A facile synthesis of saxagliptin intermediate N-Boc-3-hydroxyadamantylglycine

Article abstract of DOI:10.1007/s11164-013-1515-3

Commercial-scale synthesis of (S)-N-Boc-3-hydroxyadamantylglycine (I), which is the key intermediate of saxagliptin, can be achieved from 2-(3-hydroxy-1-adamantyl)-2-oxoacetic acid (6) by reductive amination with phenylalanine dehydrogenase. The biological enzyme used in the original medicinal chemistry route is expensive and not easily obtained, making it unsuitable for further development. In this work, we developed a conventional chemical approach to give the corresponding oxime from oxoacetic acid followed by reduction of oxime and protected amino with (Boc)₂O to afford the racemic mixture of N-Boc-3-hydroxyadamantylglycine (III). Utilizing this route, N-Boc-3-hydroxyadamantylglycine was prepared in six linear chemical steps with 38 % overall yield.

Full text of DOI:10.1007/s11164-013-1515-3

Res Chem Intermed
DOI 10.1007/s11164-013-1515-3
A facile synthesis of saxagliptin intermediate
N-Boc-3-hydroxyadamantylglycine
Yingjie Chen
Xiangnan Hu
•
Anmin Wang
•
Zhu Tao
•
Yu Deng
•
Received: 19 July 2013 / Accepted: 15 December 2013
Ó Springer Science+Business Media Dordrecht 2014
Abstract Commercial-scale synthesis of (S)-N-Boc-3-hydroxyadamantylglycine
I), which is the key intermediate of saxagliptin, can be achieved from 2-(3-hydroxy-
-adamantyl)-2-oxoacetic acid (6) by reductive amination with phenylalanine
(
1
dehydrogenase. The biological enzyme used in the original medicinal chemistry
route is expensive and not easily obtained, making it unsuitable for further devel-
opment. In this work, we developed a conventional chemical approach to give the
corresponding oxime from oxoacetic acid followed by reduction of oxime and
protected amino with (Boc) O to afford the racemic mixture of N-Boc-3-hydro-
2
xyadamantylglycine (III). Utilizing this route, N-Boc-3-hydroxyadamantylglycine
was prepared in six linear chemical steps with 38 % overall yield.
Keywords Saxagliptin Á Intermediate Á Phenylalanine dehydrogenase Á
Oxime
Introduction
Type 2 diabetes mellitus (T2DM) is a global epidemic with estimated prevalence of
5
% (346 million people) worldwide in 2012; without intervention this number is
likely to more than double, and around 470 million people are projected to have
prediabetes by 2030 [1]. The health, social, and economic burdens of T2DM are
significant [2–4]. Consequently, T2DM presents a major challenge to healthcare
systems around the world.
Saxagliptin (Onglyza) is a highly potent, orally available reversible dipeptidyl
peptidase IV (DPP-IV) inhibitor (in comparison with other dipeptidyl peptidase
Y. Chen Á A. Wang Á Z. Tao Á Y. Deng Á X. Hu (&)
Department of Pharmacy, Chongqing Medical University, Chongqing 400016,
People’s Republic of China
e-mail: huxiangnan62@163.com
123

Y. Chen et al.
enzymes) which is given once daily, being approved by the Food and Drug
Administration (FDA) for treatment of type 2 diabetes mellitus in 2009. (S)-N-Boc-
3
-Hydroxyadamantylglycine (I) as a key intermediate of saxagliptin (Scheme 1)
was originally prepared using an expensive and technically demanding enzyme-
catalyzed reaction from oxoacetic acid 6 by reductive amination with phenylalanine
dehydrogenase (Scheme 2) [5, 6]. Thereafter, various new routes without using
enzymes were developed (Scheme 3). Vu et al. [5] (route 1) reported a new method
to synthesize the racemic mixture of N-Boc-3-hydroxyadamantylglycine (III),
followed by resolution of III to afford I. Route 2 [7] uses an asymmetric synthesis
to obtain I, requiring harsh conditions and highly toxic reagent. A relatively simple
route has also been developed to prepare III, as shown in route 3 [8], representing
the latest research progress regarding III, but the use of expensive metal catalysts
[
Cp*RhCl2]2 made it unsuitable for large-scale production. A new synthetic
approach to III was developed starting from the inexpensive and commercially
available 1-adamantanecarboxylic acid (1) (Scheme 5). The synthesis of 1-ada-
mantyl methyl ketone (2) had been reported by our team [9], and we had optimized
the synthesis conditions to improve the yield. Compound 2 was converted into 2-(1-
adamantyl)-2-oxoacetic acid (3) by oxidation with potassium permanganate in
aqueous NaOH. Then, we have two routes to get the final product. In one, the keto
acid 3 is treated with hydroxylamine hydrochloride to get 1-adamantyl-2-
hydroxyimino acetic acid (4), with reduction of oxime 4 with Raney-Ni and
oxidation of 2-(1-adamantyl)glycine (5) with H SO /HNO to give the racemic
2
4
3
mixture of 3-hydroxyadamantylglycine (8). In another, preferred embodiment, 8 is
directly obtained from 3 by hydroxylation of the adamantane ring using KMnO in
4
Scheme 1 Retrosynthetic analysis
Scheme 2 Original synthesis of intermediate I
1
23

