Froc: A new fluorous protective group for peptide and oligosaccharide synthesis

Article abstract of DOI:10.1021/ol060006e

The synthesis of a new fluorous protecting group, Froc, is described. This new fluorous tag has been used in peptide and carbohydrate synthesis by our group and readily allows us to fully characterize each product (NMR, MS) and monitor each synthetic step by TLC. Purification of the products is generally performed by standard fluorous solid-phase extraction techniques (e.g., F-SPE), but standard chromatographic purifications are also possible if required.

Full text of DOI:10.1021/ol060006e

ORGANIC
LETTERS
2006
Vol. 8, No. 5
955-957
Froc: A New Fluorous Protective Group
for Peptide and Oligosaccharide
Synthesis†
Leonardo Manzoni* and Riccardo Castelli
C.N.R. - Istituto di Scienze e Tecnologie Molecolari (ISTM) and Centro
Interdisciplinare Studi Biomolecolari e applicazioni Industriali (CISI),
Via Venezian 21, I-20133 Milano, Italy
leonardo.manzoni@istm.cnr.it
Received January 2, 2006
ABSTRACT
The synthesis of a new fluorous protecting group, Froc, is described. This new fluorous tag has been used in peptide and carbohydrate
synthesis by our group and readily allows us to fully characterize each product (NMR, MS) and monitor each synthetic step by TLC. Purification
of the products is generally performed by standard fluorous solid-phase extraction techniques (e.g., F-SPE), but standard chromatographic
purifications are also possible if required.
The use of fluorous techniques1 for the separation of reaction
mixtures has found wide attraction in synthetic organic
disciplines in recent years.2 Fluorous chemistry has been
studied in several fields such as catalytic chemistry, com-
binatorial chemistry, parallel synthesis,3 and very recently
in carbohydrate microarrays.4 An important methodology is
the so-called “light-fluorous” strategy developed by Curran
and co-workers,1a,5 in which fluorous compounds (40% or
less fluorine content by MW) are readily separated from
nonfluorinated compounds by a simple extraction (F-SPE).
For these reasons, an array of fluorinated protecting group
have become available in recent years,6 including fluorous
versions of the Z,7 Boc,8 t-Bu,9 Bn,10 THP,11 Msc,12 acyl-
based,13 silyl-based,14 alkoxyethyl ether,15 and alcohol16
protecting groups. Peptides and carbohydrates can be easily
prepared by solid-phase syntheses that allow a very simple
(3) (a) Barrett, A. G. M.; Braddock, D. C.; Catterick, D.; Chadwick, D.;
Henschke, J. P.; McKinnell, R. M. Synlett 2000, 847. (b) Curran, D. P.
Pure Appl. Chem., 2000, 72, 1649. (c) Curran, D. P. Angew. Chem., Int.
Ed. 1998, 37, 1174.
(4) Ko, K.-S.; Jaipuri, F. A.; Pohl, N. L. J. Am. Chem. Soc. 2005, 127,
13162.
(5) Curran, D. P.; Luo, Z. Y. J. Am. Chem. Soc. 1999, 121, 9069.
(6) (a) Read, R. W.; Zhang, C. Tetrahedron Lett. 2003, 44, 7045. (b)
Zhang, W. Curr. Opin. Drug Discuss. DeV. 2004, 7, 784. (c) Zhang, W. In
Handbook of Fluorous Chemistry; Gladysz, J. A., Curran, D. P., Horvath,
I. T., Eds.; Wiley-VCH: New York, 2004; pp 222-236. (d) Nakamura,
Y.; Okumura, K.; Kojima, M.; Takeuchi, S. Tetrahedron Lett. 2006, 47,
239.
† Dedicated to Professor Carlo Scolastico on the occasion of his 70th
birthday.
(1) (a) Zhang, W. Tetrahedron 2003, 59, 4475 and references cited
therein. (b) Gladysz, J. A.; Curran, D. P. Tetrahedron 2002, 58, 3823. (c)
Horva´th, I. T.; Ra´bai, J. Science 1994, 266, 72. (d) Zhang, W. Chem. ReV.
