Db. Millward et al., Ring-opening reactions of oxabicyclic alkene compounds: enantioselective hydride and ethyl additions catalyzed by group 4 metals, J ORG CHEM, 65(13), 2000, pp. 3902-3909
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.