The practical synthesis of β-acyl glucuronides by using allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-glucuronate and 1-chloro-N,N,2-trimethyl-1-propenylamine

Article abstract of DOI:10.1016/j.tetlet.2016.06.057

We described the practical synthesis of β-acyl glucuronide from allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-glucuronate (5) mediated by 1-chloro-N,N,2-trimethyl-1-propenylamine (TMCE). A wide range of bulky carboxylic acids (aryl carboxylic acids or tertiary-carbon-linked carboxylic acids) were employed to give the corresponding β-acyl glucuronate in good yields. The side products of this reaction are only N,N-dimethylisobutyramide and HCl. As the resulting acyl glucuronates show sufficient solubility to organic solvent, it can be easily purified by conventional silica gel column chromatography and/or crystallization, even in multigram quantities. The allyloxycarbonyl group and allyl group are cleanly removed by Pd(0), and thus the protocol can provide multigram quantities of β-acyl glucuronides with high purity.

Full text of DOI:10.1016/j.tetlet.2016.06.057

Accepted Manuscript
The practical synthesis of β-acyl glucuronides by using allyl 2,3,4-tri-(O-ally-
loxycarbonyl)-D-glucuronate and 1-chloro-N,N,2-trimethyl-1-propenylamine
Muneki Nagao, Masashi Suzuki, Yasuo Takano
PII:
S0040-4039(16)30726-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.06.057
TETL 47784
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 May 2016
9 June 2016
14 June 2016
Please cite this article as: Nagao, M., Suzuki, M., Takano, Y., The practical synthesis of β-acyl glucuronides by
using allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-glucuronate and 1-chloro-N,N,2-trimethyl-1-propenylamine,
Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.06.057
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1
Tetrahedron Letters
The practical synthesis of β-acyl glucuronides by using allyl 2,3,4-tri-(O-
allyloxycarbonyl)-D-glucuronate and 1-chloro-N,N,2-trimethyl-1-propenylamine
Muneki Nagao a,*, Masashi Suzuki b, and Yasuo Takano a
aChemistry Research Laboratory I, Watarase Research Center, Kyorin Pharmaceutical Co., Ltd., 1848, Nogi, Nogi-machi, Shimotsuga-gun, Tochigi 329-0114,
Japan
bChemistry Research Laboratory II, Watarase Research Center, Kyorin Pharmaceutical Co., Ltd., 1848, Nogi, Nogi-machi, Shimotsuga-gun, Tochigi 329-0114,
Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
We described the practical synthesis of β-acyl glucuronide from
allyl 2,3,4-tri-(O-
allyloxycarbonyl)-D-glucuronate (5) mediated by 1-chloro-N,N,2-trimethyl-1-propenylamine
(TMCE). A wide range of bulky carboxylic acids (aryl carboxylic acids or tertiary-carbon-linked
carboxylic acids) were employed to give the corresponding β-acyl glucuronate in good yields.
The side products of this reaction are only N,N-dimethylisobutyramide and HCl. As the resulting
acyl glucuronates show sufficient solubility to organic solvent, it can be easily purified by
conventional silica gel column chromatography and/or crystallization, even in multigram
quantities. The allyloxycarbonyl group and allyl group are cleanly removed by Pd(0), and thus
