Synthesis of the glucuronide metabolite of ABT-751

Article abstract of DOI:10.1016/j.tetlet.2006.12.114

A linear synthesis of the glucuronide metabolite of ABT-751 was replaced with a convergent synthesis that features direct glycosidic coupling between the aglycone and a trichloroacetimidate glucuronyl donor. Structural elucidation of a unique and unexpect

Full text of DOI:10.1016/j.tetlet.2006.12.114

Tetrahedron Letters 48 (2007) 1359–1362
Synthesis of the glucuronide metabolite of ABT-751
Kenneth M. Engstrom,* Rodger F. Henry and Ian Marsden
Abbott Laboratories, 1401 Sheridan Road, North Chicago, IL 60064-6290, USA
Received 14 November 2006; revised 18 December 2006; accepted 19 December 2006
Available online 23 December 2006
Abstract—A linear synthesis of the glucuronide metabolite of ABT-751 was replaced with a convergent synthesis that features direct
glycosidic coupling between the aglycone and a trichloroacetimidate glucuronyl donor. Structural elucidation of a unique and unex-
pected difluoroboron complex of the desired glycosidic coupling product along with optimization of the synthetic steps is described.
Ó 2006 Elsevier Ltd. All rights reserved.
Glucuronidation is a major pathway for drug metabo-
lism.1 As a drug candidate advances through clinical
N
O
development, a synthesis of its glucuronide metabolite(s)
RO
NH HN
S
OMe
often becomes necessary to verify its structure, to pro-
vide an analytical standard for use in quantification of
metabolite levels in clinical samples, and to provide
material for further pharmacological evaluation. The
synthesis of structurally complex glucuronides is not
always straightforward.2 The efficiency and scalability
of such syntheses is often limited by low yielding or
unselective glycosidic couplings, complex protecting
group strategies, tedious isolations, or impractical enzy-
matic reactions.
O
1 = H
2 = SO3H
3 =
HO2C
O
O
HO
OH
OH
Figure 1. ABT-751 (1) and its metabolites (2 and 3).
ple basic functional groups.6 However, we were aware
that the use of the Schmidt trichloroacetimidate meth-
odology might overcome this difficulty.7 Indeed,
2.0 equiv of trichloroacetimidate 58 reacts with 1 pro-
moted by 4.0 equiv of BF3ÆOEt2 in CH2Cl2 to give
nearly quantitative conversion of 1 to a 1:1 mixture of
two products as determined by HPLC analysis (Scheme
2). After quench of the reaction mixture with aqueous
sodium bicarbonate, the two products were separated
by silica gel chromatography. One product proved to
be the desired compound 6, whose structure was
assigned through X-ray crystallography (Fig. 2).9
Unfortunately, all attempts to produce crystals of the
other product suitable for X-ray crystallography were
unsuccessful. However, its structure was determined to
be boron complex 7 as deduced from NMR data.
ABT-751 (1, formerly E70103) is currently being evalu-
ated as a treatment for pediatric neuroblastoma.4 Com-
pound 1 is metabolized in humans to both sulfate 2 and
glucuronide 3 and therefore a synthesis of both com-
pounds was recently required to support Phase II clini-
cal studies (Fig. 1). The initial synthesis of glucuronide
3 used p-nitrophenol and 1-bromo-2,3,4-triacetylglucu-
ronic acid methyl ester 4 to form the key glycosidic bond
followed by five steps to reach the target molecule
(Scheme 1).5 Although this route provided the quantities
of material initially required for Phase I clinical studies,
a more convergent synthesis that uses the aglycone as
the glucuronyl acceptor would be more efficient for
supplying larger amounts since 1 is readily available in
multi-kilogram quantities.
Glucuronidation of glycosyl acceptors similar to 1 can
be particularly problematic due to the presence of multi-
The NMR spectra of 6 and 7 in DMSO-d6 are signifi-
cantly different and initially it was assumed that a differ-
ence in the glucuronide regiochemistry was responsible
for the different NMR spectra.10 However, a full assign-
ment of the two compounds using 2D NMR methods
*
Corresponding author. Tel.: +1 847 938 3183; fax: +1 847 938
2258; e-mail: kenneth.engstrom@abbott.com
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.12.114

