Protected sphingosine from phytosphingosine as an efficient acceptor in glycosylation reaction

Article abstract of DOI:10.1021/ol403688t

A convenient, simple, and high-yielding five-step synthesis of a sphingosine acceptor from phytosphingosine is reported, and its behavior in glycosylation reactions is described. Different synthetic paths to sphingosine acceptors using tetrachlorophthalimide as a protecting group for the sphingosine amino function and different glycosylation methods have been explored. Among the acceptors tested, the easiest accessible acceptor, unprotected on the two hydroxyl groups in positions 1 and 3, was regioselectively glycosylated on the primary position, the regioselectivity depending on the donor used.

Full text of DOI:10.1021/ol403688t

Letter
pubs.acs.org/OrgLett
Protected Sphingosine from Phytosphingosine as an Efficient
Acceptor in Glycosylation Reaction
Roberta Di Benedetto, Luca Zanetti, Monica Varese, Mehdi Rajabi,† Riccardo Di Brisco, and Luigi Panza*
̀
Dipartimento di Scienze del Farmaco, Universita del Piemonte Orientale A. Avogadro, L.go Donegani, 2-28100 Novara, Italy
S
* Supporting Information
ABSTRACT: A convenient, simple, and high-yielding five-step
synthesis of a sphingosine acceptor from phytosphingosine is
reported, and its behavior in glycosylation reactions is described.
Different synthetic paths to sphingosine acceptors using tetrachlor-
ophthalimide as a protecting group for the sphingosine amino
function and different glycosylation methods have been explored.
Among the acceptors tested, the easiest accessible acceptor,
unprotected on the two hydroxyl groups in positions 1 and 3, was
regioselectively glycosylated on the primary position, the regiose-
lectivity depending on the donor used.
t is well-known that glycosphingolipids (GSLs), components
of the plasma membrane where their hydrophilic portions
withdraws the electron density from this oxygen, making the
primary hydroxyl group less nucleophilic.4
Therefore, we decided to choose a protecting group that
should allow us to avoid this drawback and, possibly, to have a
positive effect, and we selected tetrachlorophthalimide7 (TCP)
(Figure 1) for this purpose.
I
are exposed toward the cell surface and the hydrophobic
moieties are inserted into the membrane layer, play essential
roles in cellular trafficking, signaling functions,1 and interactions
of the cells with various agents,2 proliferation, differentiation,
apoptosis, and cellular embryogenesis.3 Most GSLs are
composed of a hydrophobic moiety, ceramide, and a hydro-
philic group of core monosaccharides.
Ceramide, in turn, is composed of sphingosine, a long-chain
2-amino-1,3-diol in the D-erythro configuration containing a
C4−C5 trans double bond, linked to a fatty acid, usually with a
long chain, which is sometimes hydroxylated or unsaturated.
The biological properties of GLS and the need for pure,
structurally well-defined compounds prompted the develop-
ment of various synthetic ways for their preparation.
Figure 1. Hydrogen bonding in sphingosines.
The two most relevant aspects in GLS synthesis are the
availability of sphingosine and the glycosylation of either
sphingosine or ceramide precursors.4 A number of sphingosine
syntheses are described in the literature.5 Among these, efficient
preparations of sphingosine from much cheaper phytosphingo-
sine have been recently described.6 We therefore decided to
exploit this approach, not with the major aim of developing an
efficient route to sphingosine but to obtain a properly protected
sphingosine acceptor for effective glycosylation by an accurate
choice of the protecting groups, minimizing the protection−
deprotection steps and therefore shortening the synthesis.
We here describe the results obtained from application of
different protection strategies to phytosphingosine, two differ-
ent conversions of phytosphingosine to sphingosine and the
glycosylation properties of the so-obtained protected sphingo-
sines.
TCP is safer in comparison to the most commonly used
azide, does not require the use of harmful triflic azide, and was
also expected to be easy to remove at the end of the synthesis.
Our first approach to the synthesis of sphingosine from
phytosphingosine is shown in Scheme 1.
Treatment of phytosphingosine 1 with tetrachlorophthalic
anhydride in DMF at 150 °C, after pouring the mixture in
water and filtration of the precipitate, gave almost pure 2 in a
very satisfactory yield.8
Silylation of the primary hydroxyl group with TBDPSCl
furnished 3, again in excellent yield. Compound 3 was
efficiently converted in two steps into the corresponding cyclic
sulfate 4 by reacting the remaining 3,4-hydroxyl groups with
SOCl2 and oxidation of the intermediate cyclic sulfite with
RuCl3/NaIO4.6
We started by selecting the protecting group to be
introduced on the amino function. It is now commonly
accepted that the presence of a hydrogen bond between the
electron pair on the C-1 oxygen and the amide hydrogen
Received: December 20, 2013
© XXXX American Chemical Society
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Scheme 1. First Synthesis of Sphingosine Acceptors
Scheme 2. Second Synthesis of a Sphingosine Acceptor
Both glycosylation reactions on 7a gave satisfactory results
without relevant differences between the two galactosyl donors
affording product 13 in good yield (Scheme 3).