the protocol can provide multigram quantities of β-acyl glucuronides with high purity.
Available online
Keywords:
Acyl glucuronide
1-Chloro-N,N,2-trimethylpropenylamine
Allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-glucuronate
Aryl carboxylic acid
2009 Elsevier Ltd. All rights reserved.
Acyl glucuronides (AGs) are one of the major metabolites for
drugs that contain carboxylic acid.1 Several studies have revealed
that AGs can undergo intermolecular reactions with plasma
proteins, leading to covalent drug protein adducts; thus AGs have
been considered to induce idiosyncratic drug toxicity.2 As the
ICH M3 guidelines show, it is preferable to evaluate the toxic
profiles of reactive AGs in the early stages of drug development.
synthesizing AGs with high purity. We also report on the
stereoselectivity for the acylation step in our new synthetic
protocol.
Toxicity studies require sufficient quantities of AGs with high
purity thus robust and easy synthetic method that can provide
them is demanded. Plausible procedures for the preparation of
AGs are 1) stereoselective ester bond formation between the
corresponding acid and the anomeric hydroxyl group in the
protected D-glucuronic acid, and 2) chemical or enzymatic
deprotection of the D-glucuronic acid moiety.1 It was shown that
the ester bond of AG is labile for acidic and/or basic conditions.
Thus the reaction conditions for esterification and deprotection
are both limited for the preparation of AGs.3 However, both of
those reports1, 3 succeeded in milligram-scale preparation of AGs.
To the best of our knowledge, the synthesis of multigram
quantities of AGs has not been reported.
Figure 1. Structures of KRP-105 (1) and its β-acyl glucuronide 2.
An established method of preparing AGs is to employ allyl D-
glucuronate (3, Scheme 1). Under Mitsunobu reaction conditions
using Ph3P and diisopropyl azodicarboxylate (DIAD), 3 reacts
with carboxylic acids to give a corresponding conjugate.5 The
resulting conjugate was transformed to AG by treatment with
Pd(Ph3P)4 in conjunction with pyrrolidine. Another useful
method for constructing the β-acyl bond is the condensation
reaction by using O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-
tetramethyluronium hexafluorophosphate (HATU).6 Under these
protocols, AGs of active pharmaceutical ingredients have been
synthesized on a milligram scale.7 Consequently, we tried to
apply our substrate 1 by using the previous method.
In a clinical study of KRP-105 (1, Figure 1), which is a highly
selective peroxisome proliferator-activated receptor-γ agonist, as
a candidate for anti-dyslipidemia agent,4 it was shown that KRP-
105 is readily metabolized to β-acyl glucuronide (2, Figure 1) in
the human body. Therefore, we planned to prepare the β-acyl
glucuronide of KRP-105 in multigram quantities with high purity
for toxicity studies.
In this study, we investigate a synthetic method of preparing
AGs in multigram quantities and describe a practical protocol for
Corresponding author. Tel.: +81-280-57-1551; fax: +81-280-57-2336; e-mail: muneki.nagao@mb.kyorin-pharm.co.jp

