Phenylcoumarone structures

Treatment of lignin with acid as is the situation, for example, in acidolysis (Chap. 6.1) gives rise to stilbene and phenylcoumarone structures (Adler et al. 1966). Stilbenes similar to those identified in acid sulfite pulping liquors have been detected in the steam hydrolyzate of aspen wood (Bardet et al. 1985), and a coniferyl aldehyde structure was formed when a yS-aryl ether-type lignin model compound was heated with slightly acidified water (Kratzl et al. 1959). 

Strong analytical support for the presence of the phenylcoumaran system (I) in lignin was obtained a few years ago (5) (Figure 1). Under the conditions of acidolysis, models for system I, namely dihydrodehydro-diconiferyl alcohol (III) 13) and its methyl ether (III, OCH3) were converted into phenylcoumarone derivatives (VIII and VIII, OCH3). The structure of the phenolic coumarone (VIII) was established by an inde-... 

Phenylcoumarone (VIII) has a characteristic ultraviolet and ioniza-tion-Ae spectrum, which enabled us to detect dimeric structures of this type in reaction mixtures obtained when Bjorkman spruce lignin was subjected to acidolysis for 20 hours. From the spectrophotometric estimation of the amount of the phenylcoumarone systems formed, we concluded that from a total of 100 phenylpropane units of Bjorkman lignin, about 20 are involved in phenylcoumaran systems (I) in other words, about every 10th phenylpropane unit is linked to one of its neighbors by the cyclic benzyl aryl ether linkage characteristic of I. 

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. 

The structure of the phenylcoumarone (XXII) 26) was derived from analytical and spectral investigation and was confirmed by a synthesis 29) starting from dehydrodiconiferyl alcohol (XXVII). The latter compound (XXVII) was converted by monoperphthalic acid into an epoxide whose side chain was equivalent to that of an arylglycerol. By properly performed acidolysis, the epoxide side chain therefore was converted into the primary ketol structure, and at the same time the hydroxymethyl-substituted phenylcoumaran system see XXVII) was converted into the methyl-substituted phenylcoumarone system of XXII (Figure 8). 

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. 

The occurrence of small amounts of lignin units attached to each other by a-P linkages has been suggested on the basis of thioacetolysis studies [52]. However, a-P linkages may possibly be formed during the alkaline step involved in the thioacetolysis procedure [4]. It has been suggested, based on examinations by fluorescence spectroscopy [114], that small amounts of P-5 structures of the phenylcoumarone type are present in lignins. 

C Noutary, P Former de Violet, J Vercanteren, and A Castellan. Photochemical Sudies on a Phenoliuc Phenylcoumarone Lignin Model in Relation to the Photodegradation of Lignocellulosic Materials. Part 1. Structure of Photoproducts. Res Chem Intermed 21(3-5) 223-245, 1995. 

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