A facile synthesis of saxagliptin intermediate
Scheme 3 Routes without using enzyme
hot aqueous NaOH, with reduction of 2-(3-hydroxy-1-adamantyl)-2-hydroxyimino
acetic acid (7) to get 8. Alternatively, the Boc-protected amino acid III can be
prepared by treatment of 8 with (Boc) O and NaOH.
2
Experimental
Method for preparation of 1-adamantyl methyl ketone (2)
A mixture of 20 g (0.11 mol) 1 and 40 mL thionyl chloride was stirred under reflux
for 4 h, the excess SOCl was removed under reduced pressure, and the residue was
2
1
1
1
-adamantanecarbonyl chloride. To a solution of 3.83 g (0.17 mol) sodium in
00 mL petroleum ether at room temperature, we added dropwise over a period of
h a mixture of 26.7 mL (0.17 mol) diethyl malonate and 50 mL petroleum ether,
123

Y. Chen et al.
and the mixture was stirred until sodium was consumed completely. A solution of
1
-adamantanecarbonyl chloride in 50 mL petroleum ether was added slowly
duration 1 h) to the resulting suspension. The mixture was stirred overnight at room
(
temperature. Distilled water (200 mL) was added, and the organic layer was
separated; after concentration under vacuum, a mixture of acetic acid (80 mL),
distilled water (24 mL), and concentrated sulfuric acid (8 mL) was added and reflux
for 6 h, then the reaction solution was poured into 600 mL ice-water for
crystallization; the white needle crystal was filtered off and dried to afford
1
-adamantyl methyl ketone (2) (yield 18.41 g, 93 %); m.p. 53–54 °C (53–55 °C
10]); IR (KBr, cm ): 2,930, 2,913, 2,851, 1,698, 1,451, 1,345; ESI-MS: m/z 177
-1
[
[
-
1
M-H] ; H NMR (300 MHz, CDCl ) d: 2.09 (s, 3H), 2.07–1.56 (m, 15H).
3
Method for preparation of 2-(1-adamantyl)-2-oxoacetic acid (3)
Sodium hydroxide solution (2 %, 100 mL), t-butanol (30 mL), and 2 (8.9 g,
0
.05 mol) were charged into a 250-mL three-neck round-bottom flask; the contents
were heated with an oil bath to 40–45 °C, then potassium permanganate (15.8 g,
.1 mol) was added over a 1-h period; following the addition, the reaction was
stirred for an additional 2 h at 40 °C. The mixture was quenched with 10 % Na SO
0
2
3
(
10 mL) and filtered through Celite; the filter cake and reaction flask were rinsed
with 20 mL hot water. The filtrate was adjusted to pH 1 using concentrated
hydrochloric acid, and a large amount of white solid appeared; the white solid was
filtered off and dried to afford 3 (yield 9.37 g, 90 %); m.p. 98–101 °C (102–104 °C
-
1
[
11]); IR (KBr, cm ): 2,906, 2,887, 2,848, 2,675, 2,557, 1,717; ESI-MS: m/z 207
-
1
[
M-H] ; H NMR (300 MHz, MEOH-d4) d: 2.05 (s, 3H), 1.92–1.739 (m, 12H).
Method for preparation of 1-adamantyl-2-hydroxyimino acetic acid (4)
3
(8.32 g, 0.04 mol) and sodium hydroxide (1.6 g, 0.04 mol) were added to 500 mL
aqueous solution and stirred at room temperature until completely dissolved to give
a clear solution, then hydroxylamine hydrochloride (3.34 g, 0.048 mol) was added
and the reaction solution was stirred in an ice-water bath for 2 h; the white solid was
filtered off and dried to afford 4 (yield 7.93 g, 89 %); m.p. 147–149 °C
-
1
(
152–153 °C [12]); IR (KBr, cm ): 3,280, 2,905, 2,850, 2,527, 1,651, 1,702;
- 1
ESI-MS: m/z 222 [M-H] ; H NMR (300 MHz, DMSO) d: 2.51–2.44 (m, 1H), 1.93
s, 3H), 1.79–1.56 (m, 12H).
(
Method for preparation of 2-(1-adamantyl)glycine (5)
A solution of 4 (4.46 g, 0.02 mol) in alcohol (40 mL) was shaken with hydrogen in
the presence of Raney-Ni as catalyst [prepared from 2 g Ni–Al alloy (50–50)] at
5
filtration, the filtrate was dried over anhydrous Na SO and the solvent was removed
0 °C under ordinary pressure for 7 h. After the catalyst had been removed by
2
4
under vacuum to obtain the crude product, then 20 mL methanol was added in and
concentrated hydrochloric acid was added dropwise until the solution became clear;
the pH of the solution was adjusted to the isoelectric point with 10 % NaOH, and
1
23

