Abstract
An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.
Highlights
Over the last two decades, ynamides,[1−3] readily prepared by a range of efficient methods,[4−11] have emerged as remarkably versatile building blocks enabling the development of a variety of processes based on their unique reactivity
These activated keteniminium ions, readily generated upon reaction between an ynamide and an acid, an electrophile, or a π-acidic metal complex, are among the most electrophilic intermediates known to date and readily react even with the worst nucleophiles.[23−28] They have been shown to be reactive enough to promote hydrogen and hydride shifts,[29−33] even from relatively nonactivated positions,[34] which have been used to develop a series of innovative and efficient processes to access a variety of building blocks and molecules, ranging from the simplest ones to remarkably complex nitrogen-containing heterocycles
With this goal in mind, ynamide 1a, selected as a model substrate, was reacted with catalytic amounts (5 mol %) of a set of representative Brønsted acids and π-acidic metal complexes at room temperature for 20 h in dichloromethane. These catalysts were selected based on the need for their conjugated bases or counterions to be as poorly nucleophilic as possible to avoid trapping the transient activated keteniminium ion, and dichloromethane was chosen as the solvent for the same reason
Summary
Over the last two decades, ynamides,[1−3] readily prepared by a range of efficient methods,[4−11] have emerged as remarkably versatile building blocks enabling the development of a variety of processes based on their unique reactivity. They have been shown to display an exceptional level of reactivity and to participate in anionic,[12] carbocationic,[13,14] radical,[15,16] and metal-catalyzed reactions[17,18] as well as cycloadditions[19] with exquisite levels of regioselectivity.[20] They have been shown to be excellent precursors of carbenoids,[3] to provide new opportunities in asymmetric synthesis,[18,21] and to enable the design of efficient and innovative routes to a variety of natural products.[1−3] Among all reactions developed from these unique building blocks, the cationic ones have received much attention in recent years They rely on the formation of highly electrophilic activated keteniminium ions[13,14,22] whose reactivity has enabled the development of processes that would fail with other alkynes and/or less activated keteniminium ions. ACS Organic & Inorganic Au in a variety of biologically relevant products,[41−45] we first evaluated the feasibility of this process
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