Activation of Some Condensed Tannins via Facile Ring Isomerizations

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Chapter 16 Activation of Some Condensed Tannins via Facile Ring Isomerizations Potential Adhesive Applications D a v i d G. R o u x Department of Chemistry University of the Orange Free State P . O . B o x 339, Bloemfontein, 9300 Republic of South A f r i c a

[4,8]-2,3-trans-3,4-trans-(-)-Robinetinidol-2,3-irans-(+)-catechin(1b) and its (-)-fisetinidol homologue (1a) as prototypes of mimosa and quebracho tannins, respectively, are to a varying degree subject to positional isomerization) in N a H C O - Na CO buffer under nitrogen, with "liberation" of reactive nucleophilic resorcinol units to form a range of bifunctional phlobatannins. The potential of this efficient method of inducing strong bifunctionality is discussed in relation to the cold-set adhesive application of these commercially available extracts. 3

2

3

T h e use o f c o m m e r c i a l m i m o s a e x t r a c t i n adhesive a p p l i c a t i o n s is firmly establ i s h e d i n S o u t h A f r i c a , a n d i t i s i n fact c o m p l e t e l y d o m i n a n t i n p a r t i c l e b o a r d a n d p l y w o o d m a n u f a c t u r e (1). S i m i l a r f o r m u l a t i o n s are c u r r e n t l y also i n use i n A u s t r a l i a a n d elsewhere, w h i l e f o r t i f i c a t i o n w i t h r e s o r c i n o l p e r m i t s a p p l i c a t i o n of m i m o s a e x t r a c t i n fingerjointing a n d b e a m l a m i n a t i o n (2). C h e m i c a l differences are apparent between the condensed t a n n i n s t h a t c o n s t i t u t e u p w a r d o f 7 0 % o f m i m o s a a n d quebracho e x t r a c t s o n the one h a n d a n d those c o m m o n l y encountered elsewhere outside the respective L e g u m i n o s e a e a n d A n a c a r d i a c e a e . T h e s e devolve m a i n l y o n s u b t l e differences i n f u n c t i o n a l i t y affecting b o t h the degree o f c o n d e n s a t i o n o f the t a n n i n s a n d their r e a c t i v i t y , a n d hence their p h y s i c a l b e h a v i o r d u r i n g c o n d e n s a t i o n w i t h f o r m a l d e h y d e . T h i s is obvious when comparing prototype dimeric, trimeric, a n d tetrameric oligoflav a n o i d u n i t s ( 1 ) t o ( 3 ) f r o m m i m o s a (3,^,5) a n d quebracho (6,7) e x t r a c t s w i t h t h e i r p r o c y a n i d i n homologues ( 4 ) present, for e x a m p l e , i n the b a r k s o f western 0097-6156/89/0385-0217$06.00/0

c

1989 American Chemical Society

Hemingway et al.; Adhesives from Renewable Resources ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

ADHESIVES F R O M RENEWABLE RESOURCES


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Activation of Condensed Tannins

hemlock, Douglas-fir, southern pine, and redwood from N o r t h A m e r i c a . Salient differences between these t w o groups m a y be s u m m a r i z e d as follows:


1) P r o t o t y p e s (1) t o (3) possess r e s o r c i n o l - t y p e flavanyl u n i t s a t t a c h e d to ( + ) - c a t e c h i n or ( - f ) - g a l l o c a t e c h i n , whereas, p r o c y a n i d i n p r o t o t y p e s (4a) are u n i f o r m l y based o n p h l o r o g l u c i n o l - t y p e flavanyl u n i t s . 2) D u e t o the greater p r e d i s p o s i t i o n o f t h e i r e l e c t r o p h i l i c flavanyl precursors ( l e u c o c y a n i d i n s ) t o c o n d e n s a t i o n (8, 0 ) , the p r o c y a n i d i n t a n n i n m i x t u r e s [(4a) η = 0 t o 10] e x h i b i t general emphasis o n the m o r e h i g h l y condensed u n i t s , whereas, d i m e r i c a n d t r i m e r i c p r o t o t y p e s (1) a n d ( 2 ) , respectively, are p r o m i n e n t i f not dominant among profisetinidins ( R = R = H ) and prorobinetinidins ( R = H , R = Η or O H ) . P r o c y a n i d i n s m a y , therefore, be generally considered t o occur n a t u r a l l y i n m o r e h i g h l y condensed m i x t u r e s . 1

