Abstract
Most of the synthetic approaches to the benzo-fused heterocycles containing an oxygen atom have involved the use of phenol derivatives as a starting material. This review highlights the new synthetic approaches involving the aromatic C-O bond-forming process using arynes. The insertion of arynes into the C=O bond gives the unstable intermediates, [2 + 2] cycloaddition-type adducts, which can be easily converted into a variety of oxygen atom-containing heterocycles in a single operation. In this review, the syntheses of oxygen heterocycles, such as coumarin, chromene, xanthene, dihydrobenzofuran and benzofuran derivatives, via the insertion of arynes into the C=O bond of aldehydes or formamides are summarized.
Highlights
Oxygen atom-containing heterocycles are an important class of the organic heterocyclic compounds.In particular, the benzo-fused oxygen heterocycles, in which oxygen heterocyclic ring is fused to benzene ring, are found as a key structural unit in natural products, pharmaceuticals and biologically active compounds (Figure 1) [1,2,3]
Yu reported the oxidative approach to dihydrobenzofurans 2 from alcohols 1 via Pd(II)-catalyzed and hydroxyl-directed C-H bond activation followed by C-O bond formation [18]
This review highlights the new synthetic approaches to oxygen heterocycles via the aromatic C-O bond-forming process based on the insertion of arynes into the C=O bond (Figure 3)
Summary
Oxygen atom-containing heterocycles are an important class of the organic heterocyclic compounds. Most of the synthetic approaches are based on the construction of oxygen heterocyclic ring from various phenol derivatives. Many reported approaches have involved the use of oxygen atom-containing arenes such as phenol, 2-bromophenol or salicylaldehyde derivatives as a starting material [4,5,6,7,8]. This review highlights the new synthetic approaches to oxygen heterocycles via the aromatic C-O bond-forming process based on the insertion of arynes into the C=O bond (Figure 3). The insertion of aryne A into the C=O bond giving the [2 + 2] cycloaddition-type adduct D is assumed to proceed via the stepwise [2 + 2] mechanism involving the zwitterionic specie as an intermediate
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