2H-Chromenes (2H-1-benzopyran derivatives) display a broad spectrum of biological activities. The 2H-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 2H-chromenes and is structured around the three main approaches applied in catalytic 2H-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 2H-chromene synthesis. However, for a complete overview, synthetic routes involving some stoichiometric steps and reactions producing ring-scaffolds that are closely related to 2H-chromenes are also included.
A new route to the chromene ring system has been developed which involves the reaction of an α,β-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. This transformation involves a cascade of reactions that begins with a benzannulation reaction and is followed by the formation of an o-quinone methide and finally, results in the emergence of a chromene upon an electrocyclization. This reaction was extended to the provide access to by employing an aryl carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a single step and is highlighted in the synthesis of lapachenole and Vitamin E.
Catalytic asymmetric transformation is a powerful strategy for the introduction of chirality to synthesize diverse nonracemic molecular entities. Chiral molecules play a significant role in the pharmaceutical area, as 50% of current drugs that are in use and 80% of the developing drugs are chiral. Asymmetric catalysis has been explored tremendously by synthetic organic chemists, and still continuous efforts are being directed toward advanced discoveries. The role of carboxylic acids as building blocks in catalytic asymmetric reactions is an emerging area. Carboxylic acids are known for their robustness and thus low reactivity. They are widely available, yet quite challenging substrates for catalytic asymmetric synthesis. The catalyst required for this kind of transformation must withstand acidic as well as oxidizing conditions. Catalyst and reaction design are required to break the glass ceiling of low reactivity of the acid substrates to ensure the useful transformation becomes successful. At the same time, high stereoselectivity needs to be achieved with complete control. Because of these challenges, carboxylic acids are highly intriguing substrates in the asymmetric catalysis research area, and more interesting methods are developing. There have been many important advancements in the last three decades. These literature reports show that carboxylic acids can act as both C- and O-nucleophiles. Furthermore, they can also be employed as electrophiles in asymmetric reactions under catalytic conditions. Brilliant application of catalyst and reaction designs made these transformations possible. This review article summarizes all these important developments on the use of carboxylic acids as building blocks in asymmetric catalysis.
Catalytic Synthesis of 2H-Chromenes -[114 refs.]. -(MAJUMDAR, N.; PAUL, N. D.; MANDAL, S.; DE BRUIN, B.; WULFF, W. D.; ACS Catal. 5 (2015) 4, 2329-2366, http://dx.doi.org/10.1021/acscatal.5b00026 ; Dep. Chem., Mich. State Univ., East Lansing, MI 48824, USA; Eng.) -Koehler 22-291
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