Retro Diels–Alder reaction in synthesis of corallopyronin A
Here is my assessment of the mechanism. You are correct about the Dess-Martin periodinane (DMP) oxidation of 1 to 2. The resulting ketone 2 can also exist as the enols (E)-3 and (Z)-4. Any one of the three can undergo a retro-Diels-Alder-like thermolysis, but I have chosen (Z)-4 for the convenience of cyclizing ketene 5, which has lost acetone in the thermolysis. Finally, the ketene 5 affords pyranone 6.
ADDENDUM: Citation for the &quot;retro-Diels-Alder&quot;. A. R. Katritzky, Z. Wang, M. Wang, C. D. Hall and K. Suzuki, J. Org. Chem. 2005, 70, 4854. https://doi.org/10.1021/jo050307m