2
Tetrahedron Letters
that regioselective deacylation of peracylated glycopyranoses
with NH3 in THF gave the corresponding α-1-OH sugar
derivatives.11 Therefore, we envisioned that regioselective
ammonolysis would occur on anomeric allyl carbonate
predominantly to give 5 selectively. As we expected, stirring 6
with ammonium hydroxide in THF at 0 ºC for 2 h afforded 5 in
58% yield (Scheme 3).
Scheme 1. General methods for the synthesis of -acyl glucuronide.
First, we employed Mitsunobu reaction conditions for the
synthesis of 2 (Scheme 2, Condition A). In the presence of Ph3P
and DIAD in THF, KRP-105 (1) reacts with 3 to give
corresponding acyl glucuronate 4 in moderate β-selectivity (β/α =
70/30) with low conversion (31%). Severe silica gel column
chromatography followed by reslurry washing in ethyl acetate
gave 4 with satisfactory quality (β/α = 99.4/0.6), although the
isolated yield of 4 was poor (8%).
Scheme 3. Synthesis of glucuronate 5.
Having developed a practical route to 5, we then focused on
the diastereoselective condensation of 5 with KRP-105 (1).
Under the Mitsunobu reaction conditions, 5 reacts with 1 to give
acyl glucuronate 8 in moderate β-selectivity with low yield
(Table 1, entry 1). The β-selectivity diminished when the reaction
Conversely, the condensation reaction using HATU and N-
methylmorpholine (NMM) in acetonitrile provided excellent β/α
selectivity (β/α = 93/7). However, the yield of 4 was also poor
(19%) because of the low rate of conversion along with the
occurrence of regioisomers such as 4-O-acyl glucuronate
(Scheme 2, Condition B). Extensive investigations to improve the
yield were unsuccessful.8 Moreover, both the desired glucuronate
4 and its byproducts have low solubility in organic solvents
and/or water. In the preparation of multigram quantities of 4, it
was assumed that purification of 4 by column chromatography or
recrystallization would be problematic.
was
carried
out
using
1-ethyl-3-(3-dimethylamino-
propyl)carbodiimide hydrochloride (EDCI•HCl) and 1-hydroxy-
7-azabenzotriazole (HOAt) (entry 2). These results required us to
find another reactive intermediate. It has been reported that 1-
chloro-N,N,2-trimethyl-1-propenylamine (TMCE) would form
reactive carboxymethyleneiminium chloride with carboxylic acid,
and that it reacts with organometallic compounds to give ketones
in good yield.12 As expected, treatment of KRP-105 (1) with
TMCE at room temperature for 4 h followed by stirring with 5 in
the presence of Et3N gave the desired acyl conjugate 8 with high
β-selectivity (β/α = 97/3) (entry 3). The yield of 8 was improved
when 2 eq. of 1 and 2.5 eq. of TMCE were used (entry 4). It is
conceivable that the activated iminium intermediate derived from
1 and TMCE showed not only appropriate reactivity but also the
desired stereoselectivity against the anomeric β-1-OH of 5.13
Table 1. Condensation reaction with KRP-105 (1) and 5 using
TMCE.
Scheme 2. Synthesis of acyl conjugate 4.
It was expected that the 2,3,4-O-tri-protected analog of 4
would increase solubility into organic solvents, and thus the
purification of the conjugate would be easier because of its
higher lipophilicity. There is also no doubt that the 2,3,4-O-tri-
protected analog of 3 would react with acids at 1-OH to afford
the desired 1-acyl conjugate exclusively. Among the protecting
groups of 2,3,4-OH in 4, the allyloxycarbonyl group seems to be
more suitable for the synthesis of AGs than other protecting
groups, such as acetyl, benzoyl, and benzyl, as it can be
introduced in a few steps and removed under neutral conditions.9
entry
conditions (equiv)
8 / 9 / 10 a
yield
(%) b
36
Therefore,
we
considered
using
allyl
2,3,4-tri-(O-
1
2
1 (1.5), 5 (1.0), Ph3P (1.5), DIAD
(1.5), THF, rt,48 h
86.7 / 11.5 / 1.8
50.1 / 30.1 / 19.8
allyloxycarbonyl)-D-glucuronate (5, Figure 2) instead of allyl D-
glucuronate 3. It has been reported that 5 reacts with aromatic
isocyanate to afford the corresponding β-O-glycosyl carbamate.10
However, unexpectedly, the literature contains no example of the
synthesis of β-acyl glucuronides by using 5.
1 (1.2), 5 (1.0), EDCI•HCl (1.2),
HOAt (1.2), NMM (2.0), DMF, rt,
20 h; DMAP (0.03), rt, 53 h
1 (1.3), TMCE (1.6), CH2Cl2, THF, rt,
4 h; 5 (1.0), Et3N (1.5), CH2Cl2, THF,
rt, 18 h
55
40
73
3
96.9 / 3.0 / 0.1
95.8 / 4.0 / 0.2
4
1 (2.0), TMCE (2.5), CH2Cl2, THF, 40
ºC, 1 h; 5 (1.0), Et3N (2.0), rt, 2 h
a
The ratio was determined by HPLC analysis of the reaction mixture.
Anomeric stereochemistry of 8 and 9 was determined by coupling constant
analysis of 1H NMR.
Figure 2. Structures of glucuronate 5.
b The yield was a mixture of isolated β-anomer (8), α-anomer (9), and 4,5-ene
conjugate (10).
For the preparation of 5, we examined the selective removal of
the anomeric allyl carbonate in the fully protected glucuronate 6.
According to Schmidt and co-workers, bis(tributyltin) oxide is an
effective reagent for selective cleavage of the anomeric allyl
carbonate.10 However, in light of its toxicity, this reagent was not
suitable for the multigram preparation of 5. It has been reported
Having developed a reliable method for the -selective
acylation of KRP-105 (1), we applied a multigram preparation of
its -acyl glucuronide (Scheme 5). First, allyl 1,2,3,4-tetra-O-
(allyloxycarbonyl)-D-glucuronate (6) was prepared in two steps

