Efficient synthesis of α-galactosyl ceramide analogues using glycosyl iodide donors

Article abstract of DOI:10.1021/ol050659f

(Chemical Equation Presented) The combination of reactive galactosyl iodide donors with electron-rich acceptor lipids provides highly stereoselective and efficient routes to α GalCer analogues. Using per-O-silylated donors, key intermediates can be obtain

Full text of DOI:10.1021/ol050659f

ORGANIC
LETTERS
2005
Vol. 7, No. 10
2063-2065
Efficient Synthesis of
Ceramide Analogues Using Glycosyl
r-Galactosyl
Iodide Donors
Wenjun Du and Jacquelyn Gervay-Hague*
UniVersity of California, DaVis, Department of Chemistry, One Shields AVenue,
DaVis, California 95616
gerVay@chem.ucdaVis.edu
Received March 28, 2005
ABSTRACT
The combination of reactive galactosyl iodide donors with electron-rich acceptor lipids provides highly stereoselective and efficient routes to
GalCer analogues. Using per-O-silylated donors, key intermediates can be obtained in a three-step, one-pot sequence providing useful
r
constructs for analogue development.
Since the discovery of R O-galactosyl ceramides from marine
sponge in 1993,1 several studies of KRN7000 (1) (Figure 1)
and its derivatives have been reported. These investigations
have revealed that tumor-associated cell-surface glycopro-
teins such as CD1d present exogenous lipids to natural killer
T (NKT) cells causing the release of chemokines that regulate
immune response to cancer.2 The potent immunostimulatory
activities of lipid antigens such as KRN7000 have led to the
development of anticancer chemotherapeutics that are cur-
rently in clinical trials.3 Moreover, R O-galactosyl ceramide
analogues have provided important tools for elucidating
signal transduction pathways involved in CD1d-mediated
antigen presentation.4
been most successfully employed in the formation of R
O-glycosidic linkages. Yields for the fluoride typically range
between 30 and 60% and are complicated by the formation
of the â anomer, which is difficult to separate.5 Stereose-
lectivity can be improved with the use of galactosyl trichlo-
roacetimidates,6 but the reaction requires portionwise addition
of both the donor and acceptor, otherwise yields drop and
approximately one-half of the unreacted acceptor is recov-
ered.7 Thioglycosides and glycosyl bromides8 have also been
utilized, and while relatively easy to employ, both yields and
stereoselectivities are compromised with these donors.
Here, we report remarkably efficient syntheses of R
O-galactosyl ceramides using galactosyl iodide donors. We
recently demonstrated the utility of glycosyl iodides in the
synthesis of R-glycosides.9 Stereoselectivity is achieved from
in situ anomerization of the R-iodide to a more reactive
Several syntheses of KRN7000 analogues have been
reported. However, current glycosidation methods suffer from
relatively low yields and sometimes poor R/â selectivities.
Galactosyl fluoride and trichloroacetimidate donors have
(1) Natori, N.; Koezuka, Y., Higa, T. Tetrahedron Lett. 1993, 34, 5591-
5592.
(2) (a) Brutkiewicz, R. R.; Sriram, V. Crit. ReV. Oncol./Hematol. 2002,
41, 287-298. (b) Brutkiewicz, R. R.; Lin, Y.; Cho, S.; Hwang, Y. K.;
Sriram, V.; Roberts, T. J. Crit. ReV. Immunol. 2003, 23, 403-419. (c) Zhou,
D.; Cantu, C., 3rd; Sagiv, Y.; Schrantz, N.; Kulkarni, A. B.; Qi, X.; Mahuran,
D. J.; Morales, C. R.; Grabowski, G. A.; Benlagha, K.; Savage, P. B.;
Bendelac, A.; Teyton, L. Science 2004, 303, 523-527.
(3) Crul, M,.; Mathor, R. A. A.; Giaccone, G.; Punt, C. J. A.; Rosing,
H.; Hillebrand, M. J. X.; Ando, Y.; Nishi, N.; Tanaka, H.; Schellens, J. H.
M.; Beijnen, J. H. Cancer Chemother, Pharmacol. 2002, 49, 287-293.
Figure 1. KRN7000 (1).
10.1021/ol050659f CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/14/2005

