Tandem intramolecular hydroalkoxylation-hydroarylation reactions:: Synthesis of enantiopure benzofused cyclic ethers from the chiral pool

A highly efficient and general method for the diastereoselective synthesis of benzo-fused eight-membered carbo- and heterocycles was studied. The method is based on a new tandem gold- or platinum catalyzed hydroalkoxytion- hydroarylation reaction. The use of [(cod)PtCl2] (cod=1,5- cyclooctadiene) was completely ineffective and the starting material was recovered after 24 hours at room temperature. The use of the cationic platinum(II) complex formed in situ by mixing [(cod)PtCl2] and AgOTf gave excellent results, allowing the complete transformation of alkynol 1 a into bicyclic compound 3a after only 1 h. No reaction was observed in the absence of platinum or gold complexes or by treatment of 1d with Brønsted acids, which demonstrate the necessity of the platinum or gold complexes at least for the hydroalkoxylation reaction. The number of points of diversity present in the reaction products, together with the simplicity of the starting materials, makes this reaction a powerful method for target synthesis.