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In the presence of BF 3 ‚Et 2 O, tungsten η 1 -propargyl complexes reacted smoothly with aldehydes to give tungsten η 1 -2,5-dihydro-3-furyl complexes 1-3. Treatment of 1-3 with Ph 3 CBF 4 in CH 2 Cl 2 (-60 °C, 1 h) delivered tungsten η 1 -3-furylidene complexes 4-6 which underwent skeletal rearrangement to η 1 -2-furylidene 13-15 isomers when warmed to 5 °C. Treatment of 4-6 with Et 3 N at -60 °C produced tungsten η 1 -3-furyl complexes 7-9 which were isomerized to η 1 -2-furyl isomers 10-12 by a strong Bronsted acid. Protonation of 10-12 by CF 3 CO 2 H yielded η 1 -2-furylidene complexes 13-15. The two exceptional isomerizations were examined by in situ 1 H NMR studies. Further oxidation of η 1 -2-furyl compounds 10-12 with m-chloroperbenzoic acid gave tungsten η 1 -∆ 3 -butenolides 16-18. Tungsten η 1 -3-furyl complexes 7 and 9 underwent alkylation with trimethoxymethane and aldehydes at -60 °C to yield η 1 -3-furylidene cationic precipitates that were subsequently reduced with NaBH 3 CN to yield complex η 1 -2,5-dihydro-3-furyl compounds in good diastereoselectivity; the stereochemical courses were distinct for 7 and 9. Tungsten η 1 -2,5-dihydro-3-furyl, η 1 -2-furyl, η 1 -3-furyl, and η 1 -∆ 3 -butenolide complexes were decomplexed with appropriate reagents to liberate 2,5-dihydrofurans, furans, and ∆ 3 -and ∆ 2 -butenolides in reasonable yields.

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