A facile synthesis of saxagliptin intermediate
the precipitate was filtered off and washed with alcohol; the white solid was dried to
obtain 5 (yield 3.68 g, 88 %). 5 (0.5 g) was suspended in 10 mL methanol, then
concentrated hydrochloric acid was added until giving a clear solution; the solvent
was removed to obtain the hydrochloride salt of 5. IR (as hydrochloride salt, KBr,
-
1
- 1
cm ): 3,449, 3,173, 3,053, 2,903, 2,849, 1,724; ESI-MS: m/z 208 [M-H] ; H
NMR (as hydrochloride salt, 300 MHz, DMSO-d6) d: 8.32(s, 3H), 3.25 (d,
J = 5.1 Hz, 1H), 1.96 (s, 3H), 1.66–1.51 (m, 12H).
Method for preparation of 2-(3-hydroxy-1-adamantyl)-2-oxoacetic acid (6)
3
(8.32 g, 0.04 mol) was added to a solution of 2 % sodium hydroxide (360 mL).
The mixture was stirred and heated to 80 °C, then potassium permanganate (11.4 g,
.072 mol) was added portionwise to the mixture over a 1-h period; following the
0
addition, the reaction was stirred for an additional 3 h at this temperature. The
mixture was quenched with 10 % Na SO (10 mL) and was filtered through Celite;
2
3
the filter cake and reaction flask were rinsed with 20 mL hot water. The filtrate was
adjusted to pH 1 by concentrated hydrochloric acid and extracted with ethyl acetate
(3 9 200 mL). The extracts were washed with brine, dried over anhydrous sodium
sulfate, and concentrated to give colorless oil, which was recrystallized from ethyl
acetate and heptane to give 6 as colorless solid (yield 6.09 g, 68 %); m.p.
-
1
1
1
62–163 °C (164–165 °C [13]); IR (KBr, cm ): 3,401, 2,932, 2,861, 2,482, 1,876,
- 1
,713, 1689; ESI-MS: m/z 223 [M-H] ; H NMR (300 MHz, DMSO-d6) d: 3.32 (br
s, 1H), 2.16 (s, 2H), 1.74–1.42 (m, 12H).
Method for preparation of 2-(3-hydroxy-1-adamantyl)-2-hydroxyimino
acetic acid (7)
6
(8.96 g, 0.04 mol) and sodium hydroxide (1.6 g, 0.04 mol) were added to 100 mL
aqueous solution and stirred at room temperature until completely dissolved to give
a clear solution, then hydroxylamine hydrochloride (3.34 g, 0.048 mol) was added
and the reaction solution was stirred in an ice-water bath for 2 h; the white solid was
filtered off and dried to afford 7 (yield 8.63 g, 90 %). m.p. 172–174 °C; IR (KBr,
-
1
- 1
cm ): 3,429, 3,282, 2,913, 2,864, 1,744, 1,706; ESI-MS: m/z 238 [M-H] ; H
NMR (300 MHz, DMSO-d6) d: 10.70 (br s, 1H), 2.13 (s, 2H), 1.71–1.41 (m, 12H).
Method for preparation of 3-hydroxyadamantylglycine (8)
5
1
0
8
(2.09 g, 0.01 mol) was added portionwise to a cooled mixture of HNO (65 %,
3
.33 mL, 0.019 mol) and H SO (98 %, 32 mL, 0.59 mol) over a 0.5-h period at
2
4
°C; the reaction was stirred at room temperature overnight and added dropwise to
0 mL ice-water; the pH of the solution was set to 1 by concentrated hydrochloric
acid, and the solvent was removed under vacuum to obtain a white solid; 30 mL
anhydrous ethanol was added, after which the precipitated inorganic salts were
filtered off, the pH of the filtrate was adjusted to the isoelectric point with 10 %
NaOH, and the precipitate was filtered off to afford 8 (yield 1.69 g, 75 %).
123

