Palladium-Assisted Formation of Carbon-Carbon Bonds. 3. Study of Reactions of Bis{.eta.2-(2,3,4-trimethoxy-6- acetylphenyl-C,O)}di(.mu.-chloro)dipalladium(II) with Symmetrical and Unsymmetrical Internal Alkynes. A Stoichiometric Route to Highly Functionalized Spirocyclic and Benzofulvene Compounds. Isolation of a .pi.-Allylic Palladium Intermediate. X-ray Structure of 10-(1-Hydroxyethyl)-6,7-dimethoxy-1,2,3,4-tetraphenylspiro[4.5]1,3,6,9-decatetraen-8-one