Abstract
Benzopyran and benzodihydropyran (chromane) nuclei are the core structure of many natural products, in particular flavonoids. Many compounds possessing this structure are nutraceuticals, pharmaceutical nutrients. Therefore, benzopyran and chromane scaffolds are important building blocks in organic synthesis and many efforts have been made to set up efficient methods for their synthesis. In particular, asymmetric methods are of great importance, being natural products, and generally chiral substances. This review aims to cover literature in the range 2017–first half of 2019.
Highlights
Chiral chromanes are a class of privileged structural motif present in a plethora of natural products and synthetic analogs, displaying a wide range of biological activities and used as drugs [1,2].the chromane structure is the core nucleus of the natural class of compounds named flavonoids
As most of the natural products, flavonoids are chiral; the asymmetric synthesis of these compounds has been extensively studied over the years and a plethora of methodologies are available: cycloaddition of ortho-hydroxystyrene derivatives or of ortho-quinone methides, cascade reactions, and reaction involving oxocarbenium ions
Tetrahydrofuran/tetrahydropyran-fused 3,4-dihydrocoumarin was prepared by a two-step-one-pot synthesis from 2-hydroxycinnamaldehydes and γ/δ-hydroxy enones activated by iminium ion catalysis and followed by pyridinium chlorochromate (PCC) oxidation (Scheme 14) [25]
Summary
Chiral chromanes are a class of privileged structural motif present in a plethora of natural products and synthetic analogs, displaying a wide range of biological activities and used as drugs [1,2]. As most of the natural products, flavonoids are chiral; the asymmetric synthesis of these compounds has been extensively studied over the years and a plethora of methodologies are available: cycloaddition of ortho-hydroxystyrene derivatives or of ortho-quinone methides, cascade reactions, and reaction involving oxocarbenium ions. Asymmetric syntheses can be performed by chiral metal catalysts or by organocatalysis. Organocatalysis provides many advantages, such as the employment of small organic molecules (overcoming the toxicity of transition metal ions), the various and simultaneous activation modes for substrates and reagents (allowing different catalytic loops in the same reaction, favoring cascade reactions), the easy availability and low cost of these catalysts, and the very mild operating conditions.
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