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The Lignans: A Family of Biologically Active Polyphenolic Secondary Metabolites

Anna K.F. Albertson and Jean-Philip Lumb

Department of Chemistry, McGill University, Montreal, Québec, Canada

1.1 Introduction

Nature has long served as an important source of therapeutics, and lignans represent a large class of pharmacologically active compounds (Cunha et al. 2012). This family of molecules demonstrates a wide range of biological activities, which plants use as a front-line chemical defence against pathogens (Figure 1.1). Additionally, the anticancer, antimiotic, antiangiogenesis and antiviral properties possessed by lignans have made them appealing drug candidates, as well as starting points for drug discovery. Lignans currently employed for healthcare include (-)-podophyllotoxin (1), a treatment for warts, and its derivatives (-)-etoposide (2) and (-)-teniposide (3), two potent chemotherapeutic agents (Liu et al. 2007). Other members of this class with promising biological activities include (+)-gomisin J (4) and (+)-pinoresinol (5). Due to the established benefits of the lignans, both their biosynthesis and synthetic strategies to access them have been areas of extensive research.

Figure 1.1 Selected biologically active lignan natural products.

In addition to their varied biological activities, lignans comprise a vast array of structurally distinct skeletons (Figure 1.2), including 6- and 8-membered carbocycles (6, 7), linear dibenzylbutanes (8), and diversely oxidized tetrahydrofurans (9-11). Remarkably, their biosynthesis originates from a regio- and stereoselective, oxidative coupling of relatively simple monolignols (propenyl phenols) (12), to form the key 8-8 bond that serves to characterize all lignan natural products. Subsequent transformations, including cyclization and oxidation of the parent scaffold, convert the initially formed dimer to various family members, imparting unique functionalities. While this blueprint has served as a key source of inspiration for decades of biomimetic synthetic approaches to the lignans, issues of selectivity in the oxidative coupling have led researchers to alternative, target-oriented routes, which are often specific for an individual structural class. In this review, we summarize these recent efforts from 2009 to 2016, and provide an overview of contemporary research efforts interrogating the lignans. Previous reviews on this subject cover 2000-2004 (Saleem et al. 2005), 2005-2008 (Pan et al. 2009), and 2009-2015 (Teponno et al. 2016).

Figure 1.2 Structural classes of lignans.

1.2 Biosynthesis of Lignans

Due to their biological activity and fundamental importance to plant biology, significant efforts have been made to elucidate lignan biosynthesis (Suzuki and Umezawa 2007; Umezawa 2009; Petersen et al. 2010). Lignans originate from cinnamic acids, which are themselves biosynthesized from phenylalanine (Scheme 1.1). The shikimate pathway, which produces several aromatic amino acids including phenylalanine (16), is preceded by the synthesis of shikimic acid (15) from phosphoenolpyruvate (13) and erythrose-4-phosphate (14). The conversion of phenylalanine to cinnamic acid (17) is carried out by phenylalanine ammonia-lyase (). Substitution of the aromatic ring is performed by cinnamate hydroxylases (C4H and C3H), to access coumaric acid (18) and caffeic acid (19). The methyl ether found in ferulic acid (20) is installed by caffeic acid O-methyltransferase (). Several additional steps convert the carboxylic acid to the primary alcohol, affording coniferyl alcohol (21). This propenyl phenol undergoes an oxidative coupling, the first step in the biosynthesis of pinoresinol (5). The oxidative coupling has been extensively investigated (Hapiot et al. 1994; Gavin and Huai-Bing 1997; Halls et al. 2004; Pickel et al. 2010), and involves a unique mechanism, starting with a one-electron oxidation of the phenol, believed to be carried out by a laccase. Two phenoxyl radicals (22) are then proposed to combine in the presence of a dirigent protein to form a bis-para-quinone methide (23), which undergoes subsequent cyclization to provide the furofuran 5.

Scheme 1.1 Biosynthesis of (+)-pinoresinol.

Several dirigent proteins have been isolated, including those that are selective for either enantiomer of pinoresinol. They display a unique ability to control the regio- and stereoselectivity of phenoxyl C-C coupling, despite not having any oxidative activity themselves. This has led to a biosynthetic proposal that requires an exogenous oxidant, followed by diffusion of the phenoxyl radicals into the dirigent protein's active site. In their absence, the oxidative coupling of coniferyl alcohol leads to a complex mixture (Scheme 1.2), from which pinoresinol is isolated in only trace quantities. The first crystal structure of such proteins was obtained from a pea plant, Pisum sativum (Figure 1.3), affording (+)-pinoresinol (Kim et al. 2015). While it was not co-crystallized with the substrate, several aspects of the protein are consistent with the proposed biosynthesis. A trimer structure was determined, which was observed to have six conserved residues in the proposed active site with other proteins that produce (+)-pinoresinol. These include arginine and aspartic acid residues that are on opposite sides of the pocket but are sufficiently close to co-ordinate to the phenolic and primary hydroxylic oxygens of the oxidized substrate. However, since several loops surrounding the potential binding cavity were not resolved in the structure, alternative modes of substrate binding and coupling could not be confirmed.

Scheme 1.2 (a) Main coupling pathways for oxidative coupling of coniferyl alcohol. (b) Atom labelling of coniferyl alcohol. (c) Calculated spin density for atoms contributing most to coniferyl radical. (d) Conversion of radical-coupled products to neolignans.

Figure 1.3 Crystal structure of dirigent protein from Pisum sativum.

While the exact mechanistic steps involved in the dimerization have not been conclusively determined, it is now accepted that the dirigent protein is critical for controlling selectivity during the oxidative coupling. This is readily apparent from numerous studies on the free radical coupling of monolignols (Table 1.1). In the presence of various oxidants, coniferyl alcohol rarely forms pinoresinol but instead affords dimers arising from radical coupling at carbon 8 with carbon 5 and oxygen 4 (Scheme 1.2a and b), along with extensive polymerization and decomposition. Attempts at directly mimicking the biosynthetic pathway by employing laccases (Wan et al. 2007; Lu and Miyakoshi 2012) (Table 1.1, entries 1-4) and peroxidases (Chioccara et al. 1993; Mitsuhashi et al. 2008; Matsutomo et al. 2013) (entries 5-7) afford mixtures that vary significantly depending on the specific enzyme used, as well as the method of isolation and purification of the oxidase. Due to the sensitivity of the enzymes, temperature and pH play a large role in the product distribution. More traditional synthetic oxidants, such as peroxides (Dellagreca et al. 2008) (entry 8) and metal salts (Brezný and Alföldi 1982; Vermes et al. 1991; Kasahara et al. 2006; Lancefield and Westwood 2015) (entries 9-12), have been utilized and suffer from similar challenges with regioselectivity and decomposition.

Table 1.1 Synthetic oxidative couplings of coniferyl alcohol.