LEONARD J ~ R Di-O-methyl-3,3 ’-di-0-benzylellagic acid separated in colorless needles, n1.p. 239-241”. Anal. Calcd. for CaoH2208: C, 70.5; H, 4.35; 2 Me&, 12.2. Found: C, 71.8; H, 4.45; MeO-, 10.0. The crude di-0-methyl-di-0-benzylellagic acid (0.7 g. ] was suspended in acetic anhydride (20.0 ml.). Concentrated sulfuric acid (1.0 ml.) was added and the solution was heated on a steam-bath for 2 hours. It was allowed to stand a t room temperature for 24 hours, diluted with water and filtered. T h e solid thus obtained was washed with acetone and recrystallized from dioxane-methanol. 4,4‘Di-0-methylellagic acid diacetate thus was obtained in colorless needles, m.p. 324’ (0.4 g,). Anal. Calcd. for C20H14010:C, 57.95; H, 3.41; 2 MeO-, 15.0; 2 CHaCO-, 20.8. Found: C, 58.1; H , 3.42; MeO-, 11.7; CHICO-, 21.6.
Vol. 81
D
T h e diacetate was hydrolyzed by heating it with dioxane (10.0 ml.), methanol (10 ml.) a n d 10% aqueous sodium hydroxide (5.0 ml.) for 10 minutes. After dilution with water a n d acidification the solid product mas collected and recrystallized from a large volume of N,N-dimethylform.Imide. 4,4’-Di-O-meth>-lellagic acid was thereby obtained in cnlorless needles, m.p. >360° (lit.4m.p. > 320’).
Anal. Calcd. for C~eHloOs: C, 58.2; H , 3.27; 3 MeO-, 18.8. Found: C, 58.0; H, 3.22; MeO-, 18.2.
Acknowledgment.-The author is indebted to L. A I . \X7hite for performing the elementary analyses. PASADENA, CALIF,
[ COXTRIBUTED FROM THE FRUIT A S D VEGETABLE CHEMISTRY LABORATORY, h LABORATORY O F THE W E S T E R S UTILIZATIOX
c.s. DEPARTMENT O F AGRICULTURE] Plant Polyphenols. VII. The Structure of Ellagorubin’
RESEARCH AND DEVELOPMEST DIVISIOX,-1GRlCULTURAL RESEARCH SERVICE,
BY LEOXARD JURD RECEIVED JAXUARY 20, 1959 Hydrolysis of di-0-methylellagorubin gives a di-O-methyl-5,5‘-di-C-benzylellagic acid I :( j . Hydrolysis of di-0-benzylellagorubin forms the corresponding di-O-ben~~.l-~,5’-di-C-benzylellagic acid. Methylation and subsequent debenzylation of this gives a di-O-methyl-5,5’-di-C-benzylellagic acid ( B ) . Comparison of the ultraviolet spectra of these ethers, (.\) and ( B ) , with the spectra of synthetic 3,3’-di-O-methylellagic acid (IT’) and 4,4’-di-O-methylellagic acid (V)establishes the constitution uf (A\)as 3,3’-di-0-metli~l-~,5‘-di-C-benzylellagic acid ( X ) and of ( B ) as 4,4 -di-O-methyl-5,5’-di-C-benzylellagic acid ( S I ) . From these data it follows that ellagorubin has the structure S I 1 and not I1 as previously reported.
The widespread distribution of ellagic acid derivatives in the plant has resulted in a considerable current interest in the chemistry of this acid.5-7 The extensive investigations of Schmidt and his co-workers a t Heidelberg are particularly significant? Ellagic acid is a polyphenolic dilactone (I, R = H) which reacts normally with benzyl chloride in acetophenone to give the colorless tetra-0-benzyl derivative ( I , K = C G H ~ C H ~ - In ) . ~aqueous alkali, however, Schmidt, Voigt and Bernauer’O found that ellagic acid and benzyl chloride react to form a deep red pigment. ellagorubin, for which they proposed the quinoidal structure I I.
