DIASTEREOSELECTIVITIES ASSOCIATED WITH THE 1,2-ADDITION OF CHIRAL (RACEMIC) CYCLOPENTENYL ORGANOMETALLICS TO BICYCLO[2.2.2]OCTENONES

The levels of diastereoselection attainable during condensation of the bicyclo[2.2.2]octenones 5-7 with organometallics derived from a selection of the vinyl bromides 8-12 have been determined. The 1,2-additions involving 6 exhibit excellent molecular recognition (>98:2) during syn (endo) addition to the carbonyl group. The diastereoselectivities associated with the capture of 5 and 7 are lower and more variable, although still respectable in several examples. In all cases, the dominant product is the alcohol in which the C-5 substituent on the nucleophilic subunit is beta-oriented. The structural assignments to the numerous alcohols follow from correlations of olefinic carbon chemical shifts, X-ray crystallography in selected examples, and anionic oxy-Cope rearrangement to generate polycyclic ketones whose stereochemical features were defined by NMR methods. The findings provide insight into the transition State geometries adopted in the course of these reactions. The model which has been formulated is consistent with the greater discriminatory power of 6 and conforms closely to earlier proposals for related reactions.