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

Various protonated chiral glyoxyloyl-alpha -imino-N-benzyl hetero-dienophil es have been examined in the diastereoselective exo-cycloaddition to cyclop entadiene at -78 degreesC promoted by CF3CO2-BF3, a dissociated non-nucleop hilic counter ion. The best pi -facial selectivities were observed with (2R )-bornane-10,2-sultam (76% d.e.) and (2R)-10-dicyclohexylsulfonamoyl-isobor neol (80% d.e.) as chiral auxiliaries. These conditions were found to be su perior in terms of yields and selectivities as compared to analogous aza-di enophiles treated with simple Lewis acids or under thermal conditions. Abso lute 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 c orresponding new amino alcohol (-)-(3S)-4a. Plausible transition states are briefly discussed on the basis of PM3 conformational calculations. (C) 200 1 Elsevier Science Ltd. All rights reserved.