Synthesis of glycosyl phosphates from 1,2-orthoesters and application to in situ glycosylation reactions

Article abstract of DOI:10.1021/ol0603155

A series of glycosyl phosphates were prepared in high yield by treatment of the corresponding 1,2-orthoesters with dibutyl phosphate. Glycosyl phosphates are efficient glycosylating agents even when used in crude form or when generated in situ. The immuno

Full text of DOI:10.1021/ol0603155

ORGANIC
LETTERS
2006
Vol. 8, No. 9
1815-1818
Synthesis of Glycosyl Phosphates from
1,2-Orthoesters and Application to in
Situ Glycosylation Reactions
Alessandra Ravida`,† Xinyu Liu, Linda Kovacs,‡ and Peter H. Seeberger*
Laboratorium fu¨r Organische Chemie, Swiss Federal Institute of Technology,
ETH Zu¨rich, HCI F315, Wolfgang-Pauli Strasse 10, 8093 Zu¨rich, Switzerland
seeberger@org.chem.ethz.ch
Received February 6, 2006
ABSTRACT
A series of glycosyl phosphates were prepared in high yield by treatment of the corresponding 1,2-orthoesters with dibutyl phosphate. Glycosyl
phosphates are efficient glycosylating agents even when used in crude form or when generated in situ. The immunodominant epitope
trirhamnoside of group B Streptococcus was prepared to demonstrate the synthetic utility of the method.
The chemical synthesis of oligosaccharides is of utmost
importance to procure tools for glycobiology in sufficient
amounts.1 A plethora of glycosylating agents carrying a
variety of anomeric leaving groups have been developed to
construct glycosidic linkages.2 Glycosyl chlorides, bromides,
iodides, trichloroacetimidates, fluorides, n-pentenyl glyco-
sides, anhydro sugars, as well as anomeric aryl sulfoxides,
and thioglycosides have been applied to the construction of
complex oligosaccharides and glycosylated natural products.3
Glycosyl phosphate triesters are effective glycosylating
agents for the chemical synthesis of carbohydrates.4 Dif-
ficulties associated with the synthesis of anomeric phosphates
from either anomeric lactols5 or other glycosylating agents
such as glycosyl trichloroacetimidates, halides, or n-pentenyl-
and thioglycosides6 diminished the synthetic utility of this
class of glycosylating agents. Introduction of a one-pot
protocol7 gave ready access to glycosyl phosphate building
blocks that now serve as key monomers for the synthesis of
complex oligosaccharides in solution and by automated solid-
phase synthesis.8 The conversion of glycals to glucosyl and
galactosyl phosphates is efficient but requires the use of
dimethyldioxirane (DMDO). DMDO has to be prepared
freshly, can be dangerous to handle, and makes reaction
scaleup difficult.9 The need to access a host of glycosyl
† Visiting Ph.D. student from Dipartimento di Chimica Organica e
Biochimica, Universita` degli Studi di Napoli Federico II, Italy.
‡ Eramus exchange student from Imperial College London, U.K.
(1) (a) Garegg, P. J. Med. Res. ReV. 1996, 16, 345. (b) Seeberger, P. H.
Chem. Commun. 2003, 1115.
(4) Hashimoto, S.; Honda, T.; Ikegami, S. J. Chem. Soc., Chem. Commun.
1989, 685.
(2) Toshima, K.; Tatsuta, K. Chem. ReV. 1993, 93, 1503.
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B. A.; Gin, D. Y. Org. Lett. 2000, 2, 2135.
(3) (a) Koenigs, W.; Knorr, E. Chem. Ber. 1901, 34, 957. (b) Lemieux,
R. U.; Hayami, J. Can. J. Chem. 1965, 43, 2162. (c) Paulsen, H.; Lockhoff,
O. Chem. Ber. 1981, 114, 3102. (d) Schmidt, R. R.; Kinzy, W. AdV.
Carbohydr. Chem. Biochem. 1994, 50, 21. (e) Mukaiama, T.; Murai, Y.;
Shoda, S. Chem. Lett. 1981, 431. (f) Fraser-Reid, B.; Konradsson, P.;
Mootoo, D. R.; Udodong, U. J. Chem. Soc., Chem. Commun. 1988, 823.
(g) Seeberger, P. H.; Bilodeau, M. T.; Danishefsky, S. J. Aldrichimica Acta
1997, 30, 75. (h) Kahne, D.; Walker, S.; Cheng, Y.; Van Engen, D. J. Am.
Chem. Soc. 1989, 111, 6881. (i) Garegg, P. J. AdV. Carbohydr. Chem.
Biochem. 1997, 52, 179.
(6) (a) Schmidt, R. R.; Stumpp, M. Liebigs Ann. Chem. 1984, 680. (b)
Adelhorst, K.; Whitesides, G. M. Carbohydr. Res. 1993, 242, 69. (c) Boons,
G.-J.; Burton, A.; Wyatt, P. Synlett 1996, 310. (d) Pale, P.; Whitesides, G.
M. J. Org. Chem. 1991, 56, 4547. (e) Timmers, C. M.; van Straten, N. C.
R.; van der Marel, G. A.; van Boom, J. H. J. Carbohydr. Chem. 1998, 17, 471.
(7) (a) Plante, O. J.; Andrade, R. B.; Seeberger, P. H. Org. Lett. 1999,
1, 211. (b) Plante, O. J.; Palmacci, E. R.; Andrade, R. B.; Seeberger, P. H.
J. Am. Chem. Soc. 2001, 123, 9545.
(8) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Science 2001, 291, 1523.
10.1021/ol0603155 CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/07/2006

