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The first [2+2+1] cycloaddition reaction of a 1,3-diene, an allene, and carbon monoxide has been reported by chemists at Stanford University (Angew. Chem. Int. Ed. 2006, 45, 2459). Paul A. Wender, Mitchell P. Croatt, and Nicole M. Deschamps developed the reaction (shown), which offers an efficient and selective route to highly substituted alkylidenyl cyclopentanones. Others have attempted similar cycloadditions in the past—using an unactivated alkene rather than a diene—with little success. Rather than undergo cycloaddition, in those instances the alkene and allene moieties simply cycloisomerized to a seven-membered ring. By replacing the alkene with a 1,3-diene, the Stanford group was able to coax the cycloaddition reaction to take place under mild conditions in the presence of a rhodium catalyst. The reaction is chemoselective, targeting only the olefin closest to the allene, and appears to be accelerated in the presence of a Brønsted acid. The chemists report excellent yields and no evidence of the cycloisomerization that plagued earlier efforts with alkenes.
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