Regioselective Inverse Electron Demand Diels-Alder Reactions of N-Acyl 6-Amino-3-(methylthio)-1,2,4,5-tetrazines

The regioselective inverse electron demand Diels-Alder reactions of 6-[(tert-butyloxycarbonyl)amino]-3-(methylthio)-1,2,4,5-tetrazine (2), 6-(acetylamino)-3-(methylthio)-1,2,4,5-tetrazine (3), and 6-(benzyloxycarbonyl)amino-3-(methylthio)-1,2,4,5-tetrazine (4) are disclosed. All three underwent regioselective [4 + 2] cycloaddition with electron-rich dienophiles to form the corresponding functionalized 1,2-diazines in excellent yields. An order of reactivity with electron-rich dienophiles was observed with both 2 and 3 being more reactive than 3,6-bis(methylthio)-1,2,4,5-tetrazine (1, i.e. 3 > 2 > 1), and both 3 and 4 were shown to be more robust than 2 at the higher temperatures necessary for [4 + 2] cycloaddition with less reactive dienophiles. The cycloaddition regioselectivity is consistent with the polarization of the diene and the ability of the methylthio group to stabilize a partial negative charge at C-3, and the N-acylamino group to stabilize a partial positive charge at C-6. While intermolecular reactions of unactivated alkynes either did not proceed or required high temperatures and long reaction times, intramolecular Diels-Alder reactions utilizing tethered unactivated acetylenes led to five- and six-membered bicyclic 1,2-diazines under mild conditions.