title: Recent advances in "formal" ruthenium-catalyzed [2+2+2] cycloaddition reactions of diynes to alkenes.
abstract: "Formal" and standard RuII-catalyzed [2+2+2] cycloaddition of 1,6-diynes to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. When terminal 1,6-diynes 1 were used, two isomeric bicyclic 1,3-cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6-diynes 7 were used, the reaction with acyclic alkenes took place regio- and stereoselectively to afford bicyclic 1,3-cyclohexadienes 8. A cascade process that behaves as a "formal" RuII-catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru-catalyzed linear coupling of 1,6-diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5-trienes of type 3, which after a thermal disrotatory 6e(-) pi-electrocyclization led to the final 1,3-cyclohexadienes 4 and 8. When disubstituted 1,6-diyne 10 was used with electron-deficient alkenes, new exo-methylene cyclohexadienes 12 arose from a competitive reaction pathway.
authors.affiliation: Departamento de Química Orgánica, Facultad de Química, Universidad de Santiago de Compostela, 15782 Santiago de Compostela (Spain).
authors.forename: Silvia
authors.initials: S
authors.lastname: García-Rubín
authors.forename: Jesús A
authors.initials: JA
authors.lastname: Varela
authors.forename: Luis
authors.initials: L
authors.lastname: Castedo
authors.forename: Carlos
authors.initials: C
authors.lastname: Saá