It is thought that the 2-butenals containing a functional substituent in the 4-position may be valuable as new type dienophiles for the practical use of the Diels-Alder reaction. No work on the synthesis of such r-substituted crotonaldehydes had been found in the literature of the subject, till the formation of 4-dimethylamino-2-butenal was first reported by the authors, 0 who were unable to isolate the free aldehyde, but succeeded in isolating the 2, 4- ...