A general method for the facile synthesis of optically active 2-substituted piperazines via functionalized 2,5-diketopiperazines

Article abstract of DOI:10.1016/j.tetlet.2014.06.058

Upon utilization of some common methods described in the literature for the synthesis of chiral, 2-substituted 2,5-diketopiperazines, extensive racemization was observed. Further investigation showed that heating in the presence of a mild base racemized the chiral center in the product diketopiperazines. A generalized, readily scalable route was sought and, after investigating the effect of base and temperature, conditions were identified that promoted cyclization without erosion of enantiomeric excess. An array of functionalization was tolerated and this procedure serves as a useful and reliable method for the facile synthesis of this important class of compounds.

Full text of DOI:10.1016/j.tetlet.2014.06.058

Tetrahedron Letters 55 (2014) 4501–4504
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A general method for the facile synthesis of optically active
2-substituted piperazines via functionalized 2,5-diketopiperazines
b
a
a
Kate S. Ashton a, , Mitchell Denti , Mark H. Norman , David J. St. Jean Jr.
⇑
a Department of Therapeutic Discovery—Medicinal Chemistry, Amgen, Inc., One Amgen Center Drive, Thousand Oaks, CA 91320, USA
b Polyera Corporation, 8045 Lamon Ave., Skokie, IL 60077, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Upon utilization of some common methods described in the literature for the synthesis of chiral, 2-
substituted 2,5-diketopiperazines, extensive racemization was observed. Further investigation showed
that heating in the presence of a mild base racemized the chiral center in the product diketopiperazines.
A generalized, readily scalable route was sought and, after investigating the effect of base and tempera-
ture, conditions were identified that promoted cyclization without erosion of enantiomeric excess. An
array of functionalization was tolerated and this procedure serves as a useful and reliable method for
the facile synthesis of this important class of compounds.
Received 5 June 2014
Revised 9 June 2014
Accepted 13 June 2014
Available online 21 June 2014
Keywords:
2-Substituted 2,5-diketopiperazine
2-Substituted piperazine
Fischer method
Ó 2014 Elsevier Ltd. All rights reserved.
Amino acid cyclization
Piperazines with substituents at the 2-position are useful inter-
mediates in the synthesis of numerous pharmacologically active
molecules (Fig. 1).1 While there are multiple methods reported to
access 2-substituted piperazines via installation of a chiral substi-
tuent at a late stage, many of these methods have drawbacks such
as low enantiomeric purities or the use of chiral auxiliaries.2
During the course of the synthesis of AMG-1694 (4, Fig. 1),3 we
had to not only quickly access varied substitution of the piperazine
H
N
OH
O
N
O
O
N
O
N
H
N
OH
N
N
H2N
O
2
1
(L-745631)
(Indinavir)
SH
O
N
O
CF3
S
S
N
O
N
N
N
N
N
H
CF3
CF3
OH
N
NH
O
4
3
(AMG-1694)
(FK-355)
Figure 1. Representative examples of biologically active molecules containing 2-substituted piperazines.
⇑
Corresponding author. Tel.: +1 805 313 5532; fax: +1 805 480 1337.
E-mail address: katea@amgen.com (K.S. Ashton).
http://dx.doi.org/10.1016/j.tetlet.2014.06.058
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

4502
K. S. Ashton et al. / Tetrahedron Letters 55 (2014) 4501–4504
.TFA
O
enantioenriched 2-substituted piperazines had not been fully
1. glycine methyl
ester HCl, HATU,
DIPEA, DMF
OBn
OBn
OH
explored. Therefore, we examined various reaction conditions for
the cyclization of compound 6, not only to optimize the prepara-
tion of compound 7, but also to gain a better understanding of
the factors influencing the enantiomeric outcome of the reaction
(Table 1). As a result of this investigation, we identified a general
and simple procedure for the synthesis of 2-substituted 2,5-diketo-
piperazines while maintaining high stereochemical integrity.
