JARQ 28, 70-78 (1994)

Chemistry and Utilization of Condensed from Tree Barks

Seip .. OHARA. ... . Wood Chemistry Division, Forestry and Forest Products Research Institute (Tsukub a, Ibaraki, 305 Japan)

Abstract Chemical characteristics, protein-adsorbing capacity, chemical modifications and utilization of condensed tannis from tree barks were reviewed. Various aspects relating to the chemistry of these compounds and prospects for their utilization were discussed. Condensed tannins from Quercusjalcata, Salix sieboldiana and most of the Japanese conifers consist mainly of and those from Acacia mearn­ sii of prorobinetinidins. The protein-adsorbing capacity of condensed tannins is closely related to their molecular weights and the positions of phenolic hydroxyl groups in the B-ring. The most effective hydroxyl group is located in the para­ position. In the reactions of , the pyran ring is cleaved by alkaline treatment at 40°C or phenolation in the presence of BF3. The latter reaction can be effective for opening the pyran ring selectively. The tannins can be processed for utilization as cold-setting adhesives, polyurethane foams and adsorbents of heavy metal ions. Prospects for uses of condensed tannins include applications to biologically active compounds and functional materials.

Discipline: Forestry and forest products Additional keywords: catechin, , protein-adsorbing capacity, pyran ring, -derived polyurethane foams

dicotyledoneae. They are present at very high Introduction concentrations in the bark of timber species such as conifers, eucalypts and leguminous Vegetable tannins are widely distributed in hardwoods. Their molecular weights cover a plants and have been used as tanning agents, wider range than those of hydrolysable tannins dyestuffs and drugs since older times. They with the highest one being 20,000. Recently, can be divided into two major groups, con­ many investigations on new forms of utiliza­ densed tannins and hydrolysable tannins. The tion and biological activities have been reported. former group consists of proanthocyanidins and In this paper the classification, chemical the latter of polyesters based on gallic acid or characteristics, protein-adsorbing capacity and hexahydroxydifenic acid and their derivatives. chemical modifications of condensed tannins, Condensed tannins are distributed in ferns and as well as several fields of utilization are allied species, and both monocotyledoneae and reviewed. Furthermore, various aspects relating

This work was supported in part by the Integrated Research Program for Bio Renaissance Program from the Ministry of Agriculture, Forestry and Fisheries (BRP93 - V - B-I). Ohara: Chemistry a11d U1iliw1io11 of Tannins from Tree Barks 71 to the chemistry or these compounds and ponding (2R, 3R) isomers are designated by the prospects for their utilization are discussed. addition of the prefix 'cpi' to the beginning of each monomer name, and (2S, 3R) or (2S, Classification 3S) isomers are indicated by the addition of the prefix ent to the beginning of the appropri­ Condensed tannins belong to pro•an­ ate monomer name. thocyanidins as plant secondary metabolites. Proanthocyanidins which are polymers of sever­ Chemical characteristics al kinds or flavan-3-ols are classified as shown in Fig. I, according to the hydroxylation pat­ J) Quercus falcata terns or A- and B-rings of the monomer Q. falcata (southern red oak) is the most im­ units8>. Procyanidins are widely distributed portant species of red oak in the forests of the and , profisetinidins and southern United States. Related compounds of prorobinetinidins are also found frequently in condensed tannins isolated from the bark of many plants. As each flavan-3-ol has two the southern red oak are listed in Fig. 25>. The asymmetric carbon atoms in the C-ring, there bark of southern red oak is a rich source of are four stereoisomers. , quercetin-3-rhamnoside (1) and it contains only , , , catechin low concentrations of catechin (2). The three and gallocatechin are the monomer units with major dimeric proanthocyanidins are a (2R, 3S) absolute stereochemistry correspond­ epicatechin-(4.8-8)-catechin (3), catechin-(4a-8)­ ing to progibourtinidin, profisetinidin, catechin (5) and the 3-gallatc ester of prorobinetinidin, propelagonidin, epicalechin-(4~·8)-calcchin ( 4). The acetone­ and , respectively. The corres- water soluble tannins with a higher molecular weight consist of polymeric procyanidins predominantly made up of 2,3-cis chain ex­ R2 OH tender units with a 2,3-trans catcchin terminal. I The polymers contain only small amounts of HO~O2, 0 R3 I A C 3 2,3-trans procyanidin chain extender units and ~ only traces of prodelphinidin units. R, r OH 2) Salix sieboldiana There is only one report on the chemical n analysis of proanthocyanidins in the Salix spe­ cies grown in Japan. Proanthocyanidins and their derivatives from the bark of S. sieboldi­ ana, a shrub common in Japan, arc shown in Fig. 3 2>. This species contains a homologous series of acylated procyanidins (7), (9) and

