π-Facial selectivities and proximal/distal regioselectivities in Diels–Alder reactions of unsymmetrical, cage-annulated 1,3-cyclohexadienes

Diels–Alder cycloaddition of hexacyclo[7.5.1.01,6.06,13.08,12.010,14]pentadeca-2,4-diene-7,15-dione (1) to ethyl propiolate proceeds with π-facial selectivity to afford two cycloadducts, 4b and 5b (product ratio: 4b/5b=80:20). Thermal [4+2] cycloaddition of hexacyclo[7.5.2.01,6.06,13.08,12.010,14]hexadeca-2,4-diene-7,16-dione (2) to ethyl propiolate affords two isomeric cycloadducts, i.e. 6c and 6d (product ratio: 6c/6d=40:60). Cycloadducts 6c and 6d are formed with a high degree of endo π-facial selectivity but with only modest proximal/distal regioselectivity. The corresponding [4+2] cycloaddition of 2 (diene) to dimethyl acetylenedicarboxylate (dienophile) produced a single cycloadduct, 7b. These observations have been further examined by employing Hartree–Fock relative energies of transition states presumably formed enroute to the various products. Interestingly, filled orbital electrostatic repulsion between the incoming dienophile and the CO groups in the diene substrate does not appear to be a significant factor that might influence π-facial selection in this [4+2] cycloaddition. Finally, thermal [4+2] cycloaddition of cyclopentadiene (diene) to dienophile 3 proceeded with a high degree of π-facial selectivity to afford a single cycloadduct, 9b.