Because of the novel nature of this benzylation product, Schmidt’s work has been extended in this (1) Financial support fur this work \\.a. provided b y t h e Diamond Walnut Growers, I n c . ( 2 ) A . G. Perkin a n d 31. Sierenqtein. .i, < ‘ h e i ? i . .Cor. !7’i,or7c ) , 81, 1112 (1905). (3) E. C. Rate-Smith. C ‘ h e i i i t s l i ~ yir I i z d u / r y , K :3’2 ( 1 9 X ) . (4) 0. T. Schmidt and W. S l a y e r , A i i g e w . C h r m . . 68, 103 (19:li). ( 5 ) D. E. H a t h w a y , S a t u ~ e 177, , 747 (l95fi). (6) D. E. H a t h w a y , Biochrin. J., 67, 445 (1957). (7) D . E. H a t h w a y , J . CIieiii. Soc., ,519 (1957). (8) 0. T. Schmidt, E. Komarek and H. Kent&, A n n . , 602, 50 (1957), a n d previous papers in this series. (9) 0 . T. Schmidt, H. Voigt, W. Puff and 11. Kiister, ibid., 686. 165 (1954). ( I O ) 0. 1‘. Schmidt, H. Voigt and K . Bernauer, C h e m . Rw.,88, 91 (19\55)
Laboratory. It already has been reported11 that, although the benzylation of ellagic acid gives ellagorubin under the conditions described by Schmidt, the presence of small amounts of pyridine inhibits the formation of ellagorubin and produces the colorless compound, 5,5’-di-C-benzyl-tetra0-benzylellagic acid (111, R = CoH5CHz-), together with a small quantity of a yellow pigment which is partially quinoidal and partially aromatic. In the process of identifying the ellagorubin formed in these reactions its dimethyl ether was hydrolyzed acid to the di-O-methyl-5,5’-di-C-benzylellagic described by Schmidt and his co-workers as the 1,4’-di-O-methyl derivative XI. The ultraviolet spectra of this dimethylellagic acid derivative in various media, however, could not be satisfactorily accounted for on the basis of the orientation of methoxyl and hydroxyl groups suggested by these authors. The reactions of ellagorubin have therefore been re-examined. I n this paper chemical evidence and ultraviolet spectral data are presented which establish structure XI1 for ellagorubin. In the following paper’? structure XII, but not structure 11, is shown to be compatible with infrared and nuclear magnetic resonance spectral data.
R? RO*O\CO CoHSCH,+CH~GH,
oc,o+OR .~~ ~
I11 OR ~
L. J u r d , TKISJ O U R N A I . , IS. 604.3 (1R57). (12) P a r t V l I I , F. S t i t t , E. Gong, K . J. Palmer and J . N. Shoolery, ihiif.. 81, 4fi15 (19.59). (11) P a r t 11,
Sept. 5, 19;i9
STRUCTURE OF ELLAGORURIK
4611
Ellagorubin, C4?HsoOe,forms a dimethyl ether and a diacetate and therefore contains two free Ho% Me0% hydroxyl groups. It differs from ellagic acid, C14H608, in that four hydrogen atoms of the latter have been replaced by benzyl groups. Two of / OH OC0 OMe ‘0 these benzyl groups are extremely labile. Thus OMe catalytic hydrogenation or acid hydrolysis of v OH IV ellagorubin and ellagorubin dimethyl ether results in the loss of two benzyl groups and the formation Ultraviolet Spectra in Ethanol.-In ethanol the of 5,5’-di-C-benzylellagic acid (111, R = H) and ultraviolet spectra of these ellagic acid derivatives its di-0-methyl derivative, respectively. It is showed characteristic differences. The spectrum clear, therefore, that the conversion of the almost Amax, mp colorless ellagic acid into the red ellagorubin does Ellagic acid 255 Xm.,,mr 35ZQ 3R(i 5,:’-Di-C-benzylnot involve an oxidation process but is due to the ellagic acid 256 303 378 stabilization of a potentially tautomeric form of IV 248 3 5 g a 372 A 249 371 387 V 253 3 l l i ” 3lil I\ 2.52 34;7 :15H ellagic acid by replacement of the active hydrogen Inflection. atoms by benzyl groups. The production of 5,5’di-C-benzylellagic acid proves that a t least two of IV relative to ellagic acid is closely similar to that of these are C-benzyl groups. The ease of hydro- of A relative to 5j5’-di-C-benzylellagic acid; ie., genation and hydrolysis of the two remaining the conversion of ellagic acid into 3,3’-di-Obenzyl groups (with subsequent rearrangement to methylellagic acid (IV) and of 5,5’-di-C-benzylthe phenolic lactone form I of ellagic acid) led ellagic acid into 9results in a considerable hypsoSchmidt, et al., to assume that these benzyl groups chromic shift (7 m p ) of