112. Asymmetric Diels-Alder Reactions of Neopentyl-Ether-Shielded Acrylates and Allenic Esters: Syntheses of (–)-Norbornenone and (–)-β-Santalene
     Oppolzer, W.; Chapuis, C.; Dupuis, D.; Guo, M.
     Helv. Chim. Acta 1985, 68, 2100-2114

Starting from (+)- or (−)-camphor, the antipodal alcohols 14 and 18, respectively, have been prepared; the corresponding acrylates 15 and19 underwent TiCl₂(i-PrO)₂-mediated Diels-Alder additions to cyclopentadiene to give adducts 20a and 22a respectively, with 95 % endo- and 99.2% πpH-face selectivities. Adduct 22a was converted to enantiomerically pure norbornenone 26. Addition of 1,3-butadiene to acrylate 15 in the presence of TiCl₄ afforded 3-cyclohexenyl carboxylate 29 with > 95.6 % stereodifferentiation. The TiCl₂(i-PrO)₂-promoted [4 + 2] cycloaddition of cyclopentadiene to allenic ester 43 proceeding with 99 % 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.

DOI : 10.1002/hlca.19850680803