Abstract
Numerous oxidative transformations of lignan structures have been reported in the literature. In this paper we present an overview on the current findings in the field. The focus is put on transformations targeting a specific structure, a specific reaction, or an interconversion of the lignan skeleton. Oxidative transformations related to biosynthesis, antioxidant measurements, and total syntheses are mostly excluded. Non-metal mediated as well as metal mediated oxidations are reported, and mechanisms based on hydrogen abstractions, epoxidations, hydroxylations, and radical reactions are discussed for the transformation and interconversion of lignan structures. Enzymatic oxidations, photooxidation, and electrochemical oxidations are also briefly reported.
Highlights
Lignans constitute a group of natural phenolics found in plant species
We report an overview on oxidative transformations of lignan structures, where the focus is put on transformations targeting a specific structure, a specific reaction, or an interconversion of the lignan skeleton
Lignans are oxidatively transformed in a number of processes and conditions
Summary
Lignans constitute a group of natural phenolics found in plant species. They have been identified in around 70 families in the plant kingdom, for example in trees, grasses, grains, and vegetables. Lignans are found in roots, rhizomes, stems, leaves, seeds, and fruits, from where they are usually isolated through extraction with an appropriate solvent They have a diverse structure built up from two phenyl propane units with different degrees of oxidation in the propane moiety and different substitution patterns in the aromatic rings. The structure and occurrence of lignans have previously been extensively reviewed in the literature In some of these reviews the chemical transformations and oxidative degradations have partially been described, mainly for determination of structures [6,11,12,13,14,15,16]. Oxidation of benzylic alcohol to ketone phenolic hydroxyl oxidation to quinone type structures, Ar-Ar-coupling to dibenzocyclooctadienes, nucleophilic attack at ipso- or benzylic position. Benzylic: cleavage/alcohol oxidation to ketone/ring closure/nucleophilic attack benzylic ring closure
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