The absolute configuration of polycephalin C from the slime mold Physarum polycephalum (Myxomycetes)

Article abstract of DOI:10.1016/S0040-4020(02)01051-7

The absolute configuration of polycephalin C (1) isolated from plasmodia of the slime mold Physarum polycephalum is supposed to be 3″R,4″R as found by comparison of CD spectra of the natural product polycephalin C (1) and CD spectra of synthetic bis-benzo

Full text of DOI:10.1016/S0040-4020(02)01051-7

Tetrahedron 58 (2002) 8433–8437
The absolute configuration of polycephalin C from the slime mold
Physarum polycephalum (Myxomycetes)
*
Florian Blumenthal, Kurt Polborn and Bert Steffan
¨
Department Chemie der Ludwig-Maximilians-Universitat, Butenandtstr. 5-13, Haus F, 81377 Munich, Germany
Received 22 April 2002; revised 13 August 2002; accepted 20 August 2002
Abstract—The absolute configuration of polycephalin C (1) isolated from plasmodia of the slime mold Physarum polycephalum is supposed
to be 300R,400R as found by comparison of CD spectra of the natural product polycephalin C (1) and CD spectra of synthetic bis-benzoates (2)-
4 and (þ)-4. An X-ray structure of the corresponding bis-camphanate (þ)-3 shows that the exciton chirality method may not be applied on
these 1,4-diols in the usual manner. q 2002 Published by Elsevier Science Ltd.
1. Introduction
Possible configurations at00the branchings in the cyclohexene
ring of 1 are 300R,400R or 3 S,400S. Because of the complexity
of polycephalin C (1) the diols 2 were used for the
determination of the absolute configuration. By comparison
of the CD spectra of suitable derivatives and the natural
compound a conclusion on the absolute configuration of 1
should become possible by application of the exciton
chirality method (Scheme 1).3
Recently we reported about the isolation and structure
elucidation of the tetramic acid polycephalin C (1) from
plasmodia of the slime mold Physarum polycephalum.1 This
interesting compound is described by the formula
C32H36N2O8 and consists of N-methyltetramic acid units
that are linked by triene chains, which lead vicinally to a
cyclohexene0 ring. The absolute configuration at the atoms
C-5 and C-5 of the two tetramic acid units was determined
through comparison of CD spectra of the hydrogenated
natural compound and a synthetic tetramic acid.2 Whereas
the trans-configuration of the polyene chains was found by
comparison of coupling constants in cis- and trans-3,4-di-
(hydroxymethyl)cyclohexene (2) and the natural product 1,
the absolute configuration at the atoms C-300 and C-400 of the
cyclohexene remained unknown.
2. Results
trans-3,4-Di-(hydroxymethyl)cyclohexene (2) was syn-
thesized according to the literature4 by Diels–Alder
reaction of 2,4-pentadienol with methyl acrylate followed
by reduction with LiAlH4 and is obtained as a racemate after
separation of the isomers.
Scheme 1. Structures of polycephalin C (1) and model compounds 2–4.
Keywords: slime mold; myxomycetes; Physarum polycephalum; exciton chirality method; absolute configuration; polyene; camphanic acid.
*
Corresponding author. Tel.: þ49-89-2180-7752; fax: þ49-89-2180-7640; e-mail: bst@cup.uni-muenchen.de
0040–4020/02/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0040-4020(02)01051-7

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F. Blumenthal et al. / Tetrahedron 58 (2002) 8433–8437
corroborates the correctness of the assignment of the trans-
and cis-configuration to the two isomeric diols 2 by means
of NMR spectra and thus the trans-configuration of the two
polyene substituents in the natural compound polycephalin
C (1). With the X-ray structure at hand the absolute
configuration of the (þ)-bis-camphanate (þ)-3 can be
assigned as 3R,4R and that of (2)-bis-camphanate (2)-3
as 3S,4S as shown in the illustrations.