2004, 104, 2531.
(2) (a) Zhang, W.; Lu, Y. M. Org. Lett. 2003, 5, 2555. (b) Mizuno, M.;
Goto, K.; Miura, T.; Hosaka, D.; Inazu, T. Chem. Commun. 2003, 972. (c)
Zhang, W.; Curran, D. P.; Chen, C. H. T. Tetrahedron 2002, 58, 3871. (d)
Luo, Z.; Zhang, Q.; Oderaotoshi, Y.; Curran, D. P. Science 2001, 291, 1766.
(e) Studer, A.; Hadida, S.; Ferritto, R.; Kim, S. Y.; Jeger, P.; Wipf, P.;
Curran, D. P. Science 1997, 275, 823.
10.1021/ol060006e CCC: $33.50
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Published on Web 02/04/2006

purification by filtration. However, the method suffers from
some serious disadvantages, such as reduced reactivity and
the inability to monitor the coupling reactions by TLC, NMR,
and mass spectrometry.
With the fluorous tag in hand, introduction and removal
of the Froc protecting group was explored.
To demonstrate the versatility and utility of the Froc
protecting group in peptide synthesis, we prepared the Gly-
Gly dipeptide and the bioactive peptide RGD. The peptides
were prepared following the general procedure shown in
Scheme 2. The amino acids were commercial or easily
We reasoned that a reducible fluorous protecting group
would be an attractive alternative tag for standard SPPS
(solid-phase peptide synthesis) and solid-phase carbohydrate
synthesis. Trichloethoxycarbonyl (Troc) is frequently used
in organic synthesis,17 especially where amino sugar deriva-
tives18 are involved. This is due to its stability under mild
acidic and basic conditions and its ease of removal under
specific conditions.18a-c,19 Accordingly, we set out to design,
synthesize and apply a fluorous version of the trichloethoxy-
carbonyl (Figure 1) protecting group for amines. The new
Scheme 2. General Scheme for the Peptide Synthesis Using
Froc as Amino Protecting Group
Figure 1. Troc protecting group and the Froc tag 1.
fluorous protecting group 1 was named Froc by analogy with
its nonfluorous Troc counterpart. The synthetic route for the
preparation of the fluorous tag 1 is shown in Scheme 1.
prepared from a commercial source. The first amino acid
was protected at the amino group with the freshly prepared
FrocCl to give the Froc-protected amino acid. As a proof of
concept, treatment of FrocHN-Gly-OMe with Zn/Ac2O/Et3N
quantitatively gave the AcHN-Gly-OMe derivative. With this
result in hand, we started the peptide synthesis. After every
step, it was possible to characterize all the intermediates,
and if starting materials were revealed, a second cycle of
reaction could be performed to drive the reaction to comple-
tion. Just like in solid-phase peptide synthesis, where an
Scheme 1. Synthesis of the FrocCl
(13) (a) Miura, T.; Hirose, Y.; Ohmae, M.; Inazu, T. Org. Lett. 2001, 3,
3947. (b) Miura, T.; Inazu, T. Tetrahedron Lett. 2003, 44, 1819. (c) Mizuno,
M.; Goto; Miura, T.; Hosaka, D.; Inazu, T. Chem. Commun. 2003, 972. (e)
Miura, T.; Goto, K.; Hosaka, D.; Inazu, T. Angew. Chem., Int. Ed. 2003,
42, 2047. (f) Mizuno, M.; Goto, K.; Miura, T.; Matsuura, T.; Inazu, T.
Tetrahedron Lett. 2004, 3425. (g) Miura, T.; Satoh, A.; Goto, K.; Muratami,
Y.; Imai, N.; Inazu, T. Tetrahedron: Asymmetry 2005, 16, 3.
(14) (a) Ro¨ver, S.; Wipf, P. Tetrahedron Lett. 1999, 40, 5667. (b)
Palmacci, E. R.; Hewitt, M. C.; Seeberger, P. H. Angew. Chem., Int. Ed.