the low wave length band were attached to oxygen. If this assumption is and a considerable bathochromic shift (6-9 mp) of valid then the only formula possible for ellagorubin the long wave length band in each case. On the is that which they proposed, aiz., 11. I t follows other hand, the conversion of ellagic acid into 4,4’that the di-O-methyl-5,5’-di-C-benzylellagic acid di-0-methylellagic acid (V) and of 5,5’-di-Cobtained from ellagorubin dimethyl ether must then benzylellagic acid into B results in a hypsochromic be 4,4’-di-O-methyl-5,5’-di-C-benzylellagic acid shift (3-4 mp) of the low wave length band and a (XI). This structure, however, was not verified hypsochromic shift (5-19 m p ) of the long wave in any other way. length band. To establish the constitution of ellagorubin it is Spectra in Sodium Acetate.-In previous studies clearly necessary to confirm the structure of the on flavonoid compounds14it has been shown that acid. El- sodium acetate selectively ionizes the more acidic above di-O-niethyl-5,5’-di-C-benzylellagic lagorubin dimethyl ether was therefore hydrolyzed phenolic groups of polyphenols, e.g., those which are by Schmidt’s processlo to give this di-0-methylconjugated with a carbonyl group. Unconjugated On hydroxyls ellagic acid derivative (A), m.p. 317-315’. are not normally sufficiently acidic to acetylation, =1 formed a diacetate, m.p. 294be ionized by sodium acetate. The 3,3’-hydroxyls 295”, which also was obtained directly from el- of ellagic acid and 5,5’-di-C-benzylellagic acid are lagorubin dimethyl ether by reaction with acetic conjugated with the carbonyl groups of the lactone anhydride and sulfuric acid. Benzylation of A rings. Addition of sodium acetate to alcoholic gave a di-O-benzyl-di-O-methyl-5,5/-di-C-benzylsolutions of those acids selectively ionizes the ellagic acid, m.p. 240”. 3,3‘-hydroxyls (VI), therefore, and results in a Attempts to benzylate ellagorubin in acetone significant alteration in the spectra of the acids solution with benzyl chloride and potassium carbonate were unsuccessful. However i t was later (Fig. l), the low wave length band being characterfound that in the presence of large quantities of istically divided into two bands a t 256 and 278 potassium iodide ellagorubin reacted smoothly to mp. It would be anticipated that the spectrum give ellagorubin dibenzyl ether, m.p. 217’. Mild acid hydrolysis of the ellagorubin dibenzyl ether HO% converted i t into a di-O-benzyl-5,5‘-di-C-benzylellagic acid, m.p. 315-320”. This formed a diacetate, m.p. 292”, which also was obtained directly OCxo OH from ellagorubin dibenzyl ether on brief treatment 0with acetic anhydride and sulfuric acid. MethylaVI tion of the di-0-benzylellagic acid then gave a diO-benzyl-di-O-methyl-5,5’-di-C-benzylellagic acid, of 3,3’-di-O-methylellagic acid (IV) would not be m.p. 212’, which was debenzylated to yield a di- appreciably affected by sodium acetate while the O-methyl-5,5’-di-C-benzylellagicacid (B), m.p. 4,4’-di-O-methylellagic acid would be ionized and 360” (diacetate, m.p. 325”). Thus the orientation show the same division of the low wave length of methoxy and hydroxy groups in B is the reverse band as ellagic acid. This was found to be the of that in A. The orientations of these groups in case. The spectra of 3,3’-di-O-methylellagic acid A and B were then determined by direct comparison (IV) and of d were not appreciably affected on the of their spectra with those of synthetic13 3,3’-di-0- addition of sodium acetate’: (Fig. 2) while those of 4, methylellagic acid (IV) and 4,4’-di-O-methylel(14) L. Jurd a n d R. M. Horowitz, J . Org. Chem., 22, 1618 (1957). lagic acid (V). (15) Although sodium acetate does not essentially alter t h e ultra/ ,
(131 P a r t VI, L. J u r d . THISJ O U R X A L ,81, 4606 (19S9).
violet spectra uf I V and A , t h e solutions of these compounds in alcoholic
LEONARDJURD
4612
Vol. 81
A
\
I
1
I
250
300
350
400
mtL.
mtL*
Fig. 1.--Ultraviolet absorption spectra in ethanol of: ( A ) , ellagic acid; (B) - - -, ellagic acid sodium acetate; (C) - --, 5,5’-di-C-benzylellagic acid; ( D ) - -, 5,5’-di-C-benzylellagic acid sodium acetate.
