A convergent ring-closing metathesis approach to carbohydrate-based macrolides with potential antibiotic activity

Article abstract of DOI:10.1021/jo051021k

An efficient convergent approach has been developed for the construction of novel, non-natural, carbohydrate-based macrolides. The key step in the synthesis is the formation of the macrocyclic ring via a ring-closing metathesis reaction. The obtained macrolide analogues have been screened for biological activity against Gram-positive and Gram-negative bacteria, including resistant strains, yeasts, and molds.

Full text of DOI:10.1021/jo051021k

A Convergent Ring-Closing Metathesis
Approach to Carbohydrate-Based
Macrolides with Potential Antibiotic
Activity
Petra Blom, Bart Ruttens, Steven Van Hoof,
Idzi Hubrecht, and Johan Van der Eycken*
Laboratory for Organic and Bioorganic Synthesis,
Department of Organic Chemistry, Ghent University,
Krijgslaan 281 (S.4), B-9000 Gent, Belgium
FIGURE 1. General structure of the carbohydrate-based
macrolides.
thus imposing a conformational restriction, pyranose and
furanose sugars are ideally suited for this purpose.
We now report the synthesis of several new carbohy-
drate-based macrolides and their biological activity against
resistant microorganisms. The compounds presented
were selected to demonstrate the flexibility of our ap-
proach by displaying structural variation in (i) the
substituents R1, R2, and R3 on the carbohydrate scaffold,
(ii) the type of bond (R4 and R5) between the scaffold and
the side chains, and (iii) the size of the macrocyclic ring
(Figure 1).
Benedikt Sas, Johan Van hemel, and
Jan Vandenkerckhove
Kemin Pharma Europe, Atealaan 4H, B-2200 Herentals,
Belgium
johan.vandereycken@ugent.be
Received May 22, 2005
Commercially available 1,2,3,4,6-penta-O-acetyl-â-D-
glucopyranose (1a) was chosen as starting material for
the preparation of several carbohydrate scaffolds (Scheme
1). SnCl4-catalyzed exchange with thiophenol exclusively
gave 1b (78% yield; JH1-H2 ) 10.1 Hz).6 Solvolysis of the
remaining acetates under Zemple´n conditions, followed
by acid-catalyzed formation of the 4,6-O-benzylidene
acetal, afforded 27 (57% from 1a). Methylation of the free
hydroxyl functions of compound 2 gave dimethyl ether
3a,8 while benzylation furnished compound 3b.7 Acid-
catalyzed solvolysis finally provided the scaffolds 4a and
4b in 55% and 32% overall yield, respectively. Reductive
cleavage of the thiophenyl moiety in 4a with Raney nickel
An efficient convergent approach has been developed for the
construction of novel, non-natural, carbohydrate-based mac-
rolides. The key step in the synthesis is the formation of
the macrocyclic ring via a ring-closing metathesis reaction.
The obtained macrolide analogues have been screened for
biological activity against Gram-positive and Gram-negative
bacteria, including resistant strains, yeasts, and molds.
Because of the intensive use of antibiotics, resistant
strains of bacteria have developed, and cross-resistance
to different macrolides has been generally observed.1 This
increasing bacterial drug resistance is driving a greater
focus on the development of new, efficacious bactericides.
We have developed a flexible convergent ring-closing
metathesis (RCM) approach for the construction of a new
class of molecules consisting of a carbohydrate scaffold
fused to a macrocyclic ring system (Figure 1),2,3 classify-
ing these compounds as macrolides and ketolides.4
Carbohydrate substructures often occur in natural
macrolides and can contribute significantly to their
biological activity. Further, the structural complexity of
carbohydrates can be used to introduce wide chemical
and stereochemical variation. In addition, as the orienta-
tion of the side chains plays an important role in
macrocyclizations,5 one of the functions of the carbohy-
drate scaffold during the synthesis is to keep the side
chains in the correct position. Due to their cyclic nature,
(3) (a) Recently, the synthesis of novel macrocycles by RCM of
structurally preorganized bivalent carbohydrates was reported: Ve-
lasco-Torrijos, T.; Murphy, P. V. Org. Lett. 2004, 6, 3961-3964. (b)
For a very recent approach towards novel carbohydrate-based sym-
metrical macroheterocycles by RCM, see: Biswas, G.; Sengupta, J.;
Nath, M.; Bhattacharjya, A. Carbohydr. Res. 2005, 340, 567-578. (c)
For an approach towards macrocyclic polyethers by RCM, see: Rod-
r´ıguez, R. M.; Morales, E. Q.; Delgado, M.; Esp´ınola, C. G.; Alvarez,
E.; Pe´rez, R.; Mart´ın, J. D. Org. Lett. 1999, 1, 725-728. Delgado, M.;
Mart´ın, J. D. J. Org. Chem. 1999, 64, 4798-4816. (d) For synthetic
approaches towards resin glycosides such as Woodrosin I and Tricolorin
A, macrocyclic glycolipids with interesting biological activities, see:
Fu¨rstner, A.; Jeanjean, F.; Razon, P.; Wirtz, C.; Minott, R. Chem. Eur.
