Tech Bull Fac Agt Kagawa Univ
ON THE PERMANGANATE OXIDATION OF BARK PHENOLIC ACID,
TANNIN, AND BJoRKMAN LIGNIN ISOLATED FROM BARK OF
SOME GYMNOSPERM AND ANGIOSPERM TREESMurao SOGO
The bark of trees contains considerable amount of a reddish brown, amorphous polyphenolic substance which have been termed as bark phenolic acid. On the basis of results of some investigations (I"), the bark phenolic acid was supposed to be probably a mixture or a
copolymer of polymerized tannin and a little of lignin. Recentlyc4), bark phenolic acid, tannin, phlobaphen, and bark B jorkman lignin were isolated from some gymnosperm and angiosperm trees, and studied on their chemical properties to make a comparison of each other. The results indicated that bark phenolic acid is more similar t o tannin than to wood ligsin. Now, in order t o obtain more detailed information concerned t o nuclei of bark phenolic acid, these samples were ethylated and subjected t o perrnanganate oxidation, and then the oxidation products were studied by paper chromatography.
Permanganate oxidation of wood lignin has been well studied by many workers to investigate the lignin structurec6). However, concerning to the oxidation of bark phenolic acid, very little information i s available. ERMAN and LYNESS(~) studied on the permanganate oxidation of the methylated bark phenolic acid of slash pine bark, and, they identified only vexatric acid in the oxidized products. CATRAVAS and KIRBY(~) and Roux(*) reported that the methylated tannins of mimosa and wattle produced veratric and trimethylgallic acids by the permanganate oxidation. I t i s a well-known fact that wood lignin has 4-hydroxyphenyl (A), guaiacvl, (B)
,and syringyl (C) nuclei i n i t s molecule, while tannin has catechol (D) and pyrogallol ( E ) nuclei. When subjected to permanganate oxidation, the ethylated materials which consist of each of A W E nuclei should produce each substance of I
--V as shown in the following scheme.
A 4-Ethoxybenzoic acid H O O C - ~ O C B ~ ( I ) OCHJ
B 3-Methoxy-4-ethoxybenzoic acid HOOC O C ~ H ~ ( 11) Ethylation
C oxidation 3,s-Dimethoxy-4-ethoxybenzoic acid HOOC
D 3,4-Diethoxybenzoic acid HOOC
aOC,Hr( I v )
3,4,5,-Triethoxybenzoic acid H O O C @ c L s
Therefore, if the bark phenolic acids consist of the structural units which present in tannin, IV and/or V should be detected in the oxidation products of the ethylated bark phenolic acids. This study was carried in order to clear this problem.
The authentic compounds ( I
-V ) were synthesized and some properties of them were studied (cf. Table 1). After considerable trial, i t was found that these authentic
Table 1 Rf-values on paper chromatogram and properties of the authentic compounds
Compound I 11 111 I V
E l e m e n t a ~ y composition Melting point Carbon Hydrogen
Anal Calcd Anal Calcd
I% % E x p e ~ i m e n t Literature 6 5 1 9 6 5 0 5 61 34 61 21 58 54 58 40 6 3 1 0 6 2 8 4 61 61 61 40
*I Developing solvent, butanol saturated with 2N- (NH4)2C08 *2 Developing solvent, ligr oin -dimethylf or mamide (1 000 : 1 v/v) ;
Rf -value means t h e m i g ~ a t i o n relative to 3,4,5-triethoxybenzoic acid(V) 6 1 8 6 07 6 34 6 17 6 46 6 24 6 9 6 6 71 7 31 7 14
compounds could be separated by paper
chromatography using the developing sol-
Ivents shown i n the experimental part. 1 6 1 , -
A Bark phenolic acids, tannins, and bark
Bjorkman lignins were ethylated with die -
thylsulfate and then were subjected t o the
permanganate oxidation. B jorkman lignin
,,-0 of wood (Akamatsu and kuri) was examined _" identically, for making a comparison with
0" bark lignin. The amount of ether -soluble 0 6- parts of oxidation mixture was determined
(cf. Table 3). T h e ether extract was studied by paper chromatography to esti- 0 2-
mate the existence of I-V. The results
are given in Table 3. A few unknown
M a b e length (mu) acidic compounds were found on the chro- Solvent : 95 % ethanol..
