A mild and convenient base-catalysed approach to disubstituted epidithiodiketopiperazines

Article abstract of DOI:10.1055/s-0033-1340161

A novel base-catalysed approach to disubstituted epidithiodiketopiperazines (ETP) is described. The synthetic route involves a multicomponent reaction of a diacetoxyacetamide with a range of different amines and p- methoxybenzylmercaptan to generate the p

Full text of DOI:10.1055/s-0033-1340161

LETTER
▌2563
lAetteMr ild and Convenient Base-Catalysed Approach to Disubstituted Epi-
dithiodiketopiperazines
Base-Catalysed Approach to Disubstituted Epidithiodiketopiperazines
Bruno C. Sil, Stephen T. Hilton*
UCL School of Pharmacy, 29-39 Brunswick Square, London, WC1N 1AX, UK
Fax +44(20)7535964; E-mail: s.hilton@ucl.ac.uk
Received: 22.08.2013; Accepted after revision: 29.09.2013
simple yet effective approach therefore reduces the over-
all number of steps required to access the ETP core whilst
simultaneously avoiding the notorious solubility prob-
lems associated with diketopiperazine formation. The dif-
fering strategies are outlined below (Scheme 1).
Abstract: A novel base-catalysed approach to disubstituted epi-
dithiodiketopiperazines (ETP) is described. The synthetic route in-
volves a multicomponent reaction of a diacetoxyacetamide with a
range of different amines and p-methoxybenzylmercaptan to gener-
ate the protected ETP core.
Key words: epidithiodiketopiperazine, multicomponent reactions,
chaetocin, gliotoxin, chetomin
O
R2
R1
R3
R4
N
S2
N
The epidithiodiketopiperazines (ETPs) are a structurally
diverse group of natural products that are classified by the
presence of a disulfide bridged diketopiperazine core 1.1,2
All members of this class of compound display potent bi-
ological activity, ranging from nanomolar anticancer ac-
tivity through to antiviral and antibacterial activity.1–3
There has been widespread interest in these molecules fo-
cusing on chetomin (2) and chaetocin (3) in particular, due
to their structural complexity and reported ability to act on
epigenetic targets and to inhibit protein–protein interac-
tions (Figure 1).4 As part of an ongoing research pro-
gramme into the biological activity and mechanism of
action of this class of compound, we needed to be able to
access highly substituted derivatives with diverse func-
tionality. In our previous approach towards the ETP
core,1,5 we had described an acid-catalysed cyclisation ap-
proach, but had found that it proved to be intolerant of a
number of functional groups. We therefore decided to ex-
plore whether we could carry out the cyclisation under
milder conditions. Herein, we now report our initial re-
sults towards functionalised derivatives of the ETP core.
O
ETP core (1)
OH
Me
O
H
O
H
N
N
S2
N
HO
N
Me
O
S2
O
N
O
Me
Me
N
O
S2
N
N
OH
Me
N
S2
N
H
N
O
H
N
OH
H
H
O
chaetocin (3)
chetomin (2)
Figure 1 Examples of ETP natural products
O
R3
N
S
S
nucleophilic
sulfur
approach
electrophilic
sulfur
N
R1
R
approach
S
O
O
O
5
Br
R1
R3
R3
Most reported approaches towards either the ETP core or
the parent natural product have relied on formation of the
diketopiperazine ring prior to addition of sulfur either via
displacement of a leaving group, or addition of sulfur to
an intermediate iminium ion.6 As an example, recent ap-
proaches described by both Nicolaou and Reisman have
shown that simple ETP derivatives can be obtained by re-
action of the diketopiperazine ring with sulfur or sulfur
electrophiles under basic conditions.7 However, one main
drawback to these approaches, is the need to form the dik-
etopiperazine ring prior to incorporation of sulfur. In our
research programme, we have developed a novel alterna-
tive approach, whereby the diketopiperazine ring is
formed at the same time as sulfur is incorporated. This
N
N
this
work
N
N
Br
R1
O
4
S
O
6
R
O
S
Et
R
O
N
OAc
OAc
R1
O
7
Scheme 1 Retrosynthetic approaches to the ETP core
In our previous reports, cyclisation of the diacetate pre-
cursor 7 to the protected ETP core 8 required trifluoro-
acetic acid (TFA) to be used as a catalyst in the presence
of a number of amines and p-methoxybenzylmercaptan as
shown below (Scheme 2).
SYNLETT 2013, 24, 2563–2566
Advanced online publication: 05.11.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340161; Art ID: ST-2013-D0810-L
© Georg Thieme Verlag Stuttgart · New York

