Abstract
Treatment of tungsten-η 1 -4-(triethylsiloxy)propargyl and η 1 -4-(tripropylsiloxy)propargyl compounds 8a-d and 9a-d with CF 3 CO 2 H in cold CH 2 Cl 2 yielded a mixture of tungsten η 1 -2,5-dihydrofurans 1a-d and η 3 -syn-γ-lactones 4a-d. The product ratios depend on the types of alkyl and siloxy substituents of the propargyl ligand; η 1 -2,5-dihydrofurans 10a-d were formed in significant amount when the substituent is a secondary alkyl group. This cyclization pathway is in sharp contrast to our previous report that the dimethyl-tertbutylsiloxy group yielded only η 3 -syn-γ-lactones 4a-d. To validate this reaction on a complex system, we prepared the chiral tungsten-η 1 -4-(triethylsiloxy)propargyl species 12b, which ultimately gave optically active tungsten η 1 -2,5-dihydrofuran 13 in 68% yields. In connection with our previous report, we elaborated three types of cyclization reactions on chiral tungsten η 1 -propargylic triols 12a-c, to afford chiral η 1 -2,5-dihydrofurans and unsaturated γ- and δ-lactones in reasonable yields.