Diastereoselectivity in the hetero [4+2] cycloaddition of cyclopentadiene to N-benzyliminoacetyl derivatives of (2 R)-bornane-10,2-sultam and other chiral secondary alcohols

Abstract

Various protonated chiral glyoxyloyl-α-imino- N-benzyl hetero-dienophiles have been examined in the diastereoselective exo-cycloaddition to cyclopentadiene at −78°C promoted by CF 3CO 2 −BF 3, a dissociated non-nucleophilic counter ion. The best π-facial selectivities were observed with (2 R)-bornane-10,2-sultam (76% d.e.) and (2 R)-10-dicyclohexylsulfonamoyl-isoborneol (80% d.e.) as chiral auxiliaries. These conditions were found to be superior in terms of yields and selectivities as compared to analogous aza-dienophiles treated with simple Lewis acids or under thermal conditions. Absolute configurations were assigned on the basis of an X-ray analysis of the major cycloadduct (3′ S)- 3a as well as by chiroptical comparison with the corresponding new amino alcohol (−)-(3 S)- 4a. Plausible transition states are briefly discussed on the basis of PM3 conformational calculations.