Fig. 2.--Ultraviolet absorption spectra in ethanol of: (A) ____ , 3,3’-di-O-methylellagic acid (IV); ( B ) - - - -, I V 4- sodium acetate; ( C ) - --, product A ; ( D ) -. -, product A sodium acetate.
4’-di-O-methyleIlagic acid (Y)and B showed the characteristic division into two peaks a t approximately 255 and 280 mp (Fig. 3). Spectra in 0.02 AI Sodium Ethylate.-Sodium ethylate normally ionizes all free phenolic groups. Since the 3,3‘-hydroxyl groups of 4,4‘-di-O-methylellagic acid (V) and of B are ionized by sodium acetate it was expected and found (Fig. 3) that these compounds have identical spectra in either sodium acetate or sodium ethylate solutions. Unlike sodium acetate, however, sodium ethylate ionizes the 4,4‘-hydroxyIs of 3,3’-di-O-methylellagic acid (IV) and of A; IV and A produce intensely yellow solutions in sodium ethylate with strong absorption bands a t 438 and 460 mp, respectively; i . e . , the long wave length bands of
IV and A show bathochromic shifts of 66 and 73 mp, respectively (Fig. 4). It is noteworthy that the solutions of V and B in sodium ethylate are almost colorless whereas those of IV and A are yellow. The intense color of A in sodium ethylate provides further strong evidence of the existence of 4,4’-hydroxyl groups since ionization of these hydroxyl groups (VII) would give rise to two (identical) extended quinoidal resonance forms as VIII, whereas ionization of B produces only one completely quinoidal resonance form (IX) and in this form the chromophores are separate.
+
-
+
-o%
-0% Ale0
o”c‘o 1’11
.,o.
0Me
Am.ax,
IV
248
IV
245
Diacetateof A
249 247
A
VI I I
IX
Spectra of Acetates.-Acetylation of the ellagic acid derivatives significantly alters their spectra. Diacetateof
-O/c‘o
--/
+
-..
0 OlLIe
0
sodium acetate become slightly yellow and develop a low intensity absorption band above 400 mp. T h i s is due t o some ionization of t h e 4,4’hydroxyls.
mr
359 333 371
315
372 317 387 302
A,,.
v
253
Diacetate of V B Diacetate of B
239
252 23s
mp 361 354 368 345 359 333 371 34(i
Acetylation of 3,3’-di-O-methylellagic acid (IV) and of A has an identical effect on their spectra; it causes hypsochromic shifts of 2 ( 3 ) , 26, 25 mp of their respective bands. The spectra of 4,4’di-0-methylellagic acid (V) and of B are also affected similarly on acetylation; it produces a hypsochromic shift (14 mp) of the low wave length band, a bathochromic shift (12, 11 mp) of the intermediate band and a bathochromic shift (7, 12 mp) of the long wave length band. It is apparent from these spectral data that the orientation of methoxyl and hydroxyl groups in -4 and B is identical with that in IV and V, respectively; A, therefore, has the structure X and B the structure XI. The quinoidal oxygen atoms
,
4613
STRUCTURE OF ELLAGORUBIN
1959
260
300
380
340
420
460
500
Fig. 4.--Ultraviolet absorption spectra in 0.002 M sodium ethylate of: ( A ) , 3,3'-di-O-methylellagic acid; ( B ) ---, product A.
ellagic acid (111,R = C6H6CH2-), formed together with the ellagorubin in the benzylation of ellagic acid. 0
OXa
Fig. 3.-Ultraviolet absorption spectra in ethanol of: (A), , product B ; ( B ) ---, product B sodium acetate; ( C ) - - -, 4,4'-di-O-methylellagic acid (V); ( D ) _ . .- , V sodium acetate.
