Regio- and Stereoselectivity in Monolignol Oxidative Coupling: Single-Electron Oxidation, Radical Coupling, and Dirigent Proteins
Regio- and Stereoselectivity in Monolignol Oxidative Coupling: Single-Electron Oxidation, Radical Coupling, and Dirigent Proteins
1. Single-Electron Oxidation of Monolignols and Formation of Phenoxy Radicals
Oxidative coupling of monolignols begins with radical formation from phenolic precursors. Coniferyl alcohol is a classical model substrate in studies of lignan oxidative coupling. Its aromatic ring contains a phenolic hydroxyl group at C4 and a methoxy group at C3, while its side chain remains conjugated with the aromatic ring. Oxidative systems such as laccases and peroxidases can oxidize coniferyl alcohol through single-electron oxidation to form a resonance-stabilized phenoxy radical.[1–3]
Different oxidative systems employ different electron acceptors. Laccases use molecular oxygen to complete the redox cycle, whereas peroxidases generally depend on hydrogen peroxide; the coniferyl alcohol radicals generated by either system can subsequently undergo radical coupling.[3] In classical in vitro reactions of coniferyl alcohol, the oxidation step establishes radical reactivity, whereas the specific bond-forming positions and spatial orientations are determined during the subsequent coupling process. Ando et al. also summarized monolignol oxidation and showed that coniferyl alcohol radicals generated by single-electron oxidation can dimerize through multiple regioisomeric linkages.
Dirigent proteins (DIRs) participate in selective control after radical formation. In classical DIR experiments, the oxidase is responsible for generating coniferyl alcohol radicals, whereas the DIR itself lacks the corresponding oxidative activity.[1]
2. Resonance Delocalization and Coupling Sites of Phenoxy Radicals
2.1 Radical Reactivity at O4, C5, and C8
The radical formed after coniferyl alcohol loses one electron exhibits pronounced resonance delocalization. The unpaired electron is not confined to the phenoxy oxygen atom but can be redistributed through the aryloxy group, aromatic ring, and the side chain conjugated with the aromatic ring. Radical-reactive sites associated with the major dimerization pathways include phenoxy O4, aromatic C5, and side-chain C8.[3,4]
These resonance structures represent different limiting contributors of the same delocalized radical. In lignin chemistry, the three carbon atoms of the coniferyl alcohol side chain are generally designated α, β, and γ, corresponding to C7, C8, and C9, respectively, in lignan numbering. Accordingly, the β–β, β–5, and β–O–4 couplings described in the relevant literature correspond to C8–C8′, C8–C5′, and C8–O–C4′ bond formation, respectively.
During lignification, monolignol radicals can not only dimerize with one another but also undergo cross-coupling with growing oligolignol radicals. For coniferyl alcohol, monolignol dimerization can generate β–O–4, β–5, and β–β linkages; end-wise cross-coupling between a monolignol and an oligolignol mainly generates β–O–4 and β–5 linkages, whereas coupling between two oligolignol radicals can additionally generate 5–5 and 4–O–5 linkages.[3] Collectively, these reactions illustrate the chemical basis by which phenolic radicals undergo regioselective coupling through different reactive sites.
Figure 1. Single-electron oxidation of coniferyl alcohol, resonance delocalization of the phenoxy radical, and radical coupling. Coniferyl alcohol undergoes single-electron oxidation to form a resonance-stabilized phenoxy radical; two monolignol radicals can couple through β–O–4, β–5, and β–β modes to generate β-ether, dihydrobenzofuran-type, and resinol-type structures, respectively. The original figure also shows radical cross-coupling between monolignols and oligolignols and between oligolignols. The α/β/γ side-chain numbering convention used in lignin chemistry is adopted in the figure, with β corresponding to C8 in lignan numbering. Source: Ando et al.[3], Figure 1A, CC BY 3.0.
2.2 Electronic Structure, Substitution Pattern, and Coupling Propensity
Density functional theory (DFT) calculations have compared multiple initial dimerization modes of monolignol radicals. Among the six major linkages examined, 8–O–4, 8–8, and 8–5 coupling exhibit relatively favorable reaction enthalpies; in calculations of cross-coupling reactions, the more reactive radical tends to participate in bond formation through positions with higher spin density.[4]
Radical resonance delocalization provides multiple sites capable of participating in bond formation, while spin density and reaction energetics influence the relative reactivity of these positions. Aromatic substitution, steric hindrance, the approach geometry of the two radicals, and the local reaction medium further alter competition among the different coupling pathways. For example, C5 of coniferyl alcohol retains a hydrogen atom and can participate in 8–5′ coupling, whereas the corresponding C5 position of sinapyl alcohol is occupied by a methoxy group, altering the available regioselective coupling modes according to the substitution pattern.
