ASYMMETRIC DIELS-ALDER REACTIONS OF CHIRAL ACRYLATES OF CHOLIC-ACID DERIVATIVES

New steroid-based chiral auxiliaries 6, 9, and 12 have been synthesize d from readily available cholic acid. These new chiral auxiliaries pla ce the reactive and the shielding sites in a 1,5 relationship to each other. Diels-Alder reaction of cyclopentadiene with corresponding acry late esters (7, 10, and 13) have been examined. Acrylates 7 and 10 yie lded cycloadducts with 29-88% diastereomeric excess with excellent end o selectivity in the presence of an excess of Lewis acids such as AlCl 3, BF3.OEt(2), FeCl3, SnCl4, TiCl4, and ZnCl2. Treatment of acrylate 7 with cyclopentadiene in the presence of BF3.OEt(2) at -80 degrees C g ave the endo adduct (>99%) with 88% de. Lewis acid catalyzed and uncat alyzed reactions of acrylates 7 and 10 with cyclopentadiene yielded cy cloadducts with opposite stereochemistry. The chiral auxiliary was rec overed in a nondestructive manner only via iodolactonization. Acrylate ester of alcohol 12 did not show any selectivity in either catalyzed and uncatalyzed reactions with cyclopentadiene. The presence of a flat aromatic surface at C-7 of the steroid was found to be essential to e ffect high diastereoselection.