Three Heterocyclic Rings Fused (5-6-5)

Abstract

The chapter gives an updated overview of the many chemically and structurally diverse 5-6-5 fused tricyclic heterocyclic systems that are reported in the literature. These include fully conjugated systems, some of which have attracted interest for their electronic properties, e.g., dipyrazolopyridines, and non-conjugated systems such as the biologically interesting fused diketopiperazines. The systems outlined in the chapter are classified according to the central six-membered ring. Six-membered rings with one or more nitrogens are the most abundant and are further classified according to the number and positioning of the nitrogen(s) and ring saturation. Rings containing O, P, S or mixed heteroatoms are less abundant, but represent some very interesting and diverse chemistry. Theoretical and spectral methods and the physical properties of these ring systems are explored. Reactivity of ring carbons and heteroatoms and ring substituents are described. Synthesis of the numerous classes of compounds is reviewed for linear, angular, peri- and epi-fused ring systems. Routes from acyclic, monocyclic, and 5,6-bicyclic precursors, or by ring transformations, are also reviewed. Common methods are synthesis by cyclization onto a central six-membered ring or formation of the central six-membered ring by a wide variety of reaction types, making this an interesting and varied area of heterocyclic synthetic chemistry.