J. 2003, 9, 320-326 and references therein. Fu¨rstner, A.; Mu¨ller, T.
J. Am. Chem. Soc. 1999, 121, 7814-7821 and references therein.
(4) Farma, C.; Gagliardi, S. Seminars in Organic Synthesis; XIX
Summer School ,A Corbella., Gargnano, Italy, June 20-24, 1994;
pp 199-225.
(5) (a) Fu¨rstner, A.; Langemann, K. J. Org. Chem. 1996, 61, 3942-
3943. (b) Fu¨rstner, A.; Langemann, K. Synthesis 1997, 792-803. (c)
Fu¨rstner, A. Synlett 1999, 10, 1523-1533. For review articles, see:
(d) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413-4450. (e)
Armstrong, S. K. J. Chem. Soc., Perkin Trans. 1 1998, 371-388. (f)
Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36,
2036-2055. (g) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3013-
3043. (h) Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199-2238.
(6) Landge, A. B.; Ingle, T. R.; Bose, J. L. Ind. J. Chem. 1969, 7
(12), 1200-1202.
(1) See, for example: (a) Arthur, M.; Brissonnoel, A.; Courvalin, P.
J. Antimicrob. Chemother. 1987, 20(6), 783-802. (b) O’Hara, K.;
Kanda, T.; Ohmiya, K.; Ebisu, T.; Kono, M. Antimicrob. Agents
Chemother. 1989, 33 (8), 1354-1357. (c) Sutcliffe, J.; TaitKamradt,
A.; Wondrack, L. Antimicrob. Agents Chemother. 1996, 40 (8), 1817-
1824. (d) Wondrack, L.; Massa, M.; Yang, B. V.; Sutcliffe, J. Antimicrob.
Agents Chemother. 1996, 40 (4), 992-998.
(7) (a) Liptak, A.; Jodal, I.; Harangi, J.; Nanasi, P. Acta Chim. Hung.
1983, 113 (4), 415-422. (b) Nicolaou, K. C.; Randall, J. L.; Furst, G.
T. J. Am. Chem. Soc. 1985, 107, 5556-5558. (c) Nicolaou, K. C.;
Wissinger, N.; Pastor, J.; Deroose, F. J. Am. Chem. Soc. 1997, 119,
449-450.
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kerckhove, J.; Ruttens, B. PCT Int. Appl. WO2003032905, 2003.
10.1021/jo051021k CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/22/2005
J. Org. Chem. 2005, 70, 10109-10112
10109

SCHEME 1. Synthesis of the Scaffoldsa
a Reaction conditions: (a) SnCl4, PhSH, CH2Cl2, 0 °C to rt, 24 h; (b) K2CO3, THF/MeOH (1/1), rt; (c) benzaldehyde dimethyl acetal,
10-CSA, DMF, 110 °C; (d) (i) NaH, DME, 0 °C, 30 min, (ii) MeI, 0 °C to rt; (e) (i) NaH, THF, 0 °C, 30 min, (ii) BnBr, TBAI, 0 °C to rt, 48
h; (f) 10-CSA, MeOH, rt; (g) Raney-Ni, EtOH, rt, 1 h; (h) TsCl, Et3N, CH2Cl2, rt, 24h; (i) NaN3, DMF, 60 °C, 48h; (j) PPh3, THF/H2O
(100/1), rt, 48 h; (k) HBr/HOAc (30wt %), rt, 30 min; (l) (i) PhMgBr, Et2O, 0 °C to rt, 48 h, (ii) Ac2O, pyridine, rt, 18 h; (m) oxalyl bromide,
CH2Cl2/DMF (20/1), rt, 1 h; (n) BnMgBr, Et2O, 0 °C to rt, 18 h; (o) H2, Pd/C, EtOH, 3-4 atm, rt, 2 h.
