This part of Volume 24 in the series completes the review of fused pyrimidines in which the second ring is six-membered and contains one or more of the elements of nitrogen, oxygen or sulphur. It is a critical survey of synthetic methods and reactions of each class of compound. The Chemistry of Heterocyclic Compounds, since its inception, has been recognized as a cornerstone of heterocyclic chemistry. Each volume attempts to discuss all aspects – properties, synthesis, reactions, physiological and industrial significance – of a specific ring system. To keep the series up-to-date, supplementary volumes covering the recent literature on each individual ring system have been published. Many ring systems (such as pyridines and oxazoles) are treated in distinct books, each consisting of separate volumes or parts dealing with different individual topics. With all authors are recognized authorities, the Chemistry of Heterocyclic Chemistry is considered worldwide as the indispensable resource for organic, bioorganic, and medicinal chemists.