2

1

2

3) P r o c y a n i d i n oligomers [(4a) η = 0 t o 10] a p p a r e n t l y possess m a i n l y " l i n ear" c o n f o r m a t i o n s , c o m p a r e d w i t h the proven " a n g u l a r i t y " o f p r o f i s e t i n i d i n a n d p r o r o b i n e t i n i d i n analogues (2) a n d ( 3 ) . 4) P r o b a b l y m a i n l y as the result of a m o r e u n i f o r m mass d i s t r i b u t i o n a n d w i t h considerable emphasis o n lower mass u n i t s , p r o r o b i n e t i n i d i n s a n d profise t i n i d i n s (1) t o (3) occur i n m i x t u r e s t h a t are m o r e r e a d i l y w a t e r - s o l u b l e t h a n those o f p r o c y a n i d i n s (4a) a n d p r o d e l p h i n i d i n s (4b). H o w e v e r , o x i d a t i v e ef fects as the result o f w e a t h e r i n g of b a r k s m a y also p l a y a role i n r e d u c i n g the s o l u b i l i t y o f the l a t t e r g r o u p . D e s p i t e the aforementioned differences, b o t h t a n n i n types e x h i b i t levels of r e a c t i v i t y w i t h f o r m a l d e h y d e under n e u t r a l or m i l d l y a l k a l i n e c o n d i t i o n s w h i c h are i n t e r m e d i a t e between those o f resorcinol a n d p h e n o l or p h e n o l - r e s o r c i n o l f o r m a l d e h y d e ( P R F ) or p h e n o l - f o r m a l d e h y d e ( P F ) adhesives. T h i s follows l o g i c a l l y f r o m the m o r e reactive p h e n o l i c u n i t s present i n each t y p e of t a n n i n i.e., p r e d o m i n a n t resorcinol m o n o e t h e r A , G , a n d J u n i t s , for e x a m p l e , i n the oligoflavanoids (1) t o ( 3 ) , [the p h l o r o g l u c i n o l m o n o e t h e r D - r i n g of (1) b e i n g a n exception], a n d also the e x c l u s i v e l y p h l o r o g l u c i n o l m o n o e t h e r A , D a n d G - r i n g s of p r o c y a n i d i n s / p r o d e l p h i n i d i n s of t y p e (4), w h e n b o t h types are c o m p a r e d w i t h the h i g h l y reactive resorcinol or resorcinol " t e r m i n a l " u n i t s i n P R F adhesives o n the one h a n d a n d the r e l a t i v e l y u n r e a c t i v e p h e n o l or p h e n o l u n i t s i n P F resin o n the other. T h e last m e n t i o n e d require i o n i z a t i o n ( h i g h p H ) t o p r o m o t e reactivity. T h e i n t e r m e d i a t e order o f r e a c t i v i t y o f u n m o d i f i e d m i m o s a a n d quebracho t a n n i n s makes t h e m u n s u i t a b l e for cold-set a p p l i c a t i o n s at n e u t r a l p H . However, p r o c y a n i d i n s offer the prospect o f higher r e a c t i v i t y c o m p a t i b l e w i t h t h a t of resorcinol or P R F adhesives w i t h f o r m a l d e h y d e . U n f o r t u n a t e l y , low s o l u b i l i t y at n e u t r a l p H c o u p l e d w i t h ( a n d as a f u n c t i o n of) t h e i r h i g h n u m b e r average mass leads t o p r e m a t u r e g e l l i n g p h e n o m e n a . S t r u c t u r a l r i g i d i t y o f these oligomers w o u l d u n d o u b t e d l y also l i m i t the desirable degree of c r o s s l i n k i n g t h a t c a n occur, r e s u l t i n g i n brittleness. I n cold-set a p p l i c a t i o n s , the cost of resorcinol as a c o m m e r c i a l c o m m o d i t y has l e d t o its w e l l - k n o w n " g r a f t i n g " o n t o p h e n o l - f o r m a l d e h y d e resols to p r o v i d e


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reactive terminal units as represented in the idealized P R F formulae (5) and (6). Highly reactive bifunctionality (5) and polyfunctionality (6) are, accordingly, provided under neutral cold-set conditions during final crosslinking with formaldehyde. W i t h this type of application i n mind, attempts were made to liberate resorcinol units in mimosa and quebracho prototypes (1) and (2) by fission of the heterocyclic ether rings C and I, while accommodating recyclization elsewhere.

