Iron-Catalyzed Reductive Cyclization of Alkenyl Vinylogous Carbonates for Stereoselective Synthesis of Substituted Tetrahydrofurans, Tetrahydropyrans, and Chromans

A method for the stereoselective synthesis of substituted tetrahydrofurans (THFs), tetrahydropyrans (THPs), as well as chromans using intermolecular hydrogen atom transfer (HAT) followed by intramolecular radical cyclization of alkenyl vinylogous carbonates is described. This Fe(acac)(3)-catalyzed reaction uses PhSiH3 as H-source and works under ambient conditions in HFIP/ethylene glycol (5:1 mixture) as solvent. The methodology developed has broad substrate scope and is amenable to gram-scale synthesis. The method gives ready access to even hexa-substituted cyclic ethers containing two quaternary carbons. This strategy is used in the construction of oxaspirocyclic systems. Diastereoselective synthesis of fused bicyclic as well as tricyclic motifs could be achieved. The stereochemistry of the products was assigned using NOE experiments and was further confirmed by single-crystal X-ray diffraction studies on two derivatives.

Automated summary

This method allows for the stereoselective synthesis of various substituted tetrahydrofurans, tetrahydropyrans, and chromans using palladium catalyzed intramolecular radical cyclization, with broad substrate scope and applicability for gram-scale synthesis under suitable solvent conditions.