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23.12.3.2.5 Method 5: Cyclobutanones and Other Products by Cycloaddition Reactions with Alkenes and Dienes

DOI: 10.1055/sos-SD-023-00394

Tidwell, T. T.Science of Synthesis, (200623455.

Cycloaddition reactions of diphenylketene (1) are the most characteristic and most studied reaction of this species, and have been thoroughly reviewed.[‌92‌,‌93‌,‌95‌‌97‌] The reaction with alkenes and dienes typically results in a net [2+2] cycloaddition between the C=C bonds of the ketene and of the alkene forming a cyclobutanone. However, it has now been shown that for reaction with cyclopenta-1,3-diene with preformed 1 at low temperature, the initial reaction occurs by a [4+2] cycloaddition on the C=O bond of the ketene forming 3-(diphenylmethylene)-2-oxabicyclo[2.2.1]hept-5-ene (296), and that this subsequently rearranges to the cyclobutanone 297 at 0°C in 98% yield (Scheme 112).[‌243‌] The 2-oxabicyclo[2.2.1]hept-5-ene 296 has been purified by chromatography at 50°C and the structure was fully characterized by NMR methods. The course of the rearrangement was monitored by NMR spectroscopy. Rearrangement of 296 to 297 also occurs in the solid state, with a half-life of approximately 6hours at 0°C.[‌243‌]

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References