Diels-Alder reactions of tetradehydrodianthracene with electron-deficient dienes

Theoretical calculations and simple MO considerations predict that a coplanar and spatially close arrangement of double bonds in structures such as tetradehydrodianthracene 1, should both lower the LUMO and raise the HOMO energy of the pi system. Thus the reactivity in Diels-Alder reactions towards electron-rich, as well as electron-deficient, dienes should be enhanced. The first effect has already been shown in a previous paper, the latter statement is now also confirmed by the reactivity of 1 in Diels-Alder reactions with inverse electron demand. In [4 + 2] cycloaddition reactions of 1 with 1,2,4,5-tetrazines 2 the electronic effect of 3,6 substituents in tetrazines is far outweighed by steric factors resulting in an unexpected reactivity sequence: 2a much greater than 2b approximate to 2e > 2d.