R1 (I/), together with the widespread procyanidin 8-1(6), B-3(8) and trimer (10). Their struc­ R,:H, R2:H, R3=H -·· Proglbourtlnldln R1:H, R2:0H, R3:H --· Prollsetinidin tures are assigned to the 3-0-(1-hydroxy-6-oxo- R :H, R :0H, R :0H -··· Proroblnetlnidln 1 2 3 2-cyclohexcne carboxylic acid esters) of pro­ R1: 0H, R2:H, R3=H --- Propelargonldln R1:0H, R2:0H, R3:H •··· Procyanldln cyanidins B -1, 8-3 and trimer. As 1-hydroxy- R :0H, R :0H, R : 0H ····Prodelphinldin 1 2 3 6-oxo-2-cyclohexene carboxylic acid is structur­ Fig. I. classification ally related to saligenin, it is considered to be 72 JARQ 28(1) 1994

OH

0 HO Q(X'-:::, 6 h OH H 0 (1) (2)

OH 0 HOC(X'6 OH I OH 'OR OOH h OH OOH OH I o , ::::,... I HO Gu'-:::O , ::::,...

OH h OH H

(3) A: H (5) (4) A= Galloyl

Fig. 2. Compounds isolated from the bark of southern red oak a biosynthetic precursor of salicin which is a conifers amount to 5- 18% and 2-7%; common constituent in the Salicaceae. respectively9>. The composition of the poly­ meric proanthocyanidins from 9 species of 3) Acacia mearnsii conifers is shown in Table I 10>. Although poly­ The bark of A. mearnsii is a good source meric proanthocyanidins from the bark are of tannins (phenol content 380Jo, tannin con­ composed generally of polymeric procyanidins tent 24% 1>). it contains a large amount of low in many coniferous species, Abies sachalinen­ molecular weight proanthocyanidins. About sis and Picea jezoensis contain a certain amount 30% of 70% acetone extracts from the bark of polymeric prodelphinidins. The level of are dimers or trimers. Major dimeric proan­ stereochemistry of the flavan-3-ol monomer thocyanidins are robinetinidol-(4a-8)-catechin units of the polymers varies considerably with ( 12) and robinetinidol-(4a-8) -gallocatechin ( 13) the species. As for the extender units, the poly­ (Fig. 3). The acetone-water soluble tannins with mers from Cupressaceae and Cryptomeria a higher molecular weight are proanthocyanidin japonica show nearly the same or higher ratios polymers with a 2,3-trans heterocyclic ring, the of 2,3-trans units to 2,3-cis. On the other hand, extender units of which consist mainly of the polymers from Pinaceae show extremely prorobinetinidin units. high ratios of 2,3-cis units to 2,3-trans. In the terminal units, 2,3-trans units predominate and 4) Conifers almost all the units consist of 2,3-trans in P. Many Japanese conifers contain a large jezoensis and Pinus densiflora. amount of phenolic extractives in the bark, and proanthocyanidins are the main components of Protein-adsorbing capacity these phenolic extractives. Plavanol and tan­ nin contents in the bark of the Japanese main Condensed tannins are able to precipitate Ohara: Chemistry and Utilization of Tannins from Tree Barks 73

OH OH OH OH I O HOco~0 O OH OH OH h OH O OH I OH I O HOco~

OR OH OH OR (6) A = H (8) A= H 0-x,H 0-x,OH (7)R= V (9)R= V

OH OH

HOC(XOO OH I OH h OH OOHI HO ~ R h COOH OH (12)R:H (13)R:OH

-CO OH (11)R= OD

Fig. 3. Proanthocyanidins and their derivatives from the bark of Salix sieboldiana a.nd A. meamsii