in absolute ethanol afforded scaffold 5 nearly quantita-
tively.9 Regioselective tosylation of 4a, followed by nu-
cleophilic displacement of tosylate 6a with sodium azide
and Staudinger reduction of the resulting azide 6b, gave
amino alcohol 6c (75% from 4a).10
ent offers the possibility for additional differentiation at
the C1 position by glycosidation with other sugars,
alcohols or nucleophiles.14 While the abundant appear-
ance of methoxy groups in natural macrolides prompted
us to focus our attention on methyl ethers at the C2 and
C3 position of the scaffolds, we also explored several other
functional groups at these positions including benzyl
ethers (4b). Finally, the (latent) amino function at
position C6 in scaffolds 6b and 6c enabled us to inves-
tigate different types of bonds between scaffold and side
chains. A simple two-step reaction sequence, consisting
of the attachment of the side chains and subsequent
RCM, is sufficient to convert the scaffolds to carbohydrate-
based macrolide analogues. Initially, the efficiency of this
convergent approach toward macrolide analogues was
evaluated using simple, commercially available bifunc-
tional aliphatic chains. Macrocyclic glycolipids such as
Woodrosin I and Tricolorin A also contain a long hydro-
phobic hydrocarbon chain bridging two sugar hydroxyl
groups and show interesting biological activities.3d The
synthesis of highly functionalized macrolides from car-
bohydrate scaffolds and acyclic carbohydrate side chains
is currently in progress and will be reported elsewhere.
Attachment of 4-pentenoyl chloride to scaffold 4a
provided precursor 14 in 83% yield (Scheme 2). With 4b,
the same reaction conditions led to an inseparable
mixture of compound 21 and unknown side products. In
situ activation of 4-pentenoic acid with 1,3-diisopropyl-
Glucopyranosyl bromide 1c (JH1-H2 ) 4.0 Hz) was
converted to 7a (JH1-H2 ) 9.9 Hz) following a literature
procedure.11 Zemple´n methanolysis (f 7b), introduction
of the 4,6-O-benzylidene acetal (f 8a), methylation (f
8b), and finally, 10-CSA-mediated solvolysis of the acetal
furnished scaffold 9 (36% from 1c). Intermediate 11a
(JH1-H2 ) 9.2 Hz) was prepared from 2,3,4,6-tetra-O-
benzyl-D-glucopyranose (10a) by SN2 substitution of
R-bromide 10b (JH1-H2 ) 3.7 Hz) with benzylmagnesium
bromide, following the procedure described by Panigot
and Curley.12 Catalytic hydrogenolysis of the benzyl
ethers furnished tetrol 11b. A sequence of reactions
identical to that for the preparation of compound 9 from
7b gave scaffold 13 (30% from 10a).
Scaffolds 4a, 4b, 5, 6b, 6c, 9, and 13 were used to
prepare a variety of macrolide analogues. Due to the
substitution of the anomeric oxygen by hydrogen (5),
sulfur (4a, 4b, 6b, 6c) and carbon (9, 13) an improved
metabolic stability as compared to natural carbohydrates
can be expected.13 Furthermore, the thiophenyl substitu-
(9) Pettit, G. R.; Van Tamelen, E. E. Org. React. 1962, 12 (5), 410-
529.
(10) Xie, J. Carbohydr. Res. 2003, 338, 399-406.
(11) Panigot, M. J.; Humphries, K. A.; Curley, R. W., Jr. J.
Carbohydr. Chem. 1994, 13 (2), 303-321.
(12) Panigot, M. J.; Curley, R. W., Jr. J. Carbohydr. Chem. 1994,
13(2), 293-302.
(13) Kiefel, M. J.; Thomson, R. J.; Radovanovic, M.; Von Itzstein,
M. J. Carbohydr. Chem. 1999, 18 (8), 937-959.
(14) Garegg, P. J. Adv. Carbohydr. Chem. Biochem. 1997, 52, 179-
205.
