Abstract 5-Alkenyl-2-aminothiazoles react as in-out dienes with a wide range of electron- poor dienophiles leading to the corresponding cycloaddition products in good to excellent yields. The [4+ 2] cycloadditions of 5-alkenyl-2-aminothiazoles can be classified as site- selective because only the diene moiety incorporating the formal C–C double bond of the heterocycle and that of the side-chain is involved. Calculations of the HOMO energy ...