Stereoselectivity in the Cycloaddition of Cyclopentadiene toN-Fumaroyl-[2R,S(R)]-Bornane-10,2-sulfinamide Monomethyl Ester
摘要:
The cyclic [2R,S(R)]-bornane- 10,2-sulfinamide (-)-2b, an analogue of Oppolzer's camphor-derived sultam (-)-2a, was synthesized by reduction of the known N-alkylidenesulfinamide (+)-1b with NaBH4. The uncatalyzed [4 + 2] cycloaddition of cyclopentadiene to the methyl ester (-)-3b of the N-fumaroylsulfinamide, obtained from (-)-2b, proceeds with lower endo and pi -facial selectivity as compared to dienophiles (-)-3a,c. In contrast to these latter, the diastereoselectivity is reversed either in apolar CCl4, or in the presence of TiCl4,. This inversion is explained by a competitive C(alpha)-si addition on the reactive anti-s-trans conformer.
The [4 + 2] cycloaddition of cyclopentadiene to the (2R)-bornane-10,2-sultam derivative (-)-1b of fumaric monomethyl ester proceeds with high endo and pi-facial diastereoselectivity in the presence of 0.5 mol-equiv. of TiCl4. The major diastereoisomer endo-(2R,3R)-2b, isolated in 87% yield by crystallization, was subjected to X-ray crystal-structure analysis. Steric influence of ethyl- and benzyl-ester analogues (-)-1c and (-)-1d, respectively, is also reported.