Upon treatment of the diastereomeric bis-camphanates (þ)-
3 and (2)-3 with 2N KOH in ethanol under reflux the
enantiomeric diols (þ)-2 and (2)-2 are obtained. By
reaction of hydroxy groups with activated benzoic acid
derivatives like imidazolides or triazolides natural products
or other substrates can be acylated and thus be made
available to the exciton chirality method.5 Reaction of the
triazolide of p-dimethylaminobenzoic acid with diols (þ)-2
and (2)-2, by addition of DBU and stirring at room
temperature yields bis-benzoates (þ)-4 and (2)-4 as main
products.
Figure 1. Section of the X-ray structure of (þ)-3.
For enantiomeric resolution the racemic diol 2 in pyridine
was treated with 1.5 equiv. of (S)-(2)-camphanic acid
chloride per OH-function by addition of DMAP leading to
the diastereomeric bis-camphanates (þ)-3 and (2)-3 in 80%
yield.
The CD spectra of the bis-benzoates (þ)-4 and (2)-4 show
splitted CD curves with inverse signs, from which an
exciton coupling may be inferred (Fig. 2). However, the
effect is rather weak, the amplitude A is þ1.3 and 20.7,
respectively.
While several attempts of diastereomeric resolution by
means of column chromatography and HPLC remained
unsuccessful a resolution was finally achieved by
fractionated crystallisation from methylene chloride/ethyl
acetate/cyclohexane 2:5:5 (v/v/v). Recrystallisation yielded
crystals of (þ)-3 allowing X-ray structural analysis.
3. Discussion
The splitting of the CD curves of bis-benzoates is caused by
the interaction of the transition moments of the benzoates,
which have a position parallel to the C–O bond of the
alcohol.6 In the bis-benzoate 4 this bond does not directly
branch to the ring but is flexible because of the intermediate
methylene group, so that the benzoate groups may align
themselves relatively free in space. Most probably it is
essentially a conformation, in which the two bulky aromatic
substituents hinder each other as little as possible, such as
for instance in Scheme 2 (left) perpendicular to the
cyclohexene ring. In this conformation there can be no
excitone coupling, because the two benzoate groups are too
far away from each other. However, a conformation is
conceivable in which the substituents, such as indicated in
Scheme 2 (right), arrange themselves in the plane of the
cyclohexene ring. In this case the transition moments
indicated by the bars have a positive sense of chirality and
By means of X-ray structural analysis in general it is only
possible to get a statement on the relative configuration of
chiral compounds. However, if the molecule contains a
stereogenic centre with known absolute configuration, the
absolute configuration of any other stereogenic centre can
also be determined. As the 1S-configuration of the
camphanic acid is known, a 3R,4R-configuration for the
cyclohexene ring may be inferred. Fig. 1 shows a section of
the structure in which the camphanic acid units are left aside
for the sake of clarity.
The typical torsion of the opposite bonds C1–C2 and C4–
C5 into a ‘pseudo chair’ is easily recognizable. The
substituents at C-3 and C-4 are in trans-position. This
Figure 2. CD spectra of (þ)-4 (thick line) and (2)-4 (dotted line).

F. Blumenthal et al. / Tetrahedron 58 (2002) 8433–8437
8435
Scheme 2. Position of the transition moments in various conformations of (þ)-4.
Scheme 3. Position of the transition moments in bis-benzoates of (1R,2R)-cyclohexanediol 5 (left) and (1R,2R)-cyclohexenediol 6 (right).
should show a positive splitting of the CD curves, which is
true for (þ)-4. As only part of the molecules are contained
in this conformation, the effect is not so distinct. Conse-
quently it may be concluded that in this case the exciton
chirality method cannot be applied in the usual manner to
determine the absolute configuration.
