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For over a century, the structures and reactivities of strained organic compounds have captivated the chemical community. Whereas triple-bond-containing strained intermediates have been well studied, cyclic allenes have received far less attention. Additionally, studies of cyclic allenes that bear heteroatoms in the ring are scarce. We report an experimental and computational study of azacyclic allenes, which features syntheses of stable allene precursors, the mild generation and Diels–Alder trapping of the desired cyclic allenes, and explanations of the observed regio- and diastereoselectivities. Furthermore, we show that stereochemical information can be transferred from an enantioenriched silyl triflate starting material to a Diels–Alder cycloadduct by way of a stereochemically defined azacyclic allene intermediate. These studies demonstrate that heteroatom-containing cyclic allenes, despite previously being overlooked as valuable synthetic intermediates, may be harnessed for the construction of complex molecular scaffolds bearing multiple stereogenic centres. Strained organic compounds have long fascinated the chemistry community. Heterocyclic allenes are particularly interesting strained intermediates, but their use in synthetic chemistry is rather scarce. Now, an experimental and computational study of azacyclic allenes demonstrates that heteroatom-containing cyclic allenes can be harnessed for the construction of complex molecular scaffolds, including those that bear multiple stereogenic centres.

Strained hydrocarbons are more than molecular curiosities — they often have promising materials properties, and even just making them offers challenges that push the limits of synthetic methods. Now, a short, efficient and room-temperature synthesis of [5]cycloparaphenylene, a carbon nanohoop with 119 kcal per mol of strain energy, is reported.