Separation of a mixture of paraconic acids from Cetraria islandica (L.) Ach. employing a fluorous tag-catch and release strategy

Article abstract of DOI:10.1016/j.tetlet.2007.06.077

A light-fluorous catch and release approach application has been designed to the separation of a mixture of three paraconic acids extracted from the Island moss (Cetraria islandica (L.) Ach.). The (+)-protolichesterinic acid was caught and released via a Micha?l/retro-Micha?l addition sequence with a fluorous thiol, while the resulting two other compounds were classically separated, allowing the isolation of (+)-roccellaric acid for the first time in this lichen.

Full text of DOI:10.1016/j.tetlet.2007.06.077

Tetrahedron Letters 48 (2007) 6031–6033
Separation of a mixture of paraconic acids from
Cetraria islandica (L.) Ach. employing a fluorous
tag—catch and release strategy
*
´
David Horhant, Anne-Cecile Le Lamer, Joel Boustie, Philippe Uriac and Nicolas Gouault
¨
EA 4090 ‘Substances Liche´niques et Photoprotection’, UFR Sciences Pharmaceutiques et Biologiques,
Universite´ de Rennes 1, 2 Avenue du Prof. Le´on Bernard, 35043 Rennes Cedex, France
Received 2 April 2007; revised 27 April 2007; accepted 18 June 2007
Available online 21 June 2007
Abstract—A light-fluorous catch and release approach application has been designed to the separation of a mixture of three para-
conic acids extracted from the Island moss (Cetraria islandica (L.) Ach.). The (+)-protolichesterinic acid was caught and released via
a Michae¨l/retro-Michae¨l addition sequence with a fluorous thiol, while the resulting two other compounds were classically sepa-
rated, allowing the isolation of (+)-roccellaric acid for the first time in this lichen.
Ó 2007 Elsevier Ltd. All rights reserved.
(+)-Protolichesterinic acid (1),1 one of the major second-
ary metabolites of the well known lichen Cetraria islan-
dica (L.) Ach., is the main representative of the small
class of paraconic acids.2 These chiral trisubstituted c-
butyrolactones—bearing a carboxylic acid group in the
b-position and a lipophilic alkyl chain in the c-position
(Fig. 1)—were exclusively isolated from fungi,3 espe-
cially from lichens,4 and exhibit relevant antitumor,5
antibiotic6 and anti-inflammatory7 activities. However,
at least two very closely structurally related paraconic
acids are always extracted as a mixture from a lichen
species and their separation using classical techniques
constitutes a major drawback to their study.
In the last ten years, the use of light-fluorous versus so-
lid-phase techniques has emerged as a powerful alterna-
tive strategy for the separation of reaction mixtures in
synthetic organic disciplines.8 Indeed, it combines
Figure 1. Structure of various paraconic acids.
homogeneous reaction media and convenient separation
of fluorous-tagged compounds from non-fluorous ones
Recently, fluorous scavengers11 arose as efficient
using a fluorous solid phase extraction (FSPE) over flu-
quenching reagents12 or catch and release agents13 in
the synthesis of small organic molecules or isolation of
peptides. We describe here a new application of the
light-fluorous catch and release approach to the separa-
tion of a mixture of paraconic acids extracted from C.
islandica, allowing us to isolate a minor compound from
the lichen. To the best of our knowledge, this kind of ap-
proach has never been reported previously for the puri-
fication of natural products.
orous silica gel.9 Many light-fluorous components such
as catalysts or reagents are now available, considerably
expanding the field of light fluorous chemistry.10
Keywords: Fluorous chemistry; Catch and release; Paraconic acids;
Lichens.
*
Corresponding author. Tel.: +33
0 223234825; fax: +33 0
223234425; e-mail: nicolas.gouault@univ-rennes1.fr
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.077

6032
D. Horhant et al. / Tetrahedron Letters 48 (2007) 6031–6033
1H NMR study of the precipitate obtained after cooling
of the lipophilic extract (n-heptane) from a Danish spec-
imen of C. islandica actually revealed the presence of
(+)-protolichesterinic acid (1) and (+)-lichesterinic acid
(2) (Fig. 1) as reported to date from the literature. In
addition, we also noticed the presence of a third deriva-
tive identified as (+)-roccellaric acid (3)14 (Fig. 1). This
saturated protolichesterinic derivative, found in minute
amounts, was however co-eluted with 1 in most of the
TLC systems used. Therefore, to overcome the tedious
chromatographic process required to separate these
three paraconic acids, we investigated the performance
of the light-fluorous technologies.
