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. Stereochemical studies reveal that the cycloadditions of 1 are concerted processes, even for the most electron-deficient olefins dimethyl dicyanofumarate and dimethyl dicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under our conditions (dilute solutions and temperatures ≤60 °C) 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:  E a = 14.8 ± 0.2 kcal mol-1, log A = 11.9 ± 0.1, ΔH ⧧ = 10.8 ± 0.1 kcal mol-1, ΔS ⧧ = −6.2 ± 0.1 cal mol-1 K-1, and ΔG ⧧ = 11.40 ± 0.03 kcal mol-1.

3-butadiene in (4 + 2) Cycloadditions. A Mechanistic Comparison.-In order to probe the borderline between concerted and two-step Diels-Alder reactions via zwitterions the cycloadditions of the title dienes to a series of dienophiles of increasing electrophilicity is described. Kinetic measurements and stereochemical studies are reported. -(SUSTMANN, R.; TAPPANCHAI, S.; BANDMANN, H.; J.