Synthesis and partial biological evaluation of a small library of differentially-linked β-C-disaccharides

Article abstract of DOI:10.1021/jo030039x

The synthesis of a small library of differentially-linked β-C-disaccharides has been carried out through the use of a radical allylation-RCM strategy. Acids 6 were prepared by Keck allylation of a suitable carbohydrate-based radical precursor, followed by

Full text of DOI:10.1021/jo030039x

Syn th esis a n d P a r tia l Biologica l Eva lu a tion of a Sm a ll Libr a r y of
Differ en tia lly-Lin k ed â-C-Disa cch a r id es1
Maarten H. D. Postema,* J ared L. Piper, Lei Liu, J ie Shen, Marcus Faust, and
Peter Andreana
Department of Chemistry, Wayne State University, Detroit, Michigan 48202
mpostema@chem.wayne.edu
Received J anuary 30, 2003
The synthesis of a small library of differentially-linked â-C-disaccharides has been carried out
through the use of a radical allylation-RCM strategy. Acids 6 were prepared by Keck allylation of
a suitable carbohydrate-based radical precursor, followed by oxidative cleavage of the formed alkene.
Dehydrative coupling of these acids with the known olefin alcohol 5 then gave the precursor esters
7 in excellent yield. Methylenation of the esters 7 was followed by RCM and in situ hydroboration-
oxidation of the formed glycals to furnish the protected â-C-disaccharides 10 in good overall yield.
Five examples were then deprotected and screened for their efficacy as enzyme inhibitors of
â-glycosidase and against several solid-tumor cell lines for in vitro differential cytotoxicity.
In tr od u ction
C-disaccharides, can be divided into two categories. The
(1f6)-linked derivatives such as 2 possess two carbohy-
drate rings connected by a two-atom linker, Figure 1. Any
linkage, other than (1f6), generally consists of only a
single carbon atom separating the two monosaccharide
units and these include the (1f4)-, (1f3)-, and (1f2)-
linked derivatives. The (1f4)-linked derivative 4, is
shown below.
It is generally easier to prepare (1f6)-C-disaccha-
rides8,9 such as 2, since the two-carbon linker allows for
more versatility in the type of coupling method used to
join the two pyranosides. Although several approaches
to the synthesis of a variety of differentially-linked
C-disaccharides10 have been described, few methods
provide a unified and versatile strategy for a convergent
The preparation of C-glycoside-based derivatives is a
fairly mature field2 and has seen the use of interesting
chemistry for the attachment of a variety of carbon-based
groups to the anomeric carbon atom of carbohydrates.
By definition, C-glycosides are compounds in which the
interglycosidic oxygen atom has been replaced by a
carbon atom to produce a stable glycoside derivative that
will not be prone to enzymatic or chemical hydrolysis.3
A wealth of approaches have been used for the synthesis
of both alkyl and aryl C-glycosides.4 The latter class of
compounds is particularly important due to the existence
of a number of naturally occurring aryl C-glycosides,5,6
many of which possess interesting and potentially useful
biological activity. C-Saccharide derivatives7 are the
carbon analogues of O-saccharides and the simplest class,
(8) For previous approaches to the synthesis of 1,6-linked C-
disaccharides, see: (a) Postema, M. H. D.; Piper, J . L. Tetrahedron
Lett. 2002, 43, 7095-7099. (b) Griffin, F. K.; Paterson, D. E.; Taylor,
R. J . K. Angew Chem., Int. Ed. 1999, 38, 2939-2942. (c) Leeuwen-
burgh, M. A.; Timmers, C. M.; van der Marel, G. A.; van Boom, J . H.;
Mallet, J . M.; Sinay¨, P. Tetrahedron Lett. 1997, 38, 6251-6254. (d)
Dondoni, A.; Zuurmond, H.; Boscarato, A. J . Org. Chem. 1997, 62,
8114-8124. (e) Kobertz, W. K.; Bertozzi, C. R.; Bednarski, M. D. J .
Org. Chem. 1996, 61, 1894-1895. (f) Armstrong, R. W.; Sutherlin, D.
