Synthesis of fluorous photolabile aldehyde and carbamate and alkyl carbamate protecting groups for carbohydrate-associated amines

Article abstract of DOI:10.1021/ol500023y

Two new fluorous photolabile-protecting groups (FNBC and FNB) and a new base-labile protecting group (FOC) for the masking of amines are reported. The protecting groups survive a wide range of common reaction conditions used in oligosaccharide synthesis and render the attached molecules amenable to fluorous solid-phase extraction (FSPE). A glycosyl acceptor containing the FNB group is shown to be useful in the synthesis of carbohydrates tagged with free deactivated secondary amines.

Full text of DOI:10.1021/ol500023y

Letter
pubs.acs.org/OrgLett
Synthesis of Fluorous Photolabile Aldehyde and Carbamate and
Alkyl Carbamate Protecting Groups for Carbohydrate-Associated
Amines
Rajarshi Roychoudhury and Nicola L. B. Pohl*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: Two new fluorous photolabile-protecting groups (FNBC and FNB) and a new base-labile protecting group
(FOC) for the masking of amines are reported. The protecting groups survive a wide range of common reaction conditions used
in oligosaccharide synthesis and render the attached molecules amenable to fluorous solid-phase extraction (FSPE). A glycosyl
acceptor containing the FNB group is shown to be useful in the synthesis of carbohydrates tagged with free deactivated
secondary amines.
he synthesis of oligosaccharides is complex yet often
unavoidable given that the isolation of these ubiquitous
converted to an N-acetyl group in one step using AIBN and
tributyltin hydride.5 Unfortunately, this conversion step is often
incomplete when applied to higher oligosaccharides (e.g.,
hexamers) leading to inseparable mixtures and, in the presence
of uronic acids, often leads to elimination products. Also,
removal of tin byproducts can be extremely challenging.
Phthalimido groups can pose challenges in base-catalyzed
reactions,6 while an Alloc group prohibits the use of N-
iodosuccinimide and related halonium ion-mediated activation
strategies of thioglycosides.
T
structures from natural sources, in pure form, is extremely
expensive, challenging, and time-consuming.1 The use of
soluble light fluorous tags (those that contain fluorine content
equal or less than 40% by molecular weight) has shown
promise in simplifying the process of oligosaccharide synthesis;
the fluorous-tagged carbohydrate structures can easily be
purified from nonfluorous-tagged side products and excess
reagents by fluorous solid-phase extraction (FSPE) before
continuation of each synthesis cycle.2
In light of these limitations, fluorous protecting group
versions that provide the added benefit of using the masking
group for purification are of particular interest. To date, the
synthesis and use of a fluorous alkyl carbamate-type (Troc-
type) nitrogen-protecting group has been described for the
protection and purification of amino sugars.7a However, this
protecting group synthesis involves the use of toxic chemcals
such as mercury acetate and phosgene, thereby limiting its
practical use to small scales.
Our first goal then was to design a new carbamate-type
amine-protecting group that could be made from nontoxic
reagents amenable to scale up. A fluorous alkyl carbamate-type
protecting group was chosen because alkyl carbamates not only
can be deprotected using mild conditions like methyltrichlor-
osilane8a or trimethylsilyl iodide8b but also are known to survive
most protection/deprotection conditions in carbohydrate
synthesis as well as to provide stereoselectvity during
glycosylations through neighboring group participation when
placed next to the anomeric center.
Given these advantages, a range of alcohol protecting groups
with minimal fluorous content have been designed and applied
to carbohydrate synthesis.3 Apart from hydroxyl groups, amines
are the next most common functional group found in glycans
that requires masking. The N-sulfate and N-acetyl groups found
in natural products are too polar or basic to undergo most
protection or other reaction steps in a typical oligosaccharide
synthetic sequence and therefore require amine blocking.
However, relatively little work has been reported using fluorous
variants for nitrogen protection in the realm of carbohydrates.