2001, 40, 4433. (c) Seeberger, P. H. Chem Commun. 2003, 1115.
(15) Wipf, P.; Reeves, J. T. Tetrahedron Lett. 1999, 40, 5139.
(16) Goto, K.; Miura, T.; Mizuno, M. Tetrahedron Lett. 2005, 46, 8293.
(17) Green, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic
Synthesis, 3rd ed.; Wiley: New York, 1991.
(18) (a) Dullenkopf, W.; Castro-Palomino, J. C.; Manzoni, L.; Schmidt,
R. R. Carbohydr. Res. 1996, 296, 135. (b) Saha, U. K.; Schmidt, R. R. J.
Chem. Soc., Perkin Trans 1 1997, 1855. (c) Fukase, K.; Fukase, Y.; Oikawa,
M.; Liu, W.; Suda, Y.; Kusumoto, S. Tetrahedron 1998, 54, 4033. (d)
Ellervik, U.; Magnusson, G. Tetrahedron Lett. 1997, 38, 1627. (e) Mitchell,
S. A.; Pratt, M. R.; Hruby, V. J.; Polt, R. J. Org. Chem. 2001, 61, 2327. (f)
Mong, K. T.; Wong, C. H. Angew. Chem., Int. Ed. 2002, 41, 4087. (g)
Zhu, X.; Schmidt, R. R. Synthesis 2003, 8, 1262.
(19) (a) Woodward, R. B.; Heusler, K.; Gosteli, J.; Naegeli, P.; Oppolzer,
W.; Ramage, R.; Ranganathan, S.; Vorbru¨ggen, H. J. Am. Chem. Soc. 1966,
88, 852. (b) Just, G.; Grozinger, K. Synthesis 1976, 45. (c) Dong, Q.;
Anderson, C. E.; Ciufolini, M. A. Tetrahedron Lett. 1995, 36, 5681 and
references therein. (d) Tokimoto, H.; Fukase, K. Tetrahedron Lett. 2005,
46, 6831.
(20) Coudures, C.; Pastor, R.; Cambon, A. J. Fluorine Chem. 1984, 24,
93.
(21) 5 was reacted with BnNH2 to give the benzyl carbamate.
Alcohol 4 is obtained via a small modification of a pub-
lished protocol.20 In the first step, commercially available
1H,1H,2H,2H-perfluoro-1-decene 2 is treated with Hg(OAc)2
in the presence of Br2 in AcOH to give 3. The target
compound 5 is thereafter obtained in an excellent overall
yield (84%) by chloroformylation of 4.21
(7) (a) Filippov, V.; van Zoelen, D. J.; Oldfield, S. P.; van der Marel,
G. A.; Overkleeft, H. S.; Drijfhout, J. W.; van Boom, J. H. Tetrahedron
Lett. 2002, 43, 7809. (b) Curran, D. P.; Amatore, M.; Campbell, M.; Go,
E.; Luo, Z. Y. J. Org. Chem. 2001, 66, 4643. (c) Schwinn, D.; Bannwarth,
W. HelV. Chim. Acta 2002, 85, 255.
(8) Luo, Z. Y.; Williams, J.; Read, R. W.; Curran, D. P. J. Org. Chem.
2001, 66, 4261.
(9) Pardo, J.; Cobas, A.; Guitia´n, E.; Castedo, L. Org. Lett. 1998, 39,
4937.
(10) Curran, D. P.; Ferritto, R.; Hua, Y. Tetrahedron Lett. 1998, 39, 4937.
(11) Wipf, P.; Reeves, J. T. Tetrahedron Lett. 1999, 40, 4649.
(12) De Visser, P. C.; van Helden, M.; Filippov, D. V.; van der Marel,
G. A.; Drijfhout, J. W.; van Boom, J. H.; Noort, D.; Overkleeft, H. S.
Tetrahedron Lett. 2003, 44, 9013.