P. Tetrahedron Lett. 1994, 35, 7743-7746. (g) Martin, O. R.; Lai, W.
J . Org. Chem. 1993, 58, 176-185. (h) Vauzeilles, B.; Cravo, D.; Mallet,
J .-M.; Sinay, P. Synlett 1993, 522-524. (i) Baumberger, F.; Vasella,
A. Helv. Chim. Acta 1983, 66, 2210-2222. (j) Rouzaud, D.; Sinay¨, P.
J . Chem. Soc., Chem. Commun. 1983, 1353-1354.
(1) This paper is dedicated to Professor Carl R. J ohnson on the
occasion of his retirement from Wayne State University.
(2) (a) Postema, M. H. D. C-Glycoside Synthesis, 1st ed.; CRC
Press: Boca Raton, LA, 1995; p 379. (b) Levy, D. E.; Tang, C. The
Chemistry of C-Glycosides, 1st ed.; Elsevier Science: Oxford, UK, 1995;
Vol. 13, p 290.
(3) Cleavage of the C-glycoside bond of some aryl C-glycosides is
known; see: Kumazawa, T.; Kimura, T.; Matsuba, S.; Sato, S.;
Onodera, J .-i. Carbohydr. Res. 2001, 334, 207-213 and references
therein.
(4) For reviews on C-glycoside synthesis, see: (a) Postema, M. H.
D.; Calimente, D. C-Glycoside Synthesis: Recent Developments and
Current Trends. In Glycochemistry: Principles, Synthesis and Ap-
plications; Wang, P. G., Bertozzi, C., Eds.; Marcel Dekker: New York,
2000; Chapter 4, pp 77-131. (b) Du, Y.; Lindhardt, R. J . Tetrahedron
1998, 54, 9913-9959. (c) Beau, J .-M.; Gallagher, T. Top. Curr. Chem.
1997, 187, 1-54. (d) Nicotra, F. Top. Curr. Chem. 1997, 187, 55-83.
(e) Togo, H.; He, W.; Waki, Y.; Yokoyama, M. Synlett 1998, 700-717.
(f) J aramillo, C.; Knapp, S. Synthesis 1994, 1-20. (g) Herscovici, J .;
Antonakis, K. Recent Developments in C-Glycoside Synthesis. In
Studies in Natural Product Chemistry, Stereoselective Synthesis;
Elsevier: New York, 1988; Vol. 10, Part F, pp 337-403.
(5) Krohn, K.; Rohr, J . Top. Curr. Chem. 1997, 188, 127-195.
(6) Rohr, J .; Thiericke, R. Nat. Prod. Rep. 1992, 103-137.
(7) For a comprehensive review on the synthesis of C-saccharides,
see: Liu, L.; McKee, M.; Postema, M. H. D. Curr. Org. Chem. 2001, 5,
1133-1167.
(9) For an iterative Wittig-type approach to â-(1f6)-linked C-
oligosaccharides, see: Dondoni, A.; Marra, A.; Mizuno, M.; Giovannini,
P. P. J . Org. Chem. 2002, 67, 4186-4199.
(10) For some synthetic approaches to C-disaccharides, see: (a)
Postema, M. H. D.; Calimente, D.; Liu, L.; Behrmann, T. L. J . Org.
Chem. 2000, 65, 6061-6068. (b) Griffin, F. K.; Paterson, D. E.; Taylor,
R. J . K. Angew Chem., Int. Ed. 1999, 38, 2939-2942. (c) Khan, N.;
Cheng, X.; Mootoo, D. R. J . Am. Chem. Soc. 1999, 121, 4918-4919.
(d) Leeuwenburgh, M. A.; Timmers, C. M.; van der Marel, G.; van
Boom, J . H.; Mallet, J . M.; Sinay¨, P. Tetrahedron Lett. 1997, 38, 6251-
6254. (e) Dondoni, A.; Zuurmond, H.; Boscarato, A. J . Org. Chem. 1997,
62, 8114-8124. (f) Mallet, A.; Mallet, J .-M.; Sinay, P. Tetrahedron:
Asymmetry 1994, 5, 2593-2608. (g) Sutherlin, D. P.; Armstrong, R.
W. J . Org. Chem. 1997, 62, 5267-5283. (h) Martin, O. R.; Lai, W. J .
Org. Chem. 1993, 58, 176-185.