10110 J. Org. Chem., Vol. 70, No. 24, 2005

SCHEME 2. Macrolide Analogues Based on Ester
Bonds between Scaffold and Side Chainsa
SCHEME 3. Different Bond Types between
Scaffold and the Macrocyclic Ringa
a Reaction conditions: (a) 5-bromo-1-pentene, NaH, TBAI, THF,
0 °C to rt; (b) Grubbs’ catalyst (A) (10 mol %), CH2Cl2, rt; (c) allyl
bromide, NaH, TBAI, THF, 0 °C to rt, 18 h; (d) PPh3, THF/H2O
(9/1), rt, 48 h; (e) 4-pentenoyl chloride, pyridine, DMAP, CH2Cl2,
0 °C to rt.
a Reaction conditions: (a) 4-pentenoyl chloride, pyridine, DMAP,
CH2Cl2, rt, 18 h; (b) Grubbs’ catalyst (A) (10 mol %), CH2Cl2, rt;
(c) OsO4, NMO, t-BuOH, acetone/H2O (2.5/1), rt, 1 h; (d) Raney
Ni, H2, EtOH, rt, 1 h; (e) Raney Ni, EtOH, rt, 1 h; (f) 6-heptenoic
acid, DIC, HOBt, DMAP, CH2Cl2, rt, 72 h; (g) 4-pentenoic acid,
DIC, HOBt, DMAP, CH2Cl2, rt, 120 h.
16 in 29% yield (not optimized).15 Epoxidation failed due
to the sensitivity of the thiophenyl moiety to oxidation.
Some examples of different bond types between scaffold
and the macrocyclic ring are shown in Scheme 3. Scaf-
folds 4a, 5, and 6b were efficiently alkylated with side
chains to provide compounds 23, 25, and 27. Staudinger
reduction of azide 27 furnished amine 28,10 easily acy-
lated with 4-pentenoyl chloride under basic conditions
to give the mixed diene 29. Due to excessive formation
of side products the same acylation conditions afforded
only low yields of compound 31 when applied to 6c. The
moderate yields for cyclization of dienes 23, 25, 29 and
31 can be attributed to the linkage between scaffold and
alkene side chains. RCM is one of the most efficient
entries into large rings provided that a few basic param-
eters are properly assessed.5,16 Apart from the obvious
spatial requirements, the presence of polar groups for
coordination with the metal carbene complexes of the
catalytic cycle and their orientation relative to the alkene
units are particularly crucial for the success of RCM-
based macrocyclizations. Compared to esters, ethers
possess only a moderate capacity to coordinate with the
metal carbene intermediates, resulting in a low yield of
macrolide 24.
carbodiimide (DIC) in the presence of 1-hydroxybenzo-
triazole (HOBt), and DMAP proved to be more effective
and furnished diester 21 in excellent yield. Likewise,
reaction of scaffold 4a with 6-heptenoic acid gave diene
19. Macrocyclization of 14, 19, and 21 via a RCM reaction
with Grubbs’ first generation catalyst (A)5 gave fair to
good yields of the desired macrolide analogues 15, 20 and
22, respectively. A remarkable selectivity was observed
in the case of compounds 15 and 22, where the vicinal
coupling constants of the vinylic protons indicated ex-
clusive formation of a double bond with E-configuration
(J ) 15.0 and 14.8 Hz, respectively). In contrast, the
larger macrocycle 20 was obtained as a 2/1 mixture of
the E- and Z-alkene. It should be pointed out that the
yield for ring closure of compound 19 (34%) refers to the
isolated yield of pure E- and Z-isomers and neglects a
significant mixed fraction.
Next, we explored the derivatization of the double
bond. Macrolide 15 was readily hydrogenated with Raney
nickel under hydrogen atmosphere with concomitant
reductive cleavage of the anomeric thiophenyl group,
affording 17. Application of the same catalyst under inert
atmosphere afforded compound 18 in moderate yield.
Direct dihydroxylation of 15 with 4-methylmorpholine
N-oxide (NMO) and a catalytic amount of OsO4 furnished
a diastereomeric mixture of the corresponding syn-diols
(15) (a) Akashi, K.; Palermo, R. E.; Sharpless, K. B. J. Org. Chem.
1978, 43 (10), 2063-2066. (b) Dailey, O. D. Jr.; Fuchs, P. L. J. Org.
Chem. 1980, 45 (2), 216-236. (c) Ugarkar, B. G.; DaRe, J.; Schubert,
E. M. Synthesis 1987, 715-716.
(16) Fu¨rstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann,
C. W.; Mynott, R.; Stelzer, F.; Thiel, O. R. Chem. Eur. J. 2001, 7 (15),
3236-3253.