Herein we report the design of a set of three new fluorous
amine-protecting groups and demonstrate their tolerance to
various standard conditions used in oligosaccharide synthesis.
The most common amine-containing sugars, glucosamine or
galactosamine, are often protected by 2,2,2-trichloroethyl
chloroformate (TrocCl), allyloxycarbonyl chloride (AllocCl),
phthalic anhydride (Phth), and trichloroacetyl chloride
(TCACl) to form carbamates or amides when neighboring
group participation is needed in the glycosylation step.4 After
the required oligosaccharide is obtained, the protecting groups
are cleaved and then the free amine is acetylated to obtain the
final N-acetyl or N-sulfate moiety. Trichloroacetyl chloride, one
of the most common protecting groups used to this end, is
Received: January 3, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol500023y | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Imizadolinium carbamates are known for their use in the
protection of amines.9 We hypothesized that attaching a
fluorous chain five bonds distant from the reactive center would
not alter the reactivity of this type of carbamate, and therefore,
a new fluorous version of this protecting group could be
produced. To this end, a fluorous protecting group 2, with
imidazole as a leaving group, was synthesized in one step
starting from commercially available perfluorooctylpropanol 1
(Scheme 1). Unfortunately, reagent 2 was found to efficiently
protect primary amino alcohols, but not glucosamine, even at
elevated temperatures.
and protic acid mediated benzylidine acetal formation to
eventually provide compound 6.10 The free 3-OH was then
protected by a benzylchloromethyl ether. Compounds 5−7
were all readily purified by FSPE using the fluorophilic solvent
(methanol) to successfully elute the fluorous-tagged com-
pounds from the C8F17-modified silica gel.
Because the new protecting group showed good stability
under both acidic and basic conditions, work next turned to
finding suitable conditions to remove the group. Several mild
conditions known to remove alkyl carbamates were initially
attempted (using trimethylsilyl iodide and methyl trichlor-
osilane respectively) to remove the FOC,8 but none were
successful in removing the carbamate. After witnessing the
Scheme 1. Synthesis of FOC-imidazolinium Triflate 3
́
stability of FOC under Zemplen conditions, we realized a
stronger basic medium was needed for a nucleophile-mediated
deprotection. Eventually, the free amine was generated using 6
M aqueous sodium hydroxide under reflux, which was
subsequently acetylated to provide compound 8 (Scheme 2).
Our next objective was to design a fluorous amine-protecting
group amenable to the synthesis of amine-terminated
oligosaccharides, commonly used for attachment of these
structures to arrays, and our first choice was a photolabile
group, owing to its convenient deprotection conditions.
Although o-nitrobenzyl carbamate groups are frequently used
for photolysis,11 a fluorous version of this type of protecting
group amenable to oligosaccharide synthesis is still unknown.
The fluorous component of the acceptor will enable us to
synthesize various amine-terminated glycans in our automated
solution-phase synthesis platform, where FSPE is used for
purification of intermediates. Since upon photolysis the
generated free primary amine could recombine with the o-
nitrosoaldehyde side product,12 we chose to refrain from
making an o-nitrobenzyl carbamate group. Instead we decided
to synthesize a fluorous o-nitrobenzylamine with an additional
Cbz (benzyl chloroformate) group on the amine to prevent the
lone pair of nitrogen from imine formation. With our design,
once the desired chain length is obtained, oligosaccharides with
an amine functional group at the reducing end protected with a
Cbz group could be made using UV light; the Cbz group would
then be removed during standard Pd-catalyzed hydrogenolysis
of the benzyl ethers.13 A fluorous protecting group FNB
(fluorous o-nitrobenzaldehyde) was then synthesized in one
step from 5-hydroxy-2-nitrobenzaldehyde that can be used to
mask amines through reductive amination. An acceptor 12
(Scheme 3) was made to probe the scope of photolysis of FNB
in the presence of the FOC group. Compound 13 containing
both protecting groups was synthesized from previously
synthesized 4 and 12 and it was purified using gradation
FSPE.3a As expected, given that related groups are known to be
We envisioned that due to the relatively lower nucleophilicity
and steric hindrance of the secondary amine group in
glucosamine compared to the primary amino alcohol, a better
leaving group than imidazole was needed in compound 2.
