n-Pentenyl glycosyl orthoesters as versatile intermediates in oligosaccharide synthesis. The proteoglycan linkage region

Full text of DOI:10.1021/ja9832358

468
J. Am. Chem. Soc. 1999, 121, 468-469
n-Pentenyl Glycosyl Orthoesters as Versatile
Intermediates in Oligosaccharide Synthesis. The
Proteoglycan Linkage Region1
John G. Allen†,2 and Bert Fraser-Reid*,‡
Department of Chemistry
Paul M. Gross Chemical Laboratories
Duke UniVersity, Durham, North Carolina 27708
Natural Products and Glycotechnology Research Institute, Inc.
4118 Swarthmore Road, Durham, North Carolina 27707
ReceiVed September 10, 1998
Proteoglycans3 are biologically ubiquitous highly glycosylated
glycoprotein conjugates with widely varying roles in structure4
(as in cartilage and tendons), lubrication5 (as in synovial fluid
and “Wharton’s jelly” of the placenta), blood anticoagulation,6
and light transmission through the cornea.7 Not surprisingly,
aberrations in proteoglycan metabolism can therefore have serious
consequences, leading to diseases such as rheumatoid arthritis
and cystic fibrosis.8
Figure 1. Schematic representation of a proteoglycan (1) and synthetic
linkage region construct (2).
Scheme 1
In view of their implication in such disabling disorders, interest
in proteoglycan biosynthesis has been heightened.9 Some recent
findings can be interpreted with the aid of the schematic construct
1 (Figure 1). Several different proteoglycans have in common a
highly conserved7 tetrasaccharide linkage region joining a
glycosaminoglycan (GAG) group to a core protein through entities
Y and Z. Unit Y is either serine or threonine, the xylose-serine
bond being unique in that it does not occur in other mammalian
glycoconjugates. Unit Z is GlcNAc or GalNAc,10 a critical
differentiation representing the point at which the biosynthetic
routes to glucosaminoglycans (such as heparin) and galactosami-
noglycans (such as dermatan and chondroitin sulfates) diverge.11
Recent studies by Esko11 and others12 have identified the core
protein as a regulatory factor in this biosynthetic outcome. A
tetraglycosylserine corresponding to the linkage region, and
suitably protected as in 2, is therefore of interest, since the serine
moiety allows for specific elaboration of the core protein in either
direction. In this communication we describe a synthesis13 of the
required tetrasaccharide and its direct coupling to give 2 that draws
heavily upon the chemistry of n-pentenyl donors.14
Recent studies in our laboratory have shown that n-pentenyl
orthoesters can be valuable synthetic intermediates, stemming
from their ability to undergo rearrangement to n-pentenyl gly-
cosides (NPGs) under mild conditions, thereby enabling a given
precursor to serve as a donor (e.g., 3 or 4; Scheme 1) or an
acceptor (e.g., 5).15 Orthoester formation during coupling reac-
tions, on the other hand, is an unwelcome occurrence common
to glycosyl donors of uronic acids.16 Thus, whereas 2-O-acyl
hexoses such as 6a react routinely to give 1,2-trans-products such
as 7, the uronate counterpart 6b may give an orthoester 8 as the
major product under similar conditions. Accordingly, our studies
were prompted, in part, by challenges facing the glucuronate
component of the target 2.
Retrosynthesis leads to the coupling partners 9-12 shown in
Scheme 2 and reflects insights we have gained from exploratory
experiments. Thus, the uronate retron 9 was equipped with a
trichloroacetimidate activating group so as to permit orthogonal
coupling to the NPG 10c.17 Often orthoester formation is obviated
by use of a pivaloyl protecting group at O2;16f,18 however, we
chose benzoyl19 for the case at hand to ensure â-selectivity, as
well as easier removal in the future. A convergent route bringing
together the GlcUA-Gal and Gal-Xyl disaccharides would save
steps and allow the galactoside residue 10 to serve twice.
Furthermore, the labile xylose-serine bond would be formed last.
† Duke University.
‡ Natural Products and Glycotechnology (NPG) Research Institute, Inc.
(1) Supported by NIH grants to B.F.-R. (AI-31862 and GM-40171) and a
Glaxo-Wellcome Fund Fellowship to J. G. A.
(2) From the Ph.D. Dissertation of J. G. A., Duke University, May 1998.
