Stereoselective Synthesis of Substituted Bicyclo-[3.3.1]-nonan-9-ones by Additions of Enamines of Cyclohexanones to 4-Ethoxy-1,1,1-trifluorobut-3-ene-2-one

Addition of the pyrrolidine enamine of cyclohexanone to 4-ethoxy-1,1,1-trifluorobut-3-ene-2-one affords a single diastereoisomer of 4-ethoxy-2-hydroxy-2-trifluoromethylbicyclo[3.3.1]nonan-9-one. Similarly the pyrrolidine enamine of 4-methylcyclohexanone also in a highly diastereoselective manner affords a single bicyclic ketone. In the latter case a single crystal X-ray diffraction analysis permits unambiguous determination of all stereochemical detail. Other examples of addition to 4-ethoxy-1,1,1-trifluorobut-3-ene-2-one are discussed. The mechanistic explanation of the unusually high selectivity is discussed.