COMPARATIVE-ANALYSIS OF DIASTEREOSELECTION LEVELS ATTAINABLE DURING CONTROLLED EXO AND ENDO ADDITION OF CHIRAL CYCLOPENTENYL ORGANOMETALLICS TO OPTICALLY PURE AND RACEMIC 1-VINYLNORBORNAN-2-ONES

The levels of diastereoselection attainable during endo addition to (+)-8 and of exo addition to (-)-9 and (±)-10 have been tested with five differently substituted cyclopentenyl anions. The extent of intermolecular recognition reaches useful levels when an endo attack is involved. In contrast, diastereomeric differentiation is significantly reduced when an exo trajectory is followed. These data are concisely rationalized in terms of an essentially coplanar arrangement of the reactants with appropriate stacking during the endo face-selective process. In contrast, a knife-edge alignment is adopted by the cyclopentenyl species as exo attack on the norbornanone carbonyl group begins. Structural assignments to the product alcohols were realized by direct X-ray crystallographic analysis of these or their anionic oxy-Cope products. The balance of the structural information evolved from diagnostic 1HNMR correlations involving the cyclopentenyl olefinic proton and those of the vinyl group when recorded in benzene-d6 solution. In each example, oxy-Cope rearrangement took place stereospecifically to deliver a single product. Whereas an endo chair transition state was followed by the exo alcohols, an exo boat arrangement was adopted in the endo norbornanol series. © 1990, American Chemical Society. All rights reserved.