Stereoselectivity in the cycloaddition of cyclopentadiene to N-fumaroyl-[2R,S(R)]-bornane-10,2-sulfinamide monomethyl ester

The cyclic [2R,S(R)]-bornane- 10,2-sulfinamide (-)-2b, an analogue of Oppol zer's camphor-derived sultam (-)-2a, was synthesized by reduction of the kn own N-alkylidenesulfinamide (+)-1b with NaBH4. The uncatalyzed [4 + 2] cycl oaddition of cyclopentadiene to the methyl ester (-)-3b of the N-fumaroylsu lfinamide, obtained from (-)-2b, proceeds with lower endo and pi -facial se lectivity as compared to dienophiles (-)-3a,c. In contrast to these latter, the diastereoselectivity is reversed either in apolar CCl4, or in the pres ence of TiCl4,. This inversion is explained by a competitive C(alpha)-si ad dition on the reactive anti-s-trans conformer.