3. Regioselectivity of 8–8′, 8–5′, and 8–O–4′ Coupling
Regioselectivity is defined by the atomic positions at which the new bond is formed during the initial radical coupling event. The 8–8′, 8–5′, and 8–O–4′ coupling reactions of coniferyl alcohol radicals establish C8–C8′, C8–C5′, and C8–O–C4′ bonds, respectively. These initial bond-forming events are generally followed by nucleophilic trapping, cyclization, or rearomatization of quinone methide intermediates, ultimately yielding stable dimeric structures.[3,5–7]
According to IUPAC structural classification, dimers with an 8–8′ linkage are lignans; products containing non-8–8′ C–C linkages, such as 8–5′, are neolignans; and 8–O–4′ structures in which the two C₆–C₃ units are connected solely through an ether oxygen are oxyneolignans. In this article, the three linkage types are discussed together to compare the regioselectivity of monolignol radical coupling.
Coupling Mode | Initially Formed Bond | Main Intermediate Feature | Major Subsequent Transformation | Representative Product in the Coniferyl Alcohol System |
8–8′ | C8–C8′ | Bis-quinone methide intermediate | Two intramolecular nucleophilic cyclizations and rearomatization | Pinoresinol |
8–5′ | C8–C5′ | Quinone methide intermediate | Intramolecular C–O cyclization and rearomatization | Dehydrodiconiferyl alcohol |
8–O–4′ | C8–O–C4′ | Quinone methide intermediate | Addition of water or another nucleophile to C7 followed by rearomatization | Guaiacylglycerol-8-O-4′-coniferyl alcohol ether |
3.1 8–8′ Coupling and Formation of the Bis-Quinone Methide
When both coniferyl alcohol radicals participate in bond formation through C8, a C8–C8′ bond is formed directly. Following the initial coupling event, both aromatic units acquire quinone methide character, generating a bis-quinone methide intermediate (bisQM). This intermediate retains the terminal hydroxyl groups of both side chains and can subsequently undergo intramolecular nucleophilic cyclization to ultimately form pinoresinol.[7,10]
The 8–8′ system is also one of the most extensively investigated examples of DIR-controlled lignan oxidative coupling. DIRs from Forsythia and Arabidopsis AtDIR6 can strongly bias reactions of coniferyl alcohol radicals toward 8–8′ coupling and subsequently generate specific pinoresinol enantiomers.[1,8–10]
3.2 8–5′ Coupling and Formation of the Dihydrobenzofuran Ring
Formation of a C–C bond between C8 of one coniferyl alcohol radical and C5′ of another generates a quinone methide intermediate. The O4′ phenoxy oxygen on the aromatic ring involved through C5′ subsequently attacks the quinone methide C7 intramolecularly, forming a C7–O4′ bond and restoring aromaticity, thereby generating a 2,3-dihydrobenzofuran ring.
Dehydrodiconiferyl alcohol is a typical product of this reaction sequence. Its structure is established through C8–C5′ radical coupling followed by C7–O–C4′ cyclization: the first step determines the regioselective connection between the two units, whereas the second forms the oxygen-containing five-membered ring.
3.3 8–O–4′ Coupling and Nucleophilic Trapping of the Quinone Methide
8–O–4′ coupling involves bond formation between C8 of one coniferyl alcohol radical and O4′ of another, directly establishing an intermolecular C–O linkage. The resulting quinone methide intermediate is electrophilic at C7 and can subsequently undergo attack by water or another nucleophile, followed by restoration of aromaticity. Addition of water can generate the corresponding guaiacylglycerol-8-O-4′-coniferyl alcohol ether.
