Furans, Benzofurans, and Related Compounds

A great deal of work has been reported on the synthesis of furans, benzofurans, and related compounds. For the sake of clarity, the subject matter covered in this section has been divided into four parts that are based on different synthetic approaches as presented in Section II.A. 

Michael-type addition of an enolate anion to an alkynyliodonium salt probably produces the unstable iodonium ylide 34 34a. Loss of Phi from 

Other examples of the iodonium ylide-based syntheses of furan derivatives involve cycloaddition reactions with alkenes or alkynes. Although the majority of these syntheses involve stable iodonium ylides (86JOC3453 94T11541) (e.g., Eqs. 16 and 17), in some cases the ylides are unstable and are generated in situ (92JOC2135) (e.g., Eq. 18). In the case of alkenes, dihydrofuran derivatives are obtained (Eqs. 16-18). This synthetic route is especially useful for the synthesis of dihydrobenzofuran derivatives that are related to the neolignan family of natural products (Eq. 18). 

Another important variant of the preceding approach is the cycloaddition reaction between monocarbonyl iodonium salt 47 and an alkene to give dihydrofuran 48 (88TL3703 89JOC2605). The iodonium salt 47 is generated by the oxidation of acetophenone silyl enol ether (46) with iodosobenzene in the presence of fluoboric acid. 

Lewis acid-catalyzed decomposition of iodonium ylide 49 in the presence of alkenes results in the formation of y-lactones 50 (87IZV2873) (Eq. 20). 

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