title: Titanocene-catalyzed cascade cyclization of epoxypolyprenes: straightforward synthesis of terpenoids by free-radical chemistry.
abstract: The titanocene-catalyzed cascade cyclization of epoxypolyenes, which are easily prepared from commercially available polyprenoids, has proven to be a useful procedure for the synthesis of C(10), C(15), C(20), and C(30) terpenoids, including monocyclic, bicyclic, and tricyclic natural products. Both theoretical and experimental evidence suggests that this cyclization takes place in a nonconcerted fashion via discrete carbon-centered radicals. Nevertheless, the termination step of the process seems to be subjected to a kind of water-dependent control, which is unusual in free-radical chemistry. The catalytic cycle is based on the use of the novel combination Me(3)SiCl/2,4,6-collidine to regenerate the titanocene catalyst. In practice this procedure has several advantages: it takes place at room temperature under mild conditions compatible with different functional groups, uses inexpensive reagents, and its end step can easily be controlled to give exocyclic double bonds by simply excluding water from the medium.
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Granada, Granada, 18071, Spain.
authors.forename: José
authors.initials: J
authors.lastname: Justicia
authors.forename: Antonio
authors.initials: A
authors.lastname: Rosales
authors.forename: Elena
authors.initials: E
authors.lastname: Buñuel
authors.forename: Juan L
authors.initials: JL
authors.lastname: Oller-López
authors.forename: Mónica
authors.initials: M
authors.lastname: Valdivia
authors.forename: Ali
authors.initials: A
authors.lastname: Haïdour
authors.forename: J Enrique
authors.initials: JE
authors.lastname: Oltra
authors.forename: Alejandro F
authors.initials: AF
authors.lastname: Barrero
authors.forename: Diego J
authors.initials: DJ
authors.lastname: Cárdenas
authors.forename: Juan M
authors.initials: JM
authors.lastname: Cuerva
mesh_terms: D002384:Catalysis; D003500:Cyclization; D004852:Epoxy Compounds / Q000138:chemical synthesis*; D005609:Free Radicals / Q000737:chemistry; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013729:Terpenes / Q000138:chemical synthesis*
chemical_list: D004852:Epoxy Compounds; D005609:Free Radicals; D009942:Organometallic Compounds; D013729:Terpenes; C406884:titanocene