NHC/Urea Cocatalyzed Intramolecular Stetter Reaction to Construct Bicyclic and Tricyclic Spirocarbocycles

An NHC/urea‐cocatalyzed intramolecular Stetter reaction is described. Various cyclic enone‐aldehydes and enone‐benzaldehydes were exposed to an N ‐heterocyclic carbene and urea in toluene in the presence of 4 Å molecular sieves, whereupon the intramolecular Stetter reaction proceeded smoothly at room temperature to give the corresponding bicyclic and tricyclic 1,4‐diketones in 27%−96% yields. The ring size of the spiro compounds could be easily controlled by using different cyclic enones or by altering the length of the carbon tether. Both cyclic enone‐benzaldehydes and cyclic enones tethered with a four‐carbon side chain aldehyde gave the desired products, whereas substrates bearing a five‐carbon side chain aldehyde resulted a complicated mixture of the products. The presence of a substituent in the cyclic enone moiety caused a decrease in the yields. An asymmetric intramolecular Stetter reaction was also attempted, and 94% ee was achieved when using an aromatic aldehyde as the substrate. The reaction of an aliphatic aldehyde led to poor yield and lower ee.