phosphate monomers for use in automated oligosaccharide
synthesis prompted the pursuit of an efficient synthetic means
to generate glycosyl phosphates.
Table 1. Preparation of Various Glycosyl Phosphates from
1,2-Orthoestersa
General applicability and ready scaleup of such a trans-
formation are essential. 1,2-Glycosyl orthoesters are valuable
synthetic intermediates in the preparation of carbohydrate
building blocks.10 Like 1,2-glycals, 1,2-glycosyl orthoesters
possess only three hydroxyl groups to be differentiated.
Treatment of 1,2-glycosyl orthoesters with excess alcohol
nucleophiles under Lewis acid activation generates 2-O-acyl
glycosides with 1,2-trans configuration.11 In the 1970s, per-
acetylated 1,2-orthoesters served as precursors to glycosyl
phosphate monoesters and nucleotide 5′-diphospho sugars,
albeit in highly variable yields.12
Here, we describe the stereoselective conversion of 1,2-
orthoesters to glycosyl 1-phosphate triesters by employing
phosphate diesters as both a nucleophile and an acidic
activator.
Tribenzylmannosyl 1,2-orthoacetate 1 served as a test
substrate en route to find the optimal reaction conditions.
Slow addition of a solution of 1 in dichloromethane to a
solution of dibutyl phosphate in the presence of molecular
sieves (MS) yielded the desired glycosyl phosphate 2. To
minimize the excess of dibutyl phosphate and to simplify
the workup procedures, different bases to quench the acidic
dibutyl phosphate were tested. Just 3 equiv of dibutyl
phosphate is sufficient to convert 1 to mannosyl phosphate
2 in 30 min at room temperature (Scheme 1). Activated 4 Å
Scheme 1. Conversion of 1,2-Orthoester 1 to Mannosyl
Phosphate 2a
a Conditions: 1 equiv of 1 and 3 equiv of dibutyl phosphate.
MS function as drying agent,13 and excess dibutyl phosphate
was quenched by the addition of triethylamine. Filtration
through a pad of Et3N-deactivated silica gel, followed by
column chromatography, affored 2 in 96% yield.
The protocol can be further simplified by adding dibutyl
phosphate dropwise to a solution of 1,2-orthobenzoate 4 in
CH2Cl2. The corresponding mannosyl phosphate 14 was
isolated in 97% yield. This addition sequence can also be
employed when handling orthoacetates such as 1, although
longer reaction times are required to achieve full conversion.
a 3 equiv of dibutyl phosphate was added by syringe to a CH2Cl2 solution
of sugar at room temperature. b Multigram scale. c Addition of the sugar
as solution to dibutyl phosphate (3 equiv) in CH2Cl2 at room temperature.
(9) Love, K. R.; Seeberger, P. H. Org. Synth. 2005, 81, 225.
(10) Lemieux, R. U.; Morgan, A. R. Can. J. Chem. 1965, 43, 2199.
(11) (a) Ogawa, T.; Beppu, K.; Nakabayashi, S. Carbohydr. Res. 1981,
93, C6. (b) Leroux, J.; Perlin, A. S. Carbohydr. Res. 1981, 94, 108.
(12) (a) Volkova, L. V.; Danilov, L. L.; Evstigneeva, R. P. Carbohydr.
Res. 1974, 32, 165. (b) Tsai, J.-H.; Behrman, E. J. Carbohydr. Res. 1978,
64, 297. (c) Salam, M. A.; Behrman, E. J. Carbohydr. Res. 1981, 90, 83;
Carbohydr. Res. 1982, 101, 339. Behrman, E. J. Carbohydr. Res. 1982,
102, 139.
With a suitable protocol in hand, the synthetic scope of this
transformation was explored using a range of mannose 3-6,
glucose 7 and 8, galactose 9, arabifuranose 10, rhamnose
11, and xylose 12 1,2-orthoesters (Table 1).14 The correspond-
(13) AW 300 molecular sieves were also examined, but their acidity
resulted in significant formation (5-20%) of the corresponding O-methyl-
glycoside due to the rearrangement of the 1,2-orthoester.
1816
Org. Lett., Vol. 8, No. 9, 2006