Fortunately, the addition of a reaction step to our initial design
to produce such an improved leaving group was offset by the
fact that, unlike previously reported versions of fluorous
protecting groups for amines,7 the preparation of this new
reagent did not require any chromatographic separation steps.
The resulting reagent 3 (FOC, 1-(((4,4,5,5,6,6,7,7,8,8,9,9,-
10,10,11,11,11-heptadecafluoroundecyl)oxy)carbonyl)-3-meth-
yl-1H-imidazol-3-ium trifluoromethanesulfonate) was found to
be stable at −20 °C for months under an argon atmosphere.
However, the FOC reagent 3 could be easily added to
glucosamine, and the resulting urethane was found to survive
several common protection/deprotection conditions used in
oligosaccharide and other small molecule synthesis, including
basic and acidic conditions, as shown in Scheme 2. The group
withstood a Lewis acid mediated glycosylation reaction,
́
standard ambient temperature Zemplen deacylation conditions,
Scheme 2. Stability of the FOC Protecting Group under
Various Conditions Common to Oligosaccharide Synthesis
Scheme 3. Synthesis of FNB and a FNB-Protected Acceptor
That Can Be Unmasked with Light
B
dx.doi.org/10.1021/ol500023y | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cleaved by light sources of wavelengths ranging from 300 to
365 nm,14 the Cbz-protected amine could be generated using
UV radiation at 330 nm to provide amine 14 (Scheme 4) in
We next tested the stability of this new protecting group
under common reaction conditions used for oligosaccharide
synthesis (Scheme 6). Starting from the peracetylated glucos-
Scheme 4. Photodeprotection of FNB in the Presence of
FOC
Scheme 6. Stability of FNBC under Various Conditions
Common to Oligosaccharide Synthesis
high yields while keeping the FOC protecting group intact.
Even without the FOC group, the FNB-linker can render a
molecule amenable to purification by FSPE (Scheme S7,
Supporting Information).
Since more complex sugars such as heparin contain
differentially modified amines, we next set out to test a
fluorous protecting group for amino sugars orthogonal to FOC
that could be removed by photolysis. Given the precedent of o-
nitrobenzyl carbamates, which are known to generate free
amines under UV light in a traceless manner,11 we set out to
test the feasibility of making a fluorous variant of this group,
which we named FNBC (fluorous o-nitrobenzyl carbamate). In
addition, a carbamate group was chosen in order to aid in
forming stereoselective glycosylation products through anchi-
meric assistance.
Although removal of the o-nitrobenzyl carbamates is known
to be a lower yielding reaction due to imine formation with the
o-nitrosobenzaldehyde side product produced,11d we decided to
use a primary amine in the reaction mixture. We hypothesized
that owing to the relatively low nucleophilicity and steric
hindrance of an amine group in aminosugars, imine formation
would be slower than with primary amines. This, in turn, could
make the deprotection step higher yielding.11d
Ideally, a scheme to produce such a protecting group would
also require minimal or no chromatographic separation steps.
As before, we chose to have a methylimidazolinium leaving
group on the protecting group. Starting with a nucleophilic
substitution reaction, followed by a sodium borohydride-
mediated reduction, a nucleophilic addition−elimination
reaction with carbonyl diimidazole, and formation of a
methylimidazolinium salt, we successfully synthesized FNBC
17 (Scheme 5).