Inspiration for our successful attempts at effecting the release of resorcinol came from our initial recognition (10) of a new class of ring isomerized condensed tannins, termed phlobatannins, (11) i n the heartwoods of Colophospermum mopant (the African mopane tree) and Guibourtia coleosperma (false mopane). Both African species belong to the Anacardiaceae as do Schinopsis spp. (quebracho) from South America. In these African species, the ring isomerized compounds (7) and (8) and also the partially ring isomerized products (9) and (10) occur i n association w i t h their presumed precursors ( l a ) and (2a). T h e isomeric nature of the phlobatannins with their putative precursors made structural recognition of the former by nuclear magnetic resonance spectroscopy difficult, although their spectra are significantly devoid of rotational isomerism. In order to establish the "liberation" of the resorcinol rings i n phlobatannins, it


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was, therefore, necessary t o resort t o nuclear O v e r h a u s e r effect ( n . O . e ) difference spectroscopy o f t h e i r m e t h y l ether acetates (10) as s h o w n i n F i g u r e 1.

(9) = H, R = OH (10) R = O H , R = Η 2

φι,

1

2

OH

[Nuclear O v e r h a u s e r effect difference spectroscopy is o f great significance i n the e l u c i d a t i o n o f s t r u c t u r e s . If, for e x a m p l e , t h e B o l t z m a n n d i s t r i b u t i o n ( r a t i o of n u c l e a r spins) o f one t y p e of p r o t o n is d i s r u p t e d b y t h e a p p l i c a t i o n of a second field, H2, t h a t is s t r o n g e n o u g h t o s a t u r a t e the s y s t e m p a r t i a l l y , b u t not s t r o n g enough t o cause t i c k l i n g or d e c o u p l i n g effects. S h o u l d the s a t u r a t e d p r o t o n i n t e r a c t v i a a d i p o l e - d i p o l e m e c h a n i s m w i t h a n o t h e r p r o t o n , t h e second




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F i g u r e 1. Percentage n . O . e . enhancements as d e t e r m i n e d for the m e t h y l ether acetates of (7) a n d (8). T h o s e m a r k e d * are evident b u t c a n n o t be c a l c u l a t e d o w i n g t o s i g n a l overlap [cf. (12)].



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p r o t o n w i l l experience a d i s r u p t i o n of i t s B o l t z m a n n d i s t r i b u t i o n a n d hence a change i n the i n t e n s i t y of i t s resonance l i n e . T h e u t i l i t y i n s t r u c t u r e a n d c o n f o r m a t i o n a l d e t e r m i n a t i o n lies i n the fact t h a t d i p o l e - d i p o l e i n t e r a c t i o n s v a r y inversely w i t h the c u b e o f the distance between t h e d i p o l e s , so t h a t i n f o r m a t i o n o n s p a t i a l o r i e n t a t i o n o f n u c l e i m a y be e x t r a c t e d i n favorable cases. M o d e r n N M R e q u i p m e n t p e r m i t s direct measurement o f changes i n the s p e c t r u m w i t h a p p l i c a t i o n of H2, r a t h e r t h a n the s p e c t r u m itself]. T h e a s s o c i a t i o n of p r o t o n s as s h o w n b y the percentage e n h a n c e m e n t s p r o v e d the v a r i o u s s t r u c t u r e s a n d , therefore, the " l i b e r a t i o n " of t h e resorcinol u n i t s i n the p h l o b a t a n n i n s a n d , hence, t h e desired b i f u n c t i o n a l i t y i n each i n s t a n c e . OH

