Synthesis of C -glycoside analogues of α-galactosylceramide via linear allylic C-H oxidation and allyl cyanate to isocyanate rearrangement

Article abstract of DOI:10.1021/ol2032448

C-Glycoside analogues of α-galactosylceramide were synthesized in which several significant modifications known to promote Th-1 cytokine production were included. The key transformations include C-H oxidation, Sharpless asymmetric epoxidation, olefin cros

Full text of DOI:10.1021/ol2032448

ORGANIC
LETTERS
2012
Vol. 14, No. 2
620–623
Synthesis of C-Glycoside Analogues of
R-Galactosylceramide via Linear Allylic
CꢀH Oxidation and Allyl Cyanate to
Isocyanate Rearrangement
Zheng Liu and Robert Bittman*
Department of Chemistry and Biochemistry, Queens College of The City University of
New York, Flushing, New York 11367-1597, United States
robert.bittman@qc.cuny.edu
Received December 5, 2011
ABSTRACT
C-Glycoside analogues of R-galactosylceramide were synthesized in which several significant modifications known to promote Th-1 cytokine
production were included. The key transformations include CꢀH oxidation, Sharpless asymmetric epoxidation, olefin cross metathesis, and an
allyl cyanate to isocyanate rearrangement.
R-Galactosylceramide (1, also known as R-GalCer
or KRN7000), an optimized synthetic material origi-
nating from a marine sponge, is the most widely
studied glycolipid antigen for activating invariant
natural killer T (iNKT) cells (Figure 1).1 These cells
are a subset of T lymphocytes that interact with
glycolipid antigens presented by the major histocom-
patibility complex class I-related glycoprotein CD1d.2
Immunoregulatory cytokines, such as IFN-γ (Th-1
type) and IL-4 (Th-2 type), produced by stimulated
iNKT cells hold substantial promise in immunother-
apy and for development of vaccine adjuvants.3 How-
ever, phase I clinical trials of 1 in the treatment of solid
tumors have been ineffective, perhaps as a conse-
quence of counteraction of the Th-1 and Th-2 cyto-
kines induced by 1.4
A variety of glycolipid antigens that can differentially
elicit distinct effector functions in iNKT cells have been
identified. For example, installing an OMe group at the
60-position of the galactosyl moiety gave rise to the
strong Th-1 biasing ligand RCAI-61 (2).5 Introduction of
a p-fluorophenyl group at the terminus of the fatty amide
chain led to 7DW8-5 (3), which induced selective produc-
tion of IFN-γ.6
(4) (a) Giaccone, G.; Punt, C. J.; Ando, Y.; Ruijter, R.; Nishi, N.;
Peters, M.; von Blomberg, B. M.; Scheper, R. J.; van der Vliet, H. J.;
van den Eertwegh, A. J.; Roelvink, M.; Beijnen, J.; Zwierzina, H.; Pinedo,
H. M. Clin. Cancer Res. 2002, 8, 3702. (b) Dhodapkar, M. V.; Geller,
M. D.; Chang, D. H.; Shimizu, K.; Fujii, S.; Dhodapkar, K. M.;
Krasovsky, J. J. Exp. Med. 2003, 197, 1667.
(1) For recent reviews of KRN7000 (compound 1) and its analogues,
ꢀ
see: (a) Banchet-Cadeddu, A.; Henon, E.; Dauchez, M.; Renault, J.-H.;
Monneaux, F.; Haudrechy, A. Org. Biomol. Chem. 2011, 9, 3080. (b) Mori,
K.; Tashiro, T. Heterocycles 2011, 83, 951. (c) Murphy, N.; Zhu, X.; Schmidt,
R. R. Carbohydr. Chem. 2010, 36, 64.
(5) Tashiro, T.; Nakagawa, R.; Inoue, S.; Shiozaki, M.; Watarai, H.;
Taniguchi, M.; Mori, K. Tetrahedron Lett. 2008, 49, 6827.
(2) Godfrey, D. I.; MacDonald, H. R.; Kronenberg, M.; Smyth,
M. J.; Van Kaer, L. Nat. Rev. Immunol. 2004, 4, 231.
(3) (a) Kronenberg, M. Annu. Rev. Immunol. 2005, 23, 877. (b) Van Kaer,
L.; Parekh, V. V.; Wu, L. Immunotherapy 2011, 3, 59.
(6) (a) Li, X.; Fujio, M.; Imamura, M.; Wu, D.; Vasan, S.; Wong,
C.-H.; Ho, D. D.; Tsuji, M. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 13010.
(b) Wu, T.-N.; Lin, K.-H.; Chang, Y.-J.; Huang, J.-R.; Cheng, J.-Y.; Yu,
A. L.; Wong, C.-H. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 17275.
r
10.1021/ol2032448
Published on Web 01/10/2012
2012 American Chemical Society