10.1021/jo030039x CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/14/2003
4748
J . Org. Chem. 2003, 68, 4748-4754

Synthesis of Differentially-Linked â-C-Disaccharides
SCHEME 1. RCM Ap p r oa ch to C-Sa cch a r id es
olefin alcohol 5 with a suitable carbohydrate-based acid
such as 6 to give ester 7, Scheme 1. Methylenation19
(7f8) is to be followed by RCM20 to give glycal 9.
Hydroboration21 of the formed double bond then affords
the gluco-â-C-disaccharide 10.
F IGURE 1. O-Disaccharides versus C-disaccharides.
and efficient synthesis of (1f1)-, (1f2)-, (1f3)-, (1f4)-,
and (1f6)-linked-â-C-disaccharides.
Our approach to (1f6)-linked-â-C-disaccharides17,18
relied upon a similar strategy and the needed acids were
readily prepared by Wittig-type chemistry.22 For any
linkage, other than (1f6), a suitable method for install-
ing the acetyl pendant onto the pyranose ring with the
correct regio- and stereochemistry would be needed. Keck
allylation23 seemed well-suited for this task since, in
theory, the needed radical precursor could be generated
at any position of a suitably protected glucopyranoside.
If they are to be suitable mimics, then C-saccharides
should ideally possess conformations that are similar to
those of the parent O-glycoside or conformations that still
elicit a biological response or recognition event. The
debate regarding the validity of C-saccharides as accurate
conformational mimics of O-saccharides is ongoing and
has yet to be resolved.11 The Ki values for O- and
C-lactose for the competitive inhibition of â-galactosidase
are within 2 µm of one another12 and several groups13
have convincingly shown that the substitution of the
interglycosidic oxygen atom with a carbon atom does not
greatly alter biological activity. Given the vast biological
functions that carbohydrates possess,14 it stands to
reason that stable analogues of these derivatives could
be useful as biological probes or enzyme inhibitors.
In this paper, we present full details of our RCM
methodology for the preparation of differentially-linked
â-C-disaccharides.15 Three additional examples have been
prepared and the overall yield of the three-step protocol
has been optimized. Biological data on several of the
C-saccharide derivatives are also presented.
P r ep a r a tion of th e Ca r boh yd r a te-Ba sed Acid s
The equatorial C-4 acid was prepared first, since the
4-position of gluco derivatives is generally considered to
be the most hindered one. Several radical precursors
(13-17) were prepared from the known24 tri-O-benzyl
derivative 11 and separate exposure of all of these
compounds to either thermal or photochemical allylation
conditions gave only complex reaction mixtures, Scheme
2. The inordinately large number of products was at-
tributed to 1,5-H radical abstraction of the O-6 benzylic
hydrogens.25 Deuteration studies to confirm this hypoth-
esis were not carried out, but instead, the O-6 benzyl
group was exchanged for a TIPS group. Attempted
At the outset of this work, we wished to develop a
general approach for the synthesis of C-glycosides16 and
a variety of â-C-saccharides.17,18 Our generic synthetic
approach to C-disaccharide synthesis is shown in Scheme
1 and begins with the dehydrative coupling of the generic
(17) Postema, M. H. D.; Calimente, D.; Liu, L.; Behrmann, T. L. J .
Org. Chem. 2000, 65, 6061-6068.
(18) Postema, M. H. D.; Calimente, D. Tetrahedron Lett. 1999, 40,
4755-4759.
(19) Takai, K.; Kakiuchi, T.; Kataoka, Y.; Utimoto, K. J . Org. Chem.
1994, 59, 2668-2670.
(11) (a) Wei, A.; Haudrechy, A.; Audin, C.; J un, H.-S.; Haudrechy-
Bretel, N.; Kishi, Y. J . Org. Chem. 1995, 60, 2160-2169. (b) Asensio,
J . L.; Espinosa, J . F.; Dietrich, H.; Can˜ada, F. J .; Schmidt, R. R.;
Mart´ın-Lomas, M.; Andre´, S.; Gabius, H.-J .; J ime´nez-Barbero, J . J .
Am. Chem. Soc. 1999, 121, 8995-9000. (c) Rubinstenn, G.; Sinay¨, P.;
Berthault, P. J . Chem. Phys. A 1997, 101, 2536-2540 and references
therein.
