Rh-Catalyzed Synthesis of 2-Amido-furans via Cyclopropenation of Ynamides

Significance

Reported is the synthesis of 2-amido-furans 4 and 5 via a Rh(II) cyclopropenation of ynamides 1. Although the nature of intermediate (amido-cyclopropene or not) involved in this reaction is unknown, the 2-amido-furans 4 and 5 are clearly the result of a [3+2] cycloaddition process. After optimization, in particular the crucial identification of the best catalyst [Rh₂(OAc)₄], the desired 2-amido-furans were obtained in low to good yields even with sulfonyl- or terminally substituted ynamides. A clear trend is that the diazo compounds 2a,b are better cyclopropenating agents than iodonium ylides 3a,b. An application of these de novo 2-amido-furans in N-tethered intramolecular [4+2] cycloaddition is illustrated, leading to ­dihydroindoles 7 and tetrahydroquinolines 8.