J. Org. Chem, Vol. 70, No. 24, 2005 10111

TABLE 1. Antibacterial and Antifungal Activity of Some of the Macrolides
% of growth at a dose of 25 ppm compared to the negative control
compd
E. faecalis
VRE
S. aureus
MRSA
P. aeruginosa
S. typhimurium
C. albicans
M. gypseum
control
20 (E)
20 (Z)
24 (E)
24 (Z)
32
100
100
100
100
100
100
100
100
74
58
60
54
81
76.5
98.4
87.5
85.5
68.2
61.8
81.0
82.0
86.4
93.0
102.8
97.0
98.3
100.4
56.7
114.6
104.1
107.3
110.8
75.6
29.0
79.8
91.6
67.6
83.3
76.4
87.0
90.1
90.1
96.6
98.4
85.0
93.9
103.2
78.1
SCHEME 4. Synthesis of Macrolide Analogues
with Different Ring Sizesa
Depending on the choice of the side chains, macrocyclic
rings of different sizes can be attached to the carbohy-
drate scaffolds (Scheme 4). All diene intermediates 33,
36, 38 and 40 were easily prepared by acylation of
scaffolds 9 and 13 with alkenoyl chlorides under basic
conditions, although formation of side products at el-
evated temperature decreased the yield of diester 33
significantly. Subsequent RCM with Grubbs’ catalyst (A)
of the diene precursors 33, 36, 38 and 40 yielded the
desired macrocycles 34, 37, 39 and 41, respectively.
Epoxidation of substrate 34 with 3-chloroperbenzoic acid
(mCPBA) afforded a diastereomeric mixture of epoxides
35 in 27% yield (not optimized).17
The biological activity of the macrolide analogues
towards Gram-positive bacteria (Enterococcus faecalis,
vancomycin-resistant enterococci (VRE), Staphylococcus
aureus, methicillin-resistant Staphylococcus aureus
(MRSA)), Gram-negative bacteria (Pseudomonas aerugi-
nosa and Salmonella typhimurium), a typical yeast
(Candida albicans), and a typical mold (Microsporum
gypseum) were evaluated. Some results are summarized
in Table 1. E-20 has a good overall activity against gram-
negative bacteria, especially against P. aeruginosa and
to a lesser extent against VRE. Compound 32 displays
good activity against gram-positive bacteria, including
resistant strains such as MRSA. Macrolide Z-24 shows
the best activity against M. gypseum.
In conclusion, a highly convergent approach for the
synthesis of novel, non-natural, carbohydrate-based mac-
rolide analogues has been developed. Some of the mac-
rolides and intermediates display moderate antibiotic
activity against Gram-positive and Gram-negative bac-
teria and fungi. Currently, lead optimization and prepa-
ration of highly functionalized macrolides are under
investigation.
a Reaction conditions: (a) 4-pentenoyl chloride, pyridine, DMF,
90 °C, 2 h; (b) Grubbs’ catalyst (A) (10 mol %), CH2Cl2, rt; (c)
mCPBA, CH2Cl2, rt, 72 h; (d) 6-heptenoyl chloride, pyridine,
DMAP, CH2Cl2, 0 °C to rt, 18 h; (e) 4-pentenoyl chloride, pyridine,
DMAP, CH2Cl2, 0 °C to rt, 22 h; (f) 10-undecenoyl chloride,
pyridine, DMAP, CH2Cl2, 0 °C to rt, 18 h.
Acknowledgment. The authors are greatly in-
debted to the “Instituut ter bevordering van Wetenschap
en Technologie in Vlaanderen” (IWT) for a research
grant, and to Kemin Pharma Europe for financial
support. Dirk Van Haver is acknowledged for the
conformational analysis of the macrolides.
The much better yield for the formation of compound
26 probably originates from the close spatial arrange-
ment of the shorter side chains, making cyclization
entropically less disfavorable. In contrast, the poor yields
of dienes 30 and 32 in the RCM are probably the result
of the strong chelation of the amide bonds to the metal
carbene complex, sequestering the catalyst in an unpro-
ductive form. Both cycloalkenes 24 and 26 were formed
roughly as 1/1 mixtures of E- and Z-isomers. Macrolide
analogue 30 was obtained exclusively as the E-alkene (J
) 14.9 Hz). In the case of compound 32, line broadening
of the vinylic signals, caused by the hindered rotation
around the amide bond, obstructs proper determination
of the E/Z ratio.
Supporting Information Available: Synthesis and spec-
troscopic data of all synthesized compounds, full experimental
screening protocols and results, and theoretical calculations
of the E/Z ratios based on molecular modeling. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO051021K
(17) Finch, H.; Harwood: L. M.; Highcock, R.; Jackson, B.; Prout,
K.; Robertson, G.; Sewell, R. C. Synlett 1990, 384-386.
10112 J. Org. Chem., Vol. 70, No. 24, 2005