Studies in pi-facial selectivity.

Abstract

A number of dienophiles and ketones have been prepared and the resultant $\pi$-facial selectivity of these substrates in Diels-Alder and nucleophilic additions have been investigated. The ketene equivalent dienophile 46 gave all four possible cycloadducts when reacted with cyclopentadiene under thermal conditions. The resultant $\pi$-facial selectivity was in accord with the calculated electrostatic potential surface of 46. The $\pi$-facial selectivity of the double bond isomer of dienophile 46 was also in accord with the electrostatic potential model. The use of boron trichloride at low temperatures resulted in the isolation of a single endo cycloadduct with complete $\pi$-facial selectivity in the Diels-Alder reaction of 46 with cyclopentadiene. The Cieplak model was not followed in either thermal or Lewis acid-catalyzed conditions employed. The $\pi$-facial selectivity of a number of cyclic ketones containing adjacent heteroatoms using organometallic and reducing agents was examined. The $\pi$-facial selectivity was independent of the size of reducing agent, contrary to results using cyclohexanones. The use of diisobutylaluminum hydride produced a reversal of $\pi$-facial selectivity in cyclopentanones 97, 108, 114, but no reversal was observed with spiro-ketones 129 and 133. The failure of the Cieplak model in many of the reduction reactions indicates that transition state stabilization by hyperconjugation is not an important factor. In many of the cases, electrostatic potential surfaces correctly predicted $\pi$-facial selectivity. Carbon nucleophiles such as methyllithium added to give the major diastereomer predicted by the Cieplak model in all cases examined except tetrahydrofuran 129. The levels of $\pi$-facial selectivity for both carbon nucleophile addition and reduction reactions were at synthetically useful levels for most reagents with the exception of zinc borohydride.* ftn*Please refer to the dissertation for diagrams.