This interpretation is supported by the fact that the CD
spectrum of hydrogenated polycephalin shows neither a UV
maximum nor a Cotton effect in the area around 400 nm.8
Therefore it does not make any difference if the double bond
in the cyclohexene ring is hydrogenated or not, because, as
already mentioned above, chirality is preserved also in the
saturated 3,4-substituted trans-cyclohexane system. There-
fore polycephalin C (1) may be assumed to have the
absolute configuration 300R,400R.
Cai et al.7 used trans-(1R,2R)-cyclohexanediol 5 as a test
substrate for benzoates and other chromophores. All
derivatives of this diol, contrary to the derivatives of
trans-(3R,4R)-di-(hydroxymethyl)cyclohexene (þ)-2, show
a negative CD splitting, which is to be expected because of
the position of the transition moments (Scheme 3, left). Here
the ester group is directly connected to the ring and
therefore less flexible. Like the trans-cyclohexanediol
system the unsaturated trans-cyclohexenediol system is
also chiral (Scheme 3, right). Consequently the existence of
the double bond should not change the chiroptical
properties.
Very recently, Ley et al.9 have completed a stereospecific
total synthesis of two different polycephalin-like molecules,
where00the00stereochemistry at the ring junctions is 300S,400S
and 3 R,4 R, respectively. All spectroscopic data of the
latter product are in agreement with those of the natural
product. Though, this is an outstanding confirmation for the
Having this in mind we concluded that the excitone chirality
method should also be directly applicable to polycephalin C
(1). Two polyene chains, showing a UV-maximum at
388 nm, are connected to a chiral trans-cyclohexene system.
It is to be assumed that the transition moments of the p–pp-
excitation of the two polyene systems are parallel to the
carbon chain. With regard to the 300R,400R-configuration,
shown on Scheme 4, a negative sense of chirality results for
the polyene chains and a negative CD splitting should be
expected.
Fig. 3 shows the CD spectrum of polycephalin C (1).8 The
negative Cotton effect at 250 nm is caused by the two
tetramic acid units. The following two Cotton effects at
360 nm (þ5.73) and 415 nm (23.88), i.e. a negative CD
splitting, may be traced back to the excitone coupling of the
two polyene chains.
Scheme 4. The transition moments in polycephalin C (1) show a negative
sense of chirality.

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F. Blumenthal et al. / Tetrahedron 58 (2002) 8433–8437
Figure 3. CD spectrum of polycephalin C (1).
result of the application of the exciton chirality method to
polycephalin C (1).
1.02 (s, 6H, CH3); 0.96 (s, 6H, CH3); 0.86 (s, 6H, CH3). 13
C
NMR: d¼0178.1, 167.5 (C-30, C-80); 129.5, 125.4 (C-1, C-2);
91.1 (C-1 ); 67.5, 67.0 (C-7, C-8); 54.8, 54.1 (C-40, C-70);
36.7, 34.0 (C-3, C-4); 29.8, 28.9 (C-50, C-60); 23.1, 23.0
(C-5, C-6); 16.8 (Me); 16.7 (Me); 9.7 (Me). IR (KBr): n
˜
4. Experimental
4.1. General
(cm21)¼3436 (br); 2970 (m); 2933 (m); 1788 (s); 1743 (s);
1634 (w); 1450 (w); 1398 (w); 1380 (w); 1340 (w); 1315
(m); 1273 (m); 1230 (w); 1170 (m); 1108 (m); 1064 (m);
932 (w). MS (EI): m/z (%)¼504 (4) [MþH]þ; 503 (2) [M]þ;
456 (2); 305 (3); 304 (5); 199 (6); 181 (6); 153 (19); 134
(19); 109 (19); 107 (32); 106 (100). UV (MeCN): lmax
(log 1)¼215 nm (3.54). CD (MeCN): lmax (D1)¼215
(22.00). Analysis calcd for C18H28O8: C 66.91, H 7.62,
found: C 66.87, H 7.70.
NMR: Bruker AMX2-600 (600.28 MHz for 1H and
150.9 MHz for 13C, chemical shifts are given relative to
the residual solvent signal CDCl3: dH¼7.24, dC¼77.0).