matograms of all samples, however, they Concentration : , 6 m g perloo m l , I : 4-ethoxybenzoic acid
were not studied i n detail. I1 : 3-methoxy-4-ethoxybenzoic acid The oxidation products from bark Bjork- I11 : 3,5-dimethoxy-4-ethoxybenzoic acid man lignins of gymnosperm trees contained IV : 3,4-diethoxybenzoic acid
V : 3,4,5-triethoxybenzoic acid
much of I' and a of Fig 1 UV-absorption s p e c t ~ a of the authentic while the oxidation products from those of compounds
160 Tech Bull Fac Agr Kagawa Univ Table 2 Tree species used for isolation of bark phenolic acid, tannin,
and bark Bjorkman lignin
Akamatsu, Pinus densiflora S e t Z (Kagawa p ~ e f )
Ezomatsu, Picea jezoensis CARR (Hokkaido) Angiosperm
Akacia, Acacia decurrens W var nzollisima W (Kagawa Pref ) Konara, Querus serrata THUMB (Tokushima Pref )
Kuri, Castanea crenata S et Z (Kagawa Pref )
Table 3 Total yield of the e t h e ~ -soluble oxidation products and results of paper c h ~ o m a t o g ~ a p h y for the oxidation products.
Sample Total yield
IPhenolic acid N1
IN2 Substances Bark I 11 IZZ
vL1 L2 L3 Tannin N1 N2
+ + - t t t -
+ + - t t t -i f + + t H - -
/ Bjor kman lign N l N2 L1 L2 Ls
I- - - -
Wood Bjorkman ligni N1 L3 -I-
+- itit -
$ - t - t t t t -+ + + i W r t t t t
+t + H t t i t
+,#, a n d H , degree of intensity of identified spots;
*Value based on t h e absolute-dry weight of initial sample
angiosperm trees contained comparati- vely much of
IZIand a small amount of
I. Z Vand
Vcould not be detected in all Bjorkman lignins. Namely, t h e difference in the pro- perties of bark Bjorkman lignin be- tween gymnosperm and angiosperm was similar to that i n the properties of wood lignin between the two kinds, which have been reported by many workers@) ,,
The oxidation products of tannin and bark phenolic acid f rom gymnospeIm trees, a s well a s those of angiosperm trees, contained much of
Z Vand a small amount of
II.In addition to the above mentioned three com- pounds, little of
ZIIwere detected in the oxidation products of the bark phenolic acid f r om angiosperm trees, comparatively large amount of
Vand a smaller amount of
ZZZwere detected in the oxidation products of tannins from angiosperm trees.. These results indicate the following facts : bark phenolic acids of all species of trees used here have much catechol and a small amount of guaiacyl and
Voi 18, No. 2 (1967) 161
tannin; bark phenolic acid of angiosperm trees have a very small amount of syringyl type nucleus in addition to the above mentioned three nuclei; and tannin of angiosperm trees have a large amount of pyrogallol type nucleus in addition to the nuclei of bark phenolic acid of angiosperm trees. Accordingly, for gymnosperm, the structural nuclei of bark phenolic acid was quite similar to those of tannin, however, for angiosperm, the structural nuclei of bark phenolic acid was dissimilar to those of tannin, because of lacking i n py~ogallol type nucleus. No prooves could be established on the pyrogallol type nucleus of bark phenolic acid.
The presence of I V i n the permanganate oxidation products of bark phenolic acid is in agreement with the results of nitrobenzene oxidation. That is, i n the previous works on the nitrobenzene oxidation of bark phenolic acid of some species, protocatechualdehyde was identified together with vanillin and 4-hydroxybenzaldehyde('- 2 8 "'. A s the pro- tocatechualdehyde i s unstable under the condition of nitrobenzene oxidation, i t has been considered that the catechol nucleus exists in the initial bark phenolic acid in much greater quantities than is supposed frcm the yield of protocatechualdehyde from bark phenolic acid by nitrobenzene oxidation. This time, much I V was detected in the permanganate oxidation products of bark phenolic acid which was ethylated to protect their phenolic hydroxyl groups. This indicates that bark phenolic acid has much catechol nucleus. The permanganate oxidation after ethylation seems to be of excellent to make study the structural nuclei of tannin- like materials.
