Pt-Catalyzed Cyclization of Propargylic Oxiranes to Substituted Furans

Significance

Reported here is a Pt-catalyzed synthetic methodology for the conversion of propargylic oxiranes into 2-substituted furans in aqueous media. Whereas the catalytic reaction using AuCl₃ in acetonitrile was previously found to be sluggish (21% yield in 36 h, see A. S. K. Hashmi, P. Sinha Adv. Synth. Cat. 2004, 346, 432) and the corresponding Pt-catalyzed reaction in dioxane in the absence of water required 3 h (90% yield) for completion, dioxane-H₂O (2:1) at 100 ˚C with 10 mol% Pt catalyst was found to be optimal. Using deuterated water gave the product with 97% incorporation of deuterium at C3, a result consistent with the depicted mechanism. Trapping of the furanyl-platinum species with iodine gave the 3-iodofuran, which clearly has potential for fur-ther functionalization by cross-coupling chemistry as shown in the scheme.

Comment

Substituted furans are an important class of heterocycles present in many biologically active natural products and used as synthetic building blocks (see Reviews below). An identical gold-catalyzed transformation of alkenyl epoxides to furans has been reported (see reference in significance section). The present method constitutes a very simple, fast and efficient procedure for the synthesis of substituted furans although diversity in the substitution pattern has not been studied and is limited to 4,5-cyclo furans. Fully substituted furans (two examples) have been achieved by further cross-coupling methodology.

Reviews: B. H. Lipshutz Chem. Rev. 1986, 86, 795-819; X. L. Hou et al. Tetrahedron 1998, 54, 1955-2020.