Catalytic Synthesis of 2H-Chromenes

2 H -Chromenes (2 H -1-benzopyran derivatives) display a broad spectrum of biological activities. The 2 H -chromene substructure is an important structural motif present in a variety of medicines, natural products, and materials showing unique photophysical properties. Hence, the structural importance of the benzopyran moiety has elicited a great deal of interest in the field of organic synthesis and chemical biology to develop new and improved synthesis of these molecular skeletons. This review gives an up-to-date overview of different catalytic methodologies developed for the synthesis of 2 H -chromenes and is structured around the three main approaches applied in catalytic 2 H -chromene synthesis: (I) catalysis with (transition) metals, (II) metal-free Brønsted and Lewis acid/base catalysis, which includes examples of nonenantioselective organocatalysis, and (III) enantioselective organo-catalysis. The section in which the metal-catalyzed reactions are discussed describes different ring-closing strategies based on (transition) metal catalysis, including a few enantioselective approaches. For most of these reactions, plausible mechanisms are delineated. Moreover, synthesis of some natural products and medicinally important drugs are included. Specific advantages and disadvantages of the several synthetic methodologies are discussed. The review focuses on catalytic 2 H -chromene synthesis. However, for a complete overview, synthetic routes involving some stoichiometric steps and reactions producing ring-scaffolds that are closely related to 2 H -chromenes are also included.