The Stereoselectivities of Halocyclization of γ-Furanyloxy- and γ-Pyranyloxy-alkenes to 2,5-Disubstituted Tetrahydrofurans

Methodologies based on the electrophilic halocyclization of γ-unsaturated alcohols to tetrahydrohyfurans (THFs), which are widely occurring sub-units in several classes of natural products, have been well established over the few decades.1,2 As an example, iodocyclizations of γ-hydroxy-alkenes to substituted THFs were fully investigated by Bartlett et al., in which they used benzyloxy derivatives to obtain 2,5-substituted THFs with high cis stereoselectivi-ties.3 Recently, Fraser-Reid et al. reported that 4-pentenyl acetal moiety can be hydrolyzed by the treatment with halo-nium (X+) species to produce 2-(halomethyl)furan from the n-pentenyloxy moiety (Scheme 1). 4,5 On the basis of these results, one can apply to the synthetic methodology for furan derivatives using the halocyclization of acetal derivatives of γ-hydroxyalkenes.6 Our studies i