2.4- Pentadienoic acid 3 + 2 cycloaddition reactions

The adducts obtained from acyclic dienes and cyclic dienophiles are frequently formed in accordance with the endo rule. A classic example is found in the Woodward synthesis of reserpine, which started with the Diels-Alder reaction of i -pentadienoic acid and benzoquinone (3.67). In this cycloaddition reaction, the endo adduct 78, in which the carboxylic acid and the benzoquinone carbonyl groups become cLs to one another, is obtained as the exclusive product. 

Moderate selectivities were observed in non-catalyzed cycloaddition reactions of chiral pentadienoic acid esters and amides with e.g. benzoquinone even at higher temperature and at high pressure (1 lO torr). 

---