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(CC–Ar)­OCH2CH3 1a–c (M =
Cr) and 1d–f (M = W) and (CO)5MC­(CC-cyclohexenyl)­OCH2CH3 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.