Figure 1

Possible reaction pathways of the system. a) A two-letter code is used to name products. The first refers to endo(N) or exo(X) Diels-Alder products while the second characterizes the direction of the amidopyridine recognition site (N: same side as bridge, X: opposite side of bridge). Equilibrium constants are given as association constants. The difference between A·B·NX and A·B·NX* is the relative alignment of A and B. Hydrogen atoms are omitted for clarity (except OH and NH). b) The bimolecular background reaction is shown in 2 D as an example. c) A*·B- (2D) and A·B-complex (3D) with almost no screening effect. d) A*·B (2D) and A·B-complex (3D) with a good screening effect of the top side. 3D-structures in (c) and (d) were obtained by geometry optimizations using density functional theory.