Resolution of limonene 1,2-epoxide diastereomers by mercury(II) ions

When HgCl2 was added to a diastereomeric mixture of cis- and trans-(4S)-limonene 1,2-epoxide, the Hg(II) ions stereoselectively complexed to the cis epoxide, enabling ring opening by water. The resulting mercuric salt could be demetalated by treatment with NaBH4, giving a mixture of diastereomeric (1S,2S,4S)- and (1R,2R,4S)-diols. The remaining trans-(4S)-epoxide was obtained in >98% d.e. and 40% yield. For reactions on a larger scale, the most convenient reaction system was Hg(OAc)2 in 50% acetone/tris-HCl buffer pH 7.0. The reaction rate was affected by the pH, with pH 6–8 as optimum.