1 Materzals. Bark phenolic acid, tannin, and bark and wood Bjorkman lignins were
isolated in the ordinary methodc2 4 ) . The tree species, from which the sample bark was
taken, were recorded in Table
2. Syntheszs o f authentzc compounds.
4-Ethoxybenzoic ( I ) , 3 -methoxy
-4 -ethoxybenzoic (11)
,5 - dimethoxy -4-ethoxybenzoic (111)
,4 -diethoxybenzoic ( I V )
triethoxybenzoic ( V) acids were prepared respectively from 4 -hydr oxybenzoic, vanillic, syringic, protocatechuic, and gallic acids by the ethylation with (CzHa)2S04 and KOH. Melting point of them agreed with l i t e r a t ~ r e ( ' ~ - ' ~ ) , and the results of their analysis were in a good agreement with the calculated value (cf Table 1). The UV-absorption spectrum was determined for the alcohol solution of each compounds by HITACHI 139 spectrophotometer Fig 1 shows the UV-spectra,
3. Ethylatton and potasszum permanganate oxzdatzon. Sample (0.4 g) was ethylated thrice with (C2H5) 2S04 (10 ml) and 50 % KOH solution ( 8 ml) at 85-90°C for 2 hrs. The ethylated materials were oxidized with KMn04 by the same way as the reported p r o c e d u ~ e ( ~ ~ ' ~ ) , that is,the ethylated material suspended in about 350 ml of an acetone-water ( 1 : 9v/v) mixture was refluxed adding solid KMnOa slowly until the reduction became inappreciable. The reaction mixture was always kept a t pH 6.8-7.5 by the addition of dilute H2S04 Total amount of 5
-7 g of KMn04 were added to the sample by 0.5 g at a time. The color of permanganate was discharged each time. After the reaction, the remained KMn04 of a small amount was decomposed by the addition of ethanol of a
1 62 Tech. Bull. Fac. Agr . Kagawa Univ.. few milliliter The MnO2 was filtered off and washed with 1
%KOH solution. The filtrate and washing were combined, neutralized with dilute HzS04, and concentrated to about 50ml in vacuum a t a temperature below 60°C. The concentrated solution was acidified to pH 1.5-2.0 with dilute HzS04 and then was extracted with ethylether (70m1
x5). The amount of ether -soluble parts of reaction products was determined (cf. Table 3).
4. Paper chromatograPhy The ether-soluble reaction products were examined by paper
chromatography using the two solvent systems. In the first, i t was developed at 20°C for 24 hrs. by the ascending method with the developing solvent of butanol which was saturated with 2N-(NH4)2COs. In the second, the sample was applied on the filter paper pretreated with an acetone-dimethylformamide ( 8 : 2v/v) mixture and developed by the decending method with a mixture of ligroin-dimethylf ormamide (1 000 : 1 v/v) at 20°C for 6 hrs. The developed solvent dripped from the serrated lower edge of the paper. After the development, the paper was heated a t 140°C for 5 minutes and then was sprayed with methyl red dissolved in a pH 7.0 buffer solution. The Rf-value of each component on chromatograms was compared with that of authentic compound (cf
Bark phenolic acid, tannin, and bark Bjiirkman lignin of gymnosperm trees ( 2 species) and angiosperm trees ( 3 species) were ethylated with diethylsulfate and then oxidized with potassium permanganate. The ether -soluble reaction products were examined by means of paper chromatography to examine for the existence of 4 -ethoxybenzoic, 3 -
methoxy- 4 -ethoxybenzoic, 3
,5 -dimethoxy- 4 -ethoxybenzoic, 3 , 4 -diethoxybenzoic, and 3
,5 -triethoxybenzoic acids.
The oxidation products of bark Bjorkman lignin were in accord with those of wood Bjorkman lignin, indicating that bark Bjiirkman lignin is a true lignin. Bark phenolic acids of gymnosperm trees have much catechol and a small amount of guaiacyl and 4 -
hydroxyphenyl type nuclei as tannin of the same tree species. Bark phenolic acids of angiosperm trees have a small amount of syringyl type nucleus in addition to the above mentioned three type nuclei, and probably have no pyrogallol type nucleus. Tannins of angiosperm trees have pyrogallol type nucleus together with the other type nuclei.
The author thanks to Dr. K . HATA, Professor of Kagawa University? for his kind guidance
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