title: Stereocontrolled synthesis of bicyclic lactone derivatives via tungsten-mediated [3 + 2] cycloaddition of epoxides with a tethered alkynyl group.
abstract: In the presence of BF3*Et2O, alkynyltungsten complexes underwent [3 + 2] cycloaddition with tethered epoxides to give bicyclic -lactones efficiently. Only one diastereomeric product was formed despite the presence of three stereogenic centers. A mechanism is proposed that involves formation of a tungsten-vinylidenium species via an SN2 attack of the epoxide carbon by an alkynyltungsten group to give a tungsten-enol ether species via counterattack at the central tungsten-vinylidenium carbon by the OBF3- terminus. Most of the tungsten enol ether species were too unstable for isolation and underwent hydrolysis to give only cis-fused -bicyclic lactones. This cyclization works for both cis- and trans-epoxides and tolerates various functional groups. In the case of trans-phenyl epoxide, the reaction led to an addition product via a 6-endo attack of epoxide by the tungsten fragment. This method provides a simple enantiospecific synthesis of complex bicyclic lactones if a chiral epoxide is used in the cyclization. It is also applicable to the one-pot synthesis of bicyclic unsaturated gamma-lactones if a suitable alkynyltungsten functionality is used.
authors.affiliation: Department of Chemistry, National Tsing-Hua University, Hsinchu 30043, Taiwan, ROC.
authors.forename: Reniguntala J
authors.initials: RJ
authors.lastname: Madhushaw
authors.forename: Chien-Le
authors.initials: CL
authors.lastname: Li
authors.forename: Haw-Lih
authors.initials: HL
authors.lastname: Su
authors.forename: Chu-Chen
authors.initials: CC
authors.lastname: Hu
authors.forename: Shie-Fu
authors.initials: SF
authors.lastname: Lush
authors.forename: Rai-Shung
authors.initials: RS
authors.lastname: Liu