Do allenes form rings?

I was wondering whether allenes can form rings; geometrically speaking, this seems like it would be energetically unfavorable due to their 180 degree linear geometry. 




Yes they do. Such compounds are even found in nature, e.g. [vernonallenolides](https://books.google.com/books?id=PHkJKxprCJQC&lpg=PA155&ots=eaCqZa8ycu&dq=Allenic%20germacranolides%2C&hl=de&pg=PA154#v=onepage&q=Allenic%20germacranolides,&f=false).


[![James R. Hanson, Terpenoids and Steroids: A Review of Chemical Literature, volume 12, 1983](https://i.stack.imgur.com/fqcHH.png)](https://i.stack.imgur.com/fqcHH.png)


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As small as six-membered rings with allenes can exist, albeit fleetingly. Cyclohexa-1,2-diene has been known since a 1966 report by Georg Wittig ([*Angew. Chem. Int. Ed.* **1966,** *5* (9), 846](https://doi.org/10.1002/anie.196608461)), in which it is prepared by direct elimination of HBr from a vinyl bromide using *t*-BuOK:


[![Cyclohexa-1,2-diene reactions](https://i.stack.imgur.com/8MST5.png)](https://i.stack.imgur.com/8MST5.png)


For a more modern take on the matter, see e.g. [*Nat. Chem.* **2018,** *10* (9), 953–960](https://doi.org/10.1038/s41557-018-0080-1) where Neil Garg and coworkers report the generation of azacyclic six-membered cycloallenes. The allene is generated by treatment of a 1,2-silyl triflate with CsF (this strategy will be familiar from benzyne chemistry). The cycloallene intermediates are trapped in various cycloaddition reactions.


[![Azacyclic allenes](https://i.stack.imgur.com/6wSNA.png)](https://i.stack.imgur.com/6wSNA.png)


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