Scheme 1. Synthesis of Azido Sphingosine Acceptor
Scheme 2. Synthesis of Desoxy-KRN7000
â-iodide, which subsequently undergoes SN2-like displace-
ment to give exclusively the R-glycoside. Lemieux first
introduced the concept of in situ anomerization using
glycosyl bromides.10 However, we have found that reactions
with glycosyl iodides are orders of magnitude faster,
proceeding quantitatively, with exclusive delivery of the R
anomer. These advantages prompted us to undertake the
studies reported herein. When tetrabutylammonium iodide
(TBAI) is used as a promoter, glycosidation of sphingosine
and phytosphingosine proceeds in over 90% yield giving
exclusively the R O-galactosyl linkage. The reactions are
made even more efficient when per-O-silylated sugars are
used as the iodide precursor alleviating the need for selective
functionalization of the anomeric center.
Initial investigations began with commercially available
sphingosine (2), which is also available by total synthesis.11
The amino group was first converted to an azide (Scheme
1) in 93% yield according to the procedure reported by Wong
and co-workers.12 It is preferable to protect the amine as an
azide for the glycosidation step, as amides diminish the
nucleophilicity of the primary hydroxyl through hydrogen
bond donation.13 The primary alcohol was temporarily
blocked with a trityl ether and the secondary alcohol was
protected with an electron donating ether (p-methoxybenzyl
(PMB)) to enhance the overall nucleophiliciy of the acceptor
alcohol. Polt and co-workers have reported similar advan-
tages when using electron-donating protecting groups in the
glycosidation of serine derivatives.13b After deprotection of
the trityl group with BF3‚OEt2, a sphingosine acceptor (3)
was available for glycosidation.
The iodide donor (4) was generated in situ from 2,3,4,6-
tetra-O-benzylgalactosyl acetate according to a procedure
previously reported by our laboratory.9a Using TBAI as a
promoter, the reaction of 4 (3 equiv) with the sphingosine
acceptor (3) proceeded smoothly at 65 °C in 1.5 h giving
94% yield of the R-glycoside (5) after column chromatog-
raphy (Scheme 2). Hydrogenation of 5 to arrive at the
deprotected amine was retarded, presumably due to amine
deactivation of the catalyst.14 Therefore, we decided to reduce
the azide prior to hydrogenation. Staudinger reduction of the
azide utilizing either triphenylphosphine or triethylphosphine
did not proceed efficiently, instead we found that hydrogen
sulfide in pyridine/water was preferable producing the amine
in 88% yield. The synthesis of 4-desoxy KRN7000 (6) was
completed by condensation of the amine with stearic acid
followed by hydrogenation, which resulted in global depro-
tection and concomitant reduction of the double bond.
Encouraged by the successful synthesis of 6, we proceeded
with the synthesis of KRN7000 using the same strategy.
Phytosphingosine15 was converted into an activated acceptor
(7) by amine to azide conversion and incorporation of PMB
ethers on both of the secondary alcohols. Glycosidation with
donor 4 was uneventful yielding the glycoconjugate (8) in
90% yield after column chromatography. Reduction of the
azide, amidation, and global deprotection afforded KRN7000
in excellent overall yield (Scheme 3).
(4) Sriram, V.; Du, W.; Gervay-Hague, J.; Brukiewicz, R. R. Cancer
Cell submitted.
(5) (a) Morita, M.; Motoki, K.; Akimoto, K.; Natori, T.; Sakai, T.; Sawa,
E.; Yamaji, K.; Koezuka, Y.; Kobayashi, E.; Fukushima, H. J. Med. Chem.
1995, 38, 2176-2187. (b) Takikawa, H.; Muto, S.; Mori, K. Tetrahedron
1998, 54, 3141-3150. (c) Barbieri, L.; Costantino, V.; Fattorusso, E.;
Mangoni, A.; Aru, E.; Parapini, S.; Taramelli, D. Eur. J. Org. Chem. 2004,
468-473. (d) Zhou, X.-T.; Forestier, C.; Goff, R. D.; Li, C.; Teyton, L.;
Bendelac, A.; Savage, P. B. Org. Lett. 2002, 4, 1267-1270.
(6) (a) Plettenburg, O.; Bodmer-Narkevitch, V.; Wong, C.-H. J. Org.
Chem. 2002, 67, 4559-4564. (b) Kim, S.; Song, S.; Lee, T.; Jung, S.; Kim,
D. Synthesis 2004, 6, 847-850.
(7) Fiegueroa-Perez, S.; Schmidt, R. R. J. Carbohydr. Chem. 2000, 328,
95-102.
(8) Goff, R. D.; Gao, Y.; Mattner, J.; Zhou, D.; Yin, N.; Cantu, C., III;
Teyton, L.; Bendelac, A.; Savage P. B. J. Am. Chem. Soc. 2004, 126,
13602-13603.
Because R galactosyl ceramides are lipid antigens, the 4,5-
dehydro analogue of KRN7000 is an obvious choice for
analogue synthesis. Initial attempts to achieve selective
debenzylation of 5 using Lindlar catalyst resulted in relatively
inefficient reactions that led to byproduct formation. In earlier
work, we demonstrated that per-O-silylated sugars are
excellent precursors to glycosyl iodides that readily undergo
nucleophilic addition by cyanide.16 These donors are par-
ticularly attractive due to their ease of synthesis and facile
deprotection under methanolysis conditions. Thus reaction
(9) (a) Hadd, M. J.; Gervay, J. Carbohydr. Res. 1999, 320, 61-69. (b)
Lam, S. N.; Gervay-Hague, J. Org. Lett. 2002, 4, 2039-2042. (c) Lam, S.
N.; Gervay-Hague, J. Carbohydr. Res. 2002, 337, 1953-1965.
(10) Lemieux, R. U.; Hendricks, K. B.; Stick, R. V.; James, K. J. Am.
Chem. Soc. 1975, 97, 4056-4062.
(11) Zimmermann, P.; Schmidt, R. R. Liebigs Ann. Chem. 1988, 663-
667.
(12) Alper, P. B.; Hung, S.-C.; Wong, C.-H. Tetrahedron Lett. 1996,
37, 6029-6032.
(13) (a) Schmidt, R. R.; Michel, J. Angew. Chem., Int. Ed. In Engl. 1980,
19, 731-732. (b) Szabo, L.; Li, Y.; Polt, R. Tetrahedron Lett. 1991, 32,
585-588.
(14) Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4076-4078.
(15) Chiu, H.-Y.; Tzou, D.-L.; Patkar, L. N.; Lin, C.-C. J. Org. Chem.
2003, 68, 5788-5791.
(16) Bhat, A.; Gervay-Hague, J. Org. Lett. 2001, 3, 2081-2084.
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Org. Lett., Vol. 7, No. 10, 2005