(12) Espinosa, J . F.; Montero, E.; Vian, A.; Garcia, J . L.; Dietrich,
H.; Schmidt, R. R.; Mart´ın-Lomas, M.; Imberty, A.; Can˜ada, F. J .;
J ime´nez-Barbero, J . J . Am. Chem. Soc. 1998, 120, 1309-1318 and
references therein.
(13) (a) Wei, A.; Boy, K. M.; Kishi, Y. J . Am. Chem. Soc. 1995, 117,
9432-9436. (b) Wang, J .; Kovac, P.; Sinay¨, P.; Glaudemans, C. P. J .
Carbohydr. Res. 1998, 308, 191-193. (c) Tsuruta, O.; Yuasa, H.;
Kurono, S.; Hashimoto, H. Bioorg. Med. Chem. Lett. 1999, 9, 807-
810. (d) Yang, G.; Franck, R. W.; Bittman, R.; Samadder, P.; Arthur,
G. Org. Lett. 2001, 3, 197-200.
(14) Glycochemistry. Principles, Synthesis and Applications; Wang,
P. G., Bertozzi, C. R., Eds.; Marcel Dekker Inc.: New York, 2001.
(15) A portion of this work has been communicated in preliminary
form: Liu, L.; Postema, M. H. D. J . Am. Chem. Soc. 2001, 123, 8602-
8603.
(20) For reviews on olefin metathesis chemistry, see: (a) Trnka, T.
M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29. (b) Fu¨rstner, A.
Angew. Chem., Int. Ed. 2000, 39, 3012-3043. (c) Wright, D. L. Curr.
Org. Chem. 1999, 3, 211-240. (d) Grubbs, R. H.; Chang, S. Tetrahedron
1998, 54, 4413-4450. (e) Ivin, K. J . J . Mol. Catal. A-Chem. 1998, 133,
1-16. (f) Randall, M. L.; Snapper, M. L. J . Mol. Catal. A-Chem. 1998,
133, 29-40. (g) Armstrong, S. K. J . Chem. Soc., Perkin Trans. 1 1998,
371-388. (h) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl.
1997, 36, 2036-2056. (i) Fu¨rstner, A. Top. Catal. 1997, 4, 285-299.
(j) Grubbs, R. H.; Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995, 28,
446-452. (k) Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1995, 34,
1833-1836.
(21) (a) Hanessian, S.; Martin, M.; Desai, R. C. J . Chem. Soc., Chem.
Commun. 1986, 926-927. (b) Schmidt, R. R.; Preuss, R.; Betz, R.
Tetrahedron Lett. 1987, 28, 6591-6594.
(22) Zheng, W.; DeMattei, J . A.; Wu, J . P.; Duan, J . J . W.; Cook, L.
R.; Oinuma, H.; Kishi, Y. J . Am. Chem. Soc. 1996, 118, 7946-7968.
(23) Keck, G. E.; Enholm, E. J .; Yates, J . B.; Wiley, M. R. Tetrahe-
dron 1985, 41, 4079-4094.
(24) Garegg, P. J .; Hultberg, H.; Wallin, S. Carbohydr. Res. 1982,
108, 97-101.
(25) Mootoo, D.; Wilson, P.; J ammalamadaka, V. J . Carbohydr.
Chem. 1994, 13, 841-849.
(16) Postema, M. H. D.; Calimente, D. J . Org. Chem. 1999, 64,
1770-1771.
J . Org. Chem, Vol. 68, No. 12, 2003 4749

Postema et al.
SCHEME 2. In itia l Allyla tion Attem p ts
isomer 20b from the allylation of 19a was converted to
acid 6b in a similar fashion as that for 6a (entry 2). The
C-3 acid 6c (entry 3, Table 1) was accessed from the
known methyl 2-benzoyl-4,6-di-O-benzylidene-D-glucoside
(18c).30 Thionocarbonate formation on 18c by the NHS
method31 gave 19c and radical allylation furnished 20c
as a single isomer, presumably due to the bicyclic nature
of the molecule. Exchange of the benzoate blocking group
(20cf21c) was followed by oxidative cleavage of the
alkene to furnish an intermediate aldehyde that was then
oxidized to the corresponding acid 6c (entry 3, Table 1).