956
Org. Lett., Vol. 8, No. 5, 2006

excess of coupling reagent can be used, the excess reagent
can subsequently removed by a simple F-SPE. For the
coupling of the amino acids we decided to use a standard
protocol normally applied in SPPS (HBTU, DIPEA, DCM).
anomeric hydroxy group of compound 9 by using imidazole
and DMAP to give compound 10 in 90% yield. Removal of
the acetyl groups from 10 under Zemple´n conditions23
followed by treatment of the crude product with benzalde-
hyde dimethyl acetal in the presence of CSA afforded the
fluorous glycosyl acceptor 11 (72% over two steps). The
disaccharide Galâ1,3-GlcNAc 13 was obtained by reaction
of the pure acceptor 11 with the tetraacetylated galactose
trichloroacetimidate (as glycosyl donor) in the presence of
TMSOTf in CH2Cl2 (85%). This synthetic scheme showed
that the new protecting group is capable of withstanding the
typical reaction conditions required for protection/deprotec-
tion of a glycosyl acceptor and for glycosylation reactions.
Due to the limited number of fluorines in the tag, compounds
6, 7, and 9-13 maintain a normal chromatographic behavior
on standard silica gel, and all compounds can be character-
ized by NMR and mass spectrometry. The reactions can also
be readily monitored by TLC. Furthermore, all fluorous tag
compounds can be separated from the reaction mixture by a
simple solid-phase extraction (F-SPE); the nonfluorinated
compounds can be eluted with 80% MeOH/H2O, and the
fluorous compounds can be recovered from the column with
MeOH. A second reaction cycle can be performed if some
starting material is observed (TLC, MS, and NMR). The new
protecting group can be easily removed and substituted with
an acetyl group by treatment with Zn/Ac2O/Et3N to give 14
in 82% yield.
With the aim of using the fluorous technology to speed
up the synthesis of oligosaccharides,22 we decided to use
glucosamine as a model compound. Glucosamine was reacted
with FrocCl 5 to give the corresponding Froc-protected
glucosamine 6, which was peracetylated with Ac2O in
pyridine (Scheme 3). Treatment of 7 with Zn/Ac2O quanti-
tatively gave a one-pot conversion of the starting sugar into
the N-acetylglucosamine derivative (Scheme 3).
Scheme 3. Synthesis of a Disaccharide Using Froc
In summary, we have developed and applied a new
fluorous protecting group we have named Froc for its
analogies with the nonfluorous trichloroethoxycarbonyl
(Troc) counterpart. This new protecting group can be easily
used for peptide and carbohydrate synthesis. All fluorinated
compounds synthesized retain normal chromatographic be-
havior on standard silica gel, allowing easy monitoring of
the course of reactions by TLC. Purification of the desired
products obtained from multistep synthesis was significantly
simplified by the fluorous moiety. The reaction mixtures were
subjected to F-SPE on fluorous silica gel. The fluorous
compounds could also be purified by normal silica gel
chromatography if necessary, which was advantageous,
compared to solid-phase synthesis. In addition, each synthetic
intermediate could be easily characterized by NMR and mass
spectroscopy (MALDI-TOF, ESI). The Froc group contains
a stereogenic center but this have no effect on the interpreta-
tion of the spectra of chiral compounds. The spectra look
reasonably clean and can be interpretated (see Supporting
Information).
On the basis of these results, we turned our attention to
application of the Froc protecting group to oligosaccharide
synthesis. The synthetic scheme examinated employs many
of the reagents, protecting groups manipulations and glyco-
sylation conditions that are standard in the field (Scheme
3). So, starting from the peracetylated N-Froc-protected
O-acetylglucosamine 7, the acetyl group in the anomeric
position was removed by hydrazinium acetate to give 9 in
98% yield. The texyldimethylsilyl group was attached to the
Acknowledgment. We thank CNR and CISI.
Supporting Information Available: Experimental pro-
cedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL060006E
(22) (a) Manzoni, L. Chem. Commun. 2003, 2930. (b) Manzoni, L.;
Castelli, R. Org. Lett. 2004, 6, 4195.
(23) Zemple´n, G. Ber. Dtsch. Chem. Ges. 1927, 60, 1555.
Org. Lett., Vol. 8, No. 5, 2006
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