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Abstract
Lignans are mainly dimers of 4-hydroxycinnamic acids (HCAs) and reduced analogs thereof which are produced in Nature through phenol oxidative coupling (POC) as the primary C-C or C-O bond-forming reaction under the action of the enzymes peroxidases and laccases. They present a large structural variety and particularly interesting biological activities, therefore, significant efforts has been devoted to the development of efficient methodologies for the synthesis of lignans isolated from natural sources, analogs and hybrids with other biologically interesting small molecules. We summarize in the present review those methods which mimic Nature for the assembly of the most common lignan skeleta by using either enzymes or one-electron inorganic oxidants to effect POC of HCAs and derivatives, such as esters and amides, or cross-POC of pairs of HCAs or HCAs with 4-hydrocycinnamyl alcohols. We, furthermore, provide outlines of mechanistic schemes accounting for the formation of the coupled products and, where applicable, indicate their potential application in medicine.
Highlights
CinA and its 4-hydroxysubstituted derivatives (HCAs), namely CouA, CafA, FerA and SinA, form an important family of natural products ubiquitous in the plant kingdom generally known as cinnamates [1]
phenol oxidative coupling (POC) on free hydroxycinnamic acids (HCAs) usually leads to dilactones 1, in this case, a mixture of mainly dimeric products and unreacted CafA was obtained, the main component of which was caffeicin E (49), that is a lignan of the substituted 1,2-dihydronaphthalene (CL3) type (Scheme 30) [45]
We have presented in this review an overview of past and recent applications of an old reaction, namely POC, in the bioinspired syntheses of a variety of classical lignans (CLs), and NLs with potential medicinal significance
Summary
CinA and its 4-hydroxysubstituted derivatives (HCAs), namely CouA, CafA, FerA and SinA, form an important family of natural products ubiquitous in the plant kingdom generally known as cinnamates [1]. Dimerization of phenolic compounds in Nature is effected using POC as key reaction, which allows the connection of the monomers through the formation of new C-C or C-O bonds This concept had been long recognized, it was the seminal investigation by Barton and Cohen on the structure of Pummerer’s ketone, produced by the ferricyanide-mediated POC of p-cresol in alkaline solution, which established the relation of such reactions in the laboratory with biosynthetic pathways [5]. The two families of enzymes mainly differ in the metallic cation present in their active site, Fe3+ in the former and Cu2+ in the latter Due to their side chain, one electron oxidation of phenolate anions of phenylpropanoids creates through resonance an additional site (C-8 or C-β) for radical coupling (see canonical form V in Scheme 3). A large array of oxidants, inorganic and enzymes, have been already developed for performing POC [21], a few of them have been proved useful in producing regioselectively and/or stereoselectively the desired lignan isomer or an appropriate key intermediate
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