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26.9.1.1.20 Method 20: Retro-Diels–Alder Reaction

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261101.

Enones are active dienophiles for normal-electron-demand DielsAlder reactions, and undergo reversible cycloaddition with dienes such as cyclopentadiene. The strategy of cycloaddition/retrocycloaddition has frequently been used to protect enones through the course of a synthesis, or to allow stereoselective functionalization of cyclic enones prior to retrocycloaddition.[‌250‌] Thus, for example, the enone functionality in turmerone 324 was unmasked at the final step by retrocycloaddition during reduced pressure distillation of the parent norbornene (Scheme 56).[‌251‌] Substituted cyclopentenones of type 326 can undergo alkene isomerization to the more stable adduct 327 under the conditions used for retrocycloadditions. By contrast, careful retro-DielsAlder reaction of 325 under flash-vacuum pyrolysis conditions allows efficient isolation of cyclopentenone 326, which can subsequently be isomerized to 327 by the use of base.[‌252‌] Clean retrocycloaddition of systems such as 328 can also be carried out at room temperature in the presence of a Lewis acid, provided a sacrificial reactive dienophile such as maleic anhydride is present to trap the liberated cyclopentadiene and drive the reaction toward enone 329.[‌253‌]

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