The reactions of (E)-1-methoxy-1,3-butadiene (1) and 1,1-dimethoxy-1,3-butadiene (2) with a series of dienophiles of increasing electrophilicity are described. Stereochem. studies reveal that the cycloaddns. of 1 are concerted processes, even for the most electron-deficient olefins di-Me dicyanofumarate and di-Me dicyanomaleate. 2 Reacts under our conditions (dil. solns. and temps. ? 60 Deg) only with those dienophiles which can give zwitterions out of the antiperiplanar conformation of the diene. Zwitterionic intermediates can be trapped by methanol. In the case of tetracyanoethene the kinetics of decay of an intermediate, interpreted as the zwitterion, can be followed by stopped flow techniques: Ea = 14.8 +- 0.2 kcal mol-1, log A = 11.9 +- 0.1, DH.thermod. = 10.8 +- 0.1 kcal mol-1, DS.thermod. = -6.2 +- 0.1 cal mol-1 K-1, and DG.thermod. = 11.40 +- 0.03 kcal mol-1.