(3) Functions of the Proteoglycans; Everd, D., Whelan, J., Eds.; Wiley:
New York, 1986. Proteoglycans; Jolle´s, P., Ed.; Birkha¨user Verlag: Basel,
Switzerland, 1994.
(4) Hukins, D. W. L. Tissue components. In ConnectiVe Tissue Matrix;
Hukins, D. W. L., Ed.; Verlag: Basel, Switzerland, 1984; p 1.
(5) Burton, A. F. Amino Sugars. Quest for Health Series; 1990.
(6) (a) Hirsh, J.; Levine, M. N. Blood 1992, 79, 1-17. (b) Sie, P.; Dupouy,
D.; Caranobe, C.; Petitou, M.; Boneu, B. Blood 1993, 81, 1771-1777.
(7) Rode´n, L. In The Biochemistry of Glycoproteins and Proteoglycans;
Lannarz, W. J., Ed.; Plenum: New York, 1980; p 267.
(8) Di Sant’Agnese, P. A.; Davis, P. B. New Engl. J. Med. 1976, 295,
597.
(9) (a) Prehm, P. Biochem. J. 1983, 211, 181. (b) Hascall, V. C.
Introduction. In Functions of the proteoglycans; Evered, D., Whelan, J., Eds.;
Wiley: New York, 1986; p 1.
(10) Fedarko, N. S. Isolation and purification of proteoglycans. In Pro-
teoglycans; Jolle`s, P., Ed.; Birkha¨user Verlag: Basel, Switzerland, 1994; p 9.
(11) (a) Zhang, L.; Esko, J. D. J. Biol. Chem. 1994, 269, 19295. (b) Zhang,
L.; David, G.; Esko, J. D. J. Biol. Chem. 1995, 270, 27127. (c) Lidholt, K.;
Weinke, J. L.; Kiser, C. S.; Lugemwa, F. M.; Bame, K. J.; Cheifetz, K. J.;
Massague´, J.; Lindahl, U.; Esko, J. D. Proc. Natl. Acad. Sci. U.S.A. 1992, 89,
2267.
(12) (a) Segarini, P. R., Seyedi, S. M. J. Biol. Chem. 1988, 263, 8366. (b)
Cheifetz, S.; Massague´, J. J. Biol. Chem. 1989, 264, 12025. (c) Andres, J. L.;
Ronnstrand, L.; Cheifetz, S.; Massague´, J. J. Biol. Chem. 1991, 266, 23282.
(13) For some examples, see: (a) Goto, F.; Ogawa, T. Tetrahedron Lett.
1992, 33, 5099. (b) Rio, S.; Beau, J.-M.; Jacquinet, J.-C. Carbohydr. Res.
1993, 244, 295. (c) Rio, S.; Beau, J.-M.; Jacquinet, J.-C. Carbohydr. Res.
1991, 219, 71. (d) Garegg, P. J.; Lindberg, B.; Norberg, T. Acta Chem. Scand.
B 1979, 33, 449. (e) Ekborg, G.; Curenton, T.; Krishna, N. R.; Rode´n, L. J.
Carbohydr. Chem. 1990, 9, 15. (f) Nilsson, M.; Westman, J.; Svahn, C.-M. J.
Carbohydr. Res. 1993, 12, 23.
(14) Fraser-Reid, B.; Udodong, U. E.; Wu, Z.; Ottosson, H.; Merritt, J.
R.; Rao, C. S.; Roberts, C.; Madsen R. Synlett 1992, 927.
(15) Roberts, C.; Madsen, R.; Fraser-Reid, B. J. Am. Chem. Soc. 1995,
117, 1546.
(16) (a) Ramu, K.; Baker, J. K. J. Med. Chem. 1995, 38, 1911. (b) Kanaoka,
M.; Kato, H.; Yano, S. Chem. Pharm. Bull. 1990, 38, 221. (c) Mattox, B. R.;
Goodrich, J. E.; Nelson, A. N. Steroids 1982, 40, 23. (d) Marra, A.; Bond,
X.; Fetitou, M.; Sinay, P. Carbohydr. Res. 1989, 195, 39. (e) Hashimoto, S.;
Sano, A.; Umeo, K.; Nakajima, M.; Ikegani, S. Chem. Pharm. Bull. 1995,
43, 2267. (f) van Boeckel, C. A. A.; Delbressine, L. P. C.; Kaspersen, F. M.