The reaction medium can substantially alter the competition among different coupling pathways and among quinone methide trapping reactions. When Quideau and Ralph oxidized coniferyl alcohol with Ag₂O, β–5-coupled dehydrodiconiferyl alcohol was obtained in relatively high proportion in dichloromethane; after addition of an equal volume of water, pinoresinol generated through β–β coupling became the major dimeric product. For the quinone methide generated through β–O–4 coupling, phenolic nucleophiles can compete with water, whereas acidic conditions enhance water addition.[5]
Plant-cell experiments also demonstrate the biological relevance of all three regioselective coupling modes. Beejmohun et al. supplied [^13C₂]coniferin to flax cell suspension cultures and found that the label was incorporated into 8–8′-linked lariciresinol diglucoside, 8–5′-linked dehydrodiconiferyl alcohol glucoside, and 8–O–4′-linked guaiacylglycerol-coniferyl alcohol ether glucoside, demonstrating that the same class of monolignol precursors can enter dimeric products with different regioselective linkages.[6]
4. Quinone Methide Intermediates and Structural Transformations after Radical Coupling
4.1 Electrophilicity and Rearomatization of Quinone Methides
After radical–radical coupling forms a new σ bond, the product often transiently contains one or two quinone methide structures. In a quinone methide, the aromatic system is converted into a cyclohexadienone-type conjugated system, while the benzylic C7 becomes a pronounced electrophilic reaction center. Nucleophilic addition to C7 restores the aromatic system, and the π-electron stabilization gained upon rearomatization provides an important driving force for the subsequent reaction.[7,10]
Quinone methides generated through different regioselective coupling modes differ in spatial arrangement and in the nucleophilic groups available to them, leading to different subsequent structural transformations. The 8–O–4′ system can undergo attack by an external nucleophile; the 8–5′ system can undergo intramolecular phenoxy cyclization; and the 8–8′ system simultaneously generates two quinone methide moieties and can undergo two consecutive intramolecular cyclizations.
4.2 Cyclization of the Bis-Quinone Methide to Pinoresinol
In the bisQM generated by 8–8′ coupling, both C7 and C7′ are electrophilic. The terminal C9/C9′ hydroxyl groups of the two side chains act as intramolecular nucleophiles and attack C7/C7′, respectively, forming two C–O bonds through 1,6-addition while simultaneously completing rearomatization of both aromatic rings, ultimately generating the bis-tetrahydrofuran structure of pinoresinol.[7,10]
Figure 2. Cyclization of the bis-quinone methide intermediate to pinoresinol. The terminal hydroxyl groups undergo intramolecular nucleophilic addition to C7/C7′ of the bisQM, forming oxygen-containing rings accompanied by rearomatization of the cyclohexadienone units. Acid catalysis can enhance the electrophilicity of C7 through protonation, whereas hydrogen bonding can increase the partial positive charge at C7, thereby promoting nucleophilic attack. Source: Gasper et al.[10], Figure 2, CC BY 4.0.
5. Reaction Hierarchy of Regioselectivity and Stereoselectivity
Selectivity in radical coupling comprises two principal levels: the bond-forming position and the spatial orientation. Regioselectivity determines which pair of atoms forms the new bond; within a given linkage mode, the approach faces of the radicals and the conformations of subsequent intermediates further determine enantioselectivity or diastereoselectivity.
Type of Selectivity | Structural Variable Determined | Example in the Coniferyl Alcohol System |
Regioselectivity | Which atoms form the new bond | C8–C8′, C8–C5′, or C8–O–C4′ |
Enantioselectivity | Which pair of bond-forming faces is used for the same linkage mode | re–re or si–si coupling in the 8–8′ system |
Diastereoselectivity | The relative configuration formed among multiple stereocenters | Different relative configurations generated after nucleophilic trapping or cyclization |
In 8–8′ coupling of coniferyl alcohol, regioselectivity restricts formation of the new bond to C8–C8′. Within this linkage mode, the two radicals can still approach through different faces: re–re coupling produces the S,S-configured bisQM and leads to (−)-pinoresinol, whereas si–si coupling produces the R,R-configured bisQM and leads to (+)-pinoresinol.[10]
Regioselectivity and enantioselectivity are structurally related, but the protein constraints governing them are not completely identical. The F47A/Y118A double mutant of AtDIR6 retains approximately 25% of the wild-type regioselectivity while essentially losing its enantiomeric preference for (−)-pinoresinol, indicating that the spatial constraints required to maintain preferential C8–C8′ bond formation can be partially separated from the interactions required to further define the radical coupling faces.[10]
6. DIR-Mediated Regioselectivity and Enantioselectivity in 8–8′ Coupling
6.1 Division of Roles between Radical Generation and DIR Function
