Domino Cyclization of 1,<i>n</i>‑Enynes (<i>n</i> = 7, 8, 9) Giving Derivatives of Pyrane, Chromene, Fluorene, Phenanthrene and Dibenzo[7]annulene by Ruthenium Complexes

Abstract

Cyclization of the ether enyne <b>1</b> catalyzed by [Ru]­NCCH₃⁺ ([Ru] = Cp­(PPh₃)₂Ru) in CHCl₃ generates a diastereomeric mixture of the substituted tetrahydropyran <b>11</b>. Presumably, formation of an allenylidene complex is followed by a cyclization by nucleophilic addition of the olefinic group to Cγ of the ligand giving a boat-like six-membered ring. The diastereoselectivity is controlled by the 1,3-diaxial interaction. The vinylidene complex <b>7</b>, a precursor of <b>11</b>, is obtained from <b>1</b> and [Ru]­Cl. In a mixture of MeOH/CHCl₃, the domino cyclization of <b>1</b> further affords <b>14a</b>, a chromene product catalytically. The second cyclization proceeds via nucleophilic addition of the resulting olefinic unit to Cα of <b>7</b>. But the ether enyne <b>3</b> with a cyclopentyl ring on the olefinic unit undergoes only single cyclization due to steric effect. The propargyl alcohol and the two terminal methyl groups on the olefinic unit shape the cyclization. Thus, similar all-carbon 1,<i>n</i>-enynes (<i>n</i> = 7, 8, 9) <b>4</b>–<b>6</b> each with an aromatic linker undergo direct domino cyclization catalyzed by [Ru]­NCCH₃⁺, to give derivatives of tricyclic fluorene, phenanthrene and dibenzo[7]­annulene, respectively, with no intermediate observed