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 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 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 nonsymmetrical dienophiles. Completely oxidized cycloadducts are obtained in the reactions of 5‐alkenyl‐2‐aminothiazoles with naphthoquinone or dimethyl acetylenedicarboxylate (DMAD). Unexpectedly, the reactions with 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione (PTAD) are not stereospecific. A mechanism placed at the concerted/stepwise boundary is proposed.

Ti‐Mediated Efficient Reductive Dehalogenation of Carbon–Halogen Bonds

“…( E )‐(3‐Bromoprop‐1‐en‐1‐yl)benzene (18 a) : Following Method A of the general bromination procedure for alcohols, 18 a was obtained from commercially available cinnamyl alcohol and subsequently used without further purification.…”