Efficient synthesis of bicyclic lactones via tungsten-mediated intramolecular cycloalkenation

[[abstract]]A series of tungsten-eta(1)-alkynols tethered with a dimethylacetal, methyl ketone, or trimethoxymethane group are prepared. Treatment of these functionalized tungsten-alkynols with BF3. Et2O leads to intramolecular cycloalkenation, producing bicyclic tungsten-oxacarbeniums in high yields. Air oxidation of these oxacarbenium salts produces unsaturated bicyclic lactones in good yields. The lactone products include delta- and epsilon-lactones fused with five-, six- and seven-membered carbocyclic rings. The preceding bicyclic tungsten-eta(1)-oxacarbeniums are highly reactive toward organocuprates, Grignard reagents, and diazomethane, leading to demetalation to give various derivatives of bicyclic lactones. A short synthesis of (+/-)-mitsugashiwalactone is developed based on this method.[[fileno]]2010327010087[[department]]化學