Table 1. Composition of polymeric proanthocyanidins from coniferous bark

Ratios of stereochemistry Pcy:Pdp•J (2,3 -trans: 2,3-cis) Extender units Termination units Cupressaceae Chamaecyparis obtusa 100:0 60:40 59:41 Chamaecyparis pisijera 100:0 54:46 64:36 Taxodiaceac Cryptomeria japonica 100:0 78:22 69:31 Pinaceae Ables sachalinensis 71 :29 73:27 84 : 16 Larix leptolepis 100:0 11 :89 80:20 Picea jezoensis 95:5 11: 89 100 :0 Pinus densijlora 100:0 7 :93 97:3 Tsuga sieboldii 100:0 11 :89 44:56 Pseudotsuga japonica 100:0 16:84 23:77 a): Pcy; Procyanidins, Pdp; Prodelphinidins. 74 JARQ 28(1) 1994 proteins and it is assumed that they are involved adsorbing capacity increases with the increase in the chemical defense of some plants against of the molecular weights. 3) Tetramers show predators such as insects and herbivores. In a capacity nearly equal to that of polymers. the tannin-protein interaction, phenolic hydroxyl groups in the A- and B-rings play / / important roles. Results of BSA-precipitating 3.0 / I 00<';!, 1>rcci11i1a1ion-, / tests of synthesized oligomers / with mono-, di-, and tri-hydroxylated B-rings / 6 3 / --6 < 2.0 are shown in Fig. 4 >. BSA-precipitating ca­ v> / ---- 8 pacity of oligomers D and G accounts for 83 a, ,,.---- a= "<.> /~~ and 87% of that of oligomer F, respectively. s These results indicate that the condensed tan­ l 1.0 / / J: / nins with only a 4 ' -hydroxylated B- ring show / almost a similar capacity to that of the tannins / with 3 ', 4 '-dihydroxylated or 3 ' , 4 ', 5 '-tri­ hydroxylated B-rings. Added BSA (mg) The protein-adsorbing capacity of proan­ O: ollgomor D, A: ollgomer F, O : ollgomer G. thocyanidin dimers, trimers, tetramers and poly­ Rt R2 ollgomcrD R,=H, R2:0H, R3=H mers from the bark of A. meamsii is shown HO ..._ 0 R3 lollgomer F 7 in Fig. 5 J. The results indicate the following ~,® R1:H, R2:0H, A3: 0H OH OH ohgomcr G relationships between the degree of polymeri­ I R,:OH, R2: 0H, R3: 0H zation of proanthocyanidins and protein­ l I 0 adsorbing capacity: I) Dimers hardly display Fig. 4. BSA-precipitating capacity of synthesized a protein-adsorbing capacity. 2) The protein- condensed tannin oligomcrs

3...------~

en 5 < 2 (lo•" "'"'" ' ·' V, m It- 00 ••/<>" · Polymers - Te1ramers - • o-- Trlmers - Dimers 0------0------0 o+------l 1 . 0 2.0 3.0 4 .0 Added BSA (mg)

Fig. 5. Protein-adsorbing capacity or proanthocyanidin dimers, timers, tetramers and polymers from the bark of A . meomsii Ohoro: Chemistry and U/1/iwtion of Tannins from Tree Barks 75

.:H O o::BOHOH 2s·c Ho ~ o ~ 1 ~I..-:: OH OH (15)

(16)

OH

Fig. 6. Base-catalyzed reactions of ( + )-catechin

3-(2,4,6-trihydroxyphenyl) propane-2-ol sub- Chemical modifications stituted at C- I with a catcchinic acid moiety (/7) is formed in a yield of about 200Jo. 1) Alkaline rearrangement reaction When condensed tannins are treated with 2) Phenolation by Lewis acid catalyst alkaline media, a reduction of the phlo­ As the rotational degree of freedom of con­ roglucinol ring and appearance of carbonyl densed tannins is considerably low due to the groups are observed. Therefore condensed tan­ presence of a pyran ring, it is important to de­ nins are considered to undergo some base­ velop a method for opening the pyran ring catalyzed rearrangement reaction. Reaction of selectively. Phenolation catalyzed by BF3 is a catechin (/4) at pH 12 is shown in Fig. 6 6>. very promising method. The phenol modifica­ In reactions at 25°C, the principal reaction con­ tion of catechin (14) in the presence of BF3 sists of the opening of the pyran ring to give is shown in Fig. 74 >. The reaction mechanism the quinonemethide and reclosure of the pyran is as follows: I) an unshared electron pair of ring to give catechin and ent-epicatechin (15), oxygen atoms on the pyran ring coordinates with the former predominating in a ratio of with a BF3 molecule into an excess of phenol about 3 to I. When the same reaction is car­ and (18) is formed via the Sn2 reaction, 2) ried out at 100°C, catechinic acid (16) is ob­ then, dehydration occurs between alcoholic and tained at a high yield . At 40°C, a dimeric aromatic hydroxyl groups in the C-2 position product consisting of a 1-(3,4-dihydroxyphenyl)- and in the A-ring of (18) to form (19). 76 JARQ 28(1) 1994

