Carbohydrate sulfonyl chlorides for simple, convenient access to glycoconjugates

Article abstract of DOI:10.1021/ol0502127

(Chemical Equation Presented) The use of carbohydrate sulfonyl chlorides is introduced as a new, facile glycoconjugation method which could find broad applications. We demonstrate the approach by synthesizing a number of glycosylated cholesterol absorptio

Full text of DOI:10.1021/ol0502127

ORGANIC
LETTERS
2005
Vol. 7, No. 6
1145-1148
Carbohydrate Sulfonyl Chlorides for
Simple, Convenient Access to
Glycoconjugates
Lisbet Kværnø, Moritz Werder,† Helmut Hauser,† and Erick M. Carreira*
Laboratorium fu¨r Organische Chemie, ETH Ho¨nggerberg HCI H335,
CH-8093 Zu¨rich, Switzerland
carreira@org.chem.ethz.ch
Received February 2, 2005
ABSTRACT
The use of carbohydrate sulfonyl chlorides is introduced as a new, facile glycoconjugation method which could find broad applications. We
demonstrate the approach by synthesizing a number of glycosylated cholesterol absorption inhibitors which display high inhibitory efficacies
in our recently established in vitro assay. Furthermore, we highlight an advantage of the electron-withdrawing nature of the sulfonyl linkage
which allowed the synthesis of otherwise unstable azetidine conjugates.
The chemical and analytical tools for studying the biological
functions of carbohydrates and glycoconjugates have seen
explosive development within recent years.1,2 The structural
and synthetic complexity of densely functionalized carbo-
hydrates renders the introduction of carbohydrate domains
onto molecules of interest a nontrivial exercise. Moreover,
the issues of reactivity and stereoselectivity at the anomeric
center during glycosylation adds to the synthetic challenge.1,3
We have become interested in exploring the effect of
glycosylation of small-molecule inhibitors of intestinal
cholesterol absorption.4 In this context, we initiated a study
into the development of new strategies for the synthesis of
carbohydrate conjugates (Scheme 1).
Examples from the wide range of important applications
involving glycoconjugation methods include the synthesis
of neoglycoconjugates by Staudinger ligation to cell sur-
faces,5 the use of neoglycopeptides as oligosaccharyl trans-
ferase inhibitors,6 and the enhanced biological activities of
drugs.4,7 Thus, for instance, ezetimibe (10, Scheme 2)8
† Present address (M.W., H.H.): Lipideon Biotechnology, Hermann-
Hiltbrunnerweg 25, 8713 Uerikon, Switzerland.
(1) (a) Glycoscience; Fraser-Reid, B., Tatsuta, K., Thiem, J., Eds.;
Springer-Verlag: Berlin, Heidelberg, New York, 2001. (b) Bioorganic
Chemistry: Carbohydrates; Hecht, S. M., Ed.; Oxford University Press:
New York, Oxford, 1999.
(4) Kværnø, L.; Ritter, T.; Werder, M.; Hauser, H.; Carreira, E. M.
Angew. Chem., Int. Ed. 2004, 43, 4653-4656.
(5) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007-2010.
(6) Peluso, S.; Ufret, M. D.; O’Reilly, M. K.; Imperiali, B. Chem. Biol.
2002, 9, 1323-1328.
(2) (a) Carbohydrate-based Drug DiscoVery; Wong, C.-H., Ed.; Wiley-
VCH: Weinheim, 2003. (b) Glycochemistry. Principles, Synthesis and
Applications; Wang, G., Bertozzi, C. R., Eds.; Marcel Dekker Inc.: New
York, Basel, 2001. (c) Palmacci, E. R.; Plante, O. J.; Hewitt, M. C.;
Seeberger, P. H. HelV. Chim. Acta 2003, 86, 3975-3990.
(7) Vaccaro, W. D.; Davis, H. R. Bioorg. Med. Chem. Lett. 1998, 8,
313-318.
(8) (a) Clader, J. W. J. Med. Chem. 2004, 47, 1-9. (b) Rosenblum, S.
B.; Huynh, T.; Afonso, A.; Davis, H. R.; Yumibe, N.; Clader, J. W.; Burnett,
D. A. J. Med. Chem. 1998, 41, 973-980.
(3) Toshima, K.; Tatsuta, K. Chem. ReV. 1993, 93, 1503-1531.
10.1021/ol0502127 CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/22/2005