ing glycosyl phosphates were obtained in high yield. 1,2-
Orthobenzoates (4, 6, 8, and 10) reacted faster with dibutyl
phosphate (30 min) than 1,2-orthoacetates (1, 3, 5, 7, 9, 11,
and 12) that required reaction times of 2-3 h. These differ-
ences in reactivity can be rationalized when considering that
the phenyl moiety at the orthoester helps to stabilize the
carboxonium intermediate during orthoester ring opening.
Because the procurement of large amounts of monosac-
charide building blocks is required to supply the starting
materials for automated oligosaccharide synthesis, process
scalability is important. The proof-of-principle reactions were
carried out on a 0.2-0.5 mmol scale, but the process can be
readily scaled up, as demonstrated for the preparation of
several grams of 14.
Then, a solution of glucoside 23 (1.0 equiv) in CH2Cl2 was
added. After cooling the reaction mixture to -30 °C, 3 equiv
of TMSOTf was required to activate the glycosyl phosphate.
Partial quenching of TMSOTf by some remaining phosphoric
acid necessitated this increased amount of activator. Disac-
charide 27 was isolated in 98% yield (Scheme 2). The
Scheme 2. Glycosidations Using in Situ Generated Glycosyl
Phosphatesa
The workup and purification procedure called for filtration
through a plug of silica followed by column chromatography.
NMR spectra of the crude glycosyl phosphates obtained after
simple silica gel filtration indicated very high purity of the
desired products. Therefore, we began to examine the effic-
iency of crude glycosyl phosphates as glycosylating agents.
The crude mannosyl and glucosyl phosphates 2, 17, and
18 derived from the corresponding orthoesters (1.2 equiv)
were coupled to methyl glucoside 23 (1.0 equiv) at -30 °C
by TMSOTf activation (Table 2). The desired 1,2-trans-
a Conditions: 1.2 equiv of 4, 1 equiv of 23, 3.6 equiv of dibutyl
phosphate, and 3.6 equiv of TMSOTf.
formation of O-methyl glycoside was not observed, probably
because the molecular sieves that are present in the reaction
mixture act as a methanol scavenger. A further simplification
of the reaction protocol was achieved when orthoester 4 was
treated with dibutyl phosphate in the presence of the coupling
partner, nucleophile 23. The in situ generated glycosyl
phosphate was activated by simple addition of TMSOTf to
furnish disaccharide 27 in excellent yield.
Table 2. Glycosidations with Crude Glycosyl Phosphatesa
1,2-Glycosyl orthoesters had been used previously as
glycosylating agents.15 Direct coupling generally resulted in
poor yield, and excess orthoester was required to improve
coupling yields. Only a limited range of substrates such as
1,2-pentenol and -thio orthoesters were effective in the
assembly of complex carbohydrates.16 On the basis of the
new procedure reported here, 1,2-orthoesters may be viewed
as latent glycosylating agents similar to 1,2-glycals whereby
the in situ conversion to glycosyl phosphates is employed
for efficient couplings.
a Glycosidations were carried out with 1.2 equiv of donor, 1.0 equiv of
acceptor, and 1.2 equiv of TMSOTf at -30 °C in CH2Cl2, within 1 h.
The synthetic utility of the new approach was further
demonstrated by the rapid assembly of a group B Strepto-
coccus trirhamnoside epitope (Scheme 3).17 Rhamnose
orthoester 11 served as the only building block for both
glycosylating agent 21 and nucleophile 28 via chain elonga-
linked disaccharides 24-26 were obtained in excellent yield
and complete stereoselectivity.4 The formation of disaccha-
ride orthoester byproducts was not observed.
The carbohydrate impurities generated during glycosyl
phosphate synthesis did not interfere with the glycosylations.
Thus, a one-pot coupling following the in situ generation of
glycosyl phosphates should be possible because excess acidic
dibutyl phosphate was not expected to negatively affect the
glycosylation.
(14) See Supporting Information for the syntheses of 1 and 3-12.
(15) (a) Kochetkov, N. K.; Khorlin, A. J.; Bochkov, A. F. Tetrahedron
1967, 23, 693. (b) Kochetkov, N. K.; Bochkov, A. F.; Sokolovskaya, T.
A.; Snyatkova, V. J. Carbohydr. Res. 1971, 16, 17. (c) Wang, W.; Kong,
F. J Org. Chem. 1998, 63, 5744.
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Lett. 2002, 43, 6953. (c) Kochetkov, N. K.; Backinowsky, L. V.; Tsvetkov,
Y. E. Tetrahedron Lett. 1977, 18, 3681. (d) Backinowsky, L. V.; Tsvetkov,
Y. E.; Balan, N. F.; Byramova, N. E.; Kochetkov, N. K. Carbohydr. Res.
1980, 85, 209. (e) Wang, W.; Kong, F. Angew. Chem., Int. Ed. Engl. 1999,
38, 1247.
To evaluate the in situ route, mannosyl phosphate 14 was
generated from orthoester 4 (1.2 equiv) as described above.
Org. Lett., Vol. 8, No. 9, 2006
1817