The preparation of C-2 gluco-acid began with tri-O-
acetyl-D-glucal (18d ). Exposure of 18d to iodine in
methanol gave 19d in 85% yield,32 which was then
allylated in the usual manner to give, in this case, a 1:1.5
ratio of gluco 20d to manno 20e isomers in 64% combined
yield (entries 4 and 5, Table 1). Once again, the two
formed olefins were inseparable. The acetates were
exchanged for benzyl groups (on the mixture of allylated
compounds) and the double bond cleaved to reveal two
separable aldehydes that were then separately oxidized
to the corresponding C-2 gluco and manno acids 6d and
6e, respectively. In this case, the adjacent groups exert
an opposing stereochemical bias on the allylation reaction
with the axial anomeric substituent dominating over the
equatorial C-3 substituent.
SCHEME 3. F or m a tion of th e C-4 Acid 6a
The needed â- and R-C-1 acids 6f and 6g (entries 6 and
7, Table 1) were readily accessible since the correspond-
ing allyl derivatives are known compounds and are both
easily prepared. Compound 21f, previously prepared by
Kishi,33 was converted to the â-derivative 6f in two steps
(53%) and the known R-allyl derivative 21g34 was con-
verted to the R-acid derivative 6g in 62% yield.
radical allylation of 16 gave similar results as with the
fully benzylated derivatives. Presumably, the formed
radical exists in a conformation or conformations where
other benzylic hydrogen atoms are accessible for radical
abstraction. The ultimate solution was to remove the
abstractable hydrogen atoms by replacing all of the
benzyl groups with acetates. These would, however,
eventually need to be exchanged for benzyl groups due
to their incompatability with the methylenation step.
The known tri-O-acetyl derivative 18a 26 was con-
verted27 to iodide 19a in good yield and in this case,
radical allylation23 of 19a proceeded to deliver a mixture
of equatorial and axial allylation products in a 6:1 ratio.
The major isomer, 20a , was formed in 64% yield, and
resulted from radical addition from the bottom R-face of
the molecule, opposite the adjacent OAc and CH2OAc
groups. The two isomers were inseparable under a
variety of chromatographic conditions and the mixture
was, therefore, carried on to the next step. The acetates
were removed and replaced with benzyl groups and, once
again, the formed compounds 21a and 21b were found
to be inseparable. Only after oxidative cleavage of the
double bond to give the corresponding aldehydes 22a and
22b was separation of the isomers possible. At this point,
the aldehydes were separated and fully characterized.28
Pinnick oxidation29 of the major aldehyde 22a then gave
the needed C-4 equatorial acid 6a , Scheme 3.
The equatorial C-3 acid of methyl R-D-galactoside 6h
was also prepared (entry 8, Table 1). The known deriva-
tive 18h 35 was converted to 19h and, as expected, the
axial group directed the radical allylation reaction to give
the equatorial isomer 20h as the minor compound. It was
1
formed in a 1:1.7 ratio as shown by H NMR analysis of
the crude reaction mixture. The acetylated products were
not separable, but in this case the corresponding benzyl
derivatives were separable. Accordingly, compound 21h
was oxidized to acid 6h in 60% yield over the two steps.
For all of the entries in Table 1 (save for compounds
21f and 21g), the stereochemistry of the allylation step
was confirmed by H-H decoupling experiments and NOE
studies.28
Ester F or m a tion , Meth ylen a tion , a n d Rin g-
Closin g Meta th esis
With all the acids in hand, the RCM sequence was then
examined. DCC-mediated coupling of alcohol 5a 36 and
acid 6a proceeded uneventfully to afford ester 7a in good
Table 1 outlines the preparation of all the intermedi-
ates en route to the required carboxylic acids 6. Entry 1
outlines the 4-gluco case discussed above. The minor
(30) Szeja, W. Carbohydr. Res. 1983, 115, 240-242.
(31) Barton, D. H. R.; J aszberenyi, J . C. Tetrahedron Lett. 1989,
30, 2619-2622.
(32) Chen, S.-H.; Horvath, R. F.; J oglar, J .; Fisher, M. J .; Danishef-
sky, S. J . J . Org. Chem. 1991, 56, 5834-5845.
(33) Lewis, M. D.; Cha, J . K.; Kishi, Y. J . Am. Chem. Soc. 1982,
104, 4976-4978.