Scheme 1. Synthesis of Carbohydrate Sulfonyl Chlorides
constitutes a notable recent example of a small-molecule
interested in the rapid introduction of a variety of carbohy-
drate domains onto molecules of interest. Given the inherent
complexity that can be associated with traditional glycosyl-
ation methods,1,3 we examined a new conjugation strategy
that would obviate issues relating to reactivity and stereo-
chemistry during the introduction of the carbohydrate. In this
regard, we focused on carbohydrate sulfonyl chlorides be-
cause a conjugation strategy based on sulfonate ester forma-
tion would be convenient.10 The use of a sulfonylation reac-
tion to facilitate the construction of analogues of biopolymers
finds some precedence in the preparation of sulfonate-linked
oligonucleotides.11 However, to the best of our knowledge,
the use of sulfonylation reactions for the purposes of
glycoconjugation has not been previously reported. As a
proof-of-concept study, we have prepared three different
carbohydrate-derived sulfonyl chlorides and examined their
conjugation to ezetimibe, given our expertise with an assay
for in Vitro screening of cholesterol absorption inhibitors.4
The synthetic phase of our investigations commenced with
the synthesis of sulfonyl chlorides 3, 6, and 9 from the known
O-benzylated glucose derivates 1,12 4,13 and disaccharide 7.14
The sulfonyl chlorides were prepared in high yields through
a short sequence of reactions involving oxidation15 of the
intermediate thioacetates 2,16 5,17 and 8 (Scheme 1).
pharmaceutical whose in ViVo activity can be increased
through glycosylation.7,9 In the context of our ongoing
research on cholesterol absorption inhibitors,4 we became
Scheme 2. Synthesis of Sulfonylated â-Lactam Glycosides
(9) van Heek, M.; Farley, C.; Compton, D. S.; Hoos, L.; Alton, K. B.;
Sybertz, E. J.; Davis, H. R. Br. J. Pharmacol. 2000, 129, 1748-1754.
(10) Although there is literature precedence for the synthesis of a sugar
sulfonyl chloride, its use was limited to the preparation of simple esters
(from 2-propanol, 2-methyl-2-propanol, methanol) and amides (from
N-butylamine and an amino sugar), see: Ulgar, V.; Maya, I.; Fuentes, J.;
Fernandez-Bolanos, J. G. Tetrahedron 2002, 58, 7967-7973.
(11) (a) Johnson, D. C.; Widlanski, T. S. Synthesis 2002, 809-815. (b)
Huang, J. X.; McElroy, E. B.; Widlanski, T. S. J. Org. Chem. 1994, 59,
3520-3521. (c) Huang, J. X.; Widlanski, T. S. Tetrahedron Lett. 1992,
33, 2657-2660.
(12) Jaramillo, C.; Chiara, J. L.; Martinlomas, M. J. Org. Chem. 1994,
59, 3135-3141.
(13) RajanBabu, T. V.; Reddy, G. S. J. Org. Chem. 1986, 51, 5458-
5461.
(14) Spak, S. J.; Martin, O. R. Tetrahedron 2000, 56, 217-224.
(15) Reddie, R. N. Synth. Commun. 1987, 17, 1129-1139.
(16) Paterson, D. E.; Griffin, F. K.; Alcaraz, M. L.; Taylor, R. J. K.
Eur. J. Org. Chem. 2002, 1323-1336
1146
Org. Lett., Vol. 7, No. 6, 2005

Previous glycosylations of ezetimibe (10) have been
carried out on the phenol and furnished conjugates with
increased activity profiles in ViVo7,9 and in Vitro.4 As a means
of testing our premise involving conjugation of carbohydrates
via sulfonate ester formation, we prepared sulfonylated
derivatives 12-14 as outlined in Scheme 2.18
Conjugates 12-14 were subsequently evaluated for inhibi-
tion of intestinal cholesterol uptake at fixed substrate
concentrations of 6 µM using the brush border membrane
vesicle assay (Figure 1).4 Compared to ezetimibe (10) (16%
Figure 2. Pictorial representation of the overlay of ezetimibe (10)
and the azetidine 15. Geometry-optimized overlay at right with the
flexible C3 side chain omitted for clarity.22
â-lactam ring is critical for activity,20,21 with a related open
â-amino acid being inactive.21 As shown in an overlay of
the geometry optimized structures (ab initio minimization,
B3LYP/6-31G*)22 of the â-lactam in 10 with azetidine 15,
the puckered azetidine ring results in only minor perturba-
tions of the positioning of the side chains compared to the
flatter â-lactam. On the basis of this analysis it might be
anticipated that the azetidine ring could serve as an appropri-
ate replacement of the â-lactam.
In our initial attempts to prepare an azetidine possessing
the side chains found optimal in ezetimibe, we observed that
the electron-rich phenol at C-4 renders the azetidine unstable.
Attempts at reduction of the â-lactam with AlH2Cl23 led to
the isolation of rearranged products including predominantly
1824 (Scheme 3). Reduction of 16 to the corresponding 1,3-
Scheme 3. Attempted Synthesis of Azetidine 15
Figure 1. Inhibition of intestinal cholesterol absorption in the brush
border membrane vesicle assay.4
inhibition), the sulfonylated â-lactam glycosides displayed
interesting activities as cholesterol absorption inhibitors. The
highest efficacy was observed for the cellobiose derivative
14 (41% inhibition) compared to the monosaccharide deriva-
tives 12 (20% inhibition) and 13 (15% inhibition).19
We have observed an additional important advantage to
the use of sulfonyl glyconjugation in the context of a
structure-activity relationship study of ezetimibe. Specifi-
cally, we sought to address whether replacement of the
â-lactam ring by an azetidine (see 15, Figure 2) would furnish
structures retaining activity as cholesterol absorption inhibi-
tors. In this respect, previous studies had suggested that the
amino alcohol and closure to the azetidine 17 (LiAlH4, then
CBr4, Ph3P) permitted its isolation. However, upon depro-
tection azetidine 15 proved unstable.
(17) Gervay, J.; Flaherty, T. M.; Holmes, D. Tetrahedron 1997, 53,
16355-16364.
(18) For the synthesis of ezetimibe (10), see: Wu, G. Z.; Wong, Y. S.;
Chen, X.; Ding, Z. X. J. Org. Chem. 1999, 64, 3714-3718.
(19) Comparable in Vitro results were obtained for the corresponding
O-glycosides lacking the sulfonyl linkage, see ref 4.
(20) Dugar, S.; Kirkup, M. P.; Clader, J. W.; Lin, S. I.; Rizvi, R.; Snow,
M. E.; Davis, H. R.; McCombie, S. W. Bioorg. Med. Chem. Lett. 1995, 5,
2947-2952.
(21) Clader, J. W.; Burnett, D. A.; Caplen, M. A.; Domalski, M. S.;
Dugar, S.; Vaccaro, W.; Sher, R.; Browne, M. E.; Zhao, H. R.; Burrier, R.
E.; Salisbury, B.; Davis, H. R. J. Med. Chem. 1996, 39, 3684-3693.
Org. Lett., Vol. 7, No. 6, 2005
1147