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K. M. Engstrom et al. / Tetrahedron Letters 48 (2007) 1359–1362
1) H2, Pd/C
2) NaHCO3, O2N
Cl
MeO2C
AcO
O
Br
MeO2C
AcO
O
O
NO2
MeO2C
AcO
O
O
NH NO2
N
Ag2O
73%
OAc
OAc
OAc
HO
NO2
OAc
OAc
OAc
Cl
N
Cl
4
68% for two steps
H2
Pd/C
100%
O
S
pyridine
ClO2S
aq NaOH
MeOH
MeO2C
AcO
O
O
NH NH2
N
MeO2C
AcO
O
O
NH HN
N
OMe
3
O
OMe
OAc
OAc
34%
OAc
OAc
100%
Scheme 1. Initial route to 3.
O
MeO2C
O
O
NH HN
S
OMe
O
N
AcO
OAc
NH
CCl3
OAc
.
MeO2C
AcO
O
O
BF3 OEt2
6
+
1
CH2Cl2
OAc
F
F
OAc
O
S
B
MeO2C
AcO
O
O
N
N
OMe
5
O
HN
OAc
OAc
7
Scheme 2. Glucuronidation of 1.
nance is observed for 7 in DMSO-d6 (vide infra), in con-
trast to 6 where two are observed. Therefore, it is
postulated that the structure of 7 is a difluoroboron
complex with the BF2 moiety coordinated to both of
the exocyclic nitrogen atoms on the central pyridine
ring. This is also consistent with the chemical shift
values of the 11B (3.97 ppm in CD3CN) and 19F
(À137.2 and À137.3 ppm in CD3CN)10 coordinated to
nitrogen, since similar values were observed in a BF2
moiety complexed to two nitrogen heterocycles in mod-
ified BODIPY dyes.11 Such structures are common for
BF2 complexed to substituted dipyrromethene deriva-
tives (BODIPY), which are typically used as laser dyes12
or fluorophores in molecular probes.13 However, this
structure is somewhat unusual given that the complexa-
tion occurs between two exocyclic nitrogen atoms as
opposed to nitrogen heterocycles as is the case of
BODIPY dyes and pyridylpyrrolide complexes.14
Figure 2. ORTEP of 6 (black = C, green = H, red = O, blue = N,
yellow = S).
showed that the glucuronide was attached to the pheno-
lic hydroxyl moiety in both molecules and also that the
rest of the carbon skeleton was the same. This led to the
hypothesis that 7 might be a boron-fluoro adduct, which
was subsequently confirmed by the observation of reso-
nances in the 19F and 11B NMR spectra. The 11B spec-
trum showed a triplet in CD3CN (1JBF ꢀ 26 Hz), while
the 19F spectra showed two closely spaced doublets.
These data suggest the presence of a BF2 moiety.
Several tautomers for 7 can be envisaged. However, the
structure shown above is likely the lowest energy struc-
ture since the only exchangeable proton observed in the
spectrum resides primarily on the pyridine nitrogen
(12.99 ppm),10 as shown by observed coupling between
this proton and the adjacent proton on the pyridine ring
when the sample is cooled in CD3CN.
1
The H and 13C spectra for 6 and 7 show the greatest
Under optimized conditions, greater than 99% conver-
sion of 1 was achieved using 1.3 equiv of trichloro-
acetimidate 5 and 3.0 equiv of BF3ÆOEt2 (Scheme 2).
chemical shift changes (Dd ppm) for resonances in the
pyridine moiety and only one exchangeable NH reso-