3
from D-glucuronic acid (11) according to Schmidt’s
procedure.10Regioselective ammonolysis of anomeric carbonate
then furnished 5 in 66% yield. The TMCE-mediated -selective
acylation of 5 with 1 proceeded to provide the desired
glucuronate 8 with high stereoselectivity (β/α = 95.5/4.5). The
remaining 1 was precipitated after adding Me3N•HCl followed by
H2O, thus it was removed easily from the reaction mixture. As
glucuronate 8 has good solubility to organic solvent, multigram
quantities (14.5 g) of 8 were prepared with sufficient quality (β/α
= 99.9/0.1, HPLC 99.1area%) by using conventional silica gel
column chromatography followed by crystallization. Removal of
the allyloxycarbonyl group and allyl group of 8 was best effected
results suggested that bulky carboxylic acid tends to show better
β-selectivity in the condensation reaction with 5.
Table 2. Substrate scope for acylation of 5.
with
Pd(Ph3P)4
and
5,5-dimethyl-1,3-cyclohexanedione
(dimedone) in THF to afford glucuronide 2. The crude product
was purified by flush column chromatography on acidic silica gel
followed by crystallization to give 1.20 g of 2 in 67% yield with
high purity (HPLC 99.9area%).14
entry
carboxylic acid: R
13a: Ph
product ratio a
yield
(%) b
73
50
45
1
13b / 13c / 13d = 91.2 / 5.8 / 3.0
13b / 13c / 13d = 86.2 / 13.6 / 0.2
13b / 13c / 13d = 84.6 / 11.4 / 4.0
14b / 14c / 14d = 98.4 / 1.0 / 0.6
15b / 15c / 15d = 84.7 / 12.1 / 3.1
2c)
3d)
4
14a: 4-CH3-Ph
15a: 4-CH3O-Ph
16a: 4-Ac-Ph
77
58
5
6
16b / 16c / 16d = 75.8 / 10.9 / 13.3 78
7
8
9
17a: 4-Cl-Ph
17b / 17c / 17d = 86.8 / 8.4 / 4.8
18b / 18c / 18d = 86.8 / 5.0 / 8.2
19b / 19c / 19d = 94.2 / 5.4 / 0.4
NT
21b / 21c / 21d = 72.9 / 20.1 / 7.0
22b / 22c / 22d = 86.4 / 6.8 / 6.9
23b / 23c / 23d = 36.0 / 61.1 / 2.9
24b / 24c / 24d = 84.5 / 14.5 / 1.0
79
78
70
trace
85
77
77
79
18a: 2-CH3-Ph
19a: 3-(CH3)2N-Ph
20a: 2,6-di-CH3-Ph
21a: 4-pyridyl
22a: 3-thienyl
23a: 4-CH3-PhCH2
24a: Ph-C(CH3)2
10
11
12
13
14
a The product ratio was determined by HPLC analysis of the reaction mixture.
Anomeric stereochemistry of the products was determined by coupling
constant analysis of 1H NMR.
NT: not tested.
b The yield was a mixture of isolated in β-anomer (b), α-anomer (c), and 4,5-
ene conjugate (d).
c 13a (1.0 eq.), 5 (2.0 eq.), DIAD (2.0 eq.), Ph3P (2.0 eq), THF, 0ºC, 2 h.
c 13a (1.0 eq.), 5 (1.0 eq.), HATU (1.0 eq.),NMM (2.0 eq), THF, rt, 4 h.
It has been reported that TMCE transforms carboxylic acids
Scheme 5. Synthesis of -acyl glucuronide 2.
into
the
corresponding
acyl
chlorides
via
carboxymethyleneiminium chloride in CH2Cl2.15 On the other
hand, in a more polar solvent, such as a mixture of THF and