+
M e b h I e
+
in ellagorubin, consequently, are located in the 4,4'-positions and not in the 3,3'-positions as pro-
x
OMe
V6HiCH,
OH
XI
/
OMe
OH
Xv
XI\'
CHzCeH, CHzCeH;
Me@
It already has been pointed out that Schmidt and his co-workers assumed that the labile benzyl groups in ellagorubin were 0-benzyl groups since the facile cleavage of such linkages is well known. The structure XI1 now assigned t o ellagorubin indicates that a C-benzyl linkage in the grouping XVI must be cleaved as readily as an 0-benzyl linkage. This may be accounted for by the mechanism OOCY:C&
' OC,~
XI11
OCH&,H,
CH,C,H,
L I e b M e
\ 6+
XVI
a
CH2CtjH5 d
H'
CH~CBH~
Q+cH,c,H_ OH
X-
+
OH
XCH,C,H,
Cleavage of such C-benzyl linkages does not seem to have been described previously. It is hoped, therefore, to prepare and study the reactions of a number of model C-dibenzyl compounds a t a future HO date. I n the course of this investigation an interesting reaction involving the addition of methanol to the conjugated system of ellagorubin was found. Ellagorubin dimethyl ether (XVII, R = Me) warmed for a short time with a solution of sodium XI1 OH methoxide in methanol adds the elements of two I t is of interest to note that Curtin, Crawford molecules of methanol to form an almost colorless, and \Vilhelm16 recently found that the benzylation crystalline addition product (XVIII, R = Me), of the sodium salt of 2,6-dimethylphenol (XIII) C46H42010, m.p. 159". Ellagorubin dibenzyl ether gives both XIV and XV. The hexadienone system (XVII, R = CaH&H2-) similarly forms an addiin XIV is identical with that in ellagorubin (XII). tion product, CssHmO~o,m.p. 179-180' (XVIII, Furthermore, the ether XV corresponds to the R = C~H~CHS-).These products are not esters colorless product, 5,5'-di-C-benzyltetra-O-benzyl-formed by the opening of the lactone rings since they can be recovered unchanged after prolonged (16) D. Y . Curtin, K.J. Crawford and M. Wilhelm, THISJOURNAL, treatment with aqueous alkali. Catalytic hydro80, 1381 (1958). posed by Schmidt. Ellagorubin must, therefore have the structure XII.
4614
LEONARDJURL)
Vol. 81
( 5 1) g , ) , benzyl chloride (2.0 ml.) and acetone (80 1111.) was refluxed for 17 hours. The solid ( A ) was collected. T h e acetone filtrate was evaporated to a n oil which was suspended in hexane thereby giving a solid. This was combined with the solid ( A ) and suspended in water (100 m l , ) . The undissolved solid was collected and recrystallized from benzenehexane. 3,3'-Di-0-methyl-4,4'-di-O-henz~-l-5,5'-di-C-he11zylellagic acid was thus obtained in colorless needles, m.p. 240a. A n u l . Calcd. for C,,H,,O.: C, 7ti.3; 13, 4.96; 2 MeO-, 8.99. Found: C, 76.7; H , 3.03; MeO-, 8.81. Action of Acetic Anhydride and Sulfuric Acid on Di-Omethylellagorubin.--;\ mixture of di-O-methylellagor~ibi~~ (0.05 g.), acetic anhl-dride (2.0 m l . ) and concentrated sulfuric acid (2 drops) was hented on a steam-hat11 for 1 0 RO minutes. A'ater was added and the solid product was collected. Recrystallized froin d i o s a ~ i e - ~ n e t ~ l a.'3,8'-dill~~~, O-methyl-5,5'-di-C-benz~-lellagic acid diacetate, i i i . p , and mixed 1n.p. TTith above diacetate 394-29j3. w i 3 thus obtained. d ? d . CdCd flJr C~rHmO!a: c, 68.7; H , -4.41; 2 lICo--, 10.4; 2 CH3CO-, 11.5. Found: C, 68.3; I f , 4.72; 5IeO-, 10.1; CHICO-, 13.4. Di-0-benzylel1agorubin.--.\ inixturc nf finely powdered ellagorubin (2.0 g ), anhydrous potassium carbonate ( 10 g.), potassium iodide (10 g , ) , benz1.l chloride (10.0 nil.) and dry acetone (120 m1.l was heated under reflux for 30 hours. The undissolved solids were collected arid suspended iii water (300 1111.) thcreby giving an orange-red cr)-stallinc: solid ( A ) . The :icetone filtrate \\-\-as evaporated to an i d . \vas added and the solution was cooled. of orange-red crystalline solid was thcreby s combined with A and recrystallized froiii chloroform-hcxaiie. I)i-0-henz~lellagorubitl separated in orange-red prisms, 111.p. 2 1 7 O , which did not dissolve i n aqueous sodium hydroxide (2.1 g . ) . Anal. Calcd. for Ci6HllO3: C , 79.8; H , Z.O;j. F i ~ u n d : C, 79.5; H ,
