Facile one-step synthesis of 2,5-diketopiperazines

Article abstract of DOI:10.1016/j.tetlet.2014.01.133

We report a one-step protocol for the general synthesis of 2,5-diketopiperazines from an Fmoc-protected amino acid and an amino acid ester. The application of the method is highlighted by rapid and efficient preparation of various 2,5-diketopiperazines.

Full text of DOI:10.1016/j.tetlet.2014.01.133

Tetrahedron Letters 55 (2014) 1905–1908
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile one-step synthesis of 2,5-diketopiperazines
Xianyu Sun a,c, Rachita Rai a, Alexander D. MacKerell Jr. a, Alan I. Faden b, Fengtian Xue a,b,
⇑
a Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, 20 Penn Street, Baltimore, MD 21201, United States
b Department of Anesthesiology, University of Maryland School of Medicine, Baltimore, MD 21201, United States
c College of Animal Science and Technique, Heilongjiang Bayi Agriculture University, Daqing 163319, Heilongjiang, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a one-step protocol for the general synthesis of 2,5-diketopiperazines from an Fmoc-protected
amino acid and an amino acid ester. The application of the method is highlighted by rapid and efficient
preparation of various 2,5-diketopiperazines.
Received 17 December 2013
Revised 27 January 2014
Accepted 28 January 2014
Available online 5 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Diketopiperazine
Fmoc deprotection
Intramolecular cyclization
Amide formation
Neuroprotection
2,5-Diketopiperazine (DKP) is a privileged scaffold for numer-
ous bioactive molecules.1 DKPs have been used as a key structural
fragment for anticancer agents by different mechanisms such as
cell-cycle inhibitors,2 breast cancer resistance protein inhibitors,3
plasminogen activator inhibitors,4 and DNA binders.5 Substituted
DKPs have also been used in the development of anti-infective
therapeutics including antiviral,6 antibacterial,7 and antifungal
agents.8 Moreover, DKPs have found application as inhibitors of
phosphodiesterase-5 for the treatment of erectile dysfunction,9
and antagonists of oxytocin for the treatment of preterm labor.10
Recently, experimental evidence has highlighted the intriguing
neuroprotective properties of DKPs in various cellular and animal
models.1c,11
Tertiary amines such as TEA and DIPEA are ubiquitous catalysts
used for the amide bond formation.13 They are also known to
slowly remove the Fmoc group from an amino group.14 Moreover,
tertiary amines are among the common choices of bases to assist
the formation of DKPs for the cyclization of the dipeptide esters.15
Therefore, we hypothesized that a one-pot synthesis of DKPs may
be achieved using Fmoc-protected amino acid and amino acid ester
in the presence of a tertiary amine, which may provide a rapid and
efficient synthetic method for DKPs (Scheme 1B).
To implement this method, a solution of Fmoc-Trp(Boc)-OH
(1a) and H2N-Ala-OEtꢀHCl (2a) in DMF was treated with HBTU in
A. Previous methods
Many methods are known for the preparation of DKPs.1a
A
O
R1
*
O
R1
H
N
i
common route includes a three-step process: (i) coupling of an
N-protected amino acid to an amino acid ester, (ii) deprotection of
the N-protecting group (e.g., Boc, Fmoc, and Cbz), and (iii) base-
assisted cyclization of the dipeptide ester (Scheme 1A). Despite the
tremendous utility of this method in producing DKPs, it requires
multiple-step synthesis with tedious purification processes. To date,
one-pot procedure for the synthesis of DKPs is limited.12 In the
course of our research in developing multifunctional neuroprotec-
tive agents, we performed a series of reactions intended to
synthesize tryptophan/tyrosine-containing DKPs as potential neu-
roprotective agents for the treatment of traumatic brain injury (TBI).
H2N
PG
PG
OH
+
*
R2
OR
*
R2
N
H
OR
O
N
H
coupling
*
O
O
R1
R1
O
H
HN
ii
iii
cyclization
*
*
R2
N
*
R2
H2N
OR
NH
deprotection
O
O
B.
Current work
R1
R1
O
O
conditions
H2N
HN
Fmoc
OH
*
*
+
*
R2
OR
N
H
*
NH
O
O
R2
⇑
Corresponding author. Tel.: +1 410 706 8521.
E-mail address: fxue@rx.umaryland.edu (F. Xue).
Scheme 1. Synthetic methods to DKPs. (A) classical method and (B) current work.
http://dx.doi.org/10.1016/j.tetlet.2014.01.133
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

