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Establishment of Configurations for the Two Diastereomeric C-Glucosylanthrones Aloin A and Aloin B.

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Establishment of Configurations
for the Two Diastereomeric C-Glucosylanthrones
Aloin A and Aloin B
By Hans K Rauwuld,* Karsten Lohse, and Jan W Bats
Aloin [lo-(P-D-glucopyranosyl)-l ,8-dihydroxy-3-hydroxymethyl-9( 1OH)-anthracenone] is the principal component
and principal active ingredient in the world-wide used pharmaceutical agents Aloe burbadensis and Aloe rapensis. It is
known to be present in at least 68 aloe species.”] Aloin was
first isolated in 1851,[’] but the correct molecular formula
was not established until 1945,[31and only in 1952 was the
compound shown to be a C-glucosylanthrone derivative.I41
The suggestion that aloin might exist in stereoisomeric
was confirmed by HPLC separation into the two
diastereomers aloin B (1) and aloin A (2) in 1979.[5961Partial
synthesis“] and ‘H NMR a n a l y ~ i s ”proved
~
that the anthrone system is attached @ with respect to the D-glucosyl
residues in both aloins. It follows that 1 and 2 must differ
0
?H
2
1
.
CH20H
only with respect to the configuration at CIO itself. Recent
exhaustive studies have demonstrated that a configurational
assignment like that described
and based on ‘H N M R
data is not possible with the aloins because spectra of the two
compounds show no significant differences in either chemical shifts or coupling constants.[’] It thus became necessary
to establish the configurations by X-ray analysis.”] Since
aloin A and aloin B are readily separable in quantity by
droplet countercurrent chromatography (DCCC),’81 largescale crystallization experiments were feasible with both. The
work had to be performed quickly, however, since the aloins
are unstable in solution, where they are subject to interconversion via the tautomeric anthranol. Experiments with
aloin A 2 resulted in needle-shaped crystals, but these were
too slender to utilize for X-ray analysis. By contrast, aloin B
1 gave crystals that were quite adequate in size. In vivo
experinients on the biosynthesis of the aloins have shown
that 1 is actively synthesized by plants, whereas the
isomer 2 arises in a secondary way through transformations
of 1.[91 Structural analysis on crystalline I provided unanibiguous proof of the configurations at C10 for both
diastereomers.
Figure 1 shows the molecular structure[‘*] of 1 in the
crystalline state. Since the glucose is bound to the anthrone
system as a @-D-glUCOSylresidue, the absolute configuration
of I can be assigned as 10R, 1’s.The glucosyl residue has a
chair conformation in which all substituents are equatorial
03’v
Fig. 1 . Perspective drawing of the molecular structure of crystalline aloin B.
and all hydrogen atoms are axial. The A and C rings in the
1.8-dihydroxyanthrone system may be described as planar
within the limits of experimental accuracy. Ring 8, on the
other hand, has a “sofa” or “envelope” conformation, in
which C10 is raised out of the plane of the other five ring
atoms by 0.29(2) A. The glucopyranosyl residue at C10 is in
axial, the hydrogen atom in equatorial position. The nonplanarity of ring B results in an angle of 16(2)’ between rings A
and C. This nonplanarity of the anthrone skeleton is a consequence of substitution at C10: unsubstituted I&dihydroxyanthrone itself is planar.“ ‘I The observed deviation from
planarity in 1,1’,8,8’-tetrahydroxy-lO.lO‘-bis-anthrone
corresponds to that in aloin B 1[lZ1.The torsion angles about
the CIO-C1’ bond [C14-C10-C1’-05’: -79(2)”; C11-C10-
[*] Prof. Dr. H. W. Rauwdd. K. Lohse
Institut fur Pharmazeutische Biologie der Universitht
Georg-Voigt-Strdsse 16. D-6000 Frankfurt a m Main 11 (FRG)
Dr. J. W.Bats
Institut fur Orgdnische Chemie der Universitht
Niederurseler Hang, D-6000 Frankfurt am Main 50 (FRG)
1528
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VCH Verlu~gesellschufrmhH, D-6940 Weinheim. I989
057(1-(i833/89il
Ill-I528$OZ.SOjO
Angel$..Chem. h i .
Ed. Engl. 28 ( 1989) N o . I I
C1'-05': 48(2)"; HlO-C10-C1'-05':158"] reveal deviations
from the staggered arrangement of about 15", which may be
a result of repulsion between H5 and H2'.
The crystal packing (Fig. 2) shows channels at least 4 8, in
diameter along the c axis, where no atoms were found. The
volume defined by such a channel is sufficient to permit
incorporation of at least five positionally disordered water
molecules. The resulting disorder within the channel is transmitted to some extent to adjacent atoms in the aloin B molecule (C15.C3',02',03', and 04').
Establishment of the configuration at C10 of aloin B as R
simultaneously defines the configuration of aloin A
(1OS.l'S). Since the two substances may be regarded as parent structures for other naturally occuring diastereomeric
10-glucosylanthrones they should prove useful for making
future comparisons within this class of compounds.
scribed as "probably the most difficult structure proof to
that point accomplished"."b1 The existence of chelate complexes ML, 1 ( M = N i 2 @ ,Co2@,Cu2@,ZnZ@;L = m onoanion of indigo and certain derivatives) was demonstrated
~ ]far as we
by Kunz''] as well as by Kuhn and M a c h ~ m e r . [So
are aware, however, little attention has been paid to these
complexes since 1930.[41
The complexes I show little solubility in organic solvents.
Despite numerous attempts we were unable to prepare crystals that were suitable for X-ray analysis. The synthesis of
soluble, crystalline indigo complexes was finally accomplished by introduction of the ligand tri(n-buty1)phosphane.