Recl. TraV. Chim. Pays-Bas 1985, 104, 259.
(17) This compound previously prepared: Udodong, U. E.; Rao, C. S.;
Fraser-Reid, B. Tetrahedron 1992, 48, 4713.
(18) (a) Vlahov, J.; Snatzke, G. Liebigs Ann. Chem. 1983, 570. (b) Garegg,
P. J.; Olsson, L.; Oscarson, S. J. Org. Chem. 1995, 60, 2200.
(19) (a) Arsequell, G.; Sarries, N.; Valencia, G. Tetrahedron Lett. 1995,
36, 7323. (b) Tabeur, C.; Machetto, F.; Mallet, J.-M.; Duchaussoy, P.; Petitou,
M.; Sinay, P. Carbohydr. Res. 1996, 281, 253. (c) Garegg., P. J.; Olsson, L.;
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10.1021/ja9832358 CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/05/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 2, 1999 469
Scheme 2a
Scheme 3a
a (a) 1.2 equiv of 9, TESOTf, CH2Cl2, 0 °C, 15 min (89%, 3:1 mixture
of 18 and corresponding orthoester). (b) ClAcCl, pyridine (89%). (c) 1.27
equiv of 11c, NIS, TESOTf, -20 °C, 1.25 h (56%). (d) CS2, H4N2,
i-Pr2EtN, rt, 20 min (86%). (e) 1.8 equiv of 18, NIS, TESOTf, 0 °C, 15
min (52%). (f) NaI, MEK, 50 °C, 12 h (83%). (g) 2 equiv of 12, NIS,
TESOTf, 0 °C, 15 min (35%). PentBr2 ) 4,5-dibromopentanyl.
revealed in the coupling with 9. As is shown in Scheme 3, a short
reaction time at 0 °C, to minimize orthoester formation and acid-
catalyzed decomposition of the benzylidene group, led over-
whelmingly to the desired product 18, ready to serve directly as
the disaccharide donor.
The second galactoside synthon 19 was obtained from 10c by
chloroacetylation. Coupling with sidetracked NPG 11c then
afforded disaccharide 20a.22 Standard dechloroacetylation of 20a
with thiourea in hot ethanol caused decomposition; however, the
use of hydrazine dithiocarbonate, as pioneered by van Boeckel,23
succeeded in providing 20b in 86% yield after only 20 min at
room temperature.
NIS-TESOTf-promoted coupling of disaccharides 18 and 20b
proceeded smoothly at 0 °C in 15 min, giving tetrasaccharide
21a in the respectable yield of 52%. Mild reductive debromina-
tion, effected by warming with excess sodium iodide in methyl-
ethyl ketone,24 restored n-pentenyl activation to 21b (80% yield).25
The direct26 coupling of tetrasaccharide donor 21b with serine
acceptor 1227 was carried out at 0 °C in 15 min to give the desired
material 2 in 35% yield. By this short and efficient route, ∼100
mg of the target material 2 was prepared. Incorporation of this
synthetic probe into peptides for biological investigations is
underway and will be reported in due course.
a (a) TBDPSCl and DMAP in pyridine for 48 h, then BzCl (92%). (b)
(i) HF-pyridine; (ii) Jones oxidation, then CH2N2 (86%, three steps). (c)
(i) Pd-C, H2; (ii) Cl3CCN, DBU, 0 °C, 20 min (80%, two steps). (d) (i)
Bu4NI, lutidine, room temperature (rt), 24 h; (ii) NaOMe, 0 °C, 4 h. (e)
TESOTf, PentOH, rt, 4 h (41%, three steps). (f) PhCH(OMe)2, CSA
(94%). (g) PentOH, lutidine, rt, 10 d (87%). (h) (i) NaOMe, 0 °C, 3 h
(74%); (ii) PentOH, CSA (80%). (i) (i) 1.05 equiv of TBSOTf, lutidine,
CH2Cl2, -78 °C, 1 h (83%); (ii) BzCl, pyridine, CH2Cl2; (iii) TBAF,
THF, 0 °C (80%, two steps). (j) Et4NBr, Br2 (90%). Pent ) pent-4-enyl.
