Synthetic Urushiols from Biorenewable Carbon Resources: Chemical conversion of enzymatic degradation products of wood lignin to an ancient yet future coating material

Sequential enzymatic and chemical transformations convert woody lignin to artificial urushi compounds, demonstrating the potential of renewable biomass in materials science and technology. The elaborated enzyme system biodegrades the woody...

0][11][12][13][14] The durability, high stability and aesthetic beauty of urushi, combined with its renewable nature, have made urushi a good candidate to become a building block for a more sustainable society.Nevertheless, several limitations exist to using natural urushi due to its time-consuming and labor-intensive collection and processing steps that require highly skilled labor.
3][14][15][16][17][18][19][20][21] Most of the recent reports focus on the synthesis of urushiol analogs (catechols bearing C15 or C17 unsaturated aliphatic side chains at the 3-position) since the enzymatic oxidative polymerization of this mixture is responsible for the hardness, solvent resistance and durability of this coating.][19][20][21] Nevertheless, to preserve the sustainability of articial urushi, it is crucial to take advantage of abundant and renewable starting materials for its synthesis.7][28][29] We previously reported an enzymatic reaction degrading woody lignin efficiently to phenylpropanoid monomers, such as guaiacylhydroxypropanone (GHP) and syringylhydroxypropanone (SHP), by marine microorganisms. 30,31erein, we report the rst synthesis of lignin-derived urushiol analogs by installing vegetable oil-derived polyunsaturated side chains on Gand S-urushiol through a 5-step chemical transformation.The preparation and characterization of articial urushi lms conrmed the similar curing/ polymerization properties of lignin-derived urushiol analogs to natural urushiol.
The synthesis of lignin-derived urushiol analogs started by preparing the trienyl side chain and the catechol/pyrogallol moiety and was completed in 5 steps.First, the lignin-derived aromatic monomer guaiacyl-and syringylhydroxypropanone GHP and SHP were dehydrated and protected by the TBS group according to a previously reported method, 32 affording the corresponding TBS-protected enone products G-2 and S-2 (Scheme 1).In order to introduce the trienyl side chains, a natural trienyl fatty acid, linolenic acid, was converted to the corresponding Grignard reagent, (9Z,12Z,15Z)-octadeca-9,12,15-trienylmagnesium bromide (1) via linolenyl bromide (Scheme 1).The 1,4-addition of the Grignard reagent 1 was performed in the presence of chlorotrimethylsilane, copper bromide dimethyl sulde complex, and lithium chloride, followed by acidic treatment giving the corresponding trienyl side chain-bearing products G-3 and S-3 in 56% and 52% yield, respectively.Both the reduction of the ketone group and the deprotection of the TBS group were performed at the same time by the reaction with triethylsilane in triuoroacetic acid solution to afford the methyl-protected articial urushiol analogs Me-Gand Me-S-urushiol in 65% and 72% yield, respectively.
Finally, reuxing of a mesitylene solution of methylmagnesium iodide resulted in the removal of methyl groups, producing the guaiacyl (G)-and syringyl (S)-type articial urushiol analogs Gand S-urushiol in 23% and 51% yield, respectively.It is noteworthy that 1 H and 13 C NMR revealed that no isomerization of the trienyl side chains had taken place under these reaction conditions.
The synthesized urushiol analogs were applied for the preparation of articial urushi lms.The raw urushi-like liquids were prepared by mixing the urushiol analogs, protein hydrolysate, and water in the ratio of approximately 70, 7, and 23 wt% in the presence of laccase.The prepared articial urushi liquids were coated on glass substrates and were kept at 30 °C in 80% relative humidity for several days.The lignin-derived S-type urushiol analog S-urushiol bearing the C21 trienyl side chain at the 5-position of pyrogallol was instantaneously cured aer the addition of laccase, indicating the extremely high reactivity of the electron-rich pyrogallol core toward oxidation (Table 1, entry 1).This curing reaction was slowed down by the addition of terpene oil (Table 1, entry 2), suggesting a possible application of S-urushiol for the preparation of rapidly curing coating/ adhesive materials.In the case of the lignin-derived G-type urushiol analog G-urushiol (Table 1, entry 3), the curing reaction successfully proceeded showing a hardening prole relatively similar to that of natural urushi.In the natural lacquer, the curing mechanism relies on the formation of micelles where the oil phase, or the continuous phase, contains catechols, while the water phase, or the dispersed phase, contains the enzymes.Radicals are rst formed by the laccase-catalyzed oxidation of urushiol at the interface layer, and then transferred to the urushiol oil phase, enabling its polymerization.Molecular oxygen can continuously penetrate the aqueous phase to oxidize the reduced laccase, allowing the urushiol polymerization to continue smoothly, mostly leading to the formation of C-C and C-O-C nuclei crosslinks involving nuclei and side chains.Given the high number of polyunsaturations on the catechol side chains, auto-oxidative radical chain reactions proceed as well, enabling the formation of chemical crosslinks between the aliphatic chains. 33As can be seen from Scheme 1 Synthesis of lignin-derived urushiol analogs.