Green chelate complexes 2 and 3, respectively, result from
Bu,P
\ /C'
Received: June 7, 1989 [Z 3382 IE]
German version: Angiw. Chm7. 101 (1989) 1539
111 Q. J. Groom. T. Reynolds. Plunro Mid. 53 (1987) 345- 348.
[2] T. Smith H. Smith. Phurn7. J 11 (1851) 23.
[4R. Eder. W. Zinn. Phurni. Acru H e h . 20 (1945) 410. 485.
141 H . Muhlemann. Phurm. Acru H r h . 27(1952) 17.
[ 5 ) M. Grun, G. Franz. Phurmaric. 34 (1979) 669.
I61 H. Auterhoff. E. Graf, G. Eurisch. M. Alexa, Arch. Phurm3/3 (Wcinhaim,
(1980) 113.
[7] H . W. Rauwald, K . Roth. Arch. Phurm. (Wc+iheim, Ger.) 3/7(1984) 362.
[8] H. W. Rauwald. Arch. Phurm. lWernh<,;m, Grr.) 315 (1982) 769.
191 M Grun. G. Frdnz, Arch. Phurm. (Weinheirn, G w . ) 315 (1982) 231 -241
[lo] Detail!, related t o the crystal structure analysis of aloin B I ( C 2 , H 2 2 0 9 ) :
yellow-brown. trigonal crystals from water/methanol, space group P321,
u = 17.67(1). c = 11.381(4)& V = 3079(5)A3. % = 6. Enraf-NoniusCAD4 diffr;ictometer. Cu,, radiation. 1474 independent reflections t o
2 0 = 110 very broad reflections, (0 scan with A(o = 9 R ( f ) = 0.164,
R,(I.) = 0.146 for 1187 reflections with I > ~ ( 0
structure
.
determination
by direct methods (SHELXS-86). hydrogen atoms o n C-atoms calculated,
those o n 0 - a t o m s not locaiized. S D P program system. The high R values
are a consequence of disordered solvent. The residual density is less than
0.6 e k' Further details of the crystal structure investigation are
available on request from the Fachinformationszentrum Karlsruhe.
Gesellschaft fur wissenschaftlich-technische Information mbH, D-7514
Eggcnstein-Leopoldshafen 2 (FRG). o n quoting the depository number
CSD-53855. the names of the authors, and the journal citation.
[I 11 F. R . Ahmed. Acru Crysfallo~r.B36 (1980) 3184.
1121 M. Whitefeld, K . Henrich. P. G. Owston. Actu Crwullogr. B38 (1982)
1248: E R. Ahmed. G . A. Neville. ihid. E38 (1982) 2930.
.
By Wolf&rng Beck *, Christoph Schmidt, Rolf Wienold,
Manfred Steimunn, and Barbara Wagner
Dedicated t o Professor Wolfgang Liittke on the occasion
of his 70th birthdq
It was Ado// von Bueyer, who, in Munich in 1883, firmly
established the structure of indigo""] in what has been de[*) Prof. Dr. w. Beck. Dip1:Chem. C. Schmidt, R. Wienold, M. Steimann [ '1.
Dip1:Chem
B. Wagner [ '1
lnstitut I'ur Anorganische Chemie der Universitit
Meiserstralk 1, D-8000 Munchen 2
X-ray structure analysis
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. We thank Prof. W Liifrkc.
Gottingen. for valuable suggestions and encouragement; H. Srhiinmunn.
Consortium fur elektrochemische Industrie GmbH, Munchen. for "Computer Molecular Modeling"; and BASF AG, Ludwigshafen, for chemicals.
Angew. Chrm. In!. Ed. Enxl. 28 ( t 9 8 9 ) N o . I 1
:M=Pd
2b:M=Fi
.
Indigo-Metal Complexes: Synthesis and Structure
of Pd" and Pt" Compounds Containing
the Anions of Indigo and Octahydroindigo
as Mono- and Bis-Chelate Ligands **
[ '1
[**I
2a
$3 VCH
PBUS
3
treatment of chlorine-bridged palladium(n) and platinum(ir)
compounds [(nBu,P)(Cl)M(p-CI)], ( M = Pd,Pt) with indigo
or octahydroindigo in THF in the presence of sodium hydride o r silver acetate.
Absorption bands in the UVjVIS spectrum of the complexes 2a and 3 show bathochromic shifts relative to those of
indigo itself.[51According to X-ray structural analyses, the
atoms palladium, imide nitrogen, and keto oxygen in 2 a
(Fig. 1) and 3 (Fig. 2) lie in nearly planar six-membered rings
(average deviation from planarity: 11 and 13 pm in 2a and
3, respectively). The monoanion of indigo functions as a
monochelate ligand in 2a, the dianion of octahydroindigo as
a bischelate ligand in 3. The four ligdnd atoms together with
the central Pd deviate from planarity up to 6 pm (2a) and
30 pm (3), respectively. The two planar five-membered
chelate rings are twisted out of the plane of the central C = C
bond by 9.6" (2a) and 15" (3). Replacement of the bridging
hydrogen atoms in indigof"' by Pd producei virtually no
change in the bond lengths in the five-membered rings o r in
the coordinated carbonyl groups (Table 1). The same is true
of a structurally related Pd" chelate complex of an a,P-ketoiminate.['I
It is interesting to note that the bond of the free carbonyl
group in 2a [C2-01 122.4(9) pm] is shorter than those of the
coordinated carbonyl groups in 2a, 3, and indigo itself. This
suggests that the inter- and intramolecular N - H...O bridging in free
has been disrupted. Crystals of 2 a con-
Verlagsgesel~.~chuJr
mbH, 0-6940 Weinheim, 1989
0570-OR33/89/i111-15298 02.SO/O
1529
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two, aloin, glucosylanthrone, configuration, diastereomeric, establishments
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