Abstract
We report a simple protocol for the transition metal-free, visible-light-driven conversion of 1,3-diketones to tetra-substituted furan skeleton compounds in carbon dioxide (CO2) atmosphere under mild conditions. It was found that CO2 could be incorporated at the diketone enolic OH position, which was key to enabling the cleavage of a C–O bond during the rearrangement of a cyclopropane intermediate. This method allows for the same-pot construction of two isomers of the high-value tetra-substituted furan scaffold. The synthetic scope and preliminary mechanistic investigations are presented.
Highlights
Polysubstituted furans are of great importance in the avor and fragrance industry, pharmaceutical industry and materials chemistry, and are valuable building blocks in organic synthesis
The desired tetra-substituted furans were produced by using 4CzIPN as the photocatalyst, N,N-dimethylformamide (DMF) as solvent, Cs2CO3 as a base, at 25 C under 455 nm light irradiation, and in CO2 atmosphere
Mechanistic investigations indicated that CO2 was catalytically incorporated in order to create a better leaving group from the enolic OH group
Summary
Polysubstituted furans are of great importance in the avor and fragrance industry, pharmaceutical industry and materials chemistry, and are valuable building blocks in organic synthesis. the preparation of such polysubstituted furans is o en complicated and circuitous. Scheme 1 Approaches to access highly substituted furans via: (a) successive direct functionalization of furan C–H; (b) condensation; (c) annulation of unsaturated substrates with ketones or imines; (d) migratory cycloisomerization and rearrangements. Annulation of unsaturated substrates with ketones or imines (Scheme 1c) as well as other cross-coupling approaches could afford polysubstituted furans, but migratory cycloisomerization and rearrangements are the most straightforward and convergent routes (Scheme 1d).. We envisioned utilizing CO2 together with organic dyes to catalyze the cyclizations of 1,3-diketones1b,16,17 and subsequent cyclopropane rearrangement events en route to valuable tetra-substituted furan products, enabled by the reversible interaction of CO2 with the enol forms that can provide favorable carbonate leaving groups.10b,c,11g The. Scheme 2 CO2-promoted organic transformations: (a) rearrangement of propargyl alcohols to unsaturated ketones; (b) cross-coupling using native allylic alcohol; (c) oxidation of allylic alcohols to unsaturated aldehydes. Optimization survey, substrate scope, and preliminary mechanistic studies of this transition metal-free, photocatalytic, and CO2-promoted furan synthesis strategy are presented
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