Cycloaddition photoenolization/Diels Alder

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

The cyclization reaction of a biradical intermediate to form cyclobutanol (304 see also Yang cyclization Section 6.3.5) and the Diels Alder cycloaddition of the photoenols with dienophiles are common subsequent processes (Scheme 6.134).981,982 Both processes are often stereospecific, typically involving a single, long-lived ( )-photoenol. 

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