Two Novel 1,2,4,5-Tetrazines that Participate in Inverse Electron Demand Diels−Alder Reactions with an Unexpected Regioselectivity

[reaction: see text] Two new unsymmetrical 1,2,4,5-tetrazines, 3-methylsulfinyl-6-methylthio-1,2,4,5-tetrazine (4) and 3-(benzyloxycarbonyl)amino-6-methylsulfinyl-1,2,4,5-tetrazine (5), were prepared, and the scope of their participation in intermolecular inverse electron demand Diels-Alder reactions was defined. As anticipated, sulfoxides 4 and 5 (4 > 5) display a reactivity that is substantially greater than that of their corresponding sulfides (2 and 3), being derived from their enhanced electron-deficient character and resulting in a wider range of potential dienophile choices or the use of milder reaction conditions. The cycloaddition reactions were expectedly regioselective, typically producing a single cycloadduct, ensuring their synthetic utility, but both were found to proceed with a regioselectivity opposite what would be anticipated and complementary to that observed with 2 and 3.