Gresham and Steadmana reported the first instance of a carbonyl group (formaldehyde) acting as a dienophile. The reaction was successful for 2-methyl-l, 3-pentadiene with paraformaldehyde yielding 2, 4-dimethyl-5, G-dihydro-2H-pyran but was unsuccessful with 1, 3-butadiene or piperylene. Dale and Sisti4 have shown that chloral will act as a dienophile with certain dienes including 23-dimethylbutadiene, isoprene, and piperylene ...
[Smith et al. Journal of the American Chemical Society, 1951 , vol. 73, p. 5267,5269 Journal of the American Chemical Society, 1952 , vol. 74, p. 2018]