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Synthesis

Azacyclic allenes harnessed

Strained system participates in Diels-Alder cycloaddition to construct complex scaffolds

by Bethany Halford
August 5, 2018 | APPEARED IN VOLUME 96, ISSUE 32

 

09632-scicon9-reaction.jpg

The allene motif—in which two double bonds join three carbons together—tends to be an oddball among functional groups. Squeeze an allene into a ring system, and it becomes even more unusual. A team at the University of California, Los Angeles, led by Neil K. Garg and Kendall N. Houk, hopes to change the allene’s reputation with a report that azacyclic allenes can be useful synthetic tools (Nat. Chem. 2018, DOI: 10.1038/s41557-018-0080-1). The researchers developed silyl triflate precursors that readily generate azacyclic allenes by adding CsF under mild conditions—an advance over past syntheses that required harsh conditions and gave poor yields. These azacyclic allenes undergo Diels-Alder reactions with dienes (example shown). Computational studies of the transition states of these reactions show how substituents are able to control which double bond reacts. The methodology provides access to substituted piperidines—which are often skeletons in drug molecules—by forming two new bonds and three stereocenters with high levels of diastereoselectivity. The chemists also report [3+2] and [2+2] cycloadditions with the azacyclic allenes. “Our present study demonstrates that strained azacyclic allenes, although largely neglected, serve as valuable building blocks for the construction of complex molecular scaffolds bearing multiple stereogenic centers,” they note.

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