Several Wittig olefinations between α-heterosubstituted acetones and a phosphorus ylide were investigated concerning the product stereoselectivity. Benzyloxyacetone and tetrahydropyranyloxyacetone furnished trisubstituted (Z)-olefins exclusively. The stereoselective preparation of oxygenated acyclic terpenoids was practical by use of the direct Wittig olefination toward α-alkoxy ketones, and it facilitated a total synthesis of ...