(26) Koch, K.; Chambers, R. J . Carbohydr. Res. 1993, 241, 295-
299.
(27) Garegg, P. J .; Samuelsson, B. J . Chem. Soc., Perkin Trans. 1
1980, 2866-2869.
(28) See Supporting Information.
(29) Bal, B. S.; Childers, W. E., J r.; Pinnick, H. W. Tetrahedron 1981,
37, 2091-2096.
(34) Hosomi, A.; Sakata, Y.; Sakurai, H. Tetrahedron Lett. 1984, 25,
2383-2386.
(35) Kajihara, Y.; Kodama, H.; Endo, T.; Hashimoto, H. Carbohydr.
Res. 1998, 306, 361-378.
(36) Freeman, F.; Robarge, K. D. Carbohydr. Res. 1987, 171, 1-11.
4750 J . Org. Chem., Vol. 68, No. 12, 2003

Synthesis of Differentially-Linked â-C-Disaccharides
TABLE 1. Syn th esis of Ca r boh yd r a te-Ba sed Acid s 6a
a
b
Yields refer to chromatographically and spectroscopically homogeneous materials. Prepared by the method of Szeja.30 c Prepared
by the method of Hashimoto.35 Any epimeric allylated derivatives 20 were separated after benzylation and oxidative cleavage to the
d
corresponding aldehydes. e Reaction carried out with NaOMe in MeOH/THF followed by benzylation (BnBr/NaH/DMF). f Prepared by
the method of Kishi.33 Prepared by the method of Hosomi.34 The corresponding axial isomer was formed in 40% yield. Oxidative
g
h
i
j
cleavage of the olefin carried out in two steps. Yields are for two steps.
yield. Methylenation19 of 7a gave 8a and exposure to
catalyst 2337 in the glovebox gave a 41% yield of the
C-disaccharide glycal 9a . This was surprising, since TLC
analysis showed clean conversion of the starting material
to the product. We reasoned that the glycal was decom-
posing or hydrolyzing during purification and this
prompted us to explore a one-pot approach in which the
glycal was not isolated.
The RCM was carried out as before, but before removal
of the reaction from the drybox, an excess of BH3‚THF
(37) (a) Schrock, R. R.; Murdzek, J . S.; Bazan, G. C.; Robbins, J .;
DiMare, M.; O’Regan, M. J . Am. Chem. Soc. 1990, 112, 3875-3886.
(b) Feldman, J .; Murdzek, J . S.; Davis, W. M.; Schrock, R. R.
Organometallics 1989, 8, 2260-2265.
J . Org. Chem, Vol. 68, No. 12, 2003 4751

Postema et al.
SCHEME 6. RCM w ith th e Gr u bbs Ca ta lyst 26
SCHEME 4. Syn th esis of th e (1f4)-â-C-Glyca l 9a
SCHEME 5. On e-P ot P r otocol for th e RCM
Sequ en ce
SCHEME 7. RCM w ith No Lin k in g Atom
was added and the reaction was then quenched with basic
hydrogen peroxide and stirred overnight. After the usual
workup and purification by flash chromatography, a 64%
yield of the â-C-disaccharide 10a was obtained, Scheme
5. Presumably, the cyclized material 9a was hydrolyzing
on the column, even in the presence of 1-3% of triethy-
lamine. The benzyl groups were removed (10a f24a ) and
global acetylation gave the peracetylated (1f4)-â-C-
disaccharide 25a in good overall yield.
Table 2 shows the examples that were examined. The
esters 7 were all formed in good yield while methylena-
tion proceeded consistently in ∼50% yield, even with a
large excess of the methylenating reagent. RCM for
entries 1, 7, and 9 were carried out with catalyst 23 and
proceeded in good yield over the two steps, while entries
2-6, 8, and 10 were carried out with catalyst 26 in
comparable yield. In two cases (entries 1 and 7), both
catalysts 23 and 26 were employed for a side-by-side
comparison and yielded similar results.
An example in which there was no linking atom
between the two sugar units was also examined. Ester
7k , formed by DCC-mediated coupling of acid 6k 41 with
olefin alcohol 5a , was methylenated to provide 8k in 67%
yield and exposure to catalyst 2638 furnished glycal 9k ,
which in this case was isolable, in 77% yield, Scheme 7.