K. M. Engstrom et al. / Tetrahedron Letters 48 (2007) 1359–1362
1361
1) 2.5 eq LiOH.H2O, MeOH
O
2) H2O, CHCl3 wash
MeO2C
AcO
O
O
NH HN
N
S
OMe
3
O
3) Dowex OH- resin
4) AcOH,iPrOH distillation
5) lyophilization
OAc
OAc
6
Scheme 3. Deprotection of 6.
Yoshimatsu, K.; Iijima, A.; Nagasu, T.; Tsukahara, K.;
Kitoh, K. Sulfonamide Derivatives. U.S. Patent 5,250,549,
October 5, 1993.
After the reaction was complete, addition of 1.5 equiv of
water converted boron complex 7 into the desired prod-
uct 6. By HPLC analysis, no significant impurities or
isomers were detected in the crude reaction mixture. It
is possible that the excess equivalents of BF3 form a
complex with 1, which effectively deactivates the reactive
nitrogen functionality and prevents formation of N-
linked isomers. Aqueous sodium bicarbonate workup
quenched and removed the boron side products, while
silica gel chromatography and recrystallization from
methanol removed excess hydrolyzed 5 to give an 83%
isolated yield of 6.
4. Gordon, G.; Hagey, A. E.; Meek, K. A.; Rosenberg, S. H.
Treatment of Cancer in Pediatric Patients. International
Patent WO 2005/117903 A1, December 15, 2005.
5. Eisai Co., previously unpublished results.
6. For the synthesis of some glucuronides containing basic
functional groups, see: (a) Kim, S.; Wu, J. Y.; Zhang, Z.;
Tang, W.; Doss, G. A.; Dean, B. J.; DiNinno, F.;
Hammond, M. L. Org. Lett. 2005, 3, 411–414; (b) Liu,
C.-H.; Liu, H.; Han, X.-Y.; Wu, B.; Zhong, B.-H.; Gong,
Z.-H. Synth. Commun. 2005, 5, 701–710; (c) Stazi, F.;
Palmisano, G.; Turconi, M.; Clini, S.; Santagostino, M. J.
Org. Chem. 2004, 4, 1097–1103; (d) Kawamura, K.;
Horikiri, H.; Hayakawa, J.; Seki, C.; Yoshizawa, K.;
Umeuchi, H.; Nagase, H. Chem. Pharm. Bull. 2004, 6,
670–671; (e) Kuo, F.; Gillespie, T. A.; Kulanthaivel, P.;
Lantz, R. J.; Ma, T. W.; Nelson, D. L.; Threlkeld, P. G.;
Wheeler, W. J.; Yi, P.; Zmijweski, M. Bioorg. Med. Chem.
Lett. 2004, 13, 3481–3486; (f) Wang, Y.; Yuan, H.;
Wright, S. C.; Wang, H.; Larrick, J. W. Bioorg. Med.
Chem. 2003, 7, 1569–1576; (g) Hasuoka, A.; Nakayama,
Y.; Adachi, M.; Kamiguchi, H.; Kamiyama, K. Chem.
Pharm. Bull. 2001, 12, 1604–1608; (h) Rukhman, I.;
Yudovich, L.; Nisnevich, G.; Gutman, A. L. Tetrahedron
2001, 6, 1083–1092, and references therein; (i) Brown, R.
T.; Carter, N. E.; Mayalarp, S. P.; Scheinmann, F.
Tetrahedron 2000, 56, 7591–7594; (j) Suzuki, T.; Mabuchi,
K.; Fukazawa, N. Bioorg. Med. Chem. Lett. 1999, 5, 659–
662; (k) Dodge, J. A.; Lugar, C. W.; Cho, S.; Osborne, J.
J.; Philips, D. L.; Glasebrook, A. L.; Frolik, C. A. Bioorg.
Med. Chem. Lett. 1997, 8, 993–996.
Removal of the acetate groups and hydrolysis of the
methyl ester in 6 were accomplished using 2.5 equiv of
LiOHÆH2O in methanol (Scheme 3).7 Addition of water
and subsequent washing with chloroform removed
partially deprotected impurities if any. Treatment with
Dowex hydroxide resin adsorbed the carboxylate of 3
and filtration and washing of the resin with water re-
moved the lithium salts. The product was then recovered
from the resin by treatment with acetic acid, followed by
azeotropic removal of acetic acid with isopropanol.
Repeated lyophilization from water removed residual
solvents to give amorphous 3 in 72% isolated yield.
In summary, synthesis of the ABT-751 glucuronide 3
using the Schmidt trichloroacetimidate methodology
for the direct glucuronidation of the readily available
aglycone 1 resulted in an increase in overall yield from
16% using the linear route to 60% using this more con-
vergent route. This increase in efficiency and throughput
allowed for more rapid synthesis of the multigram quan-
tities of this metabolite required for clinical use.
7. Engstrom, K. M.; Daanen, J. F.; Wagaw, S.; Stewart, A.
O. J. Org. Chem. 2006, 71, 8378–8383.
8. Compound 5 can be synthesized on multigram scale from
1,2,3,4-tetra-O-acetyl-b-D-glucuronic acid methyl ester in
two steps: (a) Bollenback, G. N.; Long, J. W.; Benjamin,
D. G.; Lindquist, J. A. J. Am. Chem. Soc. 1955, 77, 3310–
3315; (b) Jacquinet, J. C. Carbohydr. Res. 1990, 199, 153–
181.
Acknowledgements
9. Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication numbers CCDC627190. Copies of the
data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax:
+44(0) 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk].
10. Data for 6: 1H NMR (DMSO-d6 (2.50 ppm), 500 MHz): d
9.48 (s, 1H, NH), d 7.94 (dd, J = 4.9, 1.7 Hz, 1H), d 7.85
(s, 1H, NH), d 7.59 (d, J = 9.0 Hz, 2H),d 7.33 (d,
J = 9.2 Hz, 2H), d 7.25 (dd, J = 7.7, 1.7 Hz, 1H), d 6.96
(d, J = 9.1 Hz, 2H), d 6.88 (d, J = 9.1 Hz, 2H), d 6.70 (dd,
J = 4.9, 7.7 Hz, 1H), d 5.53 (d, J = 7.9 Hz, 1H), d 5.48 (t,
J = 9.6 Hz, 1H), d 5.07 (dd, J = 9.6, 7.8, 1H), d 5.06 (t,
J = 9.6 Hz, 1H), d 4.68 (d, J = 9.9, 1H) d 3.71 (s, 3H), d
3.65 (s, 3H), d 2.04 (s, 3H), d 2.01 (s, 3H), d 2.00 (s, 3H).
13C NMR (DMSO-d6 (39.5 ppm), 125 MHz): d 170.0,
169.8, 169.5, 167.6, 163.0, 151.4, 151.4, 145.7, 136.5, 135.3,
We would like to thank Dr. Anthony Haight and Dr.
Seble Wagaw for helpful suggestions and discussions.
References and notes
1. For a review of the importance of glucuronides in
regulation of the biological activity of pharmaceuticals,
see: Mulder, G. J. J. Annu. Rev. Pharmacol. Toxicol. 1992,
32, 25–49.
2. For reviews on the chemical synthesis of glucuronides, see:
(a) Stachulski, A. V.; Jenkins, G. A. Nat. Prod. Rep. 1998,
173–186; (b) Kaspersen, F. M.; van Boeckel, C. A. A.
Xenobiotica 1987, 17, 1451–1471.
3. Yoshino, H.; Ueda, N.; Sugumi, H.; Niijima, J.; Kotake,
Y.; Okada, T.; Koyanagi, N.; Watanabe, T.; Asada, M.;