the time-course prole of Martens hardness (Fig. 2), the initial oxidation reaction proceeded very slowly, which is compatible with the slow enzymatic oxidation of the catechol core.][4] Finally, aer 73 days, the Martens hardness reached a plateau value of 147.4 ± 0.9 N mm −2 (Table 1, entry 3), which is comparable to the hardness of the commercially available polyvinyl chloride lm (144.6 ± 0.4 N mm −2 , Table S1 †), demonstrating a good potential for practical applications.1][12] The urushiol analog 4 bearing the C18 trienyl side chain at the 3-position of catechol 34 (for the synthetic protocol see ESI †) was also cured and resulted in a Martens hardness of 140.4 ± 1.5 N mm −2 aer 120 days (Table 1, entry 5).The comparable value of hardness for compounds 4 and G-urushiol reveals the negligible contribution of the alkyl chain length and substitution position towards the curing properties of the articial urushi lm.
The crosslinking reaction was also investigated using natural urushiol and thitsiol extracted from Chinese raw urushi and Cambodian raw thitsi, in which the major constituents are catechols bearing the C17 trienyl side chain at the 3-position and catechols bearing the C17 dienyl side chain at the 4-position, respectively. 5,35,36The structural difference is clearly represented by their different curing properties, in agreement with previous reports, 1,35,[37][38][39] resulting in hardness of 176.7 ± 2.3 for Chinese urushiol and 58.5 ± 0.6 N mm −2 for Cambodian thitsiol (Table 1, entries 6 and 7). 35,39The comparable hardness of the articial urushi lm from G-urushiol to that from Chinese urushiol suggests similar curing properties of lignin-derived urushiol analogs.Despite the comparable hardness of the articial urushi lm from G-urushiol to that from Chinese urushiol, the curing reaction proceeds slowly at the initial stage.The similar curing prole was observed with an urushiol analog synthesized from vanillyl alcohol, 20 suggesting a slightly different initial reaction step for an articial urushi prepared from synthetically pure urushiol analogs, compared to the one prepared from natural urushiols, which are a mixture of catechols bearing different side chains.The initial slow curing reaction resembles the auto-oxidation prole of vegetable oils, 40 suggesting that a similar auto-oxidation of trienyl side chains takes place from the initial stage.In addition, the lower value of hardness obtained from Cambodian thitsiol bearing mainly dienyl side chains suggests that the number of unsaturated bonds in the side chain has a much stronger inuence on the hardening process compared to the substituent position to the catechol core (Fig. 2).In order to analyze the curing reaction of lignin-derived urushiol analog G-urushiol and natural urushiol extracted from Chinese urushi the prepared urushi lms (aer 73 days of curing) were characterized by FT-IR spectra (Fig. 3).Both the articial and the natural urushi lms appear translucent (Fig. 3, inset), but the articial lm presents a paler color (yellow) compared to the natural lm (red/brown).From the IR spectra, we can notice that articial urushi and natural urushi lms differ mainly in the regions corresponding to the two vibrational bands at 1139 and 1504 cm −1 ; these two peaks are clearly observed in the articial urushi lm but not in the cured lm from Chinese urushiol.These vibrations are assigned to the outof-plane bending and aromatic stretching vibrations of 4substituted catechol 21 based on the DFT calculation (Fig. S1 †).When comparing the liquid and cured lms, we can notice that the aromatic vibrational band at 1608 cm −1 of G-urushiol was broadened aer the polymerization, in a similar way to the 1593 and 1628 cm −1 bands in natural urushiol, in agreement with a previous report. 41An additional signicant spectral change from liquid to cured lm was observed for both natural and articial urushi with the appearance of two peaks centered at 1635 and 1714 cm −1 .These two strong and broad peaks originate from the formation of quinone and the oxidation of side chains. 21Furthermore, the disappearance of the oleonic C-H vibration at 3011 cm −1 suggests the complete oxidation of the unsaturated side chains by autooxidation.These IR spectra demonstrate that the lignin-derived urushiol analog G-urushiol and natural urushiol undergo similar enzymatic polymerization reactions.
In summary, articial urushi materials were developed by utilizing lignin-derived phenylpropanoids as aromatic nuclei.Vegetable oil-derived polyunsaturated side chains were installed on lignin-derived aromatic compounds GHP and SHP to afford the lignin-derived urushiol analogs.Articial urushi lms were successfully prepared by mixing the lignin-derived urushiol analogs with protein hydrolysate and water in the presence of laccase enzyme.Time-course analysis of Martens hardness shows a stepwise oxidation reaction in the curing process, similar to natural urushi.The natural urushi-like properties were also conrmed by FT-IR analysis, suggesting remarkably similar molecular transformations during the curing process.The sufficiently high hardness of the ligninderived articial urushi lm paves the way for future applications as an environmentally benign and sustainable coating material.

Fig. 2
Fig. 2 Time-course curing properties of the film obtained from Gurushiol (red line), natural Chinese urushiol (blue line) and natural Cambodian thitsiol (yellow line).

Fig. 3
Fig. 3 IR spectra of the liquid state (dotted line) and cured film (solid line) obtained from natural Chinese urushiol (blue line) and G-urushiol (orange line).All spectra have been shifted and normalized for clarity.Inset: photo of natural Chinese urushi film (top, red film) and artificial (G-urushiol) urushi film (bottom, yellow film) deposited on glass.

Table 1
Curing properties of urushiol analogs a Terpene oil was added.