human iNKT cells and induce the maturation and activation
of human dendritic cells through iNKT-cell activation.9
Scheme 2. Synthesis of 9
Figure 1. Structures of glycolipids 1ꢀ6.
Scheme 1. Retrosynthetic Plan For 6a
As part of our ongoing investigations of C-glycoside
analogues of KRN7000,10 we have designed analogues
6a,b, which combine several significant structural modifica-
tions for selective Th-1 cytokine production. In addition to
the installation of the 60-OMe and p-fluorophenyl substi-
tutions stated above, the report that 4-deoxy-KRN7000
initiates cytokine production similarly to that induced by
1 in human iNKT cells in vitro, and in its murine counter-
part in vivo,11 prompted the synthesis of the correspond-
ing 4-deoxyphytosphingosine moiety in C-glycoside 6.
Our retrosynthetic plan is illustrated in Scheme 1. It was
reported that cross-metathesis12 (CM) of vinyl-C-galacto-
sides requires a high loading of catalysts and that the yields
of the coupling product are low and sensitive to the
R-C-GalCer (4), a C-glycoside analogue of KRN7000,7
binds more stably than 1 to dendritic cells and acts as a
more effective link between innate and adaptive immunity
invivo.8 Infact, comparisonsof 1 and 4in mouse modelsof
disease revealed that the C-glycoside 4 displayed higher
activity.7b Interestingly, 4 was found to be a weak agonist
of human iNKT cells in vitro, but C-glycoside analogues
that feature an E-alkene as a spacer between the galactose
moiety and the ceramide, such as GCK152 (5), activate
(10) (a) Lu, X.; Song, L.; Metelitsa, L. S.; Bittman, R. ChemBioChem
2006, 7, 1750. (b) Liu, Z.; Byun, H.-S.; Bittman, R. Org. Lett. 2010, 12,
2974. (c) Liu, Z.; Byun, H.-S.; Bittman, R. J. Org. Chem. 2011, 76, 8588.
^
(11) Lacone, V.; Hunault, J.; Pipelier, M.; Blot, V.; Lecourt, T.;
Rocher, J.; Turcot-Dubois, A.-L.; Marionneau, S.; Douillard, J.-Y.;
ꢀ
Clement, M.; Le Pendu, J.; Bonneville, M.; Micouin, L.; Dubreuil, D.
J. Med. Chem. 2009, 52, 4960.
(7) For a review of R-C-GalCer (compound 4), see: (a) Franck, R. W.;
Tsuji, M. Acc. Chem. Res. 2006, 39, 692. (b) Franck, R. W. C. R. Chimie
2011, in press, doi:10.1016/j.crci.2011.05.006.
(8) Fujii, S.; Shimizu, K.; Hemmi, H.; Fukui, M.; Bonito, A. J.; Chen,
G.; Franck, R. W.; Tsuji, M.; Steinman, R. M. Proc. Natl. Acad. Sci.
U.S.A. 2010, 103, 11252.
(12) (a) Blackwell, H. E.;O’Leary, D. J.;Chatterjee, A. K.;Washenfelder,
R. A.; Bussmann, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 58.
(b)Chatterjee,A.K.;Choi,T.-L.;Sanders,D.P.;Grubbs,R.H.J. Am. Chem.
Soc. 2003, 125, 11360.
(13) Kulkarni, S. S.; Gervay-Hague, J. Org. Lett. 2006, 8, 5765.
(14) Chen, G.; Schmieg, J.; Tsuji, M.; Franck, R. W. Org. Lett. 2004,
6, 4077.
(9) Li, X.; Chen, G.; Garcia-Navarro, R.; Franck, R. W.; Tsuji, M.
Immunology 2009, 127, 216.
Org. Lett., Vol. 14, No. 2, 2012
621

