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Allenic acids are found to add to dicyclohexylcarbodiimide affording, in the presence of Et2NH, the 4H‐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[5.2.2.01,5]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.
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Trifonov et al. (1987) studied this question.
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