Highly Stereoselective Synthesis of Trisubstituted γ,δ-Unsaturated Acid and Aldehyde via Ketal Claisen Rearrangement

A new sequence for the highly stereoselective (E>99%) synthesis of a trisubstituted γ,δ-unsaturated acid (3), which consists of a ketal Claisen rearrangement of a terpene allylic alcohol with 2,2-dimethoxy-3-methyl-3-butanol and a subsequent oxidative cleavage of the resulting α-ketol, is applicable even to the preparation of 3 which is difficult to synthesize by traditional methods. The related procedure for preparation of the corresponding aldehyde is also described.