Nonbenzenoid Cycloproparenes

Despite the extensive literature on cycloproparenes, compounds possessing cyclopropa‐fusion to functionalised benzenoid rings or to nonbenzenoid aromatic systems are rare. This review describes approaches to some of these novel systems. The cyclopropaquinones 29 and 30 , generated by bromodesilylation at −78 °C of precursors 47 and 70 , respectively, are too reactive to be isolated, but can be trapped as Diels−Alder adducts with furan. The high reactivity of 29 and 30 contrasts with the stability of quinones 24 and 26 in which the quinone moiety and cyclopropa‐fusion are located in different rings. The cycloaddition methodology developed for the generation of 30 has been extended to the synthesis of favelanone ( 98 ), a natural product incorporating the cyclopropa[ b ]naphthalene‐2,7‐dione structural motif. While several six‐membered heterocyclic cycloproparenes have been described, the synthesis of cyclopropafurans and cyclopropathiophenes poses significant challenges. There is tentative evidence for the generation of the cyclopropa[ c ]thiophene 160 by double dehydrobromination of 158 , but the photolysis of the pyrazolothiophene 166 yields only alkene 169 and none of the desired ring‐closed product. In contrast, irradiation of the related pyrazolofuran 165 results in clean fragmentation to give the acetylenic enone 171 . The cyclopropatropone 201 has been prepared, and the derived hydroxytropylium ion 202 represents the first example of a cyclopropa‐fused seven‐membered aromatic ring system. Finally, while the synthesis of cyclopropazulenes remains to be achieved, the potentially useful precursor 230 has been obtained by a directed transannular aldol condensation.