Heterocycles from Donor-Acceptor Interactions

The quinone ring is found commonly in nature. Quinones, including heterocyclic analogs, are involved in numerous biochemical processes because of their facile reduction-oxidation. They play an important role in electron therapeutic value as antitumor, antibacterial, antifungal, and anticancer agents. Additionally and in general, 1,1,2,2-tetracyanoethene (TCNE) as well as its derivatives containing the dicyanomethylene moieties are known as electron-deficient substances. This chapter is concerned with selected reactions of 1,1,2,2-tetracyanoethene (TCNE, 1), 2,3-dichloro-1,4-naphthoquinone (2a) and 2,3-dicyano-1,4-naphthoquinone (2b), 2-(1,3-dioxo-2,3-dihydro-1(H)-indene-2-ylidene)propanedinitrile (3), 3,4,5,6-tetrachloro-1,2-benzoquinone (4), 2,3,5,6-tetrachloro-1,4-benzoquinone (5) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (6) as pi-deficient compounds, have been used or potentially used in heterocyclic synthesis. The chemistry here is dominated by addition and addition-elimination reactions accompanied by donor-acceptor interactions that would undergo charge-transfer complex formation. Reactions of nucleophilic donors with acceptors 176 were investigated. Moreover, cycloadditions like [2+2], [4+2], and others with selective pi-acceptors serve as effective tools in the synthesis of heterocyclic compounds.