Scheme 3. Glycosylations of Acceptor 7a
Opening of the cyclic sulfate 4 with tetrabutylammonium
iodide, DBU-promoted elimination,9 and sulfate hydrolysis
afforded sphingosine 5 in excellent yield. It should be noted
that when the reaction was attempted as described,6 namely
using toluene for the sulfate opening, for unknown reasons the
overall yield was around 35−40%. We obtained good results
only by using the solvents indicated in Scheme 1.
Protection of the 3-OH group either as benzoate or as a p-
methoxybenzyl ether10 followed by desilylation of the 1-OH
afforded two different acceptors, 7a and 7b.
However, we were looking for a shorter synthetic scheme to
the sphingosine acceptor. Toward this goal, we protected the 1-
and 3-OH groups as p-methoxybenzylidene acetal. Then we
efficiently introduced the 4,5 double bond through 4-OH
substitution with an iodine atom, brought about by treatment
with I2, PPh3, and imidazole11 followed by DBU-promoted
elimination, which provided the protected sphingosine 10
(Scheme 2).
In order to obtain the acceptor 7b, we attempted to perform
a regioselective reductive ring-opening of the p-methoxybenzy-
lidene acetal using NaCNBH3 and different acidic promoters
(Me3SiCl, t-BuMe2SiCl, t-BuMe2SiOTf),12 but we invariably
obtained a mixture of 1- and 3-PMB derivatives in a ratio of 2:1.
On the other hand, Castillon has described the efficient
regioselective glycosylation of a 1,3-diol at the C-1 position
exploiting a 1,3-stannylene derivative.13 Therefore, we decided
to explore the possibility of exploiting a similar approach with
diol 11, either as such or as the stannylene derivative. Diol 11
was easily obtained by conventional removal of the p-
methoxybenzylidene acetal from compound 10.
When acceptor 7b was submitted to glycosylation using
donor 14, product 15 was obtained in 86% yield (Scheme 4),
and the PMB group was simultaneously removed under the
reaction conditions, avoiding a separate deprotection step.
The most interesting results, however, were obtained with
acceptor 11 (Scheme 5). The use of imidate 12 as donor (1.3
equiv) gave, besides the expected product 15, some unwanted
diglycosylated derivative 16. When glycosylation was performed
with donor 14 in the same conditions used for acceptors 7a and
Scheme 4. Glycosylation of Acceptor 7b
With the acceptors 7a, 7b, and 11 in hand, their properties as
glycosyl acceptors were then explored. As donors we chose the
perbenzoylated galactopyranosyl trichloroacetimidate 1214 or
SBox galactopyranoside 14.15
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aldehyde5c proceeded in 42% yield. The overall yield from 1
to 19 was 34%. Our results show the importance of a proper
tuning of the donor reactivity. In the present case, SBox donor
14 was better than donor 12 not only as it had the correct
reactivity for good selectivity but also because it is easy to
prepare and shelf-stable and the reaction conditions do not
require low temperature.
Scheme 5. Glycosylation of Acceptor 11 and Synthesis of
Galactocerebroside
We are currently extending the use of TCP to phytosphin-
gosine glycosylation en route to αGalCer derivatives.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and hard copies of NMR spectra of
new and known but not fully characterized compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: luigi.panza@unipmn.it.
Present Address
†Department of Pharmaceutical Sciences, School of Pharmacy
and Markey Cancer Center, University of Kentucky, Lexington,
KY 40536.
7b, we were delighted to obtain the expected glycosylation
product 15 on the primary hydroxyl group almost exclusively,
without the use of any tin derivative. When a small excess of
donor (1.1 equiv) was used the yield was 62% (85% based on
recovered acceptor). Investigation of the reaction mixture
revealed, as expected, that a tiny amount of diglycosylated
product 16 was also formed (together with traces of other
unidentified byproducts). Therefore the reaction was repeated
using an excess of acceptor (2 equiv) in order to avoid the
formation of 16. Under these conditions, compound 15 was
obtained in a satisfactory 69% yield (90% based on recovered
acceptor).
To demonstrate the feasibility and efficiency of the whole
scheme, we performed the TCP deprotection, the acylation,
and the final debenzoylation steps. When TCP deprotection
was attempted on compound 13, under the usual conditions16
(1,2-ethylendiamine, EtOH, 60−80 °C), we observed a
significant migration of the benzoyl group from the 3 to the
2 position to give the corresponding benzamide as main
product (data not shown). Similar behavior has already been
described in literature,17 and the use of milder conditions for
TCP deprotection did not give satisfactory results. On the
other hand, TPC deprotection on compound 15 using slightly
modified literature conditions,16 in particular working at room
temperature rather than at 60 °C, smoothly gave amine 17,
which was used directly for the next step. Acylation of 1718 and
debenzoylation19 of compound 18 were very satisfactory,
affording galacto cerebroside 1920 in good yield.