of orthoester 11 with dibutyl phosphate in the presence of
28, followed by the activation with TMSOTf, gave dirham-
noside in 88% yield. Removal of the acetate furnished 29
that served as a nucleophile in the coupling with the in situ
generated rhamnosyl phosphate to yield 87% of trirhamno-
side 30. Removal of all protective groups furnished trirham-
noside 31 in 98% yield.
Scheme 3. Synthesis of Rhamnose Trisaccharide 31
In summary, we developed a highly effective protocol to
convert 1,2-glycosyl orthoesters to C2 acyl glycosyl phos-
phates. The synthetic scope was established by preparing a
series of mannosyl, glucosyl, galactosyl, rhamnosyl, xylosyl,
and arabinofuranosyl phosphates in high yield. Pure glycosyl
phosphates are obtained following column chromatography.
In situ generation of glycosyl phosphates from 1,2-orthoesters
allows for simple and efficient glycoside formation as shown
for a trirhamnoside antigen. Straightforward access to
glycosyl phosphate building blocks will facilitate the syn-
thesis of complex carbohydrates in solution and by automated
solid-phase synthesis.
Acknowledgment. This research was supported by ETH
Zu¨rich and the Swiss National Science Foundation (SNF
Grant 200121-101593).
Supporting Information Available: Detailed experi-
mental procedures and compound characterization data,
including 1H, 13C, and 31P NMR spectral data for all described
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL0603155
tion with in situ generation of glycosyl phosphates. Allyl
rhamnoside 28 was obtained in 91% yield by selective
opening of 11 under acidic conditions with an excess of allyl
alcohol and subsequent cleavage of the acetate. Treatment
(17) (a) Michon, F.; Katzenellenbogen, E.; Kasper, D. L.; Jennings, H.
J. Biochemistry 1987, 26, 476. (b) Michon, F.; Brisson, J.-R.; Dell, A.;
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