Preparation and Diels−Alder Chemistry of 4-Vinylimidazoles

Various 4-vinylimidazole derivatives have been prepared from the corresponding 4-iodoimidazoles or
from urocanic acid. Several methods for the elaboration of these vinylimidazoles and their Diels−Alder
reactions are reported. All of the vinylimidazoles prepared in the course of this study react with
N-phenylmaleimide quite readily with mild thermal activation providing a single cycloadduct, in most
cases the initial, nonaromatic adduct. With more electron rich substrates, there is a tendency for these
initial cycloadducts to undergo aromatization, ene reaction, and oxidation although this can be circumvented
to a large extent by the choice of reaction conditions. Limited reactions were observed with other
dienophiles, providing the expected cycloadducts in most cases, although an abnormal adduct was obtained
in one case with dimethyl acetylene dicarboxylate. These substrates also participate in regioselective
Diels−Alder reactions with monoactivated dienophiles, but require fairly forcing conditions, thus only
providing the aromatized cycloadducts in modest yields. An investigation of substituent effects at the
2-position of the imidazole moiety was undertaken, in which electron-donating and weakly electron-withdrawing substituents are tolerated. In addition, several substrates with terminally substituted vinyl
moieties have been investigated.