Diels-Alder reaction of azines with olefins, isocyanates, or acetylenes leads2. 3 to bicyclic 2: l (dienophile/azine) adducts 1, 2, or 4 (or products of further thermal rearrangements2t3), respectively, rather than the expected 1: l adducts associated with butadienes. This type of reactivity is unique to azine chemistry and has been termed2 “criss-cross” cycloaddition.
![Benzenemethanol, a-[(5,6-dihydropyrano[2,3-c]pyrazol-1(4H)-yl)phenylmethylene]-结构式](https://image.chemsrc.com/caspic/458/63569-96-0.png)