Out-endo-cis rule

Stereochemistry The most electron withdrawing group on the dienophile prefers to be endo, and the out-endo-cis rule tells you that the OAc and CN groups are cis in the product. The suprafacial-suprafacial nature of the Diels-Alder reaction... 

Common error alert Be certain to preserve the stereochemistry about all three it bonds When a diene is given in its s-trans conformation, as in the preceding example, students often isomerize the it bonds inadvertently when they try to redraw it in its s-cis conformation. A correct application of the out-endo-cis rule then leads to an incorrect answer. This common error can be prevented by obeying Grossman s rule. 

The endo rule applies equally to inverse electron-demand Diels-Alder reactions. In these reactions, the most electron donating group on the dienophile is preferentially endo. The out-endo-cis rule applies, too. 

Dipolar cycloadditions give predominantly endo products, too. Again, the out-endo-cis rule applies. 

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