Diels-Alder reactions ortho" adduct formation

This means, for example, that in normal Diels-Alder reactions of 1-substituted dienes with 1-substituted ethenes, bond formation between C(4) of the diene and C(2) of the alkene, which leads to the ortho adduct, is favored over the other bond formation leading to the meta adduct. Formation of para products from 2-substituted dienes can be explained by a similar reasoning. 

Hartman et al. [39,40], however, succeeded in preparing the exo-ortho adduct by Diels-Alder reaction of 1,3-butadiene with cyclobut-3-en-l,2-dicar-boxylic acid anhydride, hydrolysis of the anhydride, esterification, bromination of the double bond, dehydrobromination, ester hydrolysis, and re-formation of the anhydride (Scheme 6). 

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