For this study we choose the benzyl protected serine glycinate 6
as the model substrate. This compound was of interest as it pro-
vided access to a key intermediate in Amgen’s GKRP program,3
and it had also proven to be a substrate that was particularly prone
to racemization. The effects of varying the base and temperature
on the enantiomeric excess of the product 7 are shown in Table 1.9
In the absence of base (entry d), the amine free base cyclized
after two days at reflux temperature with minimal racemization.
Although these conditions were attractive, since they provided
the product in high enantiomeric purity, we wanted to eliminate
the need to isolate the free amine10 to make the protocol more
amenable for large scale preparations. In addition, shorter reaction
times were preferable, and therefore, additional conditions were
examined. As previously noted, the use of triethylamine as a base
reduced the reaction time significantly (entry a); however, it also
resulted in complete erosion of the enantiomeric purity.11 Simply
running the reaction at room temperature (entry e) was sufficient
to prevent racemization, however, these conditions still necessi-
tated a long reaction time. The optimized conditions were achieved
by running the reaction at room temperature and by employing
ammonia (used as a 2 M solution in methanol) as the base (entry
f). These conditions resulted in complete conversion in 4 h and
no racemization. Continued stirring of 7 in methanolic ammonia
for 24 h at room temperature resulted in a minimal loss of enantio-
meric excess (1%). These conditions, occasionally referred to as the
‘Fischer method’,12 have been reported to promote racemization.13
However, this result was only highlighted in publications concern-
ing di-substituted diketopiperazines, and it does not appear to
translate to the mono-substituted diketopiperazines. The Fischer
method is also reported to take several days for di-substituted
diketopiperazine formation, which could contribute to the loss of
optical integrity. In addition, there are several examples in the lit-
erature of methanolic ammonia being used at high temperatures to
effect cyclization,14 so we investigated the effect of heating the
reaction. Brief exposure of substrate 6 in methanolic ammonia to
temperatures above 60 °C resulted in very efficient cyclization
(entries g and h), and the enantiomeric purity was only slightly
affected when the reaction time was less than 30 min. However,
running the reaction at lower temperatures required longer reac-
tion times to go to full conversion (entries i and j), and this resulted
in a deleterious effect on the enantioenrichment of the product.
Entry k demonstrates further that prolonged exposure of the reac-
tion to elevated temperatures results in rapid, complete erosion of
the enantiomeric excess.15
H
NEt3, MeOH,
N
Boc
H2N
O
N
H
65 °C
2. TFA, CH2Cl2
O
O
5
6
H
H
N
O
N
LAH, THF
OBn
OBn
O
N
H
N
H
65 °C
7
8
Scheme 1.
core for the purpose of rapid structure–activity relationship analy-
sis, but we also envisaged that scale-up of these compounds would
also be a requirement. We surveyed the literature in search of an
appropriate synthesis of chiral 2-substituted piperazines and the
initial route we examined involved the reduction of 2,5-diketopi-
perazine 7 to obtain a key piperazine intermediate (8).4 Diketopi-
perazine 7 was accessed by the base-promoted cyclization of
dipeptide 6, which in turn was prepared from the commercially
available protected amino acid 5 in two simple steps (Scheme 1).
Using these, literature prevalent, reaction conditions (Table 1,
entry a),5–7 the desired 2-substituted piperazine product was
obtained in good yield. However, it was found to have completely
racemized during the cyclization step. In attempts to eliminate this
racemization we also examined several other commonly reported
cyclization conditions to prepare either mono- or di-substituted
diketopiperazines. In the case of di-substituted dipeptides, there
are several publications that describe generalized conditions for
the formation of 2,5-di-substituted diketopiperazines, where rea-
sonable to good yields and high enantiomeric purities were
obtained.8 However, application of these methods to our system
did not provide the correspondingly enantiomerically pure prod-
uct. For example, heating compound 6 in water at high tempera-
tures in the presence of triethylamine resulted in extensive
hydrolysis of the ester and less than 25% cyclized product was
observed (Table 1, entry b). Alternatively, no product formation
was observed after heating the trifluoroacetic acid salt of com-
pound 6 for 16 h in MeOH in the absence of a base (Table 1, entry
c).
