TUNGSTEN ETA(1)-5-MEMBERED OXYGENATED HETEROCYCLES DERIVED FROM TUNGSTEN ETA(1)-PROPARGYL COMPOUNDS - SYSTEMATIC SYNTHESES, STRUCTURAL REARRANGEMENT, ELECTROPHILIC ALKYLATIONS, AND OXIDATIVE DEMETALATIONS

In the presence of BF3 . Et(2)O, tungsten eta(1)-propargyl complexes r eacted smoothly with aldehydes to give tungsten eta-2,5-dihydro-3-fury l complexes 1-3. Treatment of 1-3 with Ph(3)CBF(4) in CH2Cl2 (-60 degr ees C, 1 h) delivered tungsten eta(1)-3-furylidene complexes 4-6 which underwent skeletal rearrangement to eta(1)-2-furylidene 13-15 isomers when warmed to 5 degrees C. Treatment of 4-6 with Et(3)N at -60 degre es C produced tungsten eta(1)-3-furyl complexes 7-9 which were isomeri zed to eta(1)-2-furyl isomers 10-12 by a strong Bronsted acid. Protona tion of 10-12 by CF3CO2H yielded eta(1)-furylidene complexes 13-15. Th e two exceptional isomerizations were examined by in situ H-1 NMR stud ies. Further oxidation of eta(1)-2furyl compounds 10-12 with m-chlorop erbenzoic acid gave tungsten eta(1)-Delta(3)-butenolides 16-18. Tungst en eta(1)-3-furyl complexes 7 and 9 underwent alkylation with trimetho xymethane and aldehydes at -60 degrees C to yield eta(1)-3-furylidene cationic precipitates that were subsequently reduced with NaBH3CN to y ield complex eta(1)-2,5-dihydro-3-furyl compounds in good diastereosel ectivity; the stereochemical courses were distinct for 7 and 9. Tungst en eta(1)-2,5-dihydro-3-furyl, eta(1)-2-furyl, eta(1)-3-furyl, and eta (1)-Delta(3)-butenolide complexes were decomplexed with appropriate re agents to liberate 2,5-dihydrofurans, furans, and Delta(3)- and Delta( 2)-butenolides in reasonable yields.