221. Asymmetric Diels-Alder Reactions of Neopentyl-Ether-Shielded Acrylates and Allenic Esters: Syntheses of (-)-Norbornenone and (-)-β-Santalene

Article abstract of DOI:10.1002/hlca.19850680803

Starting from (+)- or (-)-camphor, the antipodal alcohols 14 and 18, respectively, have been prepared; the corresponding acrylates 15 and 19 underwent TiCl2(i-PrO)2-mediated Diels-Alder additions to cyclopentadiene to give adducts 20a and 22a respectively, with 95percent endo- and 99.2percent ?-face selectivities.Adduct 22a was converted to enentiomerically pure norbornenone 26.Addition of 1,3-butadiene to acrylate 15 in the presence of TiCl4 afforded 3-cyclohexenyl carboxylate 29 with > 95.6percent stereodifferentiation.The TiCl2(i-PrO)2-promoted <4+2> cycloaddition of cyclopentadiene to allenic ester 43 proceeding with 99percent face differentiation served as the key step for an efficient enantioselective synthesis of (-)-β-santalene ((-)-41) with concomitant recovery of the chiral control alcohol 14.