Chemistry of O-arylhydroxylamines. A novel acid-catalyzed rearrangement of O-aryl-N-acetoacetylhydroxylamines to benzofurans

[Selective synthesis of benzofurans and dihydrobenzofurans via efficient rearrangement as key reaction]

We have developed a novel and practical synthetic method for functionalized benzofurans via the route involving [3,3]-sigmatropic rearrangement of N-trifluoroacetyl enehydroxylamines, which was triggered by acylation of oxime ethers. Treatment of oxime ethers with trifluoroacetic anhydride (TFAA) gave dihydrobenzofurans while reaction of the identical oxime ethers with trifluoroacetyl triflate (TFAT) in the presence of DMAP afforded aromatized benzofurans without the isolation of enehydroxylamines. It is noteworthy that either dihydrobenzofurans or benzofurans can be formed selectively from the same substrate by changing only the reagent. TFAA has been proved to be the good reagent to induce regioselective [3,3]-sigmatropic rearrangement for the synthesis of hexahydrodibenzofurans with a quaternary carbon. On the other hand, the TFAT-DMAP system is found to be the most effective for constructing 2-arylbenzofurans. Synthetic utility of these selective constructions is demonstrated by the formal total synthesis of (+/-)-lunarine and the short synthesis of natural 2-arylbenzofurans.