Intramolecular Cyclizations of Allenic Acylureas and ‐Amides

Abstract Allenic acids are found to add to dicyclohexylcarbodiimide affording, in the presence of Et 2 NH, the 4 H ‐1,3‐oxazin‐4‐ones 5 via 4 . Under neutral conditions, they add to diaryl‐ or pyridyl(cyclohexyl)carbodiimides and triphenylketene imine to give the corresponding tricyclo[5.2.2.0 1,5 ]undeca‐4,8,10‐trien‐3‐ones 7 , 8 , 9 , and 12 . The allenic phenyl ester 13a dimerises, on heating in a [2+2] head‐to‐head fashion, to 14 but fails to undergo intramolecular Diels ‐ Alder cyclization, to 15 .