O n c e recognized, the s y n t h e t i c achievement o f the i s o m e r i z a t i o n process (12) proved a r e l a t i v e l y s i m p l e m a t t e r u s i n g F r e u d e n b e r g a n d P u r r m a n ' s (IS) e p i m e r i z a t i o n procedure developed for t h e conversion o f ( - f ) - c a t e c h i n i n t o ( + ) e p i c a t e c h i n . A p p l y i n g a N a H C 0 3 - N a o C O s buffer s y s t e m at 50 ° C u n d e r n i t r o g e n for 5 h o u r s to [4,8]-(-)-fisetinidol-(+)-catechin ( 1 1 ) gave (Scheme 1) the desired r i n g i s o m e r i z a t i o n o f the C - r i n g ( 1 2 ) b u t also e p i m e r i z a t i o n o f the ( + ) - 2 , 3 - i r a n s - c a t e c h i n m o i e t y at C - 2 ( F - r i n g ) t o give the ( + ) - e p i c a t e c h i n isomer ( 1 3 ) . T h e s e p r o d u c t s were a c c o m p a n i e d b y t h e p o s i t i o n a l isomers ( 1 4 ) , ( 1 5 ) , a n d ( 1 6 ) . T h e i s o m e r i z a t i o n c o m p o u n d s are a l l b i f u n c t i o n a l i n the sence t h a t a single h i g h l y reactive center at C - 6 ( D - r i n g ) i n the case o f ( 1 2 ) a n d ( 1 3 ) , or a t C - 8 ( D - r i n g ) for ( 1 4 ) to ( 1 6 ) o f the o r i g i n a l ( + ) - c a t e c h i n u n i t s u r v i v e s , together w i t h the enhanced n u c l e o p h i l i c i t y of the now "free" r e s o r c i n o l Α-ring i n e a c h . However, the c o n t i n u e d d o m i n a n c e of the n u c l e o p h i l i c i t y o f the p h l o r o g l u c i n o l


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Scheme 1

NaHC0 /Na C0 3

2

3

Ring-isomerization and partial inversion at 2-C(F)

(12)

1=1

(13)

!=i

50 °C/3-4 h / N

2

Migration, ring-isomerization and partial inversion at 2-C(F)

Migration and alternative ring-isomerization

J^on


(14)

f=|

(is)