We reasoned that substitution of the electron-rich aromatic
group by an electron-deficient sulfonylated aryl would lead
to a stable azetidine analogue.25 As outlined in Scheme 4,
inhibition, respectively) similar to that observed for the
â-lactams counterparts 12 and 13 (20% and 15% inhibition,
respectively). Thus, the use of a sulfonylated sugar permits
the synthesis of stable glycosylated azetidine analogues of
ezetimibe.
In summary, we have documented the synthesis of readily
available carbohydrate sulfonyl chlorides. We specifically
demonstrated the use of these by conjugation to the phenol
of ezetimibe, a potent cholesterol absorption inhibitor.
Furthermore, we highlight an advantage of the electron-
withdrawing nature of the sulfonyl linkage which allowed
the synthesis of otherwise unstable azetidine conjugates. The
novel sulfonylated glycosides all displayed high in Vitro
efficacies as cholesterol absorption inhibitors. The use of a
sulfonylation reaction for the synthesis of carbohydrate
conjugates has potentially wider applications which includes
sulfonamide linkages.26 Carbohydrate sulfonyl chlorides form
the basis of a versatile method for conjugation which could
find broader use in the syntheses of glycoconjugates.
Scheme 4. Sulfonylated Azetidine Glycoconjugates
Acknowledgment. This reseach was supported by a KTI
grant (6813.2 BTS-LS) and Lipideon AG. L.K. thanks The
Technical University of Denmark for a doctoral fellowship.
Supporting Information Available: Detailed descrip-
tions of experimental procedures and analytical data for
compounds 1-9, 11-14, and 19-21. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL0502127
(22) Ab initio calculations: Initial energy-minimization by molecular
mechanics using MacroModel v. 7.0 [pseudosystematical Monte Carlo
conformational search (100 steps, limit 30 kJ/mol) with CHCl3 as solvent,
MMFFs force field] was followed by a quantum-mechanical geometry
optimization using Jaguar v 4.2 (B3LYP/6-31G*) in Maestro v 4.1.
(23) (a) Yamashita, M.; Ojima, I. J. Am. Chem. Soc. 1983, 105, 6339-
6342. (b) Ojima, I.; Zhao, M. Z.; Yamato, T.; Nakahashi, K.; Yamashita,
M.; Abe, R. J. Org. Chem. 1991, 56, 5263-5277. (c) Ojima, I.; Yamato,
T.; Nakahashi, K. Tetrahedron Lett. 1985, 26, 2035-2038.
(24) As a working hypothesis, we believe that azetidine 17 could open
up to an intermediate benzylic carbocation greatly facilitated by electron-
donating substituents. This carbocation could subsequently undergo Friedel-
Crafts reaction with the adjacent aniline and furnish 18.
(25) Ritter, T.; Stanek, K.; Larrosa, I.; Carreira, E. M. Org. Lett. 2004,
6, 1513-1514.
(26) In contrast to phenols, conjugation via primary and secondary
alcohols may be problematic, as the sulfonate esters could suffer a number
of fates including substitution and elimination. However, in the context of
oligonucleotide analogue synthesis the work of Widlanski (ref 11) dem-
onstrates the stability of sulfonate-linked oligonucleotides derived from
secondary alcohols.
reduction of the â-lactam ring could be carried out using
AlH2Cl22 to give azetidine sulfonyl conjugates 19 and 21 in
78 and 94% yield, respectively, following deprotection.
Interestingly, the azetidine glycoconjugates 19 and 21
displayed activities as cholesterol absorption inhibitors in the
brush border membrane vesicle assay (27% and 20%
1148
Org. Lett., Vol. 7, No. 6, 2005