Reactivity of long conjugated systems: selectivity of Diels-Alder cycloaddition in oligofurans.

Taking advantage of the synthetic availability and solubility of long oligofurans, their reactivity toward dienophiles was studied as a model for the rarely investigated reactivity of long conjugated systems. Unlike oligoacenes, the reactivity of oligofurans decreases or remains constant with increasing chain length. Terminal ring cycloadducts of oligofurans are kinetically and thermodynamically favored, whereas central ring cycloadducts are preferred in oligoacenes, because of the different driving forces in the two reactions: π-conjugation in oligofurans and aromatization/dearomatization in oligoacenes.