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

Two new unsymmetrical 1,2,4,5-tetrazines, 3-methylsulfinyl-6-methylthio-1,2,4,5-tetrazine (<b>4</b>) and 3-(benzyloxycarbonyl)amino-6-methylsulfinyl-1,2,4,5-tetrazine (<b>5</b>), were prepared, and the scope of their participation in intermolecular inverse electron demand Diels−Alder reactions was defined. As anticipated, sulfoxides <b>4</b> and <b>5</b> (<b>4</b> > <b>5</b>) display a reactivity that is substantially greater than that of their corresponding sulfides (<b>2</b> and <b>3</b>), 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 <b>2</b> and <b>3</b>.