The crude reaction mixture was loaded onto a fluorous
silica gel cartridge and lactones 2 and 3 were first eluted
using a fluorophobic wash (MeOH and water in 90:10
ratio), whereas the fluorous thioether 5 was retained
onto the SPE cartridge until elution with a fluorophilic
solvent (CH2Cl2 and TFA in 95:5 ratio). It should be no-
ticed that this thioadduct was scarcely soluble in all
commonly used solvents, and addition of TFA revealed
it necessary to completely solubilise it in usual solvents.
We assume that intra and/or intermolecular hydrogen
bonds between carboxylic acid group and sulfur atom,
which can be disrupted in the presence of TFA, were in-
volved. The fluorous derivative (5) of the targeted pro-
tolichesterinic acid (1) was hence efficiently separated
from lactones 2 and 3 with a good yield of 80%, using
a simple filtration on fluorous silica gel.
For the development of our catch and release strategy,
the purpose was, in the first time, to catch with a fluor-
ous tag and then release one of the compounds of this
mixture in order to facilitate, in the second time, the sep-
aration of the other ones. The exocyclic double bond,
only present on the (+)-protolichesterinic acid (1) in this
mixture, seemed to be the most appropriate target.
Therefore the fluorous tag had to be chosen regarding
two aspects: (i) chemoselectivity towards this exocyclic
double bond (catch) and (ii) easy removal of the tag at
the end of the sequence for recovering 1 (release). Thus,
the synthetic approach selected was the Michae¨l addi-
tion15 of the commercially available fluorous thiol
CF3(CF2)7CH2CH2SH (4)16 on (+)-protolichesterinic
acid (1) followed by adduct oxidation to sulfoxide and
thermal release17 of 1 (Scheme 1).
Second, recovering of 1 was achieved after an oxida-
tion–elimination sequence. The subsequent oxidation
of 5 by m-CPBA in THF at 0°C afforded the corre-
sponding sulfoxide 6. As previously described, FSPE al-
lowed the purification of 6 in the fluorophilic wash with
an excellent yield of 90%. It seems noteworthy—unlike
solid-phase or polymer-supported catch and release
methodologies—that all the fluorous intermediates
could have been completely characterized by conven-
tional analytical methods. The detagged a-methylene-
c-lactone 1 was finally regenerated with concomitant re-
lease of a fluorous sulfenic acid 7 through a thermal
elimination in toluene. The residue was then purified
by FSPE to afford 1 in the fluorophobic wash (MeOH
and water in 80/:20 ratio) in a 60 % yield and complete
removal of the fluorous product 7 in the fluorophilic
wash (Et2O).
First, the mixture of 1, 2 and 3 (in a molar ratio of 65/
35/5, respectively) was stirred with the fluorous tag 4 in
the presence of triethylamine in DMF during 2 h at
room temperature18 (disappearance of 1 was monitored
by TLC) (Scheme 1). After acidification and removal of
DMF, purification of the thioadduct (5) of (+)-protoli-
chesterinic acid was conveniently carried out by FSPE.
With a 42% overall yield, the efficiency of this selective
chemical ‘extraction’ was threefold higher than prepara-
tive TLC purification. Thus, the commercially selected
Scheme 1. Reagents and conditions: (i) 4 (1 equiv/1), NEt3 (1.2 equiv/1+2+3), DMF, rt, 2 h; (ii) m-CPBA (1 equiv), THF, 0 °C, 2 h; (iii) toluene,
reflux, 45 min.

D. Horhant et al. / Tetrahedron Letters 48 (2007) 6031–6033
6033
Nakagawa, M.; Hirota, A.; Nakayama, M. J. Antibiot.
