Ring-opening reactions of oxabicyclic alkene compounds: enantioselective hydride and ethyl additions catalyzed by group 4 metals

Titanium and zirconium catalysts selectively catalyze either the ethyl or h ydride addition to [2.2.1] 4,5-bis(methoxymethyl)-7-oxabicycloheptene (6); the ring-opened products formed depend on catalyst, temperature, alkylalumi num reagent, and the concentration of alkylaluminum. Bis(neoisomen-thylinde nyl)zirconium dichloride catalyzes the ethyl addition ring-opening of 6 to produce (LR, 2S,3S,6R)-2,3-bis(methoxymethyl)-6-ethylcyclohex-4-enol (7) in 96% ee. Zirconium catalysts catalyze the ring-opening of [3.2.1] 2,4-dimet hyl-3-(benzyloxy)-8-oxabicyclo-6-octene (7) when ethylmagnesium bromide is used as a reagent. Both hydride and ethyl addition products are obtained at all conditions studied. Bis(neoisomenthylindenyl)zirconium dichloride cata lyzes the ethyl addition ring-opening of 7 to produce (1S,2R,3S,4S,7S)-2,4- dimethyl-3-(benzyloloxy)-7-ethyl-5-cyclohexen-1-ol (8) in 48% ee.