amine hydrochloride, we carried out carbamate formation,
glycosylation, deacetylation, benzylidene acetal formation, and
ether formation reactions; the protecting group was found to be
stable to all these conditions. Unfortunately, this deprotection
step proved to be as low yielding as prior reports of such light-
mediated cleavage reactions on nonfluorous compounds with
the formation of insoluble materials; using a primary amine as a
competing nucleophile in the reaction did not improve the
outcome.11d Resubjecting the desired product 8 to the
deprotection conditions resulted in complete reisolation of
the material, thereby demonstrating that the final product is
stable under the reaction conditions. The loss of material
appears to be the result of the formation of insoluble
byproducts. To probe whether the complex carbohydrate
substrate was contributing to the low yields, an additional
simpler FNBC-protected amino alcohol was synthesized
(Scheme S8, Supporting Information). In this case also, the
deprotection turned out to be low yielding with formation of
insoluble byproducts. Realistically, this parent FNBC group,
like its nonfluorous analogues, should be limited to the
protection of secondary deactivated amines rather than primary
amines. Given literature precedents,15 a second-generation
FNBC-type protecting group better suited for primary amines
could be envisioned with a methyl substitution at the benzylic
position. This added methyl group should render the molecule
more effective in the deprotection reaction owing to the low
electrophilicity of the generated o-nitrosoacetophenone prod-
uct.
Scheme 5. Synthesis of FNBC-imidazolinium Triflate
In summary, three new fluorous protecting groups for amine
functional groups have been designed and synthesized. These
new masking groups should find use not only in oligosaccharide
synthesis but also in the design of small molecules and in
peptide synthesis where amine functional groups are also
prevalent. Although the use of FNBC as a protecting group for
aminosugars is limited owing to low yields during depro-
tections, an efficient photolabile amine-protecting group FNB
has been developed to aid in the construction of linkers for
C
dx.doi.org/10.1021/ol500023y | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Werner, S. L.; Bernard, J. B.; Poulenat, G.; Petillo, P. A. Org. Lett.
2000, 2, 3135.
carbohydrates or other molecules. FNB was found to be
orthogonal to FOC, and the deprotection step was found to be
high yielding unlike that of FNBC. Work is ongoing to apply
FNB in the synthesis of a library of N-glycans.
(9) Upadhyay, S.; Sharma, A.; Sharma, A.; Rajagopal, V.; Datla, P.;
Bellary, A.; Prasad, S. S. PCT Int. Appl. 2009047792, 2009.
(10) (a) Grzyb, J. A.; Shen, M.; Yoshina-Ishii, C.; Chi, W.; Stanley, R.
B.; Batey, R. A. Tetrahedron 2005, 61, 7153. (b) Heller, S. T.; Sarpong,
R. Tetrahedron 2011, 67, 8851. (c) Heller, S. T.; Sarpong, R. Org. Lett.
2010, 12, 4572.
(11) (a) Prompinit, P.; Achalkumar, A. S.; Han, X.; Bushby, R. J.;
Walti, C.; Evans, S. D. J. Phys. Chem. C 2009, 113, 21642.
(b) Cameron, J. F.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113,
4303. (c) Gautier, A.; Nguyen, D. P.; Lusic, H.; An, W.; Deiters, A.;
Chin, J. W. J. Am. Chem. Soc. 2010, 132, 4086. (d) Amit, B.; Zehavi,
U.; Patchornik, A. J. Org. Chem. 1974, 39, 192.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis protocol and characterization of all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
(12) (a) Ajayaghosh, A.; Pillai, V. N. R. Tetrahedron 1988, 44, 6661.
(b) Chandra, B.; Subramaniam, R.; Mallik, S.; Srivastava, D. K. Org.
Biomol. Chem. 2006, 4, 1730.
Corresponding Author
*E-mail: npohl@indiana.edu.
(13) (a) Bergmann, M.; Zervas, L. Ber. 1932, 65, 1192. (b) Sakaitani,
M.; Hori, K.; Ohfune, Y. Tetrahedron Lett. 1988, 29, 2983.
(14) Bochet, C. G. J. Chem. Soc., Perkin Trans. 1 2002, 125.
(15) (a) Wu, J.; Tang, X. Bioorg. Med. Chem. 2013, 21, 6205−6211.
(b) Klinger, D.; Landfester, K. Macromolecules 2011, 44, 9758−9772.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
N.P. acknowledges the Joan and Marvin Carmack fund and
Indiana University. This work was also supported in part by the
National Institutes of Health (U19 A1091031).
