Unprecedented synthesis of 3-alkenyl-3-ethoxy-2-iminoketones via 1,3-dipolar cycloadditions of nitrones with alkynyl Fischer carbene complexes

The synthesis of novel and highly substituted (2Z,3Z)-3-ethoxy-1-(aryl/ cyclohexenyl)-4-aryl-2-(phenylimino)but-3-en-1-one derivatives 24a-k and 27a-e by an unexpected and previously unreported [3 + 2] cycloaddition/rearrangement and ring-opening cascade process of the Fischer carbene complexes (CO) ₅M=C(Cî -C-Ar)OCH₂CH₃ 1a-c (M = Cr) and 1d-f (M = W) and (CO)₅M=C(Cî -C-cyclohexenyl)OCH ₂CH₃ 23a (M = Cr) and 23b (M = W) with C,N-diaryl nitrones (7a-f) is described. It is likely that the unstable 2,3-dihydroisoxazole carbene complexes produced in the [3 + 2] cycloaddition undergo a rapid and new rearrangement, followed by a stereoselective electrocyclic ring-opening process at a low temperature, to give only the (Z,Z) diastereoisomers 24a-k and 27a-e as the isolated products. The stereochemical assignment of the products was supported by NOE measurements and by single-crystal X-ray diffraction.