Diethyl fumarate, as a dienophile

In 1965, Ishikawa and co-workers investigated the oxazole cycloaddition reactions of 4-methyloxazole, 5, with a variety of dienophiles (Fig. 3.3). The product distribution for these reactions was found to depend on the substituents on the dienophile as well as the reaction conditions. For example, the reaction of 4-methyloxazole with diethyl fumarate gave a mixture of pyridinols 6 and 7. 

A similar reaction has been observed with l-methyl-3-(l-benzylthioethenyl)pyrrole. Adducts can also be obtained from dienophiles such as acrylonitrile, acrolein, methyl acrylate, and diethyl fumarate <91CPB489>. In this case the initial products are tetrahydroindoles which can be aromatized with ddq (Scheme 143). 

Substituents such as carbonyl and cyano are electron-withdrawing and activate the dienophile toward cycloaddition. The electron-attracting properties of these groups arise from bond dipoles, which place a partial positive charge on the carbonyl or cyano carbon (see Section 1.5). Acrolein has one electron-withdrawing group attached to its dienophilic double bond. Diethyl fumarate and maleic anhydride, the most reactive and synthetically useful dienophiles you will encounter in this section, both have two. 

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