We examined a case where acetonide protecting groups
were present in the starting ester. Accordingly, compound
8j was exposed to catalyst 2337 and RCM ensued giving
the product glycal 9j in 40% yield while reaction with
the second generation Grubbs catalyst 2638 gave 9j in
74% yield, Scheme 6. When 9j was heated to 60 °C for 4
h in the presence of 20 mol % of (CF3)2CH3COH, TLC
analysis indicated that the glycal had undergone partial
decomposition under the reaction conditions. It would
seem that some of the alcohol ligand is lost from 23
during the RCM and this may be the cause of glycal
decomposition. This direct comparison (between 23 and
26) shows that if the glycal is the desired product, then
26 is the catalyst of choice. Application of the one-pot
protocol, this time with the second generation Grubbs
catalyst 26,38 was explored and RCM of 8j mediated by
26 was followed by hydroboration and oxidative quench
to furnish the C-disaccharide 10j in 57% overall yield,
Scheme 6. The yield for the two steps was still very
acceptable, especially since the RCM chemistry could now
be carried out on the benchtop with use of conventional
inert gas line techniques.
Op tim iza tion of th e Th r ee-Step P r otocol
Once the results in Table 2 were compiled, it became
clear that the overall yield of the C-disaccharide 10 from
ester 7 needed optimization. This is especially true if the
chemistry was to be used in an iterative sense or for two
(39) Haudrechy, A.; Sinay¨, P. J . Org. Chem. 1992, 57, 4142-4151.
(40) Lourens, G. J .; Koekemoer, J . M. Tetrahedron Lett. 1975, 3719-
3722.
(38) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956.
(41) Nicolaou, K. C.; Postema, M. H. D.; Yue, E. W.; Nadin, A. J .
Am. Chem. Soc. 1996, 118, 10335-10336.
4752 J . Org. Chem., Vol. 68, No. 12, 2003

Synthesis of Differentially-Linked â-C-Disaccharides
TABLE 2. P r ep a r a tion of Differ en tia lly-Lin k ed
SCHEME 8. Over a ll Yield Op tim iza tion w ith Ester
7l
â-C-Disa cch a r id esa
TABLE 3. Op tim iza tion Yield s (%) for Sch em e 8
overall
yield of 8l
yield of 9l
yield of 10l
yield of 10l
76a
68
90
65
90
55f
47
49
52
55
76a
not purifiedb
58c
not purifiedd
n ot p u r ified d
n ot p u r ified e
a
The acyclic enol ether was purified by flash chromatography.
b
The product glycal was not isolated, but rather the one-pot
protocol was employed. c Yield is over two steps. The acyclic enol
ether was not purified by flash chromatography, but the crude
reaction mixture was filtered through a pad of basic alumina.
e Then glycal was not isolated, but carried on crude to the next
step. f Yield is over three steps.
d
more simultaneous cyclizations. Since we had gram
amounts of ester 7l in hand, a few different methods for
the conversion of 7l to the target â-C-disaccharide 10l
were examined.
We therefore carried out the three-step sequence in a
few different ways as shown in Scheme 8 and Table 3.
The first entry shows that the overall yield of the
sequence is 47% if the acyclic enol ether 8l, the product
of RCM 9l, and the product of hydroboration 10l are all
purified by flash chromatography. The second entry 2
shows the overall yield of 10l is 49% if the glycal is not
purified. If the product of methylenation is not purified,
but merely filtered through a short plug of basic alumina,
then a 52% overall yield of 10l is obtained. If the
sequence is carried out with both crude acyclic enol ether
and crude glycal, and only the final product is purified,
the target â-C-disaccharide 10l is now obtained in 55%
overall yield for the three steps. In our initial work,16 the
Schrock catalyst 2337 was used to effect the cyclization
and we found that it was not tolerant of any impurities
in the acyclic enol ether, so it had to be purified prior to
the RCM step.
These results were then applied to esters 7a , 7c, 7d ,
7f, and 7l and Table 4 shows the results. The product
â-C-disaccharides 10 were formed in 53-59% overall
yield over the three steps and were then converted to the
fully acetylated â-C-disaccharides 25 in good yield.
a
Yields refer to chromatographically and spectroscopically
b
homogeneous materials. The corresponding olefin alcohol is
known.39 c The corresponding ethyl ester is known.40 Yields are
d
The deacetylated C-disaccharides 24a , 24c 24d 24f,
and 24l were tested against various glycosidase enzymes.
