Stereoselectivities of Diels-Alder Cycloadditions of Π-Facially Nonequivalent Dienes to MTAD, PTAD, and N-methylmaleimide: A Theoretical Study

1,2,3,4,9,9-hexachloro-1 alpha,4 alpha,4a alpha,8a beta-tetrahydro-1,4-methanonaphthalene (1) and 1,2,3,4,9,9-hexachloro-1 alpha,4 alpha,6,7-tetrahydro-1,4-methanonaphthalene (2) undergo pi-facially diastereoselective Diels-Alder reactions with 4-methyl- and 4-phenyl-1,2,4-triazoline-3,5-dione [MTAD and PTAD, respectively], and with N-methylmaleimide (NMM). In contrast with the results of AM1 calculations, those obtained via ab initio calculations performed at the HF/3-21G* level of theory predict that the computed Diels-Alder transition states are synchronous in all cases studied. Furthermore, these computational results account quantitatively for: (i) the observed pi-facial selectivities of the various Diels-Alder reactions studied, (ii) the observed relative lack of dienophilic reactivity of NMM vis-a-vis MTAD and PTAD, and (ii) the observed enhanced diene reactivity of 1 vs. 2 in the reactions studied. (C) 1998 Elsevier Science Ltd. All rights reserved.