Diastereoselectivities in the diels-alder reactions of cyclopentadiene with optically active dimenthyl fumarate and menthyl methyl fumarate in LiClO 4/ether and on Al 2O 3

Abstract

Reactions of cyclopentadiene with (−)-dimenthyl fumarate, which yield adducts 1 and 2, and with (−)-menthyl methyl fumarate, which yield adducts 3 through 6, were carried out on alumina activated at different temperatures, in LiClO 4/ether solutions, and in other media. Various diastereoselectivities - 2:1,3:4, 6:5, endo:exo [( 3+ 4): ( 5+ 6)], and % de [( 3+ 6) - ( 4+ 5)]/[ 3+ 4+ 5+ 6] × 100] - were determined for these reactions. The LiClO 4 data were analyzed using a model in which LiClO 4 reversibly binds to the dienophile, resulting in competing catalyzed and uncatalyzed Diels-Alder reactions. Even in approximately 5 M LiClO 4, there are sizable contributions from uncatalyzed reactions in all cases. Extrapolation of the data to infinite LiClO 4 concentration yields data for the catalyzed reactions alone; only modest diastereoselectivities are seen for the catalyzed reactions. The alumina results were unusual because the endo:exo ratios of adducts were large but % de's were small. The selectivities on alumina could not be correlated to the activity of the solid. This behavior will occur when varying numbers and types of catalytic sites are exposed to the surface on activation.