Some recent developments in higher-order cycloaddition reactions

Higher-order cycloaddition reactions provide an easy entry into medium size d ring systems which are often found in a number of important biologically active natural products. In this connection, our investigations on 2H-cyloh epta[b]furan-2-one, an 8 pi cross-conjugated system, have exposed some nove l reactivity patterns including a higher-order cycloadition profile. The cy cloaddition reactions of this electron-deficient heptafulvene system procee ded in an [8+2] fashion with 1,3-dienes, whereas an exclusive [4+2] pathway prevailed with aralkenes. The reactions with cyclic cross-conjugated syste ms such as pentafulvenes afforded [8+2] and [4+2] adducts, the ratio being controlled by the substitution on the exocyclic double bond of the fulvene. The observed reactivity has been rationalized by theoretical calculations.