1906
X. Sun et al. / Tetrahedron Letters 55 (2014) 1905–1908
Table 1
Optimization of reaction conditions for the generation of DKP 5aa
N
N
N
N
Boc
Boc
+
Boc
+
Boc
conditions
CH3
CO2Et
+
H
N
H
N
CO2Et
CH3
O
CO2Et
CH3
FmocHN
H2N
HN
FmocHN
CO2H H3N
Cl-
O
O
NH
CH3
O
1a
2a
3
4
5a
Entry
Reagentb
Base, equivc
Solventd
Time (h)
T (°C)
5ae (%)
1
2
3
4
5
6
7
8
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
HBTU
EDC
DIPEA (3.0)
TEA (3.0)
TEA (3.0)
TEA (3.0)
TEA (3.0)
TEA (3.0)
TEA (3.0)
TEA (2.0)
TEA (3.0)
TEA (4.0)
TEA (5.0)
TEA (6.0)
TEA (10)
TEA (2.0)
TEA (3.0)
TEA (4.0)
TEA (5.0)
TEA (6.0)
TEA (10)
TEA (5.0)
TEA (5.0)
TEA (5.0)
TEA (5.0)
TEA (5.0)
DIPEA (5.0)
NMM (5.0)
DMAP (5.0)
Lutidine (5.0)
DMF
DMF
DMA
DCM
CH3CN
THF
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
0
Tracef
Tracef
0
0
0
0
0
Dioxane
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
9
Tracef
Tracef
Tracef
Tracef
Tracef
0
73
74
75
75
75
12
0
37
82
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
rt
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
DIC
HATU
TSTU
HOBt
TSTU
TSTU
TSTU
TSTU
0
6
24
0
The significance of bold values mentioned in optimal conditions.
a
The reaction was carried out with 1.0 mmol of 1a, 1.0 mmol of 2a, and 1.0 mmol of coupling reagent in 5.0 mL of solvent.
b
HBTU = O-(benzotriazol-1-yl)-N,N,N0,N0-tetramethyluronium
hexafluorophosphate,
TSTU = N,N,N0,N0-tetramethyl-O-(N-succinimidyl)uroniumtetrafluoro
borate,
DIC = N,N0-diisopropylcarbodiimide, HOBt = 1-hydroxybenzotriazole hydrate, EDC = N-(3-dimethylamino propyl)-N0-ethylcarbodiimide hydrochloride, HATU = 1-
[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluoro-phosphate.
c
TEA = triethylamine, DIPEA = diisopropyl ethylamine, NMM = 4-methylmorpholine, DMAP = 4-(dimethylamino)pyridine.
DMF = N,N-dimethyl formamide, DMA = N,N-dimethylacetamide.
Isolated yields.
<5%.
d
e
f
the presence of 3 equiv of either DIPEA or TEA at room temperature
for 24 h (Table 1, entries 1 and 2). When DIPEA was used, dipeptide
3 was isolated as the only product in almost quantitative yields.
However in the presence of TEA, the reaction generated not only
3 (52%), but also the deprotected dipeptide 4 (44%), and a small
amount of DKP 5a (<5%). Accordingly, TEA was chosen for further
investigation in different solvents (entries 3–7). We found that
the reaction performed in DMA proceeded similarly to that in
DMF, while in other tested solvents dipeptide 3 was the only iso-
lated product. After a short screening of the equiv of TEA and reac-
tion temperature (entries 8–19), 5 equiv of TEA at 50 °C was
identified as the best combination, giving 75% isolated yields of
5a with no detectable racemization based on 1H NMR studies
(entry 17). We subsequently screened other possible coupling
reagents (entries 20–24), and found that TSTU was the most effec-
tive (82%, entry 23). It was noted that common coupling reagents,
such as DIC (entry 21) and HOBt (entry 24), did not lead to the
formation of DKP 5a in detectable amounts, while EDC (entry 20)
and HATU (entry 22) generated 5a in limited yields. Finally, various
bases were screened (entries 25–28). We found that TEA was
superior to other tested bases including DIPEA, NMM, DMAP, and
lutidine. It is interesting to note that the reaction using tertiary
amine DIPEA gave no detectable formation of DKP 5a.
Next, we explored the utility of this protocol for the prepara-
tion of DKPs using the optimized reaction conditions, 5 equiv of
TEA, and TSTU as the coupling reagent in DMF (Table 2). When
H2N-Ala-OEt (2a) was used, all four selected Fmoc-protected
amino acids (Fmoc-Trp(Boc)-OH, Fmoc-Tyr(tBu)-OH, Fmoc-
Phe-OH, and Fmoc-Ile-OH) reacted smoothly to generate DKPs
5a–d in good to excellent yields (entries 1–4). Similarly, good
yields of DKPs were obtained when H2N-Leu-OEt (2b) was used
(entries 5, 6 and 8), with the exception of Fmoc-Phe-OH, which
generated DKP 5g in modest yields along with significant amount
of uncyclized dipeptide ester H2N-Phe-Leu-OEt. Reactions
employing H2N-Val-OEt (2c) resulted in good yields for
Fmoc-Ile-OH (entry 12), modest yields for Fmoc-Trp(Boc)-OH
and Fmoc-Tyr(tBu)-OH (entries 9 and 10), and trace amount of
DKP for Fmoc-Phe-OH (entry 11). Generally decreased yields of
these reactions were likely due to the increased steric hindrance
of the isopropyl group of the valine side chain. These results

