control elements elucidated herein point the way to the possible utilization of this methodology in elaboration of diverse oxacembranolides. © 1990, American Chemical Society. All rights reserved.