Retro Diels–Alder reaction in synthesis of corallopyronin A

Question

I was looking at the synthesis of corallopyronin A in the following paper by Rentsch and Kalesse: Angew. Chem. Int. Ed. 2012, 51 (45), 11381–11384.

There is one reaction which is described as

oxidation and a retro Diels–Alder reaction generated pyrone

I understand that the DMP oxidises the secondary alcohol to a ketone, and at the end there is an intramolecular esterification to produce the lactone, but I don't quite understand how the retro Diels–Alder reaction works. Could someone explain the reaction steps?

Answer 1

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 "retro-Diels-Alder". 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