[4+2] Cyclization reactions of chiral Cβ-substituted Fischer alkenyl carbene complexes: efficient synthesis of enantiopure cyclohexenone and norbornene derivatives

Chiral alkenyl carbene complexes of tungsten(0) 1 and 2, readily available from the chiral pool, undergo the [4+2] cycloaddition with the Danishefsky diene to provide enantiopure 4-alkenyl-2-cyclohexenone adducts 5 and 7 with high stereoselectivity after treatment of the primary cycloadducts 4 and 6 with TBAF. Cyclopentadiene also cycloadds to carbenes 1 and 2 affording the expected norbornene metal carbene complexes 10 and 12 with remarkable diastereo and face selectivity. Oxidative removal of the metal pentacarbonyl fragment leads to the ester derivatives 11 and 13. The X-ray structure analysis of two cycloadducts derived from carbenes 1 and 2 has been performed.