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K. M. Engstrom et al. / Tetrahedron Letters 48 (2007) 1359–1362
131.2, 129.4, 120.9, 118.6, 117.1, 114.9, 114.7, 98.4, 71.6, 114.2, 111.4, 96.8, 71.0, 70.9, 70.4, 69.0, 67.0, 55.6, 52.6,
71.4, 71.1, 69.6, 56.0, 53.0, 20.8, 20.8, 20.7. Data for 7: 1H
NMR (DMSO-d6 (2.50 ppm), 500 MHz): d 12.99 (s, 1H,
NH), d 7.94 (d, J = 9.1 Hz, 2H), d 7.34–7.31 (m, 3H), d
7.13–7.08 (m, 5H), d 7.64 (t, J = 7.1 Hz, 1H), d 6.72 (d,
J = 7.9 Hz, 1H), d 5.49 (t, J = 9.7 Hz, 1H), d 5.13 (dd,
J = 7.9, 9.7 Hz, 1H), d 5.08 (t, J = 9.7 Hz, 1H), d 4.75 (d,
J = 9.9 Hz, 1H), d 3.82 (s, 3H), d 3.64 (s, 3H), d 2.03 (s,
3H), d 2.02 (s, 3H), d 2.01 (s, 3H). 13C NMR (DMSO-d6
(39.5 ppm), 125 MHz): d 169.5, 169.3, 169.0, 167.1, 162.5,
154.9, 146.7, 131.6, 131.4, 129.3, 126.8, 124.5, 117.3, 116.5,
20.3, 20.3, 20.2.
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