X. Sun et al. / Tetrahedron Letters 55 (2014) 1905–1908
1907
Table 2
Conditions and yields for DKPs 5a–pa
R1
TSTU, TEA
R1
R2
R3
CO2Et
, R2 = CH3, R3 = H
O
DMF, 50 oC, 24 h
HN
+
FmocHN
CO2H
H2N
2a
NH
R3
O
1a
, R1 = Trp(Boc)
1b, R1 = Tyr(tBu)
1c
R2
2b, R2 = i-Bu, R3 = H
, R2 = i-Pr, R3 = H
2d, R2-R3 = cyclopropyl
2c
, R1 = Phe
5a-p
1d, R1 = Ile
Entry
Structure
1
Product 5b
Yieldc (%)
R1
1
2
3
4
a
b
c
d
a
b
82
70
53
93
O
O
c17
d18
HN
NH
O
CH3
R1
5
6
7
8
a
b
c
d
e
f
65
68
53
88
g19
h19
HN
NH
O
R1
9
10
11
12
a
b
c
d
i
j
52
38
O
k20
l19
Traced
89
HN
NH
O
R1
13
14
15
16
a
b
c
d
m
n
34
38e
O
o21
p
Traced,e
87e
HN
NH
O
a
The reaction was carried out with 1.0 mmol of 1, 1.0 mmol of 2, 1.0 mmol of TSTU, and 5.0 mmol TEA in 5.0 mL of DMF at 50 °C for 24 h.
The NMR spectra of known compounds are identical to the reported ones in the corresponding references.
Isolated yields.
The corresponding deprotected dipeptide ester was isolated as the major product.
The compound indicated poor solubility in DMF, MeOH, H2O, and DMSO.
b
c
d
e
indicated that the rate of the final cyclization step depends
largely on the sequence of dipeptides. Finally, when 1-aminocy-
clopropyl carboxylic acid ethylester (2d) was used, the reactions
gave good yields for Fmoc-Ile-OH (entry 16), low yields for
Fmoc-Trp(Boc)-OH and Fmoc-Tyr(tBu)-OH (entries 13 and 14),
and trace amount of DKP for Fmoc-Phe-OH. It is known that
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
01.133.
coupling of
a,a
,-disubstituted amino acid is difficult.16 For the
References and notes
reactions that gave low yields, the corresponding dipeptide
esters (analogues to compound 4a) were isolated as the major
products. It is also noted that DKPs generally showed limited
solubility in various solvents including DMF, MeOH, H2O, and
DMSO.
In summary, we have reported a facile one-step route to DKPs
using Fmoc amino acid and amino acid ester in the presence of
TSTU and TEA in DMF at 50 °C.22 As a complimentary method for
the preparation of DKPs, the practical applicability of this protocol
is illustrated by rapid and efficient generation of various DKPs in
modest to good yields. The neuroprotective properties of the syn-
thesized compounds are underway in our laboratory.
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22. Typical procedure for the synthesis of DKP 5a. To a solution of Fmoc-protected
amino acid (1.0 mmol, purchased from Aapptec), amino acid ester (1.0 mmol,
purchased from Sigma–Aldrich), and TSTU (1.0 mmol, purchased from
Aapptec) in DMF (5.0 mL) was added TEA (5.0 mmol). The reaction was
allowed to stir at 50 °C for 24 h. Then, the reaction mixture was poured into
water (10 mL) and standing for overnight. The precipitate was filtered to give
the corresponding DKP 5a as a white solid (320 mg, 0.82 mmol, 82%): TLC: 10:1
CH2Cl2/MeOH, Rf = 0.5; 1H NMR (400 MHz, DMSO-d6) d 0.71–0.73 (d, J = 7.2 Hz,
3H), 1.63 (s, 9H), 3.02–3.07 (m, 1H), 3.19–3.22 (m, 1H), 3.71–3.76 (m, 1H), 4.22
(m, 1H), 7.22–7.25 (m, 1H), 7.29–7.33 (m, 1H), 7.45 (s, 1H), 7.64–7.66 (d,
J = 7.2 Hz, 1H), 8.02–8.04 (d, J = 7.6 Hz, 1H), 8.11 (s, 1H), 8.18 (s, 1H); 13C NMR
(100 MHz, CDCl3) d 18.37, 28.16, 47.74, 54.58, 61.33, 83.63, 115.24, 116.34,
119.17, 122.60, 124.16, 124.59, 130.28, 135.55, 149.55, 172.88, 173.82; LC–MS
(M+Na+) calcd for C19H23N3NaO4 390, found 390.