1=4

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D - r i n g t o b u l k y flavanyl electrophiles is d e m o n s t r a t e d (IS) b y s u b s t i t u t i o n of the p h l o b a t a n n i n ( 1 2 ) b y the flavan-3,4-diol ( - h ) - m o l l i s a c a c i d i n u n d e r m i l d l y a c i d c o n d i t i o n s t o give the [4,6]-(-)-fisetinidol derivatives ( 1 7 ) a n d ( 1 8 ) . Differ e n t i a t i o n between the entire range o f b i f l a v a n o i d a n d t r i f l a v a n o i d h o m o l o g u e s was possible o n l y v i a n . O . e . difference spectroscopy o f t h e i r respective m e t h y l ether acetates. T h e m e c h a n i s m b y w h i c h r i n g i s o m e r i z a t i o n proceeds is o b v i o u s , the d r i v i n g force b e i n g the enhanced n u c l e o p h i l i c i t y o f the p h l o r o g l u c i n o l D - r i n g r e l a t i v e t o the r e s o r c i n o l Α-ring or, better, the enhanced s t a b i l i t y of the t r a n s i t i o n state associated w i t h c y c l i z a t i o n of the former. T h e p h l o b a t a n n i n s ( 1 4 ) , ( 1 5 ) , a n d ( 1 6 ) result either v i a m i g r a t i o n of the flavanyl u n i t or (more l i k e l y ) v i a the q u i n o n e m e t h i d e i n t e r m e d i a t e ( 1 9 ) (based o n t h e Ε-ring), w h i c h is c a p a b l e of r o t a t i o n a n d r e c y c l i z a t i o n , thereby s i m u l t a n e o u s l y a c h i e v i n g p o s i t i o n a l a n d c o n f i g u r a t i o n a l i s o m e r i z a t i o n s (12). Recovery of c a . 1 0 % of the s t a r t i n g m a t e r i a l (11) u n d e r the c o n d i t i o n s e m ployed i n d i c a t e s most likely t h a t e q u i l i b r i a exist between i t a n d the v a r i o u s p r o d ucts ( 1 2 ) t o ( 1 6 ) . A l s o , m a i n t e n a n c e of the elevated t e m p e r a t u r e (50 ° C ) over the r e l a t i v e l y p r o l o n g e d p e r i o d (3-4 h ) , a l b e i t u n d e r m i l d l y a l k a l i n e c o n d i t i o n s , i n t r o d u c e s t h e p o s s i b i l i t y of significant side reactions s u c h as the conversion of the (-f)-catechin m o i e t y t o c a t e c h i n i c a c i d (14)W h e r e a s , the above describes those c o n d i t i o n s a p p l i c a b l e t o b i - a n d t r i flavanoid p r o f i s e t i n i d i n p r o t o t y p e s of quebracho t a n n i n s for the f o r m a t i o n of p h l o b a t a n n i n s w i t h the desired " l i b e r a t i o n " of resorcinol u n i t s , m u c h m i l d e r c o n d i t i o n s were f o u n d t o suffice for analogous conversions o f p r o r o b i n e t i n i d i n u n i t s t h a t p r e d o m i n a t e (4) i n m i m o s a e x t r a c t (Scheme 2). T h e r e a c t i o n ( 2 0 ) —• ( 2 1 ) , ( 2 2 ) , a n d ( 2 3 ) r u n s t o c o m p l e t i o n at a m b i e n t t e m p e r a t u r e s (ca. 20 ° C ) , a n d consequently, the c o m p o s i t i o n of the p h l o b a p h e n e m i x t u r e is m u c h s i m p l e r . T h i s is the result of enhanced electron release f r o m the p y r o g a l l o l B - r i n g , t h u s p e r m i t t i n g facile r i n g i s o m e r i z a t i o n o f the C - r i n g , a n d also subsequent i n version at C - 2 ( C - r i n g ) o f the p r o d u c t [(21) —• (23)] u n d e r the m i l d c o n d i t i o n s a p p l i e d . N o significant e p i m e r i z a t i o n of the (-h)-catechin a p p a r e n t l y occurs. O b v i o u s l y , where (-f)-gallocatechin replaces ( + ) - c a t e c h i n as i n a n u m b e r of m i m o s a oligoflavanoids (4), relative c o m p l e x i t y s i m i l a r t o t h a t i l l u s t r a t e d i n Scheme 1 m a y be a n t i c i p a t e d for phlobaphenes d e r i v e d f r o m [4,8]-(-)-robinetinidol-(+)g a l l o c a t e c h i n a n d [4,6:4,8]-bis-[(-)-robinetinidol]-(+)-gallocatechins u n d e r such m i l d c o n d i t i o n s . T h e relative ease of these reactions is f u r t h e r e m p h a s i z e d b y the o b s e r v a t i o n t h a t , under c o m m e r c i a l c o n d i t i o n s of h o t e x t r a c t i o n a n d s u b sequent s p r a y - d r y i n g , a degree of c o m p l e x i t y is i n t r o d u c e d i n m i m o s a e x t r a c t as evident f r o m c h r o m a t o g r a p h i c c o m p a r i s o n w i t h fresh b a r k e x t r a c t o b t a i n e d u n d e r a m b i e n t c o n d i t i o n s . C l o s e r e x a m i n a t i o n o f the c o m m e r c i a l e x t r a c t has revealed ( C r o n j e , Α . ; p e r s o n a l c o m m u n i c a t i o n ) t h a t a low degree of r i n g i s o m e r i z a t i o n o f the t y p e o u t l i n e d i n Scheme 2 is p r e s u m a b l y p a r t i a l l y responsible for the increase i n c o m p l e x i t y . I n order t o assess the p o t e n t i a l of resorcinol "release" v i a r i n g i s o m e r i z a t i o n ,


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Scheme 2

bH NaHC0 /Na C0 3

Ring-isomerization

2

3

OH

(20)

20 ° C / 4 - 5 h / N

2

Migration and ringisomerization

I n v e r s i o n at 2 - C ( C )