Y. Chen et al.
A solution of 7 (4.78 g, 0.02 mol) in alcohol (40 mL) was shaken with hydrogen
in the presence of Raney-Ni as catalyst [prepared from 2 g Ni–Al alloy (50–50)] at
5
filtration, the filtrate was dried over anhydrous Na SO and the solvent was removed
0 °C under ordinary pressure for 7 h. After the catalyst had been removed by
2
4
under vacuum to obtain the crude product, then 20 mL methanol was added in and
concentrated hydrochloric acid was added dropwise until the solution became clear;
the pH of the solution was adjusted to the isoelectric point with 10 % NaOH, and
the precipitate was filtered off and washed with alcohol; the white solid was dried to
obtain 8 (yield 3.86 g, 86 %). 8 (0.5 g) was suspended in 10 mL methanol, then
concentrated hydrochloric acid was added until giving a clear solution; the solvent
was removed to obtain the hydrochloride salt of 8. IR (as hydrochloride salt, KBr,
-
1
-
cm ): 3,351, 3,187, 3,050, 2,924, 2,903, 2,853, 1,724; ESI-MS: m/z 224 [M-H] ;
1
H NMR (as hydrochloride salt, 300 MHz, DMSO-d6) d: 8.32 (s, 2H), 5.52 (bs,
2
H), 3.32 (d, J = 5.2 Hz, 1H), 2.12 (s, 2H), 1.60–1.39 (m, 12H).
Method for preparation of N-Boc-3-hydroxyadamantylglycine (III)
The pH of a mixture of 8 (2.25 g, 0.01 mol) and water (40 mL) was adjusted to 10
with 10 % NaOH, then the reaction mixture was treated with (Boc) O (2.62 g,
2
0
.012 mol) in tetrahydrofuran (THF, 40 mL). The reaction was stirred at room
temperature overnight, and the THF layer was discarded; the aqueous layer was
adjusted to pH 3 with concentrated hydrochloric acid. After extracting with ethyl
acetate (3 9 30 mL), the aqueous layer was separated and the combined organic
layers were washed with brine and dried over Na SO , and the solvent removed
2
4
in vacuo to leave oil, which was recrystallized to obtain III (yield 2.86 g, 88 %).
-
1
m.p. 177–179 °C; IR (KBr, cm ): 3,407, 3,288, 2,977, 2,926, 2,595, 1,718, 1,696;
-
1
ESI-MS: m/z 324 [M-H] ; H NMR (300 MHz, CDCl ) d: 5.18 (d, J = 8.3 Hz, 1H),
3
4
.08 (d, J = 8.8 Hz, 1H), 2.25 (s, 2H), 1.59 (dd, J = 44.4, 22.8 Hz, 12H), 1.44
s, 9H).
(
Results and discussion
The original route presents a very attractive pathway based on a key biological
enzyme, phenylalanine dehydrogenase (Scheme 2). Although this enzyme catalysis
method can afford saxagliptin intermediate I from keto acid 6 efficiently and
specifically, the biological enzyme used in this method is expensive and not easily
obtained, resulting in high production costs of the final product and thereby
increasing the economic burden of diabetics. In addition, the keto acid 6, which
could be afforded by several methods [5] as described in Scheme 4, is the raw
material of the biological enzyme-catalyzed synthesis. As shown in Scheme 4, 6 can
be prepared in five linear chemical steps from 1-bromoadamantane via route 1, but
the usage of toxic reagents (such as oxalyl chloride) and harsh reaction conditions
(-78 °C) limits its further development and large-scale application. Route 2 also
needs long linear chemical steps to afford 6, and the operation is complex. Route 3
was developed as a method to afford compound 6 by reacting 1-adamantanecarboxylic
1
23