After the unsuccessful application of several literature proce-
dures it was apparent that standardized conditions to form diverse,
Table 1
Effects of solvent, base, and heat on cyclization reaction times and racemization
BnO
H
N
O
O
H
N
conditions
H N
OMe
TFA •
OBn
2
O
N
H
O
6
7
In conclusion, although the cyclization of dipeptide 6 can be run
at high temperatures without the erosion of the enantiomeric pur-
ity, it requires careful monitoring. In addition, temperature did not
reduce the reaction time significantly when using methanolic
ammonia as the base. Moreover, reaction times would likely vary
for different substrates and this would make it difficult to establish
generalized conditions involving heating of the reaction. Therefore,
we concluded that, for a broadly applicable method, the cyclization
was best conducted at room temperature in the presence of meth-
anolic ammonia. To examine the scope of the reaction we used
these general conditions to cyclize a variety of substrates (Table 2).
This method was found to be effective for both alkyl (9 and 10)
and 4-substituted phenylalanine derivatives with varied electronic
properties (11–14). In addition, the substrate with the large alkyl
Entry
Solvent
Base
Temp (°C)
Timeb (h)
ee (%)
a
b
c
da
e
f
g
h
i
MeOH
H2O
Et3N
Et3N
—
65
100
65
65
23
23
65
60
50
40
65
10–16
n/a
n/a
48
16
4
0.25
0.5
1
0
n/a
n/a
98
>99
>99
>99
97
92
91
0
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
—
Et3N
NH3
NH3
NH3
NH3
NH3
NH3
j
k
2
1
a
This reaction used the amine free base.
Time to full conversion as monitored by LCMS.
b

K. S. Ashton et al. / Tetrahedron Letters 55 (2014) 4501–4504
4503
Table 2
Synthesis of varied 2-substituted diketopiperazines
H
N
R
1. HATU, DIPEA, DMF
2. TFA, CH2Cl2
O
R
Boc
N
OH
OMe
HCl • H2N
+
3. NH3, MeOH, 23 o
C
O
N
H
R'
O
O
Compd No.
Product
Isolated yielda (%)
eeb (%)
Timec (h)
H
N
O
7
80
>99
4
O
N
H
OBn
O
H
N
9
63
60
74
78
79
79
47
>99
>99
>99
>99
>99
>99
>99
3
4
4
4
4
5
4
O
O
O
O
O
O
O
O
N
H
H
N
O
O
O
O
O
O
O
10
11
12
13
14
15
N
H
N
N
H
H
N
OMe
F
N
H
H
N
N
H
H
N
NO2
N
H
H
N
O
N
H
H
N
16
59
>99
2
N
H
a
b
c
Represents isolated yield over three steps.
Determined by SFC analysis.
Time for complete cyclization to occur as monitored by LCMS.
tetrahydropyran substituent also cyclized efficiently under the
reaction conditions to give compound 15.16 Considering the
electron-withdrawing nature of an aryl group directly attached to
the piperazine ring, we were pleasantly surprised to find that the
procedure was also applicable to phenylglycine derivative 16. For
all substrates, the cyclizations were complete in less than 5 h.
