Dimeric lignin acidolysis products

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). 

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)... 

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. 

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. 

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