OOH WT-PES-TDI : 6 HOO:X OH 0.3 I ..-: WT-PES-MOI : A OH OH <'/- WT-PES-HOI : • (14) E § 0 ,2 ~ coordlnotlon QI

0.1 BF3 OH HO~'- OH I /, ~~o. 0 .0 OH OH •oH

f S0 2 reaction 2.0 OH HOO ., ~ I .OOH eQ. t> HO~H J ..-,: -H (18) g> OH OH ..J

tdehydration

'i Fig. 7. Phenolation of ( + )-catechin e by BFJ catalyst w .3"'

Utilization

1) Adhesives • 1,0.._...___ ...... _ __ _,_ __ _._ _, Recently, it has been demonstrated that con­ I 0 IS 20 densed tannins from conifer barks can be uti­ WT conient (%) lized as adhesive materials. As conifer tannins have a phloroglucinol type A-ring, they dis­ Fig. 8. Changes in density (Q), compression play a high reactivity to formaldehyde. There­ strength (o) and compression elasticity (E) as functions of WT content of PU fore, cold-setting adhesives can be produced foams from tannins from conifer barks. The opti­ WT: Wattle tannin, mum method of preparation of tannin-based PBS: Polyester-polyol, cold-setting adhesives consists of a simple mix­ TDI : Tolylene diisocyanate, ture of bark extracts and resorcinol resin MDI: Diphenylmethane di- without prereaction. The adhesives consisting isocyanate, HD!: Hexamethylene diisocyanate. of 50 parts of methanol extract from Larix lep­ to/epis, 50 parts of resorcinol resin, 15 parts of paraformaldehyde at pH 9 exhibit a higher resorcinol resin adhesives (Dainihon Ink Co., bonding ability than that of commercial phenol- Ltd.) 11 >. Ohara: Chemi$try and Utilization of Tannins from Tree Barks 77