Scheme 3. Synthesis of KRN7000
Scheme 4. Rapid Analogue Synthesis
of per-O-trimethylsilylgalactopyranose (9) with TMSI pro-
vided the galactosyl iodide in 10 min at 0 °C. After
evaporation of the solvent, 0.33 equiv of 3, 2 equiv of TBAI,
and 1.5 equiv of DIEA in benzene were added to the iodide
and stirred for 45 min, after which time TLC indicated the
reaction was complete. The crude product was refluxed in
methanol with Dowex H+ resin for 45 min. The NMR
spectrum of the resulting glycolipid (10) was remarkably
clean, and only the R anomer was observed (Scheme 4). For
our own purposes, the azide is a useful intermediate for
analogue development so we terminated the process after
removal of the PMB protecting group, but one could easily
continue with reduction of the azide and subsequent con-
densation with a lipid of interest.
one-pot sequence providing useful intermediates for analogue
development.
Acknowledgment. This work was supported by National
Science Foundation CHE-0210807. NSF CRIF program
(CHE-9808183), NSF Grant OSTI 97-24412, and NIH Grant
RR11973 provided funding for the NMR spectrometers used
on this project.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
In summary, the combination of highly reactive galactosyl
iodide donors with electron rich acceptor lipids provides
stereoselective and efficient routes to R GalCer analogues.
Key intermediates such as 10 can be obtained in a 3-step
OL050659F
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