EI-MS: Finigan MAT 95Q (DI, 2108C, 70 eV). IR: Perkin–
Elmer Spectrum 1000. UV/Vis: HP 8452A Diode Array
Spectrophotometer. CD: Instruments S.A. Jobin Yvon
CD-6-Dichrograph. Analytical TLC: aluminium sheets
silica gel 60 F254 (Merck), 0.2 mm.
Even with several attempts in different solvent mixtures
(2)-3 could not be crystallized ([a]D¼230.1, (c¼0.562 in
chloroform)).
4.1.1. Bis-camphanates (1)-3 and (2)-3. A stirred and ice-
cooled solution of 0.072 g (0.5 mmol) (^)-trans-3,4-di-
(hydroxymethyl)cyclohexene (2) and some crystals of
DMAP in 3 ml pyridine was treated dropwise with 0.340 g
(1.5 mmol) (S)-(2)-camphanic acid chloride. After 20 min
the cooling was removed and the mixture stirred for another
2 h at room temperature. Then 3 ml water were added and
the mixture extracted with benzene. The aqueous phase was
again extracted twice with benzene and the unified organic
phases were washed twice with water, 2N hydrochloric acid,
saturated sodium hydrogen carbonate solution and dried
over magnesium sulfate. Evaporation of the solvent in
vacuo yielded a colourless oil. Yield: 0.200 g (79.6%).
4.1.2. trans-3,4-Di-(hydroxymethyl)cyclohexene (1)-2
and (2)-2. To 0.080 g (0.16 mmol) (þ)- resp. (2)-bis-
camphanate 3 in 7 ml ethanol were added 2 ml 2N
potassium hydroxide and the mixture refluxed for 16 h.
The reaction mixture was concentrated in vacuo, dissolved
in chloroform and purified by column chromatography
(chloroform/methanol 10:1 (v/v)).
Yield: 0.015 g (66.0%). (þ)-2: [a]D¼þ17.9 (c¼0.520 in
chloroform), (2)-2: [a]D¼216.5 (c¼0.340 in chloroform).
1H NMR: d¼5.85–5.81 (m, 1H, 1-H); 5.55–5.45 (m, 1H,
2-H); 3.73–3.56 (m, 4H, 7-H, 8-H); 2.21–2.17 (m, 1H,
3-H); 2.09–2.02 (m, 2H, 6-H); 1.77–1.68 (m, 2H, 4-H,
5-Ha); 1.44–1.35 (m, 1H, H-5b).
The mixture of the diastereomeric camphanates (0.200 g,
0.4 mmol) was dissolved in 8 ml methylene chloride/ethyl
acetate/cyclohexane 2:5:5 (v/v/v) and about 2 ml of the
solvent mixture removed in vacuo. While allowing to stand
over several days the (þ)-bis-camphanate (þ)-3 crystallized
in fine needles, which were used for an X-ray structure
analysis.
4.1.3. Bis-benzoates (1)-4 and (2)-4. 0.003 ml DBU were
added to a solution of 0.015 g (0.10 mmol) (þ)- resp. (2)-2
and 0.065 g (0.30 mmol) p-dimethylaminobenzoyl-1,2,4-
triazole in 8 ml methylene chloride. The mixture was
stirred over 18 h at room temperature and the reaction
monitored by TLC. After complete reaction the solvent was
removed in vacuo and the residue purified by flash
chromatography (chloroform/methanol 10:1 (v/v)). The
1
(þ)-3: [a]D¼þ25.7 (c¼0.806 in chloroform). H NMR:
d¼5.80–5.79 (m, 1H, 1-H); 5.53–5.49 (m, 1H, 2-H); 4.29–
4.04 (m, 4H, 7-H, 8-H); 2.41–1.44 (m, 14H, aliphat. H);

F. Blumenthal et al. / Tetrahedron 58 (2002) 8433–8437
8437
bis-benzoates (þ)- resp. (2)-4 were obtained as a yellowish
structure in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication numbers CCDC 175933. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (Fax: þ44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).
solid matter.
