Editorial [Hot topic: Multicomponent Reaction (Guest Editor: Song Cao)]

After nearly 20 years of extensive study and rapid development, multicomponent reactions have already been an commonly used and powerful tool for the construction of novel and complex target molecules. Up to now, numerous literatures on the development of novel MCRs have been reported and several reviews have summarized the progress on MCRs before the end of 2005 thoroughly. Recently, with the development of new methodologies of organic synthesis, MCRs have penetrated into various aspects of organic synthesis. This special thematic issue includes five reviews which devote to the research on multicomponent reaction of the specific compound classes, new catalysts and the most commonly used green reaction media, respectively. The first review, written by E. J. Yoo and S. Chang, Department of Chemistry, Korea Advanced Institute of Science and Technology, focuses on authors' and other groups' recent progresss on the new type of Cu-catalyzed multicomponent reactions which have been developed relying on the in situ generation of N-sulfonyl- or N-phosphorylketenimine intermediates. They comprehensively summarize this facile and versatile route to ketenimines which has led to the development of a range of highly efficient multicomponent reactions by employing diverse nucleophiles such as amines, alcohols, water, pyrroles, and thiolates. Alkynones are versatile three-carbon building blocks in heterocyclic chemistry and are easily prepared by a modified Sonogashira coupling of acid chlorides and terminal alkynes. The second review provided by B. Willy and T. J. J. Muller, Institute of Organic Chemistry and Makromolecular Chemistry, Heinrich-Heine University, is dedicated to their achievements in multicomponent synthesis of heterocycles initiated by Pd-catalyzed generation of alkynones as established (by themselves) within the past years. Thus, isoxazoles, indolizines, pyrazoles, pyridimines, 1,5-benzoheteroazepines, furans, oxazoles and pyrroles are readily accessible by this novel multicomponent reactions under the mild reaction conditions. P. J. Tambade, Y. P. Patil and B. M. Bhanage, Department of Chemistry, Institute of Chemical Technology (Autonomous), contribute to the third review, including more than 84 references, which deals with recent development in the fields of multicomponent reactions catalyzed by lanthanides. Some named reactions such as Biginelli, Hantzsch, Ugi, Mannich, and Povarov reaction catalyzed by various lanthanides are dicussed. The combination of water as an environmentally benign solvent for chemical transformations with multicomponent reactions has been receiving much attention. K. Kumaravel and G. Vasuki, Department of Chemistry, Pondicherry University, make a thorough literature survey of multicomponent reactions performed in water or aqueous solutions, are including nearly 108 references.