Furans in Synthesis. 3. Furans as Terminators in Cationic Cyclization

Article abstract of DOI:10.1021/jo00172a026

3-Furylmethyl-Grignard 30 is readily coupled with a variety of ω-haloalkenes to afford the corresponding 3-substituted furan in good to excellent yields.Epoxidation of the product furyl olefins was found to be effective in producing the desired cyclization substrates only when the olefin was trisubstituted.Less highly substituted epoxy furans were prepared via the coupling of (3-furylmethyl)lithium (43) with ω-iodo epoxides or protected ω-iodo diols followed by closure.The cyclizations of these epoxy furans were examined with a range of Lewis acids.Treatment with Ti(O-i-Pr)3Cl and ZnI2 led to the isolation of cyclized products 19b, 22, 24, 26, and 28 in moderate to excellent yields.Cyclization of 7,8-epoxydendrolasin (39) with Ti(O-i-Pr)3Cl and ZnI2 provided 3β-hydroxypallescensin A (49) in 62 percent and 65 percent yields, respectively.