Unprecedented Synthesis of 3‑Alkenyl-3-ethoxy-2-iminoketones via 1,3-Dipolar Cycloadditions of Nitrones with Alkynyl Fischer Carbene Complexes

Abstract

The synthesis of novel and highly substituted (2<i>Z</i>,3<i>Z</i>)-3-ethoxy-1-(aryl/cyclohexenyl)-4-aryl-2-(phenylimino)­but-3-en-1-one derivatives <b>24a</b>–<b>k</b> and <b>27a</b>–<b>e</b> 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 <b>1a</b>–<b>c</b> (M = Cr) and <b>1d</b>–<b>f</b> (M = W) and (CO)₅MC­(CC-cyclohexenyl)­OCH₂CH₃ <b>23a</b> (M = Cr) and <b>23b</b> (M = W) with <i>C</i>,<i>N</i>-diaryl nitrones (<b>7a</b>–<b>f</b>) 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 (<i>Z</i>,<i>Z</i>) diastereoisomers <b>24a</b>–<b>k</b> and <b>27a</b>–<b>e</b> as the isolated products. The stereochemical assignment of the products was supported by NOE measurements and by single-crystal X-ray diffraction