Davin et al. found that oxidation of coniferyl alcohol with an oxidase alone produced a mixture of dimeric products, including dehydrodiconiferyl alcohol, pinoresinol, and guaiacylglycerol-8-O-4′-coniferyl alcohol ether, with the pinoresinol being racemic. Addition of a DIR protein fraction from Forsythia strongly redirected the reaction toward 8–8′ coupling and generated highly enantioenriched (+)-pinoresinol.[1] DIR proteins themselves lack the active center required to catalyze single-electron oxidation, and their function occurs after formation of coniferyl alcohol radicals.[1]
Halls et al. further compared the kinetic characteristics of unoxidized coniferyl alcohol and the coniferyl alcohol radical in DIR-mediated reactions. The equilibrium dissociation constant, KD, of unoxidized coniferyl alcohol for DIR was 370 ± 65 μM; according to a kinetic model in which the DIR pathway competes with radical coupling in free solution, the apparent KM estimated with respect to the steady-state concentration of the coniferyl alcohol radical was approximately 10 nM. These experimental results support the coniferyl alcohol radical, rather than unoxidized coniferyl alcohol, as the substrate for the DIR-mediated reaction.[2]
Electron paramagnetic resonance (EPR) studies provided further evidence for radical involvement. Modolo et al. observed that AtDIR6 can stabilize transient coniferyl alcohol radicals and subsequently bias the reaction toward formation of (−)-pinoresinol, consistent with a mechanism in which radicals are first bound and stabilized by DIR and then undergo directed coupling.[11]
6.2 Different DIRs Produce Opposite Enantioselectivities
DIR control of pinoresinol formation is manifested not only in 8–8′ regioselectivity but also in the specific enantiomeric outcome.
Forsythia-derived FiDIR1 biases the reaction toward (+)-pinoresinol. By contrast, Arabidopsis AtDIR6, identified by Pickel et al., displays the opposite enantioselectivity, producing (−)-pinoresinol with 78% enantiomeric excess (ee) in an in vitro laccase-coupling system; in the absence of DIR, pinoresinol remains racemic.[8]
Kim et al. subsequently provided further support for the physiological role of AtDIR6 in formation of (−)-pinoresinol through in vitro reactions, overexpression, and RNA interference experiments with Arabidopsis DIR proteins, and also demonstrated that DIRs from Schizandra species can exhibit the opposite selectivity toward (+)-pinoresinol.[9]
The same coniferyl alcohol radicals and the same C8–C8′ linkage can therefore generate opposite enantiomeric outcomes, with the difference arising from protein-imposed constraints on the relative spatial orientation of the two radicals.
6.3 Spatial Preorganization of Two Radicals by the Dual Pockets of AtDIR6
An early kinetic model proposed that two coniferyl alcohol radicals bind separately to the two subunits of a DIR dimer. Subsequent DIR crystal structures showed that the protein forms a trimer and that the potential binding sites on different subunits are too far apart to permit radical–radical coupling. Structural studies of AtDIR6 therefore proposed that two radicals bind simultaneously within the bilobed active cavity of a single subunit and undergo directed coupling within a single binding site.[2,10]
Gasper et al. determined the crystal structure of Arabidopsis AtDIR6 at 1.4 Å resolution. An AtDIR6 monomer contains an eight-stranded antiparallel β-barrel and assembles into a trimer; the open end of each monomer forms a bilobed binding cavity containing adjacent pocket A and pocket B.[10]
Pocket A contains functionally relevant residues including Asp-137, Arg-144, and Thr-166, whereas pocket B contains Asp-49, Lys-75, Tyr-104, and Tyr-106. Blocking either pocket A or pocket B individually with a bulky tryptophan residue markedly reduced AtDIR6 activity, supporting the involvement of both pockets in positioning the reacting substrates.[10]
The investigators manually placed two coniferyl alcohol radicals into pockets A and B, respectively, and then optimized the structure by simulated annealing and energy minimization. In the optimized model, C8 and C8′ of the two radicals were brought into close proximity, producing a spatial arrangement compatible with the 8–8′, re–re coupling required by AtDIR6.[10]
Figure 3. Dual-pocket architecture of the AtDIR6 active site and a proposed binding mode for two coniferyl alcohol radicals. The upper-left panel shows the bilobed active cavity on the surface of an AtDIR6 monomer, including pocket A and pocket B; the upper-right panel shows a cross-section through the active cavity after a 90° rotation, revealing the internal structures of the two pockets. The lower-left panel shows important amino acid residues in the two pockets, with pocket A residues shown in green, pocket B residues in blue, and Tyr106 and Phe175 in red. The lower-right panel shows a proposed binding mode obtained after two coniferyl alcohol radicals (CA·) were manually placed in the pockets, followed by simulated-annealing optimization and energy minimization; in this model, C8 and C8′ are in close proximity, and their geometric arrangement is compatible with re–re 8–8′ coupling. This binding mode was constructed on the basis of the ligand-free AtDIR6 crystal structure. Source: Gasper et al.[10], Figure 3, CC BY 4.0.
