Asymmetric Diels‐Alder Reactions of Neopentyl‐Ether‐Shielded Acrylates and Allenic Esters: Syntheses of (−)‐Norbornenone and (−)‐β‐Santalene

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 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.