As a representative example, Scheme 2 shows that the optically
pure 2,5-diketopiperazine 15 can be readily reduced by lithium
aluminum hydride to access the desired 2-substituted piperazine
intermediate 17 in good yield with no erosion of the enantiomeric
excess.17
In summary, we have investigated an operationally simplistic,
general method to access enantiomerically pure, 2-substituted
2,5-diketopiperazines.18 Although variations of this protocol have
been reported previously, to the best of our knowledge the current
literature does not systemically investigate the instability of the
chiral center with regard to base and elevated temperatures. In
addition to the lowered risk of racemization, this method is appeal-
ing given that the stereochemistry is derived from readily available
amino acids, no chromatography is required, and the mild
H
N
H
N
O
1. LAH, THF, 65 °C
76% yield
O
O
N
H
17
O
N
H
15
Scheme 2. Reduction to chiral 2-substituted piperazine 16.

4504
K. S. Ashton et al. / Tetrahedron Letters 55 (2014) 4501–4504
had possibly encountered difficulty in removing either of the benzyl groups
selectively. We also evaluated the number of commercially available Boc and
benzyl protected amino acids and determined that Boc protected analogs were
more widely available. In addition, the route we ultimately optimized avoids
the use of H2 gas and flammable catalyst, an advantage when dealing with
large quantities of material.
conditions and operational simplicity lend this process to large-
scale preparations.19,20
Acknowledgment
7. Chiral diketopiperazines have been reported to racemize under basic
conditions: Smith, G. G.; Baum, R. J. Org. Chem. 1987, 52, 2248–2255.
8. (a) Suzuki, K.; Sasaki, Y.; Endo, N.; Mihara, Y. Chem. Pharm. Bull. 1981, 29, 233;
(b) Ueda, T.; Saito, M.; Kato, T.; Izumiya, N. Bull. Chem. Soc. Jpn. 1983, 56, 568;
(c) Tullberg, M.; Grotli, M.; Luthman, K. Tetrahedron 2006, 62, 7484; (d) Perez-
Picaso, L.; Escalante, J.; Olivo, H. F.; Rios, M. Y. Molecules 2009, 14, 2836. This
method (heating the Boc-precursor at 200 °C in water in a sealed tube) was not
attempted on our system, as the previous reference suggested that it was
substrate limited.
The authors wish to thank the Amgen Intern Program and
Kyung-Hyun Gahm for his help with ee analysis.
Supplementary data
Supplementary data (detailed conditions for each substrate
shown in Table 2 and characterization data) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.06.058.
9. The SFC method used was as follows: ChiralpakÒ ADH column (4.6 Â 150 mm)
using 25% ethanol in supercritical CO2 (total flow was 4 mL/min).
10. Free basing the substrate prior to cyclization required an aqueous work-up,
something we wanted to avoid on large scale.
11. These conditions were also used in the cyclization of material derived from
Boc-phenylglycine and resulted in complete racemization.
12. Fischer, E. Chem. Ber. 1906, 39, 2893.
References and notes
13. Nitecki, D. E.; Halpern, B.; Westley, J. W. J. Org. Chem. 1968, 33, 864. This Letter
highlights the issue of long reaction times which they infer leads to
racemization when using base to facilitate the cyclization. However, these
examples only examined di-substituted diketopiperazines. We have shown
that the conditions for di-substituted do not necessarily translate to the
formation of mono-substituted diketopiperazines, and that the mono-
substituted diketopiperazines are generally formed in under 5 h.
14. (a) Harrison, B. A.; Kimball, S. D.; Mabon, R.; Rawlins, D. B.; Rice, D. S.;
Voronkov, M. V. WO Patent 2009/021169, 2009. (b) Sebti, S. M.; Hamilton, A.
D.; WO Patent 2008/079945, 2008. (c) Aicher, T. D.; Chen, Z.; Le Huerou, Y.;
Martin, F. M.; Pineiro-Nunez, M. M.; Rocco, V. P.; Ruley, K. M.; Schaus, J. M.;
Spinazze, P. G.; Tupper, D. E. WO Patent 2004/014895, 2004.
1. For example, the 2-piperazine motif is contained within: (a) marketed drug
Indinavir, see: Dorsey, B. D.; Levin, R. B.; McDaniel, S. L.; Vacca, J. P.; Guare, J.