In this structural model, positioning of the aromatic rings within the two pockets reduces the number of available relative orientations of the two radicals, providing a regioselective advantage for C8–C8′ bond formation; the specific arrangement of the side chains within the confined space also helps determine the radical coupling faces. Together, the architecture of the AtDIR6 binding cavity, substrate modeling, and site-directed mutagenesis support a mechanism in which spatial constraints on both the bond-forming atoms and coupling faces generate regioselectivity and enantioselectivity.
6.4 Potential Catalytic Role of AtDIR6 in bisQM Cyclization
The structure and mutational results for AtDIR6 also support its involvement in the cyclization step following 8–8′ coupling. Asp-137, Arg-144, and Thr-166 in pocket A, together with Asp-49, Tyr-104, and Tyr-106 in pocket B, are all closely associated with protein activity; mutation of several of these residues markedly reduces regioselectivity, enantioselectivity, or overall reaction activity.[10]
Gasper et al. proposed that acidic residues such as Asp and the neighboring hydrogen-bonding network could increase the electrophilicity of C7/C7′ in the bisQM through carbonyl protonation or hydrogen bonding, thereby promoting intramolecular nucleophilic attack by the terminal hydroxyl groups. The energy-minimized bisQM model can also be accommodated within the AtDIR6 active cavity in a conformation suitable for cyclization.
This catalytic mechanism is currently supported collectively by the ligand-free AtDIR6 crystal structure, structural modeling of the substrates and intermediate, and site-directed mutagenesis experiments. High-resolution complex structures of the substrate radicals or bisQM bound to AtDIR6 could further define the specific proton-transfer and cyclization pathways.[10]
6.5 Protein-Mediated Selective Control in Different Coupling Modes
At present, the most complete structural and kinetic mechanistic evidence is concentrated on the DIR system responsible for 8–8′ coupling of coniferyl alcohol radicals to form pinoresinol. The 8–5′ and 8–O–4′ radical-coupling modes have a well-established chemical basis and can generate corresponding products in plant cells, but the specific modes of protein-mediated regulation of these linkages in different plants must be determined on the basis of the relevant biochemical and genetic evidence.
Plant natural products also include other phenolic coupling reactions in which regio- or stereoselectivity is directly controlled by cytochrome P450 enzymes, laccases, or peroxidases.[10] DIRs therefore represent an important mechanism by which plants control the spatial coupling of radicals, while different substrates and different linkage types may employ different modes of selective control.
7. Continued Processing of Stereochemical Information by Downstream Reductases
7.1 Sequential Reduction by Pinoresinol/Lariciresinol Reductase
After radical coupling and bisQM cyclization have generated pinoresinol, the established stereochemical structure continues to influence downstream enzymatic reactions. Pinoresinol/lariciresinol reductase (PLR) is a well-characterized representative enzyme acting at this stage.
PLR from Forsythia intermedia uses reduced nicotinamide adenine dinucleotide phosphate (NADPH) as a hydride donor to catalyze the sequential reactions:
(+)-Pinoresinol → (+)-Lariciresinol → (−)-Secoisolariciresinol
This process sequentially reduces the two benzylic ether structures of pinoresinol, progressively opening the bis-tetrahydrofuran framework.[12]
7.2 Stereospecificity of Hydride Transfer
Isotopic-labeling experiments showed that both hydride-transfer steps catalyzed by Forsythia PLR exhibit greater than 99% stereospecificity. The pro-R hydrogen at C4 of the NADPH nicotinamide ring is selectively transferred to specific spatial positions in lariciresinol and secoisolariciresinol.[12]
DIR and PLR function at different stages of the reaction sequence. DIR constrains the regioselective linkage and bond-forming faces of the radicals before pinoresinol formation, whereas PLR recognizes the already established stereochemical structure of the lignan and performs stereospecific reductive transformations. The stereochemical information of plant lignans can therefore be retained and further selected through successive reaction steps.