1988, 41, 751; (c) He, G.; Matsuura, H.; Yoshihara, T.
Phytochemistry 2004, 65, 2803.
fluorous thiol perfectly meets the requirements cited
above, since spectral data and HPLC analysis confirmed
that 1 was successfully recovered with an excellent pur-
ity of 99% after FSPE, and the catch step of the se-
quence was complete, affording a mixture of 2 and 3,
free of 1.
4. (a) Huneck, S.; Schreiber, K.; Ho¨fle, G.; Snatzke, G. J.
Hattori Bot. Lab. 1979, 45, 1; (b) Ghomogu, R. T.; Bodo,
B. Phytochemistry 1982, 21, 2355; (c) Huneck, S.; Tons-
berg, T.; Bohlmann, F. Phytochemistry 1986, 25, 453; (d)
David, F.; Elix, J. A.; Wahid bin Samsudin, M. Aust. J.
Chem. 1990, 43, 1297; (e) Rezanka, T.; Gushina, I. A.
Phytochemistry 2000, 54, 635; (f) Rezanka, T.; Gushina, I.
A. Phytochemistry 2001, 58, 1281; (g) Shrestha, S.;
Manandhar, M. D.; Elix, J. A. Aust. J. Chem. 2002, 55,
623.
5. (a) Hirayama, T.; Fujikawa, F.; Kasahara, T.; Otsuka, M.;
Nishida, N.; Mizuno, D. Yakugaku Zasshi 1980, 100, 755;
(b) Tan, G. T.; Lee, S.; Lee, I. S.; Chen, J.; Leitner, P.;
Besterman, J. M.; Kinghorn, A. D.; Pezzuto, J. M.
Biochem. J. 1996, 314, 993; (c) Ogmundsdottir, H. M.;
Zoega, G. M.; Gissurarson, S. R.; Ingolfsdottir, K. J.
Pharm. Pharmacol. 1998, 50, 107; (d) Haraldsdottir, S.;
Guolaugdottir, E.; Ingolfsdottir, K.; Ogmundsdottir, H.
M. Planta Med. 2004, 70, 1098; (e) Bucar, F.; Schneider,
I.; Ogmundsdottir, H.; Ingolfsdottir, K. Phytomedicine
2004, 11, 602.
Finally, this pre-purified mixture of 2 and 3 served as
starting material for isolation of 2 and especially of 3
with preparative TLC. The (+)-roccellaric acid (3),
which has only been isolated from Roccellaria mollis
(Hampe) Zahlbr.,14 was unambiguously identified by
NOESY experiments and by comparison of their spec-
tral data with those of the literature.19 Thanks to this
light-fluorous catch and release approach, it was iso-
lated for the first time in C. islandica.
In summary, we have developed an original and efficient
‘fluorous tag—catch and release’ strategy for a straight-
forward isolation of the major (+)-protolichesterinic
acid from an extract of Iceland moss, and the conse-
quent separation of the two other close compounds 2
and 3. Since paraconic acids are always described as a
mixture in lichens, and their biogenesis20 and chemotax-
onomic distribution21 are almost unknown, this meth-
odology reveals to be a significant tool for further
studies of these compounds. Furthermore, this approach
may represent an attractive method to facilitate the iso-
lation of minor secondary metabolites and would have
potential for automated purification of different natural
products in crude extracts, via the use of appropriately
functionalized fluorous probes.
6. (a) Cavallito, C. J.; Fruehauf, D. M.; Bailey, J. H. J. Am.
Chem. Soc. 1948, 70, 3724; (b) Ingolfsdottir, K.; Hjal-
marsdottir, M. A.; Sigurdsson, A.; Gudjonsdottir, G. A.;
Brynjolfsdottir, A.; Steingrimsson, O. Antimicrob. Agents
¨
Chemother. 1997, 41, 215; (c) Turk, A. O.; Yilmaz, M.;
¨
Kivanc¸, M.; Turk, H. Z. Naturforsch. 2003, 58 c, 850.
¨
7. (a) Ingolfsdottir, K.; Breu, W.; Huneck, S.; Gudjonsdottir,
G. A.; Muller-Jakic, B.; Wagner, H. Phytomedicine 1994,
¨
1, 187; (b) Kumar, K. C.; Muller, K. J. Nat. Prod. 1999,
62, 817.
