REACTIONS OF COMPLEX LIGANDS .61. OXACYCL OALKENYLIDENE COMPLEXES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN - SYNTHESIS, THEIR ANNULATION WITH ALKYNES AND DIELS-ALDER REACTION WITH DIENES

Lithiiated propargylic ether 1 reacts with M(CO)(6) (M = Cr, Mo, W) to give the 2-oxacyclic carbene complexes 2-4 containing an alpha,beta-d ouble bond and a cyclic acetal structure which serves as a protected b enzoyl group. Reaction of the chromium complex 2 with tolan leads to a n acyl hydroquinone derivative 5. The tungsten complex 3 undergoes [4 + 2] cycloaddition with cyclopentadiene to give two diastereomeric oxa tricyclic carbene complexes 6 and 7 with an endo/exo selectivity of 3: 1. The stereochemistry of the Diels-Alder reaction is controlled by th e configuration of the acetal carbon center.