The results were unimpressive and only the deacetylated
variant 24f, the (1,1)-linked derivative, showed modest
inhibitory activity with a Ki of 126 µM against â-glucosi-
dase from almonds.28
for two steps; RCM and hydroboration-oxidative workup. e Ste-
reochemistry at C-1 and C-2 determined by acetylation and
analysis of the H-2 coupling constant in 1H NMR.28 f Reaction
carried out with 20-30 mol % of 23 in a glovebox followed by
g
hydroboration. Reaction carried out with 20-30 mol % of 26 on
an argon manifold followed by hydroboration.
J . Org. Chem, Vol. 68, No. 12, 2003 4753

Postema et al.
TABLE 4. Op tim ized Con d ition s for RCM a n d
Dep r otection a
Con clu sion
The synthesis of â-C-disaccharides by RCM has proven
to be an effective means of gaining access to an array of
â-C-disaccharides. The use of the new Grubbs catalyst
26 allows for the cyclization chemistry to be readily
carried out with benchtop techniques, and coupled with
our one-pot approach has made the synthesis of these
compounds quite practical. Inhibition assays against
â-glucosidase from almonds showed that only the C-1
derivative displayed modest to weak inhibitory behavior.
Structural modification of the C-disaccharides to try and
improve the inhibitory activity is a possible next step in
the project. Application of the methodology in an iterative
sense and to multiple simultaneous cyclizations is un-
derway and will be reported in due course.
Ack n ow led gm en t. We are grateful to Professor
Chuck Winter (WSU) for unlimited use of his glovebox.
We thank Professor P. G. Wang for allowing us to carry
out the inhibition assays in his laboratory and Professor
Frederick Valeriote (J osephine Ford Health Center) for
carrying out the in vitro differential cytotoxicity assays.
We thank Professor Christopher Hadad (Ohio State
University) for HRMS spectra and Dr. M. K. Ksebati
(WSU) and Dr. L. Hryhorczuk (WSU) for assistance with
NMR and mass spectral work, respectively. Acknowl-
edgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society (#33075-G1) and the NSF (CHE-0132770),
for partial support of this research.
a
Yields refer to chromatographically and spectroscopically
Su p p or tin g In for m a tion Ava ila ble: Experimental and
spectral data listings for the major compounds, 1H NMR
spectra of 7a -k , 8a , 9a , 9j, 10a -k , 19a , 20a , 22a , 25a , 25c,
25d , and 25f, as well as details of the biological testing. This
material is available free of charge via the Internet at
http://pubs.acs.org.
b
homogeneous materials. Yield is for three steps: methylenation,
RCM, and hydroboration-oxidative workup. c 20-25 mol % of 26
d
used for the RCM reaction. In this case, 40-45 mol % of 26 was
needed to push the RCM to completion. e Reaction carried out in
MeOH with 5% Pd/C under 50 psi of H2 followed by acetylation
(Ac2O/pyridine/4-DMAP).
J O030039X
C-Disaccharide 24a and 24l were screened42 for dif-
ferential cytotoxicity43 against both murine solid tumor
colon cells (C38) and murine leukemia cells and showed
no cytotoxicity whatsoever.
(43) Corbett, T. H.; Valeriote, F. A.; Polin, L.; Panchapor, C.; Pugh,
S.; White, K.; Lowichick, N.; Knight, J .; Bissery, M.-C.; Wozniak, A.;
LoRusso, P.; Biernat, L.; Polin, D.; Knight, L.; Biggar, S.; Looney, D.;
Demchick, L.; J ones, J .; J ones, L.; Blair, S.; Palmewr, K.; Essenmacher,
S.; Lisow, L.; Mattes, L. C.; Cavenaugh, P.; Rake, J .; Baker, L. In
Cytotoxic Anticancer Drugs: Models and Concepts for Drug Discovery
and Development; Kluwer Academic Publishers: Boston, MA, 1992;
pp 35-87.
(42) We thank Dr. Frederick Valeriote of the J osephine Ford Cancer
Center for carrying out these assays.
4754 J . Org. Chem., Vol. 68, No. 12, 2003