Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Regioselectivity in cycloadditions can be observed if unsymmetrically substituted components participate in the reaction, for example in a Diels-Alder reaction in which both diene and alkene are unsymmetrically substituted .
Electron-donating and -withdrawing substituents in the product are arranged in pseudo-ortho and pseudo-para positions to each other. Although the terms ortho and para are reserved for use in aromatic compounds, they are being applied here to serve as mnemonics. The reason for regioselectivity can be simply derived from the mesomeric structures of diene and dienophile.
As in endo/exo selectivity, strict regioselectivity is not always observed. Mixtures are obtained in most cases.