Heterocycles as dienophiles

6-double bond in activated pyrimidines such as 2-acylamino-6-acetyl-4(lH)-pyrimidinones (427) undergo Diels-Alder reactions to yield on heating with l-acetoxy-3-methylbutadiene (428) hydroquinazolines (429). The yields are modest, but the regiochemistry is clean. With 1,3-butadiene and with isoprene the cycloadducts were formed in low yields (83JOC3627). 

Pyran-2-ones can give unsymmetrical dimers (430) and (431) where one molecule behaves as diene and another as dienophile. 

2-Quinolone undergoes photochemical addition of tetramethylethylene to give (434) (70AHC(ii)50), l,3-oxazin-4-ones photocycloadd ketene acetals to give (435), and irradiation of 2,6-dimethylpyran-4-one yields the cage dimer (436). 2-Pyranones form [2 + 2] photodimers whose structure is similar to that of uracil dimers (432 or 433). 

---

Ionic Liquids in Polar Diels-Alder Reactions Using Carbocycles and Heterocycles as Dienophiles... 

Wenkert, E., Moeller, P.D., Piettre, S.R. (1988). Five-membered aromatic heterocycles as dienophiles in Diels-Alder reactions. Furan, pyrrole and indole Journal of the American Chemical Society, 110, 21, 7188-7194, ISSN 0002-7863 Wenkert, E., Piettre, S.R. (1988). Reaction of a-and 3-acylated furans with conjugated dienes. Journal of Organic Chemistry, 53,5850-5853, ISSN 0022-3263 Wilkes, J.S. (2002). A short history of ionic liquids - From molten salts to neoteric solvents. Green Chemistry, 4, 73-80, ISSN 1463-9270... 

---