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the relative o r i g i n s a n d hence e x t r a c t c o m p o s i t i o n s of quebracho a n d m i m o s a e x t r a c t s m u s t be considered, a p a r t f r o m the r e l a t i v e ease of r i n g i s o m e r i z a t i o n of t h e i r p h e n o l i c components. Q u e b r a c h o e x t r a c t is derived f r o m the heartwoods of Schinopsis s p p . , trees t h a t u s u a l l y reach 80-100 years of age before sufficient h e a r t w o o d f o r m a t i o n p e r m i t s e c o n o m i c c o m m e r c i a l e x p l o i t a t i o n . D u r i n g t h i s l e n g t h y p e r i o d of a g i n g of the h e a r t w o o d , progressive c o n d e n s a t i o n occurs (15), so t h a t i n the process of e x t r a c t i o n , a h o t - w a t e r - s o l u b l e f r a c t i o n is first o b t a i n e d ( " q u e b r a c h o o r d i n a r y " ) , w h i l e the r e m a i n i n g h i g h l y condensed t a n n i n s are s u b j e c t e d t o s u l f i t a t i o n i n order t o p r o m o t e s o l u b i l i t y a n d achieve complete e x t r a c t i o n ("sulfited q u e b r a c h o " ) . T h e p r o p o r t i o n of the m o r e h i g h l y condensed p r o f i s e t i n i d i n u n i t s i n the o r d i n a r y e x t r a c t a n d t h e i r p o t e n t i a l for c o n version v i a r i n g i s o m e r i z a t i o n reactions are u n k n o w n . H o w e v e r , the s t r u c t u r e of four t e t r a m e r i c p r o f i s e t i n i d i n t a n n i n s f r o m the c h e m i c a l l y r e l a t e d h e a r t w o o d e x t r a c t of Rhus lancea ( 7 ) , w h i c h has the c o m p o s i t i o n of m o n o m e r s , d i m e r s , a n d t r i m e r s i d e n t i c a l t o t h a t of quebracho e x t r a c t (6) is k n o w n a n d also the c o n f o r m a t i o n of a derivative of one of these (16). I n these t et r amers, the J K L p r o f i s e t i n i d i n u n i t at least is capable of r i n g i s o m e r i z a t i o n . I n the case of m i m o s a b a r k e x t r a c t , the source of the t a n n i n s is u n i q u e i n t h a t a steady-state c o m p o s i t i o n of oligoflavanoids is m a i n t a i n e d at least u p t o the p o i n t of h a r v e s t i n g ( a n d also b e y o n d ) i n a n 8-year r o t a t i o n cycle. P r o r o b i n e t i n i d i n - c a t e c h i n a n d p r o r o b i n e t i n i d i n - g a l l o c a t e c h i n " d i m e r s " a n d " t r i m e r s " (also t o a l i m i t e d extent p r o f i s e t i n i d i n - c a t e c h i n " d i m e r s " ) (3,4) c o n s t i t u t e « 5 0 % of the e x t r a c t , b u t the c o m p o s i t i o n of higher oligomers t h a t i n c l u d e p r o c y a n i d i n s a n d p r o d e l p h i n i d i n s is u n k n o w n . Conclusions

P r o r o b i n e t i n i d i n s a n d profisetinidins representative of m i m o s a a n d quebracho e x t r a c t s are subject t o facile r i n g i s o m e r i z a t i o n s u n d e r m i l d l y a l k a l i n e c o n d i t i o n s . T h i s provides the first p r o o f of " l i b e r a t i o n " of resorcinol u n i t s f r o m p o l y f l a v a n o i d s present i n these t a n n i n s t o give reactive b i f u n c t i o n a l p h l o t a t a n n i n s , analogous t o the P R F resins [cf. (5,6)] c o m m o n l y used i n cold-set a p p l i c a t i o n s . A n earlier c l a i m b y P i z z i a n d D a l i n g (17) of resorcinol " l i b e r a t i o n " d u r i n g s u l f i t a t i o n is w i t h o u t direct s t r u c t u r a l proof. M o r e o v e r , t h e i r c l a i m e d i n d i r e c t evidence of enhanced r e a c t i v i t y of m i m o s a e x t r a c t i n s u p p o r t of t h i s n o t i o n is based e x c l u s i v e l y o n the adhesive properties of sulfited m i m o s a i n the presence o f large a d m i x t u r e s (27 t o 50%) of r e s o r c i n o l . T h i s effect c o u l d also be a s c r i b e d to the increased accessibility of reactive sites a n d (hence enhanced c r o s s l i n k i n g ) f o l l o w i n g reduced viscosity after degradative s u l f i t a t i o n , c o m p a r e d w i t h the relative r i g i d i t y of higher oligomers w h e n u n s u l f i t e d . However, the i m m e d i a t e prospect of t a n n i n a c t i v a t i o n v i a facile r i n g isomeri z a t i o n a n d w i t h o u t i n t r o d u c t i o n o f s o l u b i l i z i n g sulfonic groups, is the reduct i o n of resorcinol requirements i n those cold-set m i m o s a - r e s o r c i n o l - f o r m a l d e h y d e


