Condensation of the following vinyl ether heterocycles with various dienes was examined: 2,3-dihydrofuran (I), 5-methyl-2,3-dihydrofuran (II), 2-methylenetetrahydrofuran (III), and dihydro-γ-pyran (IV).Acrolein (V) reacted best among the dienes, while butadiene adducts could not be isolated.Among the dienophiles, III was most reactive, IV least.Other dienes which failed to yield isolable products were 2-methyl-1,3-pentadiene and mesityl oxide.Yields with crotonaldehyde were much lower than with V.The probable structure of the condensation product of I and V is O.CH2.CH2.CH.CH.O.CH:CH.CH2 rather than O.CH2.CH2.CH.CH.CH2.CH:CH.O.With butadiene, the chief condensation product was vinylcyclohexane.Condensations were carried out at 80-180° and 10 kg. pressure.
