Abstract
We have developed a new and efficient synthetic method for functionalized indoles and benzofurans via the route involving [3, 3] -sigmatropic rearrangement of N-trifluoroacetyl enamine.
N-Trifluoroacetyl enehydrazine carrying a cyclopentene ring smoothly underwent [3, 3] -sigmat-ropic rearrangement followed by cyclization to give indoline in excellent yield. Cyclohexenyl or acyclic N-trifluoroacetyl enehydrazines gave indoles in good yields. The rearrangement of N-trifluoroacetyl enehydrazines proceeded smoothly even under either aqueous or solvent-free conditions.
The acylation of cyclic or acyclic oxime ethers with TFAA proceeded under mild conditions to give the dihydrobenzofurans in excellent yields without the isolation of N-trifluoroacetyl enehydoxylamine. On the other hand, the reaction of oxime ethers with trifluoroacetyl triflate (TFAT) in the presence of DMAP gave the benzofurans in good yields. It is noteworthy that dihydrobenzofurans or benzofurans can be formed selectively from the same substrate by changing only the reagent. This successive method was successfully applied to the synthesis of natural products.
