Isomerizations of Cycloalkene-and Bicycloalkene-Derived Achiral
Epoxides by Enantioselective α-Deprotonation. -Enantioselective αdeprotonation-rearrangement of cis-cyclooctene oxide (IV) with organolithium compounds in the presence of (-)-sparteine (I) or (-)-isosparteine (II) affords the alcohol ((-)-V) in good yields and enantioselectivities. (-)-Isosparteine can also be successfully used in catalytical amounts. In the presence of chiral C 2 -symmetric bisoxazolines like compound (III), the corresponding enantiomeric alcohol ((+)-V) is produced. The α-deprotonation methodology is successfully extended to other cycloalkene oxides like compounds (VI) or (XI). Thereby, an unusual intramolecular insertion of a lithium alkoxide into a lithiated epoxide is observed for compound (XI) to furnish the oxabicyclic compound (XII). The lithium amide-induced transformations of rigid bicycloalkene epoxides (XV), (XVII) and (XIX) are described, providing access to the chiral ketones (XVIII) and (XX) by an α-protonation-rearrangement sequence.
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