Single-electron oxidation of coniferyl alcohol first generates a resonance-delocalized radical, and reactive sites including O4, C5, and C8 provide multiple possibilities for regioselective radical coupling. Formation of 8–8′, 8–5′, and 8–O–4′ bonds produces different quinone methide intermediates, which subsequently undergo intramolecular cyclization or nucleophilic trapping to form stable dimeric structures. In the pinoresinol system, DIR spatially preorganizes two coniferyl alcohol radicals, giving 8–8′ bond formation and a specific pair of coupling faces a marked advantage; subsequent cyclization of the bisQM and stereospecific reduction by PLR continue to transmit and process this stereochemical information. The electronic structure of the radicals provides multiple possible bond-forming pathways, whereas the three-dimensional constraints imposed by protein binding sites and downstream enzymes confer defined regio- and stereoselectivity on selected reactions.
8. Representative Chemicals Related to Monolignol Oxidative Coupling, Regio- and Stereoselectivity, and Downstream Transformations
Table 1. Monolignol Radical Precursors and Glycosylated Precursors
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Monolignol radical precursor—guaiacyl type | 458-35-5 | Coniferyl alcohol | ≥98% | A core model substrate for lignan oxidative coupling; applicable to studies of laccase- or peroxidase-mediated single-electron oxidation, competition among 8–8′/8–5′/8–O–4′ coupling pathways, and DIR-mediated regio- and enantioselective formation of pinoresinol. | |
Coniferyl alcohol glycoside—monolignol precursor pool | 531-29-3 | Coniferin | ≥98% | A glycosylated derivative of coniferyl alcohol; applicable to studies of hydrolytic release of coniferyl alcohol followed by entry into oxidative-coupling pathways, as well as metabolic relationships between monolignol precursors and dimeric lignan products. | |
Monolignol radical precursor—syringyl type | 537-33-7 | Sinapyl alcohol | Moligand™, ≥95% | Contains methoxy substituents at both C3 and C5 of the aromatic ring and can be compared with coniferyl alcohol to investigate how C5 substitution affects radical-reactive sites, regioselective coupling modes, and the structures of 8–8′ coupling products. | |
Sinapyl alcohol glycoside—monolignol precursor pool | 118-34-3 | Syringin | Analytical standard, ≥98% | A glycosylated derivative of sinapyl alcohol; applicable to studies of glycoside hydrolysis and sinapyl alcohol release, as well as entry of syringyl monolignols into oxidative-coupling pathways and downstream lignan formation. |
Table 2. Products of Regioselective Coupling, Enantioselective Coupling, and Downstream Stereoselective Transformations
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
8–8′ coupling product—dextrorotatory pinoresinol | 487-36-5 | Pinoresinol | ≥95% (HPLC) | A representative lignan formed by 8–8′ coupling of coniferyl alcohol; can be paired with (−)-Pinoresinol as an enantiomeric reference for studies of DIR-mediated regioselectivity, enantioselectivity, and downstream reduction of pinoresinol. | |
8–8′ coupling product—levorotatory pinoresinol | 81446-29-9 | (−)-Pinoresinol | — | A representative enantiomer associated with AtDIR6-directed 8–8′ coupling; can be compared with dextrorotatory pinoresinol to investigate how different DIRs select the coupling faces of two coniferyl alcohol radicals and to study stereoselectivity of downstream enzyme substrates. | |
Glycosylated product downstream of 8–8′ coupling—pinoresinol type | 63902-38-5 | Pinoresinol diglucoside | Analytical standard, Moligand™, ≥98% | A diglucoside of pinoresinol; applicable to studies of glycosylation after formation of 8–8′ coupling products and of the accumulation and metabolic fate of pinoresinol-pathway products in plants. | |
8–8′ coupling product—mixed aromatic substitution type | 40957-99-1 | 5′-Methoxypinoresinol | ≥93% | Contains a guaiacyl-substituted aromatic ring and a syringyl-substituted aromatic ring; applicable to comparisons of how different combinations of monolignol aromatic substitution affect the structures and regioselective features of 8–8′ coupling products. | |
8–8′ coupling product—levorotatory syringaresinol | 6216-81-5 | (−)-Syringaresinol | Moligand™, ≥98% | A syringyl-type 8–8′ lignan that can be paired with (+)-Syringaresinol as an enantiomeric reference for investigating stereochemical relationships in highly methoxylated monolignol coupling products and the effects of aromatic substitution on coupling structures. | |