2) Polyurethane foams sorption capacity than MT, presumably due to Tannin-derived polyurethane (PU) foams the cleavage of the pyran ring in the flavanol can be readily prepared by the following skeleton. On the other hand, resorcinolation procedure 1>. A polyester-polyol (PES, hydroxyl is not very effective i.u enhancing adsorption, groups J.35 mmol/g, carboxyl groups 0.14 due to the increased steric hindrance. mmol/g, viscosity 3800 cp) solution of watlle tannin (WT) reacts with three kinds of di­ Prospects isocyanates in the presence of triethylene diamine catalyst, silicone surfactant and Over the past ten years tannin chemistry has trimethylol propane as a crosslinking agent. made progress due to the development of chem­ Density and mechanical properties of the ob­ ical analyses, and many new compounds and tained foams are shown in Fig. s1>. Proper'ties structures have been identified. However, since generally vary directly with the tannin content. condensed tannins are widely distributed and Density increases gradually, and compression their structure is very complicated, it is neces­ strength and compression elasticity increase sary to study plants containing tannins more rapidly with the increase of the tannin content. extensively. The relationship between the Therefore, tannins seem to act as crosslinking stereochemistry and the protein-adsorbing ca­ agents and as hard segments in the PU foam pacity of condensed tannins must be clarified. structure. A form containing 25% WT is slow­ Application to biologically active compounds ly degraded by wood-destroying fungi such. as is one of the most promising areas for use of Corio/us versicolor and Tyromyces palustris. condensed tannins. Recently, various medici­ nal activities of condensed tannins have been 3) Adsorbents of heavy metal ions reported. Antimicrobial activity, antibacterial Condensed tannins can adsorb several heavy activity, antitermite activity and germination­ metal ions. The capacity for heavy-metal ad­ and growth-regulation activity against other sorption of spherical tannin resin prepared from plants are also very promising biological ac­ commercial Mimosa tannin (MT) and MT with tivities. As outlined previously, the rotational various pretreatments is shown in Table 2 12>. degree of freedom of condensed tannins is con­ Sulfonated MT (SMT) and trichloroacetic acid­ siderably low due to the presence of a pyran pretreated MT (TMT) display a greater ad- ring. Therefore if the selective cleavage of the pyran ring could be achieved, the biological ac­ Table 2. Adsorption of metal ions on pretreated tivity of the tannins could be markedly im­ tannin-resins proved. Another area of promising utilization is prob­ Amount of adsorbed metal ions ably the preparation of tannin-derived function­ Resin•> (mmol/g of resin) al materials. As it is known that some species Cr6 .,. Cd2 ... 2 Fe2 + Cu • of fungi and bacteria can degrade condensed MT 0.074 0.000 0.036 0.026 tannins, the materials derived from condensed SMT 0.067 0.025 0.060 0.069 tannins may be potentially biodegradable. TMT 0.077 0.01 I 0.051 0.065 RMT 0.071 0.000 0.029 0.046 References Metal solution: 20 ppm x 100 m /, pH= 5 (Cr and Cd), pH= 4(Cu and Fe). I) Ge, J. &Sakai, K. (1993): Compressive proper­ a) : MT; Commercial Mimosa tannin , SMT; Sul­ ties and biodegradability of polyurethane forms fonatcd MT, TMT; Trichloroacetic acid treat­ derived from condensed tannin. Mokuzai Gak­ ed MT, RMT; Resorcinolated MT. kaishi (In press). 78 JARQ 28(1) 1994

2) Hsu, F., Nonaka, G. & Nishioka, I. (1985) : annual meeting of Japan Wood Research Soci­ Acylated flavanols and procyanidins from Salix ety, 435 [In Japanese). sieboldiana. Phytochemislry, 24, 2089-2092. 8) Porter L. J. (1988): Flavans and proan­ 3) Kawamoto, H., Nakatsubo, F. & Murakami, K. thocyanidins. Chapman and Hall, London, (1990): Relationship between the 8-ring 21-62. hydroxylation pattern of condensed tannins and 9) Samejima, M. & Yoshimoto, T. (1981): General their protein-precipitating capacity. J. Wood aspects of phenolic extractives from coniferous Chem. Technol., 10, 401-409. barks. Mokuzai Gakkaishi, 27, 494-497 (In 4) Mitsunaga, T. & Abe, 1. (1992); The phenola­ Japanese with English summary] . tion of monomeric compound of condensed tan­ JO) Samejima, M. & Yoshimoto, T. (1982): Sys­ nins by Lewis acid catalyst. Mokuzai Gakkaishi, tematic studies on the stereochemical composi­ 38, 585 - 592. tion of proanthocyanidins from coniferous bark. 5) Ohara, S. & Hemingway, R. W. (1989): The Mokuzai Gakkaishi, 28, 67-74. phenolic extractives in southern red oak (Quer­ 11) Takano, R. (1991): Production of cold-setting cus falcata Michx. var.falcata) bark. Holz[or­ adhesives utilizing phenolic extracts from conifer­ sch1111g, 43, 149-154. ous barks. Res. Rep. Biomass Conversion Pro­ 6) Ohara, S. & Hemingway, R. W. (1991): Con­ gram, 29, 67-76 (In Japanese wit11 English densed tannins, the formation of a diaryl­ summary). propanol-catechinic acid dimer from base­ 12) Yamaguchi, H., Higuchi, M. & Sakata, I. catalyzed reactions of ( + )-catechin. J. Wood (1991): Adsorption of heavy metal ions on Chem. Technol., 11, 195 -208. porous spherical tannin resins. Mokuzai Gak­ 7) Ohara, S., Hishiyama, S. & Obira, T. (1992): kaishi, 37, 942-949. The biological activities of condensed tannins from A. mearnsii. In Proceedings of 42nd (Received for publication, Feb. 5, 1993)