CH2Cl2 (3:1), TMCE reacts with carboxylic acids to form the
carboxymethyleneiminium chloride without liberation of
hydrogen chloride.12 As shown in Scheme 5, treatment of 5 with
benzoyl chloride (25) in THF afforded corresponding acyl
conjugate 13b/13c with lower β-selectivity (β/α = 4/1). In
contrast, adding 5 into pre-mixed TMCE and benzoic acid (13a)
in THF gave 13b/13c with high β-selectivity (Table 2, entry 1;
β/α = 94/6). These results suggested that TMCE transforms
benzoic acid into carboxymethyleneiminium chloride rather than
acid chloride in THF (Figure 3). As its steric hindrance, the
iminium intermediate reacted to the β-anomeric OH in 5, which
is more reactive than the α-OH,13 to give β-acyl conjugate 13b
predominantly. Therefore, we consider TMCE to be an effective
activating reagent for the β-selective acylation.
After discovering an effective acylation reaction of 5 with
KRP-105 (1) by using TMCE, we evaluated the substrate scope
for the acylation of 5 with aryl carboxylic acids. Benzoic acid
(13a), which is a simpler substrate than KRP-105 (1), showed
almost the same results as 1. The TMCE-mediated acylation of 5
with 13a gave the corresponding conjugate in 73% yield with
good β-selectivity (β/α = 94/6) (Table 2, entry 1). In contrast,
both the isolated yield and the β-selectivity were diminished
under the Mitsunobu reaction conditions (entry 2) or using
HATU as a condensation reagent (entry 3). In many cases of the
use of aryl carboxylic acid, corresponding conjugates were
obtained in satisfactory yield with sufficient β-selectivity by
using TMCE (entries 4-9, 11, 12). It is also noteworthy that the
presence of an ortho substituent, as in 2-methylbenzoic acid 18a
(entry 8), does not appreciably lower the yield. In contrast, a di-
ortho substituent such as 2,6-dimethylbenzoic acid 20a (entry 10)
didn’t give the coupling conjugate, because its bulkiness kept it
away from the active iminium intermediate.
In the case of aliphatic carboxylic acid, the reaction proceeded
well, but the β-selectivity depended on the substrate. The use of a
secondary carbon directly linked to the carboxylic acid substrate,
as in 4-methylphenylacetic acid 23a, gave an α-selective
coupling conjugate (entry 13). We consider that this substrate
cannot show sufficient steric hindrance with β-selectivity. On the
other hand, the use of a tertiary carbon directly linked to the
carboxylic acid substrate, as in 2-methyl-2-phenylpropanoic acid
24a, gave the β-selective coupling conjugate (entry 14). These
Scheme 5. Reaction of 5 with benzoyl chloride (25).
Corresponding author. Tel.: +81-280-57-1551; fax: +81-280-57-2336; e-mail: muneki.nagao@mb.kyorin-pharm.co.jp