Synthesis of trichloroacetimidate 9 from commercially available
benzyl â-D-glucoside (13a) was achieved by the high-yielding
route shown in Scheme 2, featuring an efficient (92%) one-pot
silylation-acylation procedure,20 to obtain C6 differentiated 13b,
and thence methyl glucuronate 13c and 9 (69% for five steps).
With respect to the galactoside acceptor, our preliminary work
had shown that a triester 10b was so deactivated that it reacted
to give orthoester products only.21 We hoped that replacing two
of the esters with a benzylidene group would enhance reactivity.
Accordingly, the n-pentenyl orthoester 15 was prepared routinely
from galactosyl bromide 14, and rearrangement in the presence
of excess pentenyl alcohol in order to obviate self-condensation
afforded 10a in 41% yield over three steps. The desired
benzylidene derivative 10c was obtained routinely in 94% yield.
The xylose retron 11c required differentiation at O4. An
n-pentenyl orthoester, in this case 17, again proved valuable in
transforming xylosyl bromide 16 to the diol 11a. It transpired
that subtle differentiation between the C3 and C4 hydroxyl groups
of 11a was best accomplished by relying on steric factors. Thus,
monosilylation yielded 11b as the major regioisomer (8:1), and
standard operations led to the 2,3-di-O-benzoyl xyloside 11c.
Dibromination14 then provided the sidetracked dibromopentanyl
acceptor 11c (31% over five steps).
Acknowledgment. The authors thank Dr. Lars Olsson of NPG
Research Institute for helpful discussions and Dr. George Dubay of Duke
University for obtaining high-resolution mass spectral data. Duke
University is gratefully acknowledged for partial financial support.
Supporting Information Available: Text giving experimental pro-
cedures and characterization data for all compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA9832358
Increased reactivity of benzylidene-protected galactoside ac-
ceptor 10c, in contrast to the result using acceptor 10b, was
(22) The disappointing yield of 56% was due to substantial transfer of
chloroacetyl from 19 to the C4-OH of 11c, the resulting xylosyl 4-chloro-
acetate (11c, H ) ClAc) being produced in 21% yield.
(23) van Boeckel, C. A. A.; Beetz, T. Tetrahedron Lett. 1983, 24, 3775.
(24) Merritt, J. R.; Debenham, J. S.; Fraser-Reid, B. J. Carbohydr. Chem.
1996, 15, 65.
(25) Completion of the reaction was indicated by integration of the fully
resolved vinyl CH2 group in the 1H NMR spectrum and complete absence of
the dibromide in MALDI-TOF and FAB mass spectra.
(26) (a) Jansson, A. M.; Meldal, M.; Bock, K. Tetrahdron Lett. 1990, 48,
6991. (b) Meldal, M.; Jensen, K. J. J. Chem. Soc., Chem. Commun. 1990,
483. (c) Jansson, A. M.; Meldal, M.; Bock, K. Perkin Trans. 1 1992, 1699.
(27) (a) Kisfaludy, L.; Scho¨n, I. Synthesis 1983, 325. (b) Scho¨n, I.;
Kisfaludy, L. Synthesis 1986, 303.
(20) This transformation is usually achieved in two steps: (a) Ratcliffe,
A. J.; Konradsson, P.; Fraser-Reid, B. Carbohydr. Res. 1991, 216, 323. (b)
Ratcliffe, A. J.; Konradsson, P.; Fraser-Reid, B. J. Am. Chem. Soc. 1990,
112, 5665. (c) Ziegler, T.; Sutoris, H.; Glaudemans, P. J. Carbohydr. Res.
1992, 229, 271. (d) Hashimoto, S.; Sakamoto, H.; Honda, T.; Ikegami, S.
Tetrahedron Lett. 1997, 38, 5181. (e) Moradei, O.; du Mortier, C.; Cirelli, A.
F.; Thiem, J. J. Carbohydr. Chem. 1995, 14, 525. Or in two steps without
purification: (f) Nashed, E. M.; Glaudemans, C. P. J. J. Org. Chem. 1987,
52, 5255. (g) Ziegler, T.; Pavliak, V.; Lin, T.-H.; Kovac, P.; Glaudemans, C.
P. J. Carbohydr. Res.1990, 204, 167. Please see Supporting Information for
two more examples of this useful reaction performed in one pot.
(21) Please see Supporting Information for unpublished work.