Synthesis of 3‐Aminotropones from N‐Boc‐Protected Furan‐2‐amine (=tert‐Butyl Furan‐2‐ylcarbamate; Boc=(tert‐Butoxy)carbonyl) by Cycloaddition Reactions and Subsequent Rearrangement

The 3-aminotropones (=3-aminocyclohepta-2,4,6-trien-1-ones) 4 were prepared in two steps by i) a [4+3] cycloaddition reaction between a conveniently substituted α,α′-dihalo ketone 1 and a furan-2-amine derivative 2 functionalized at C(2) by a protected amino group (→3), and ii) a base-induced molecular rearrangement of the cycloadduct 3via cleavage of the O-bridge. A mechanism for the formation of 3-aminotropones is proposed on the basis of the initial deprotonation of the [(tert-butoxy)carbonyl]amino (BocNH) group of 3, followed by O-bridge opening, an acid–base equilibrium, and finally an alkoxyaluminate elimination to afford the conjugated stable troponoid system (Scheme 7).