Thioacidolysis

As in acidolysis (Chap. 6.1), thioacidolysis proceeds mainly by cleavage of arylglycerol-/ -aryl ether linkages. In contrast to acidolysis, however, thioacidolysis is performed in anhydrous media. The ether-cleaving reagent combines a hard Lewis acid, boron trifluoride etherate, and a soft nucleophile, ethanethiol (Node et al. 1976, Fuji et al. 1979). 

Springer Series in Wood Science Methods in Lignin Chemistry (Edited by S.Y. Lin and C.W. Dence) 

First the a-carbon is substituted by BF3, resulting in compound 4.20. The activated a-carbon makes this compound reactive with ethanethiol, resulting in the formation of 4.21. The thioethyl group now attacks the activated P-carbon of 4.21, resulting in the intermediate 4.22. Attack of ethanethiol results in the formation of 4.23, and through a similar mechanism the y-carbon is substituted, resulting in the formation of 1,2,3-trithioethane phenylpropanoid monomer 4.25 (Lapierre et al., 1986 Chen, 1991 Rolando etal., 1992 Lapierre, 1993). 

Thioacidolysis allows the distinction between products derived from lignin and products derived from / coumaric and ferulic acids, and the distinction between products derived from cinnamaldehydes and cinnamyl alcohols. Recent improvements have made it possible to estimate the fraction of free phenolic groups in uncondensed lignin (see Section 1.3.1), and to depolymerize the dimers, so that they can be included in the analysis of the lignin composition. 

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Ralph, J. Grabber, J. H. Dimeric (1-ether thioacidolysis products resulting from incomplete ether cleavage. Holzforschung 1996, 50, 425 128. 

Ralph, J. Kim, H. Lu, F. Grabber, J. H. Boerjan, W. Leple, J.-C. Berrio Sierra, J. Mir Derikvand, M. Jouanin, L. Lapierre, C. Identification of the structure and origin of a thioacidolysis marker compound for ferulic acid incorporation into angiosperm lignins (and an indicator for cinnamoyl-CoA reductase deficiency). Plant J. 2008, 53, 368-379. 

Tables IV and V show the monomer composition of lignins for both parietal and saponification residues. In this regard, comparison between PR and SR values allows the characterization of the lignin core, which is not solubilized after alkaline treatment (20,21). Instead of using nitrobenzene oxidation, thioacidolysis was used to characterize the non-condensed...

Table IV. Monomeric Composition of Lignin in the Parietal Residue (PR) of the Upper and Lower Internode from Stem or Normal and b.m.-Mutant of Maize shown by Thioacidolysis (G = guaiacyl and S =syringyl-trithio-ethylethers Fig. 1). (Yields are expressed as micromoles per gram of ABL in each sample of PR standard deviation less than 10% )...

In the case of saponification residues (Table V), these trends were even more pronounced for the upper internode. Total yields of thioacidolysis products, (S + G), were slightly higher for the case of PR (Table IV), than that of SR (Table V). However, the residual lignin from the SR of the lower internodes was relatively unchanged when compared with PR, with the difference between SR and PR being nearly negligible in the case of normal maize but significant in the case of the mutant. 

Vivas, N. et al.. Differentiation of proanthocyanidin tannins from seeds, skins and stems of grapes Vitis vinifera) and heartwood of Quebracho (Schinopsis balansae) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and thioacidolysis/liquid chromatography/electro-spray ionization mass spectrometry. Anal Chim. Acta, 513, 247, 2004. 

Advantages of the DFRC method over thioacidolysis include the better yield, simpler and milder reaction conditions, and the fact that esters are not cleaved. Especially in the analysis of lignin in grasses, with large amounts of esterified / -coumaric acid (1.13), this is an important consideration. 

Lapierre C Monties B, Rolando C (1985) Thioacidolysis of lignin comparison with acidolysis J Wood Chem Technol 5 277-292... 

Lapierre C, Monties B, Rolando C (1986) Thioacidolysis of poplar lignins identification of monomeric synngyl products and characterization of guaiacylsynngyl lignin fractions Holzforschung 40 113-118... 

Fig. 6.4.1A,B. Proposed mechanism of lignin thioacidolysis A Substitution at C B Substitution at Cp, then at Cy via a neighboring group participation reaction...

Table 6.4.1. Main products obtained by thioacidolysis of various phenolic structures...

Ethanethiol and HBF4 etherate, which replaces BF, etherate in the thioacidolysis of acetylated samples, are of analytical grade quality. 

The thioacidolysis reagent is prepared immediately before use 2.5ml of BF, etherate and 10 ml of ethanethiol are successively introduced into a 100-ml volumetric flask containing 20ml of dioxane and the final volume is adjusted to 100 ml with dioxane. This reagent is therefore a solution of 0.2 M BF-, etherate (Note 1) in a 8.75 1 (v/v) dioxane/ethanethiol mixture. It is a colorless reagent immediately after preparation. When an inferior grade of dioxane is used, a yellow color is observed this reagent is not suitable for analytical use. 

The entire extraction procedure preferably is performed without interruption, when kept in aqueous media overnight, the main products are slightly degraded as shown by decreased recovery In contrast, the stability of thioacidolysis products in organic media (for example CH2C12) is excellent, at least at 4°C, where standard solutions remained unchanged for more than 1 year... 

Silylation the final CH2C12 solution of the thioacidolysis sample is again carefully dried over Na2S04, 5-10/d of this dried organic solution is trimethylsilylated (TMS) at room temperature with 50/i of N,0 bis (trimethylsilyl) trifluoroacetamide (BSTFA) and 5 1 of GC-grade pyridine in a 200-/d reaction vial fitted with a Teflon-lined screwcap For soluble lignin samples, the silylation is complete within a few minutes, for lignocellulose residues, silylation is complete within 2 to 4h In both cases and with well-dried samples, silylations performed in the aforementioned vials are stable for several days (at least 48 h)... 

The main syringyl (S) and guaiacyl (G) monomeric products from the thioacidolysis of lignins are isolated by preparative-scale experiments (Lapierre et al. 1986a,b). The only difference between these and the previously described analytical procedure is that the reaction mixture (500 ml of reagent and 3-6g of lignin sample) is refluxed. The product yields are identical for both analytical and preparative procedures. 

Note 2 For soluble lignins the ratio of reagent volume to samples weighed is not critical and the same yields are observed when small (5 mg) or large (50 mg) amounts of lignin are reacted with 5 ml of the thioacidolysis... 

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