Dimeric Acidolysis Products

Acidolysis of spruce and/or birch lignins results in the formation of the dimeric products 8-21 (Fig- 6.1.5) (Lundquist 1976). Products 8-16 originate from structural elements of the / -1 type (22). Acidolysis of trimeric sequences of lignin units of type 23 gives rise to phenylcoumarone 17 and stilbene 18- Structures of fl-[i type are the sources of compounds 19-21. 

Compounds 8-16 are prominent constituents of the dimer fractions from lignin acidolysis mixtures and this finding serves as evidence for the presence of substantial amounts of fS-1 structures in the original lignins. However, other studies do not provide support for this conclusion (Lundquist 1987, Habu et al. 

Davies JL, Nakatsubo F, Murakami K, Umezawa T (1987) Organic acid pulping of wood IV Reactions of arylglycerol fi guaiacyl ethers Mokuzai Gakkaishi 33 478-486 Deobald LA, Crawford DL (1987) Activities of cellulase and other extracellular enzymes during lignin solubilization by Streptomyces vindosporus Appl Microbiol Biotechnol 26 158-163 

Ede RM, Brunow G (1989) Formic acid/peroxyformic acid pulping II Synthesis of 3 aryl 2,3 dihydro 7 methoxy-2-benzofuranmethanols - model compounds for lignin acidolysis products Holzforschung 43 127-129 

Esenther GR, Kirk TK (1974) Catabolism of aspen sapwood in Reticulitermes flavipes (Isoptera Rhinotermitidae) Ann Entomol Soc Am 67 989 - 991 Fischer HE, Hibbert H (1947) Studies on lignin and related compounds LXXXIII Synthesis of 3 hydroxy-l-(4-hydroxy-3-methoxyphenyl)-2 propanone J Am Chem Soc 69 1208-1210 Froment P, Robert A (1970) Etude des conditions d extraction par des solution dioxane-eau-acide chlorhydrique Rev ATIP 24 189-193 

---

The fraction of the Sephadex eluate, which corresponded to peak B (Figure 5), contained 5% of the weight of the Bjorkman lignin used in the acidolysis experiment. We expected it to contain dimeric acidolysis products. So far, five pure compounds (XXII-XXVI) have been obtained from fraction B. 

Lignins can be detected and characterized by analysis of their low-molecular weight acidolysis products, and it is this application of acidolysis that is the subject of the current presentation Routine analytical procedures have been developed only for monomeric acidolysis products Therefore, analysis of such compounds is the primary concern in this section Analysis of dimeric acidolysis products is, however, of great value for the characterization of lignins and is treated under a separate heading (Sect 6 16)... 

The reaction depicted in equation 43 between a nitrile and a lithium amide takes place as a 1,2-addition to the cyano group. The product crystallizes as a dimer (236) in which the lithium atoms are solvated by nitrile molecules and differently bonded to the amidine moieties, as shown by XRD analysis. Low-temperature H NMR spectrum in solution points to uniform chemical environments for both the aryl groups and for the Me—Si groups, and to rapid rearrangement of the Li—N coordination structures. Acidolysis of the dimer in solution yields the corresponding amidine (237) . The crystal structure of the THF-solvated analog of 236 shows dissimilar N—Li bond lengths for the two Li atoms... 

The crude Bjorkman lignin acidolysis mixture contained both polymeric material and a number of more or less low molecular, chromato-graphically visible products in addition to ketol (XII) and was considered to be a potential source not only of further monomeric but also of dimeric and oligomeric degradation products. The polymeric material was readily removed by filtering the crude mixture of reaction products through a short silica gel column (solvent, dioxane-benzene 1 3). 

If a reaction mixture obtained after 4 hours acidolysis of Bjorkman lignin (spruce), after neutralization and removal of the polymeric material, were filtered through a Sephadex G 25 column, the elution curve (Figure 5) exhibited three peaks (26). The effluent fractions corresponding to peaks A and B were subjected to further fractionation by chromatography on silica gel columns. We found that fraction A contained only monomeric products while fraction B was a mixture of dimeric products (Figures 6 and 7). 

Of the four remaining dimeric products, the phenylcoumarone (XXII) (m.p., 110°C.), isolated in a yield of about 0.5% of the lignin, and the 0,/> -dihydroxystilbene (XXIII) obviously are genetically interrelated both must be derived from a phenylcoumaran structure, as already shown above in the acidolysis of dihydrodehydrodiconiferyl alcohol. 

Both products could be considered to originate from dimeric phenylpropane systems by losing one of the propane side chains in addition, the stilbene (XXV) then had lost the terminal carbon of the remaining propane side chain. One therefore had to look for an appropriate dimeric phenylpropane structure which could explain the formation of these products on acidolysis. 

---