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Robinson annulation seemed somewhat like the Diels-Alder reaction to me, if all ketonic groups were to be represented as enol then cyclohexanone would be like diene and the Michael acceptor as a dienophile.

On trying out this thought, I got pyran derivatives. The doubt is attached as the following image.

"Heathcock, C. H., Ellis, J. E., McMurry, J. E., & Coppolino, A. (1971). Acid-catalyzed Robinson Annelations. Tetrahedron Letters, 12(52), 4995–4996. doi:10.1016/s0040-4039(01)97609-9 " report that Robinson annulation can be achieved using sulphuric acid instead of a base. Though no mechanism is reported in it, I think enol is formed as an intermediate.

I request you to clear this doubt of mine whether if there is any slightest possibility in Robinson annulation that basic/acidic medium would enolize the ketone then undergo diels-alder reaction on providing heat. Also, if it does then would it follow Woodward-Hoffmann rules as Robinson annulation looked like a thermally allowed cycloaddition reaction

The following paper reports such possible diels-Alder reaction with enols though not directly for reagents of Robinson annulation: "Akbulut, N., Hartsough, D., Kim, J. I., & Schuster, G. B. (1989). The triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles: scope and utility. The Journal of Organic Chemistry, 54(11), 2549–2556. doi:10.1021/jo00272a017 "

. Diels Alder reaction using reagents of Robinson annulation

( Reference: https://en.wikipedia.org/wiki/Oxo-Diels–Alder_reaction and https://en.wikipedia.org/wiki/Inverse_electron-demand_Diels–Alder_reaction )

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  • $\begingroup$ Those classes of reactions are known, but generally not via the enol. More common is via a silyl enol ether or something like Danishevsky's diene or possibly with a Lewis acid as catalyst. $\endgroup$
    – Zhe
    May 9, 2020 at 22:04
  • $\begingroup$ Thank you for your insightful comment. Is there report of diels-alder reaction between ketone and enol or enol- dienol ? $\endgroup$
    – Swastik
    May 9, 2020 at 22:11
  • $\begingroup$ Another intriguing question about the C-C bond formation in the annulation is does the carbon of the enolate add 1,4 to the enone or does the oxygen of the enolate add 1,2- to the enone and then undergo an alkoxy (hydroxy) Claisen rearrangement. There is no evidence to my knowledge that the reaction proceeds this way so there is no need to invoke it. Nonetheless, it is an interesting thought experiment. $\endgroup$
    – user55119
    May 9, 2020 at 23:20
  • $\begingroup$ I don't remember ever seeing a cycloaddition via a naked enol. $\endgroup$
    – Zhe
    May 10, 2020 at 2:18

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