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Hakomori, S. I. Glycoconjugate J. 2000, 17, 143−151.
(2) Varki, A. Glycobiology 1993, 3, 97−130.
(3) Tettamanti, G.; Bassi, R.; Viani, P.; Riboni, L. Biochimie 2003, 85,
423−437.
(4) Morales-Serna, J. A.; Boutureira, O.; Díaz, Y.; Matheu, M. I.;
Castillon
́
, S. Carbohydr. Res. 2007, 342, 1595−1612.
(5) (a) Llaveria, J.; Díaz, Y.; Matheu, M. I.; Castillon
́
, S. Org. Lett.
2009, 11, 205−208 and references cited therein. For reviews, see:
(b) Koskinen, P. M.; Koskinen, A. M. P. Synthesis 1998, 1075−1091.
(c) Morales-Serna, J. A.; Llaveria, J.; Diaz, Y.; Matheu, M. I.; Castillon,
S. Curr. Org. Chem. 2010, 14, 2483−2521.
(6) (a) Lee, Y. M.; Lee, S.; Jeon, H.; Baek, D. J.; Seo, J. H.; Kim, D.;
Kim, S. Synthesis 2011, 867−872. (b) van den Berg, R. J. B. H. N.; van
den Elst, H.; Korevaar, C. G. N.; Aerts, J. M. F. G.; van der Marel, G.
A.; Overkleeft, H. S. Eur. J. Org. Chem. 2011, 6685−6689.
(7) (a) Debenham, J. S.; Debenham, S. D.; Fraser-Reid, B. Bioorg.
Med. Chem. 1996, 4, 1909−1918. (b) Castro-Palomino, J. C.; Schmidt,
R. R. Tetrahedron Lett. 1995, 36, 5343−5346.
(8) Golub, A. G.; Yakovenko, O. Ya.; Prykhod’ko, A. O.; Lukashov, S.
S.; Bdzhola, V. G.; Yarmoluk, S. M. Biochim. Biophys. Acta 2008, 1784,
143−149.
(9) Kim, S.; Lee, N.; Lee, S.; Lee, T.; Lee, Y. M. J. Org. Chem. 2008,
73, 1379−1385.
(10) Gathriwa, J.; Maki, T. Tetrahedron 2012, 68, 370−375.
(11) (a) Garegg, P. J.; Samuelsson, B. J. Chem. Soc., Perkin Trans. 1
́
1980, 2866−2869. (b) Jackowski, O.; Chretien, F.; Didierjean, C.;
In conclusion, we have described the use of tetrachloroph-
thalimide as an effective protecting group for the amino
function of phytosphingosine, allowing its efficient conversion
to sphingosine acceptors for glycolipid synthesis. The overall
yield competes well with other sphingosine synthesis:
compound 11, from which sphingosine can be obtained in
one step, has been obtained in 72% overall yield. A recent,
efficient sphingosine synthesis6b reports a 67% overall yield,
while a more classical synthesis starting from Garner’s
Chapleur, Y. Carbohydr. Res. 2012, 356, 93−103.
(12) Gustafsson, T.; Schou, M.; Almqvist, F.; Kihlberg, J. J. Org.
Chem. 2004, 69, 8694−8701.
́
(13) (a) Morales-Serna, J. A.; Díaz, Y.; Matheu, M. I.; Castillon, S.
Eur. J. Org. Chem. 2009, 3849−3852. See also: (b) Liu, K. K.-C.;
Danishefsky, S. J. J. Org. Chem. 1994, 59, 1895−1897. (c) Liu, K. K.-
C.; Danishefsky, S. J. J. Am. Chem. Soc. 1993, 115, 4933−4934.
(14) Mbadugha, B. N. A.; Menger, F. M. Org. Lett. 2003, 5, 4041−
4044.
C
dx.doi.org/10.1021/ol403688t | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(15) Kamat, M. N.; Rath, N. P.; Demchenko, A. V. J. Org. Chem.
2007, 72, 6938−6946.
(16) Debenham, J. S.; Madsen, R.; Roberts, C.; Fraser-Reid, B. J. Am.
Chem. Soc. 1993, 117, 3302−3303.
(17) Jia, Z. J.; Kelberlau, S.; Olsson, L.; Anilkumar, G.; Fraser-Reid, B.
Synlett 1999, 565−566.
(18) Michieletti, M.; Sillani, L.; Panza, L. Synlett 2009, 2609−2612.
(19) See, e.g.: Kimura, A.; Imamura, A.; Ando, H.; Ishida, H.; Kiso,
M. Synlett 2006, 2379−2382.
(20) Compostella, F.; Franchini, L.; De Libero, G.; Palmisano, G.;
Ronchetti, F.; Panza, L. Tetrahedron 2002, 58, 8703−8708.
D
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