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resins t h a t have p r e v i o u s l y been f o u n d effective i n b e a m l a m i n a t i o n a n d fingerj o i n t i n g (2). A m b i e n t c o n d i t i o n s a p p l i c a b l e t o p r o r o b i n e t i n i d i n s s t r o n g l y favor the use o f m i m o s a e x t r a c t , since p h e n o l i c " d e g r a d a t i o n " o f t h e c a t e c h i n i c t y p e (14) a l m o s t c e r t a i n l y accompanies t h e s u s t a i n e d h i g h t e m p e r a t u r e (50 ° C ) r e quirements for p r o f i s e t i n i d i n s representing quebracho e x t r a c t .


Acknowledgment s S u p p o r t b y t h e Sentrale Navorsingsfonds o f t h i s U n i v e r s i t y , t h e F o u n d a t i o n for Research Development, P r e t o r i a , a n d the M a r k e t i n g C o m m i t t e e of the South A f r i c a n W a t t l e I n d u s t r y is acknowledged. Literature Cited 1. Roux, D. G.; Ferreira, D.; Botha, J. J. J. Agric. Food Chem., 1980, 28, 216. 2. Pizzi, Α.; Roux, D .G. J. Appl. Polym. Sci., 1978, 22, 1945; Pizzi, Α.; Rossouw, D. d u T.; Knuffel, W. E.; Singmin, M. Holzforschung und Holzverwertung, 1980, 32, 140; Pizzi, Α.; Scharfetter, H.; Kes, E . W. Holz als Roh und Werkstoff, 1981, 39, 85. 3. Botha, J. J.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1981, 1235. 4. Viviers, P. M.; Botha, J. J.; Ferreira, D.; Roux, D. G.; Saayman, H. M. J. Chem. Soc., Perkin Trans. 1, 1983, 17. 5. Young, D. Α.; Ferreira, D.; Roux, D. G.; Hull, W. E . J. Chem.Soc.,Perkin Trans, 1, 1985, 2529. 6. Viviers, P. M.; Kolodziej, H.; Young, D. Α.; Ferreira, D; Roux, D. G. J. Chem. Soc., Perkin Trans, 1, 1983, 2555. 7. Young, D. Α.; Kolodziej, H.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin 1, 1985, 2537.

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8. Delcour, J. Α.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1983, 1711; Delcour, J. Α.; Serneels, E. J.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1985, 669. 9. Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1985, 669. 10. Kolodziej, H.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1984, 343. 11. Steenkamp, J. Α.; Steynberg, J. P.; Brandt, Ε . V.; Ferreira, D.; Roux, D. G. J. Chem. Soc., Chem. Commun., 1985, 1678. 12. Young, D. Α.; Cronje, Α.; Botes, A. L.; Ferreira, D.; Roux, D. G. J. Chem. Soc., Perkin Trans. 1„ 1985, 2521. 13. Steynberg, J. P.; Young, D. Α.; Burger, J.F.W.; Ferreira, D.; Roux, D. G. J. Chem. Soc., Chem. Commun., 1986, 1013. 14. Freudenberg, K.; Purrman, L. Chem. Ber., 1923, 56, 1185; Liebigs Ann. Chem., 1924, 437, 274. 15. Sears, K. D.; Casebier, R. L.; Hergert, H. L.; Stout, G. L.; McCandlish, L. E. J. Org. Chem., 1974, 39, 3244; Kiatgrajai, P.; Wellons, J. D.; Gollob, L.; White, J. D. J. Org. Chem., 1982, 47, 2910. 16. Roux, D. G.; Evelyn, S. R. Biochem. J., 1958, 70, 344. 17. Brandt, Ε . V.; Young, D. Α.; Kolodziej, H.; Ferreira, D.; Roux, D. G. J. Chem. Soc., Chem. Commun., 1986, 913; Brandt, Ε . V.; Young, D. Α.; Ferreira, D.; Roux, D. G. J. Chem.Soc.,Perkin Trans. 1, 1987, 2353. 18. Pizzi, Α.; Daling, G.M.E. J. Appl. Polym. Sci., 1980, 25, 1039. R E C E I V E D September 29, 1 9 8 8


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