Chemistry of electron‐rich conjugated polyenes V. Regio‐ and stereo‐selectivity in Diels‐Alder reactions of 1‐alkoxy‐1,3‐butadienes and β‐cyanostyrenes

Effects of substituents in diene and dienophile on the regio‐ and stereo‐selectivity of Diels‐Alder reactions have been studied on cycloadditions of alkoxy‐substituted 1,3‐butadienes (1) and 2‐substituted cyanoethenes (2). The regioselectivity is always very high, and is determined by the magnitude of the orbital coefficients of the interacting carbon atoms according to the frontier orbital theory. The stereoselectivity is more determined by steric factors than by secondary orbital overlap. The results point to a transition state, in which the planes of diene and dienophile, are non‐parallel (Figure 1).