Scheme 3. Synthesis of 6a
protecting groups on the vinyl-C-galactoside.13,14 There-
fore, a CM disconnection of 8 was deemed more practical
than for 7, leading to the simple synthons 9 and 13. The
required amide at C-3 could potentially be delivered with
stereocontrol from C-1 via an allyl cyanate to isocyanate
rearrangement15 (8 to 7). The concerted nature of the
sigmatropic rearrangement would secure a high level of
[1,3]-chirality transfer during the CꢀO to CꢀN bond
reorganization. The stereocontrolled construction of the
hydroxyl group at the allylic position in 9 could be fulfilled
via zinc-mediated reductive elimination of the asymmetric
epoxide halide 10, which might be accessible from ter-
minal alkene 11 via linear CꢀH oxidation16 followed by
Sharpless asymmetric epoxidation (SAE).17 R-Allyl galac-
toside 11 could possibly be prepared from β-galactoside
12 through straightforward transformations according
to known protocols.18 Similarly, allylic ester 13, the lipid
olefin for CM, could be obtained through a sequence of
SAE and reductive elimination from known epoxy alcohol
14,19,20 which is accessible from palmitaldehyde (15).
As shown in Scheme 2, our synthesis commenced with
the preparation of R-allyl-C-galactoside 11 from commer-
cially available penta-O-acetyl-β-D-galactose (12) and in-
volved straightforward protecting group manipulations
using 16 and 17.18 Linear CꢀH oxidation of 11 under
conditions developed by Chen and White16a [Pd(OAc)2,
benzoquinone (BQ), DMSO/AcOH (1:1), 50 °C, 100 h]
afforded 18 in low yield (17%) and incomplete conversion
(71%). However, treatment of 11 under conditions16b
revised by White et al., followed by acetate deprotection
under basic conditions, provided allylic alcohol 19 in 40%
yield (two steps) with high E-selectivity (E/Z ratio >20:1).
SAE of 19 smoothly afforded epoxy alcohol 20 in 81% yield.
Iodination of 20 afforded 10, which upon zinc-mediated
reductive elimination gave rise to 9 (71% yield, two steps).
The R configuration of C-1 in 9 was established by the
advanced Mosher method (see Supporting Information),21
and the dr value was determined on the basis of the 1H NMR
spectra of the corresponding Mosher esters.
(15) For a review of the allyl cyanate to isocyanate rearrangement,
see: Ichikawa, Y. Synlett 2007, 2927.
(16) (a) Chen, M. S.; White, M. C. J. Am. Chem. Soc. 2004, 126, 1346.
(b) Covell, D. J.; Vermeulen, N. A.; Labenz, N. A.; White, M. C. Angew.
Chem., Int. Ed. 2006, 45, 8217.
(17) For a review of SAE, see: Katsuki, T.; Martin, V. S. Org. React.
1996, 48, 1.
(18) Cabaret, D.; Wakselman, M. J. Carbohydr. Chem. 1991, 10, 55.
(19) Liu, Z.; Gong, Y.; Byun, H.-S.; Bittman, R. New J. Chem. 2010,
34, 470.
(20) Roush, W. R.; Adam, M. A. J. Org. Chem. 1985, 50, 3752.
(21) (a) Kusumi, T.; Fukushima, T.; Ohtani, I.; Kakisawa, H. Tetra-
hedron Lett. 1991, 32, 2939. (b) For a review of the advanced Mosher
~ ꢀ
method, see: Seco, J. M.; Quinoa, E.; Riguera, R. Chem. Rev. 2004, 104,
17. The 1H NMR spectra of all MTPA-esters/amides are included in the
Supporting Information.
622
Org. Lett., Vol. 14, No. 2, 2012