REFERENCES
■
(1) (a) Seeberger, P. H. Chem. Soc. Rev. 2008, 37, 19. (b) Ratner, D.
M.; Adams, E. W.; Disney, M. D.; Seeberger, P. H. ChemBioChem
2004, 5, 1375.
(2) (a) Curran, D. P.; Luo, Z. J. Am. Chem. Soc. 1999, 121, 9069.
(b) Zhang, W. Chem. Rev. 2009, 109, 749. (c) Zhang, W. Curr. Opin.
Drug Discovery Dev. 2004, 7, 784. (d) Roychoudhury, R.; Pohl, N. L. B.
Light fluorous tag assisted oligosaccharide synthesis. In Modern
Synthetic Methods in Carbohydrate Chemistry-From Monosaccharides to
Complex Glycoconjugates; Werz, D. B., Vidal, S., Eds.; Wiley-VCH,
2013; p 221. (e) Zhang, W.; Curran, D. P. Tetrahedron 2006, 62,
11837.
(3) (a) Park, G.; Ko, K.-S.; Zakharova, A.; Pohl, N. L. J. Fluorine
Chem. 2008, 129, 978. (b) Zhang, F.; Zhang, W.; Zhang, Y.; Curran,
D. P.; Liu, G. J. Org. Chem. 2009, 74, 2594. (c) Manzoni, L. Chem.
Commun. 2003, 23, 2930. (d) Kojima, M.; Nakamura, Y.; Takeuchi, S.
Tetrahedron Lett. 2007, 48, 4431.
(4) (a) Wang, Z.; Chinoy, Z. S.; Ambre, S. G.; Peng, W.; McBride, R.;
de Vries, R. P.; Glushka, J.; Paulson, J. C.; Boons, G.-J. Science 2013,
341, 379. (b) Davis, B. G. J. Chem. Soc., Perkin Trans. 1 2000, 2137.
(c) Mitchell, S. A.; Pratt, M. R.; Hruby, V. J.; Polt, R. J. Org. Chem.
2001, 61, 2327. (d) Mong, K. T.; Wong, C. H. Angew. Chem., Int. Ed.
2002, 41, 4087. (e) Oberli, M. A.; Bindschadler, P.; Werz, D. B.;
Seeberger, P. H. Org. Lett. 2008, 10, 905. (f) Love, K. R.; Seeberger, P.
H. J. Org. Chem. 2005, 70, 3168.
(5) Pauls, H. W.; Fraser-Reid, B. Carbohydr. Res. 1986, 150, 111.
(6) (a) Sasaki, T.; Minamoto, K.; Itoh, H. J. Org. Chem. 1978, 43,
2320. (b) Costello, C. A.; Kreuzman, A. J.; Zmijewski, M. J.
Tetrahedron Lett. 1996, 37, 7469.
(7) (a) Manzoni, L.; Castelli, R. Org. Lett. 2006, 8, 955. (b) Zong, C.;
Venot, A.; Dhamale, O.; Boons, G.-J. Org. Lett. 2013, 15, 342. (c) Luo,
Z.; Williams, J.; Read, R. W.; Curran, D. P. J. Org. Chem. 2001, 66,
4261. (d) Filippov, D. 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. (e) Matsugi, M.; Yamanaka, K.;
Inomata, I.; Takekoshi, N.; Hasegawa, M.; Curran, D. P. QSAR Comb.
Sci. 2006, 25, 713. (f) 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.
(8) (a) Lott, R. S.; Chauhan, V. S.; Stammer, C. H. J. Chem. Soc.,
Chem. Commun. 1979, 495. (b) Raucher, S.; Bray, B. L.; Lawrence, R.
F. J. Am. Chem. Soc. 1987, 109, 442. (c) Yeung, B. K. S.; Adamski-
D
dx.doi.org/10.1021/ol500023y | Org. Lett. XXXX, XXX, XXX−XXX