P.; Darke, P. L.; Zugay, J. A.; Emini, E. A.; Schleif, W. A.; Quintero, J. C.; Lin, J. H.;
Chen, I.-W.; Holloway, M. K.; Fitzgerald, P. M. D.; Axel, M. G.; Ostovic, D.;
Anderson, P. S.; Huff, J. R. J. Med. Chem. 1994, 37, 3443; L-745631 (a ras
farnesyltransferase inhibitor), see: (b) Leonard, D. M. J. Med. Chem. 1997, 40,
2971–2990; (c) FK-335 (neurokinin-1 [NK-1] inhibitor), see: Matsuo, M.;
Hagiwara, D.; Manabe, T.; Konishi, N.; Shigenaga, S.; Murano, K.; Matsuda, H.;
Miyake, H. PCT Int. Appl., to Fujisawa Pharmaceuticals, WO Patent 9637489
A1, 1996.; AMG-1694 (a GK-GKRP disruptor), see: (d) Lloyd, D. J.; St. Jean, D. J.,
Jr.; Kurzeja, R. J. M.; Wahl, R. C.; Michelsen, K.; Cupples, R.; Chen, M.; Wu, J.;
Sivits, G.; Helmering, J.; Ashton, K. S.; Pennington, L. D.; Fotsch, C. H.; Vazir,
M.; Chen, K.; Chmait, S.; Zhang, J.; Liu, L.; Norman, M. H.; Andrews, K. A.;
Bartberger, M. D.; Van, G.; Galbreath, E. J.; Vonderfecht, S. L.; Wang, M.;
Jordan, S. R.; Véniant, M. M.; Hale, C. Nature 2013, 504, 437.
2. (a) Anderson, H.; Banchelin, T. S.; Das, S.; Gustafsson, M.; Olsson, R.; Almqvist,
F. Org. Lett. 2010, 12, 284; (b) Schanen, V.; Cherrier, M.; Sebastião, J.; Quirion, J.;
Husson, H. Synthesis 1996, 833; (c) Jin, S.; Liebscher, J. Synlett 1999, 459; (d)
Crestey, F.; Witt, M.; Jaroszewski, J. W.; Franzyk, H. J. Org. Chem. 2009, 74, 5652;
(e) Gao, H.; Renslo, A. R. J. Org. Chem. 2007, 72, 8591.
3. Ashton, K. S.; Andrews, K. L.; Bryan, M. C.; Chen, J.; Chen, K.; Chen, M.; Chmait,
S.; Croghan, M.; Cupples, R.; Fotsch, C.; Helmering, J.; Jordan, S. R.; Kurzeja, R. J.
M.; Michelsen, K.; Pennington, L. D.; Poon, S. F.; Sivits, G.; Van, G.; Vonderfecht,
S. L.; Wahl, R. C.; Zhang, J.; Lloyd, D. J.; Hale, C.; St. Jean, D. J., Jr. J. Med. Chem.
2014, 57, 309.
15. Subjection of a single enantiomer of the diketopiperazine 7 to the reaction
conditions led to erosion of ee with continued heating.
16. The slightly lower yield obtained in this reaction can be attributed to the fact
that the diketopiperazine product was partially soluble in the solvent used to
remove the salt by-products. Due to the ease of procedure however, this
purification method was deemed superior to column chromatography, even
with the slight loss of yield.
17. For a detailed procedure see: Ashton, K.; Bartberger, M. D.; Bo, Y.; Bryan, M. C.;
Croghan, M.; Fotsch, C. H.; Hale, C. H.; Kunz, R. K.; Liu, L.; Nishimura, N.;
Norman, M. H.; Pennington, L. D.; Poon, S. F.; Stec, M. M.; St. Jean, D. J., Jr.;
Tamayo, N. A.; Tegley, C. M.; Yang, K. WO Patent 2012/027261, 2012.
