Evaluation of the Claisen Rearrangement of 2-Cyclohexenols for the Stereoselective Construction of a Terpene Synthon

Article abstract of DOI:10.1021/jo00332a027

A variety of stereocontrolled propanoate Claisen rearrangement procedures was applied to 2-cyclohexenol and the trans-6-methyl-substituted derivative.The ester-enolate procedure afforded the RS,SR diastereomers 6a and 8a predominantly, regardless of enolate geometry, indicating a preference for the boatlike conformation in rearrangement of the ketene acetals in which the methyl and cyclohexenyloxy groups are trans.Similar boat specificity is seen on rearrangement of (E)-ketene N,O-acetals.A model is proposed to explain the alternating chair-boat specificity.Evaluation of these reactions with cis-6-methyl-2-cyclohexenol was foiled by poor yields of rearranged products.An alternative, highly stereoselective route to both diastereomers of p-menth-1-en-9-ol was developed by using kinetically controlled alkylation and epimerization of a bicyclic lactone intermediate to control the stereochemistry.