The synthesis of lipid olefin 21 was accomplished in
a similar way, using SAE to introduce the chirality
(Scheme 3). Unlike the strategy involving zinc-mediated
reductive elimination of an epoxide halide, the trans-
formation of 2,3-epoxy alcohol 14 to allylic alcohol
21 was achieved in one step by using the titanocene-
induced regioselective deoxygenation protocol devel-
oped by Yadav and co-workers.22 CM with 15 mol %
Grubbs second generation catalyst (G-2) using 3.2
molar equiv of lipid olefin 13 with 9 provided 8 in
70% yield with high E-selectivity (E/Z ratio >20:1).
This reaction required reflux for only 2 h, and 48% of 13
was recovered, along with a small amount of the easily
separable homodimers of 13 (11%, based on 13).
Treatment of 8 with trichloroacetyl isocyanate15 afforded
intermediate 22, and hydrolysis with potassium carbonate in
aqueous methanol gave carbamate 24, along with 5% of 23.
Dehydration of 24 with trifluoroacetic anhydride (TFAA)
and triethylamine at 0 °C gave allyl cyanate 25, which
immediately underwent the allyl cyanate to isocyanate
rearrangement15 to afford allyl isocyanate 26. It is note-
worthy that this rearrangement can occur below room
temperature and, thus, is milder than the related Overman
rearrangement.23 Isocyanate 26 was further reacted with
methanol in the presence of a catalytic amount of tributyltin
methoxide24 in situ, providing carbamate 7. The E config-
uration of the alkene was confirmed by the coupling con-
stants of the vinylic protons (16.0 Hz). Basic hydrolysis
of 7 afforded cyclic carbamate 27, which was further
treated with 30% aqueous KOH solution at reflux in
ethanol to afford amine 28. The absolute configuration
at the C-3 position was confirmed by the advanced
Mosher method,21 which revealed the S configuration
at C-3 in 28. The fatty amide chain was then introduced
into amine 28 by using cerotyl chloride25 to afford amide
29 in 97% yield over two steps. Finally, global debenzyl-
ation using Birch reduction furnished the final analogue
6a in 91% yield.
Scheme 4. Synthesis of 6b
As shown in Scheme 4, the corresponding carboxylic
acid 33ofthe amide moiety in36 was preparedin59% yield
(two steps) from commercially available iodide 30 and
alkyne 31 via Sonogashira coupling followed by catalytic
hydrogenation of alkyne 32. For in situ N-acylation, 33
was converted to acyl chloride 34. In order to avoid
defluorination during Birch reduction, debenzylation must
be carried out prior to N-acylation. As a result, target 6b
was obtained in 29% yield over three steps from 27.
In conclusion, we have developed a highly stereocon-
trolled total synthesis of C-glycoside analogues of KRN-
7000 containing an E-alkene linker in 20 steps starting from
penta-O-acetyl-β-D-galactose (12) in 2.5% (6a) and 0.8%
(6b) overall yield. The synthesis showcases the utility of a
linear allylic CꢀH oxidation in synthetic carbohydrate
chemistry and an allyl cyanate to isocyanate rearrangement
for stereoselective construction of the stereogenic center
in the presence of a sugar moiety. These novel R-GalCer
analogues are currently undergoing biological evaluation.
(22) (a) Yadav, J. S.; Shekharam, T.; Gadgil, V. R. J. Chem. Soc.,
Chem. Commun. 1990, 843. (b) Fu, R.; Ye, J.-L.; Dai, X.-J.; Ruan, Y.-P.;
Huang, P.-Q. J. Org. Chem. 2010, 75, 4230. (c) RajanBabu, T. V.;
Nugent, W. A. J. Am. Chem. Soc. 1994, 116, 986. (d) Cuerva, J. M.;
Justicia, J.; Oller-Lopez, J. L.; Oltra, J. E. Top. Curr. Chem. 2006,
264, 63.
(23) Overman, L. E.; Carpenter, N. E. Org. React. 2005, 66, 1.
(24) Ichikawa, Y.; Osada, M.; Ohtani, I.; Isobe, M. J. Chem. Soc.,
Perkin Trans. 1 1997, 1449.
Acknowledgment. This work was supported by National
Institutes of Health Grant HL-083187.
ꢀ
Supporting Information Available. Experimental pro-
1
cedures as well as H and 13C NMR spectra for all new
compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
(25) Wipf, P.; Pierce, J. G. Org. Lett. 2006, 8, 3375.
Org. Lett., Vol. 14, No. 2, 2012
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