8. Zhang, W. Tetrahedron 2003, 59, 4475; Zhang, W. Chem.
Rev. 2004, 104, 2531.
9. (a) Zhang, W.; Curran, D. P. Tetrahedron 2006, 62, 11837;
(b) Curran, D. P.; Luo, Z. J. Am. Chem. Soc. 1999, 121,
9069; Curran, D. P. Synlett 2001, 1488.
Acknowledgement
10. (a) Gladysz, J. A.; Curran, D. P. Tetrahedron (Fluorous
Chemistry) 2002, 58, 3823; (b) Gladysz, J. A.; Curran, D.
P.; Horvath, I. T. Handbook of Fluorous Chemistry; Wiley-
VCH: Weinheim, 2004; Curran, D. P. Aldrichim. Acta
2006, 39, 3.
11. Hiu-Tung chen, C.; Zhang, L. Mol. Diversity 2005, 9, 353.
12. (a) Werner, S.; Curran, D. P. Org. Lett. 2003, 5, 3293; (b)
Zhang, W.; Chen, C. H.-T.; Nagashima, T. Tetrahedron
Lett. 2003, 44, 2065; (c) Lindsley, C. W.; Zhao, Z.; Leister,
W. H. Tetrahedron Lett. 2002, 43, 4225.
The authors gratefully acknowledge Drs. S. Tomasi and
J. Renault for helpful discussion, Dr K. Articus-Lepage
for critical reading of this manuscript; the CRMPO for
Mass Spectroscopy and 2D NMR experiments and the
´
Service de Microanalyse de l’Universite de Chatenay-
Malabry for Elemental Analysis experiments.
13. (a) Zhang, W. Org. Lett. 2003, 5, 1011; (b) McAllister, L.
A.; McCormick, R. A.; James, K. M.; Brand, S.; Willetts,
N.; Procter, D. J. Chem. Eur. J. 2007, 13, 1032; (c)
Brittain, S. M.; Ficarro, S. B.; Brock, A.; Peters, E. C. Nat.
Biotech. 2005, 23, 463.
14. Huneck, S.; Follmann, G. Z. Naturforsch. 1966, 22, 666.
15. Michae¨l, A. J. Prakt. Chem. 1887, 35, 349–356.
16. Zhang, W.; Curran, D. P.; Chen, C. H.-T. Tetrahedron
(Fluorous Chemistry) 2002, 58, 3871.
Supplementary data
Experimental procedures and spectroscopic data. Sup-
plementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.tetlet.2007.06.077.
17. Barbier, P.; Benezra, C. Tetrahedron Lett. 1982, 23, 3513.
18. Steurer, S.; Podlech, J. Eur. J. Org. Chem. 2002, 2002, 899.
19. (a) Takahata, H.; Uchida, Y.; Momose, T. J. Org. Chem.
1995, 60, 5628; (b) Mandal, P. K.; Roy, S. C. Tetrahedron
1999, 55, 11395.
References and notes
1. Zopf, W. Die Flechtenstoffe in chemischer, botanisher,
pharmakologisher and technischer Beziehung; Fischer:
Jena, 1907.
20. (a) Bloomer, J. L.; Eder, W. R.; Hoffman, W. F. Chem.
Commun. 1968, 354; (b) Bloomer, J. L.; Eder, W. R.;
Hoffman, W. F. J. Chem. Soc., Perkin Trans. 1 1970, 13,
1848.
21. Karnefelt, I.; Thell, A. Bibliotheca lichenologica 1993, 53,
115.
2. (a) Huneck, S.; Yoshimura, I. Identification of Lichen
Substances; Springer: Berlin, 1996; (b) Bandichhor, R.;
Nosse, B.; Reiser, O. Top. Curr. Chem. 2005, 243, 43
(Natural Product Synthesis I).
3. (a) Mahato, S. B.; Siddiqui, K. A. I.; Bhattacharya, G.;
Ghosal, T. J. Nat. Prod. 1987, 50, 245; (b) Park, B. K.;