PM3 analysis of nitrone cycloadditions to methyl 4,4,4-trifluoro-2-butenoate

3,4-Dihydroisoquinoline N-oxide (1) cycloadded with methyl 4,4,4-trifluoro-2-butenoate at 20 degrees C to give a mixture of the cycloadduct with a 1,10b-cis-configuration, methyl (1R*, 2R*, 10bR*)-2-trifluoromethyl-1,5,6,10b-tetrahydro-2 H-isoxazolo[3,2-a]isoquinoline-1-carboxylate, and its stereoisomer with a 1,10b-trans-configuration in a ratio of 3:1. Nitrone 1 cycloadded with methyl 2-butenoate to afford the cycloadduct with a 1,10b-cis-configuration exclusively. The stereoselectivity in the cycloadditions was reproduced by calculations of the transition structures by the PM3 method.