Yield: 0.022 g (50.4%). (þ)-4: [a]D¼þ18.9 (c¼0.721 in
chloroform), (2)-4 [a]D¼220.3 (c¼0.670 in chloroform).
1H NMR: d¼7.91 (d, 4H, J¼9.2 Hz, arom. H); 6.63 (d, 4H,
J¼8.0 Hz, arom. H); 5.89–5.82 (m, 1H, 1-H); 5.70–5.64
(m, 1H, 2-H); 4.34–4.30 (m, 4H, 7-H, 8-H); 3.03 (s, 12H,
Me); 2.58–2.54 (m, 1H, 3-H); 2.19–2.07 (m, 3H, 6-H, 4-H);
1.94–1.87 (m, 1H, 5-Ha); 1.69–1.64 (m, 1H, 5-Hb). 13C
NMR: d¼166.9; 153.2; 131.3; 129.0; 126.5; 117.4; 110.9;
Acknowledgements
66.4; 66.3; 40.1; 37.2; 34.6; 23.3; 23.2. IR (KBr): n
The authors gratefully acknowledge the support of the
Deutsche Forschungsgemeinschaft (SFB 369). We would
˜
(cm21)¼3436 (br); 2925 (m); 2897 (m); 1697 (s); 1613 (s);
1531 (m); 1445 (w); 1372 (m); 1317 (w); 1279 (s); 1233
(w); 1184 (s); 1111 (m); 947 (w); 826 (w); 770 (m); 697 (w).
MS (EI): m/z (%)¼436 (34) [M]þ; 347 (2); 271 (2); 165
(100); 148 (92); 106 (4); 77 (3). UV (MeCN): lmax
(log 1)¼230 nm (0.75); 313 nm (2.60). CD (MeCN): (þ)-
4: lmax (D1)¼230 nm (20.15); 292 nm (20.40); 320 nm
(þ0.80), (2)-4: lmax (D1)¼230 (þ0.02); 290 (þ0.20); 320
(20.50).
¨
also like to thank Dr Wolfgang Marwan, Universitat
suspension culture of Physarum
Freiburg, for
a
polycephalum.
References
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3139–3141.
4.1.4. Crystal data for (1)-3. C18H28O8, M¼502.58,
˚
orthorhombic, space group: P212121, a¼6.778(2) A,
2. Nowak, A.; Steffan, B. Liebigs Ann-Recl. 1997, 1817–1821.
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3
˚
˚
˚
b¼10.715(2) A, c¼36.135(3) A, V¼2624.4(8) A , Z¼4,
Dc¼1.272 Mg/m3, m¼0.092 mm21, crystal dimensions:
0.47£0.33£0.17 mm3,
F(000)¼1080,
T¼293(2) K,
u¼2.54–24.008, reflections measured: 4727, unique reflec-
tions: 4112, Rint¼0.0310, min. and max. transmission:
0.9803 and 0.9993, R1¼0.0649 and wR2¼0.1278 for all
2757 observed reflections with I.2s(I), R1¼0.1050,
wR2¼0.1488 for all reflections and 331 parameters. Final
4. Corey, E. J.; Petrzilka, M. Tetrahedron Lett. 1975, 2537–2540.
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6. Nakanishi, K.; Berova, N.; Woody, R. W. Circular
Dichroism—Principles and Applications. Verlag Chemie,
1994; Chapter 13, pp 361–398.
3
˚
electron density 0.149 and 20.143 e/A , S¼1.112, absolute
7. Cai, G.; Bozhkova, N.; Odingo, J. JACS 1993, 115, 7192–7198.
¨
¨
8. Nowak, A. PhD Thesis, Universitat Munchen, 1997.
structure factor 20.85 (208).
9. Ley, S.; et al. unpublished results.
Crystallographic data (excluding structure factors) for the