18. A 250-mL round-bottomed flask was charged with the N-Boc amino acid
(10.0 mmol, 1.00 equiv), glycine methyl ester hydrochloride (1.256 g,
10.0 mmol, 1.00 equiv), DMF (50 mL), and DIPEA (3.50 mL, 20.5 mmol,
2.05 equiv) and the reaction mixture cooled to 0 °C. HATU (3.802 g,
10.0 mmol, 1 equiv) was added and the reaction stirred for 3 h while
warming to room temperature. EtOAc (150 mL) was added and the organic
layer was washed with 75 mL each of 0.5 N hydrochloric acid, saturated
aqueous sodium bicarbonate, water, and brine, dried with MgSO4, filtered, and
concentrated in vacuo. The crude material was dissolved in CH2Cl2 (33 mL) and
TFA (17 mL) and stirred for 2 h at room temperature. The mixture was then
concentrated and water removed by the formation of an azeotrope with
toluene (2 Â 40 mL). To this crude material was added 2 N NH3 in methanol
(40 mL). The reaction time and workup procedure for the cyclization are
specified for each compound in the Supplementary material section.
19. The reaction to synthesize product 7 has been performed on 0.5 mol scale. The
procedure was run as described by the general method; the cyclization took
4 h and the yield over the three steps was 61%.
4. For a review on DKP syntheses, see Dinsmore, C. J.; Beshore, D. C. Tetrahedron
2002, 3297.
5. A Scifinder search for 2-substituted diketopiperazines as the product of a
reaction returned over 600 reactions, and triethylamine was the most common
reagent used (although the use of triethylamine in the reaction sequence was
not always used to effect the cyclization). Numerous examples that use
triethylamine and elevated temperatures to promote the formation of a 2-
substituted diketopiperazine were found, some are listed: (a) Aicher, T. D.;
Chen, Z.; Chen, Y.; Faul, M. M.; Krushinski Jr., J. H.; Le Hureou, Y.; Pineiro-
Nunez, M. M.; Rocco, V. P.; Ruley, K. M.; Schaus, J. M.; Thompson, D. C.; Tupper,
D. E. WO Patent 2003/082877, 2003. (over 50 different diketopiperazines with
various 2-substituents are listed in this reference, a single isomer is claimed as
the product in each instance).; (b) Su, X.; Zhu, J.; Nie, F. PCT Appl., Nanjing
University of Technology, CN201110025008, 2011.; (c) Terada, K.; Masuda, T.;
Sanda, F. Macromolecules 2009, 42, 913; (d) Li, Y.; Bacon, K.; Sugimoto, H.;
Fukushima, K.; Hashimoto, K.; Marumo, M.; Moriwaki, T.; Nunami, N.; Tsuno,
N.; Urbahns, K.; Yoshida, N.; WO Patent 2004/084898, 2004.
20. (a) St. Jean, D. J., Jr.; Ashton, K. S.; Bartberger, M. D.; Chen, J.; Chmait, S.;
Cupples, R.; Galbreath, E.; Helmering, J.; Hong, F.-T.; Jordan, S. R.; Liu, L.; Kunz,
R. K.; Michelsen, K.; Nishimura, N.; Pennington, L. D.; Poon, S. F.; Reid, D.; Sivits,
G.; Stec, M. M.; Tadesse, S.; Tamayo, N.; Van, G.; Yang, K. C.; Zhang, J.; Norman,
M. H.; Fotsch, C.; Lloyd, D. J.; Hale, C. J. Med. Chem. 2014, 57, 325; (b) Bourbeau,
M. P.; Ashton, K. S.; Yan, J.; St. Jean, D. J., Jr. J. Org. Chem. 2014, 79, 3684.
6. The second most common cyclization method utilized was hydrogenation to
remove the protecting group and cyclization of the free base. This procedure
used a benzyl group instead of Boc on the initial amino acid. We did not pursue
this method as we were using a benzyl protected serine, thus we would have