Cp*RuCl-Catalyzed Formal Intermolecular Cyclotrimerization of Three Unsymmetrical Alkynes through a Boron Temporary Tether:  Regioselective Four-Component Coupling Synthesis of Phthalides

Highly substituted phthalides were efficiently synthesized by sequential Cp(*)RuCl-catalyzed cyclotrimerization of alkynylboronates, propargyl alcohols, and terminal alkynes and palladium(II)-catalyzed carbonylation of the resultant arylboronates. The intermediate arylboronate was isolated and unambiguously characterized by X-ray crystallography. The perfect regioselectivity of the ruthenium-catalyzed formal intermolecular cyclotrimerization was discussed on the basis of the density functional calculations of a boraruthenacycle intermediate.