4
Tetrahedron Letters
Figure 3. Proposed reaction species.
In conclusion, we demonstrated a novel method for the
chemical synthesis of AGs by using TMCE-mediated β-selective
acylation of allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-glucuronate
(5). It is conceivable that the activated iminium intermediate
derived from carboxylic acid and TMCE showed not only
appropriate reactivity but also the desired stereoselectivity
against the anomeric β-1-OH of 5. As the acyl glucuronate
conjugate protected with an allyloxycarbonyl group and an allyl
group show sufficient solubility to organic solvent, it can be
easily purified by conventional silica gel column chromatography
or crystallization, even in multigram quantities. Consequently,
AGs that are difficult to synthesize in large quantities by using
existing methods could be prepared with high purity. We believe
this protocol should be applicable to the preparation of various
AGs that possess fragile functional groups and/or low solubility.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at
References and notes
1.
2.
3.
Stachulski, A. V.; Harding, J. R.; Lindon, J. C.; Maggs, J. L.;
Park, B. K.; Wilson, I. D. J. Med. Chem. 2006, 49, 6931-6945.
Bailey, M. J.; Dickinson, R. G. Chem. Biol. Interact. 2003, 145,
117-137.
Although enzymatic deprotection of glucuronate has been
reported, it was not deemed suitable for multigram synthesis of
acyl glucuronides. Thus we focused on developing a chemical
synthesis method for acyl glucuronides. Baba, A.; Yoshioka, T. J.
Org. Chem. 2007, 72, 9541-9549.
4.
5.
6.
7.
Nomura, M.; Yumoto, K.; Shinozaki, T.; Isogai, S.; Takano, Y.;
Murakami, K. Bioorg. Med. Chem. Lett. 2012, 22, 334-338.
Juteau, H.; Gareau, Y.; Labelle, M. Tetrahedron Lett. 1997, 38,
1481-1484.
Bowkett, E. R.; Harding, J. R.; Maggs, J. L.; Park, B. K.; Perrie, J.
A.; Stachulski, A. V. Tetrahedron 2007, 63, 7596-7605.
(a) Meng, X.; Maggs, J. L.; Pryde, D. C.; Planken, S.; Jenkins, R.
E.; Peakman, T. M.; Beaumont, K.; Kohl, C.; Park, B. K.;
Stachulski, A. V. J. Med. Chem. 2007, 50, 6165-6176; (b) Noort,
D.; van Zuylen, A.; Fidder, A.; van Ommen, B.; Hulst, A. G.
Chem. Res. Toxicol. 2008, 21, 1396-1406.
8.
Condensation reagents; EDCI, 2-bromo-1-ethylpyridinium
tetrafluoroborate (BEPT), bis(2-oxo-3-oxazolidinyl)phosphinic
chloride (BOPCl), 2-chloro-1,3-dimethylimidazolidinium
tetrafluoroborate (CIB), diphenylphosphorylazide (DPPA), 2-(1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate (HBTU), 2-methyl-6-nitrobenzoic anhydride
(MNBA), benzotriazol-1-yl-oxytripyrrolidinophosphonium
hexafluorophosphate (PyBOP), Additive; DMAP, HOAt, Base;
NMM, 1,4-diazabicyclo[2.2.2]octane (DABCO), Solvent; DMF,
THF, CH2Cl2.
9.
Greene’s Protective Groups in Organic Synthesis; Wuts, P. G. M.;
Greene, T. W., Ed.; Wiley: New Jersey, 2007; pp287-288.
10. Alaoui, A. E.; Schmidt, F.; Monneret, C.; Florent, J-C. J. Org.
Chem. 2006, 71, 9628-9636.
11. Fiandor, J.; García-López, M. T.; De Las Heras, F. G.; Méndez-
Castrillón, P. P. Synthesis 1985, 12, 1121-1123.
12. Fujisawa, T.; Mori, T; Higuchi, K; Sato, T. Chem. Lett. 1983,
1791-1794.
13. Schmidt, R. R. Angew Chem. Int. Ed. Engl. 1986, 25, 212-235.
14. The synthesis of 2 was carried out by using a part of 8.
Considering the yield of 2 from 8 (67%), 6.64 g of 2 would be
synthesized from the whole quantity of 8 (14.5 g).
15. Devos, A.; Remion, J.; Frisque-Hesbain, A. M.; Colens, A.;
Ghosez, L. J. Chem. Soc. Chem. Commun. 1979, 24, 1180-1181.

8
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The practical synthesis of β-acyl glucuronides
by using allyl 2,3,4-tri-(O-allyloxycarbonyl)-D-
glucuronate and 1-chloro-N,N,2-trimethyl-1-
propenylamine
Leave this area blank for abstract info.
Muneki Nagao*, Masashi Suzuki, and Yasuo Takano

9
Highlights


A practical method for the synthesis of
β-acyl glucuronides is developed.
1-chloro-N,N,2-trimethyl-1-
propylamine is a good reagent for the
condensation of aryl carboxylic acids
with D-glucuronate.

Multigram quantities of β-acyl
glucuronide could be prepared with
high purity.
Corresponding author. Tel.: +81-280-57-1551; fax: +81-280-57-2336; e-mail: muneki.nagao@mb.kyorin-pharm.co.jp