Vinyl Benzenes as Dienes in Mild Solid-Phase Diels−Alder Reactions

Wang resin-bound intermediates derived from Fmoc-l-phenylalaninal and Fmoc-l-valinal, and a resin supported Horner−Wadsworth−Emmons reaction, were treated with cinnamaldehyde derivatives, acetic acid, and borohydride to give secondary amines which were subsequently benzoylated to afford various derivatives of 3. Heating 3 at 95 °C induced cycloaddition reactions and produced 4 as the major product. Compounds 3 which were derived from 4-methoxycinnamaldehyde were more reactive, but did not give 4 and 4−7g. The direct cleavage of 3b using TFA led to the isolation of cycloaddition−demethylation product 10. The derivative of 3, which contained an electron-withdrawing nitro group on the phenyl ring, produced a single diastereomer of 4. The Diels−Alder cycloaddition between two electron-deficient counterparts showed similar reactivity to that of the reactions which have a normal complementary electron-demand.