A facile synthesis of saxagliptin intermediate
Scheme 4 Synthesis of 6
acid with methyl lithium, followed by oxidation of adamantane tertiary–H to
hydroxyl with dimethyldioxirane and oxidation of methyl to carboxyl with sodium
permanganate. Due to the above limitations, it has become a challenge for us to
develop an alternative method for rapid access to intermediate I in good yield.
Some solutions to these problems are shown in Scheme 3. There is no doubt that
these routes are not perfect and have various disadvantages, so we developed the
new route shown in Scheme 5. In our route, 6 is prepared in three linear chemical
steps with good yields from commercially available adamantane intermediate
1
-adamantanecarboxylic acid. Compared with the reported routes, our route is not
very complicated, the operation is simpler, and the reaction conditions milder; we
can further reduce the cost of production by avoiding use of biological enzyme. The
highlights of this new synthesis include readily available raw materials, low cost,
high yield, mild reaction conditions, and its potential for large-scale industrial
application. However, it also showed a few issues; e.g., we can only afford the
racemic mixture of intermediate III but not saxagliptin intermediate I. So, in further
studies, we need to choose an appropriate method for chiral separation of
intermediate III to intermediate I. Despite the issues, we feel that the oxime
123

Y. Chen et al.
Scheme 5 Synthesis of N-Boc-3-hydroxyadamantylglycine
2
?
Scheme 6 Coordination compound of amino acid with Zn
approach described in Scheme 5 still represents a very attractive and potentially
industrial approach to saxagliptin intermediate I.
Previous work showed that compound 2 was converted into compound 6 by
oxidation with potassium permanganate with yield of only 36 % [14] and 47 % [9].
Our research indicates potassium permanganate oxidation of methyl into carboxyl
with simultaneous oxidation of adamantane tertiary–H into hydroxyl. The electron-
withdrawing effect of the carbonyl group made the oxidation of methyl compar-
atively easy. Therefore, this oxidation reaction was divided into two steps: first
using potassium permanganate oxidation of compound 2 to obtain compound 3 in
9
0 % yield at 40–45 °C, then oxidation of compound 3 at 80 °C to afford compound
with yield of 68 %. The total yield of the two-step reaction reached 61.2 %,
4
representing a great improvement compared with literature. In the other route, we
used H SO /HNO to oxidize the adamantane tertiary–H of compound 5 into
2
4
3
hydroxyl to give compound 8 in 75 % yield.
Initially, we chose Zn/AcOH as the reducing agent for the reaction of the oxime
to amino because of its low cost. Moreover, we found that the reaction was complete
and almost no by-product appeared from thin-layer chromatography (TLC);
1
23

A facile synthesis of saxagliptin intermediate
however, the final separation yield was only about 50 %. We believe that maybe the
amino and carboxyl groups of the amino acid formed a coordination compound with
2
?
Zn , which led to lower yield (Scheme 6). So we chose Raney-Ni as the reducing
agent to afford the corresponding amino acid in high yield [15].
Conclusions
We have developed an efficient and chromatography-free route for preparation of
saxagliptin intermediate N-Boc-3-hydroxyadamantylglycine (III). In consideration
of the economic costs, a traditional chemical reduction process was utilized to avoid
using biological enzyme, we chose the compound 1-adamantanecarboxylic acid (1)
as the starting material, and other reagents used in this route were cheap and
commercially available. Our route considerably lowers the cost and delivery time
required to prepare saxagliptin intermediate N-Boc-3-hydroxyadamantylglycine
(III). In the optimized process, the overall yield from the proposed starting materials
was approximately 38 %.
Acknowledgments This work was supported by the Natural Science Foundation of Chongqing (CSTC,
2006BB5286). We furthermore thank Chongqing Medical University for partial financial support of this
work. We are grateful to Yue Feng and Jun Pen for helpful discussions related to this work.
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