5-Alkenylthiazoles as In-Out Dienes in Polar [4+2] Cycloaddition Reactions

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 since only the diene moiety including the formal C-C double bond of the heterocycle and that of the side-chain is involved. Calculations of the HOMO energy values of representative 5-alkenyl-2-aminothiazoles are disclosed The cycloadditions are endo-selective with N-phenylmaleimide or maleic anhydride and regioselective when the reactions are conducted with non symmetrical dienophiles. Completely oxidized cycloadducts are obtained in the reactions of 5-alkenyl-2-aminothiazoles with naphthoquinone or DMAD. Unexpectedly, the reactions with PTAD are not stereospecific. A mechanism placed at the concerted/stepwise boundary is proposed.