8–8′ coupling product—dextrorotatory syringaresinol | 21453-69-0 | (+)-Syringaresinol | ≥99% | Has the same linkage mode and oxygen-substitution pattern as (−)-syringaresinol but the opposite enantiomeric configuration; applicable to enantiomeric comparisons of 8–8′ coupling products and studies related to stereoselectivity. | |
8–5′ coupling product—dihydrobenzofuran type | 4263-87-0 | Dehydrodiconiferyl alcohol | — | A representative product formed through 8–5′ coupling of coniferyl alcohol radicals followed by intramolecular C–O cyclization; applicable to comparisons between the 8–5′ and 8–8′ regioselective branches and studies of dihydrobenzofuran formation. | |
Reduced product downstream of 8–5′ coupling—side-chain saturated type | 28199-69-1 | Dihydrodehydrodiconiferyl alcohol | ≥98% | Retains the dihydrobenzofuran framework generated by 8–5′ coupling, with further reduction of the side-chain unsaturation; can be paired with dehydrodiconiferyl alcohol to study downstream structural transformations following formation of the 8–5′ branch. | |
Pinoresinol reduction product—levorotatory lariciresinol | 83327-19-9 | Lariciresinol | Moligand™, ≥98% | An important intermediate in the pinoresinol reduction pathway; can be paired with (+)-Lariciresinol as an enantiomeric reference for investigating substrate selectivity of pinoresinol/lariciresinol reductase toward an already established 8–8′ stereochemical structure. | |
Pinoresinol reduction product—dextrorotatory lariciresinol | 27003-73-2 | (+)-Lariciresinol | ≥95% (HPLC) | A representative lignan formed by single-step reduction of pinoresinol; applicable to studies of stereochemical information transfer after 8–8′ coupling, enantioselectivity of reductase substrates, and sequential reduction. | |
Sequential reduction product—open-chain lignan type | 29388-59-8 | Secoisolariciresinol | ≥95% | An open-chain lignan generated by sequential reduction in the pinoresinol pathway; applicable to studies of the sequential pinoresinol → lariciresinol → secoisolariciresinol reduction pathway and transmission of stereochemical information during downstream enzymatic processing. | |
Glycosylated sequential reduction product—open-chain lignan type | 158932-33-3 | Secoisolariciresinol diglucoside | ≥98% | A diglucoside of secoisolariciresinol; applicable to studies of glycosylation following 8–8′ coupling and sequential reduction, as well as the accumulation forms of downstream lignans in plants. | |
Downstream oxidation product of the 8–8′ pathway—butyrolactone type | 580-72-3 | Matairesinol | ≥98% | A butyrolactone-class product formed by further oxidation of an open-chain 8–8′ lignan; applicable to studies of structural continuity from the initial 8–8′ stereochemical framework through reductive and oxidative processing and of downstream lignan transformations. |
Table 3. Reagents Related to Single-Electron Oxidation and Downstream Reduction Systems
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Biomimetic single-electron oxidant | 20667-12-3 | Silver oxide 99+ | Guaranteed reagent, suitable for analysis, ≥99% | Applicable to biomimetic oxidative-coupling systems of phenolic precursors such as coniferyl alcohol for investigating different radical regioselective coupling pathways and the distributions of 8–8′, 8–5′, and other products in the absence of a dirigent protein. | |
Reagent for peroxidase oxidation systems | 7722-84-1 | H112517 | Hydrogen peroxide solution (explosive precursor) | GR, 30 wt.% in H₂O | Applicable to construction of peroxidase/hydrogen peroxide single-electron oxidation systems for generating monolignol phenoxy radicals and investigating how radical-generation conditions affect subsequent regioselective coupling reactions. |
Cofactor for pinoresinol reductase | 2646-71-1 | Reduced coenzyme II tetrasodium salt hydrate (β-NADPH tetrasodium salt hydrate) | ≥99% | A reducing cofactor for pinoresinol/lariciresinol reductase reactions; applicable to studies of sequential reduction of pinoresinol to lariciresinol and secoisolariciresinol and of stereospecific hydride transfer. |
Note: The products listed above are representative Aladdin products relevant to scientific research. Specific applications should be determined according to the product specification, batch COA, and the intended reaction or evaluation system. Additional product specifications, grades, and COA information can be retrieved from the Aladdin website using the product name, CAS number, or catalog number.
References
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