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 3 via 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 ).