2564
B. C. Sil, S. T. Hilton
LETTER
cleophilic sulfur of p-methoxybenzylmercaptan can react,
followed by cyclisation to produce the ETP precursor 12
analogous to the mechanism with TFA (Scheme 3).
O
O
S
R
Et
S
O
R
N
Me
n-BuNH2
Ph
N
Ph
N
R
OAc
S
PMBSH, MeCN
TFA, 51 °C, 16 h
In order to investigate cyclisation of the diacetate using
these newly developed conditions, a series of reactions
were carried out towards the synthesis of the n-butyl ETP
precursor 8 as our reaction standard. We initially decided
to investigate the effects of both conventional and micro-
wave heating conditions in order to try to reduce the
length of time that it took to carry out the reaction and a
preliminary optimization study was carried with the re-
sults shown below (Table 1).
OAc
O
7
O
8
Scheme 2 Formation of the protected ETP core
However, in our hands, TFA proved to be incompatible
with amines containing either tethered esters and amines
or aromatic rings with carbamate groups attached. In our
previous proposed mechanism of cyclisation, we envis-
aged that TFA is able to catalyse the reaction by unmask-
ing the protected glyoxamide 7 in situ, which can then
react with an equivalent of amine and thiol prior to cycli-
sation to form the ETP core (Scheme 3). We therefore de-
cided to investigate whether 4-N,N-dimethylamino-
pyridine (DMAP) could also catalyse the cyclisation of
the diacetate 7 to the protected dithiodiketopiperazine 8
for the development of a novel milder synthetic strategy
for ETP production.
Table 1 Initial Screen of Reaction Conditions using DMAPa
Entry
Method
Time
Temp (°C) Yield (%)
1
2
3
4
5
A
B
B
B
B
16 h
51
130
130
130
160
80
38
40
37
19
15 min
20 min
30 min
30 min
Me
Me
N
N
a Reaction conditions: (A) MeCN, p-methoxybenzylmercaptan,
DMAP, reflux conditions; (B) MeCN, p-methoxybenzylmercaptan,
DMAP, microwave conditions. Yields are based on isolation of 8 af-
ter purification.
O
O
O
O
S
R
OEt
O
Me
Me
Me
O
N
S
Et
R3
R1
R
O
O
H2N
From our studies, the results clearly showed that optimum
conditions were achieved when using conventional heat-
ing with 80% of the protected ETP obtained after heating
the reaction overnight at 51 °C (Table 1, entry 1). Surpris-
ingly, when the reaction was carried out in the microwave,
only 40% of the protected ETP 8 was obtained. Prolonged
heating of the reaction led to reduced yields and associat-
ed product degradation. Under conventional heating,
DMAP proved to be an effective catalyst for the deprotec-
tion of diacetate 7 and formation of the ETP core as shown
above (Table 1). Following the success of our initial
screen, the synthesis of a number of different protected
dithiodiketopiperazines containing functionality that was
previously impossible was attempted using this approach
(Table 2).
N
N
N
R1
O
O
7
O
9
Me
Me
H2O
Et
O
O
R
S
S
Et
HS
R
O
R
O
S
R3
R3
N
N
R1
N
N
H
R1
H
O
O
R
10
11
O
S
R3
S
R
N
From the results obtained, it was noticeable that heating
the reaction at reflux to obtain the protected ETP was gen-
erally far superior to that of microwave-accelerated reac-
tions in terms of reaction yield. This was in contrast to the
acid-catalysed reaction, where heating the reaction under
microwave conditions did not lead to product degradation.
This may be attributed to the stability of the intermediate
glyoxamide 9, which is unstable and cannot be isolated.
During our studies, we also investigated the effects of mi-
crowave heating on the cyclisation and its effects on vari-
ous functional groups. As predicted from our initial screen
with butylamine, reaction yields were higher when the re-
action was heated under conventional conditions with
DMAP as catalyst. Only the allylamine derivative (Table
N
R
R1
O
12
Scheme 3 Proposed mechanism of deprotection and conversion to
the ETP core
We anticipated that DMAP would similarly be able to un-
mask the glyoxamide 9 by nucleophilic attack at the car-
bonyl group of one of the acetate groups of the diacetate
7, resulting in concomitant removal of the second acetate
group and formation of the intermediate glyoxamide. This
would then facilitate reaction with the nucleophilic amine
and lead to formation of an iminium ion 10 where the nu-
Synlett 2013, 24, 2563–2566
© Georg Thieme Verlag Stuttgart · New York

LETTER
Base-Catalysed Approach to Disubstituted Epidithiodiketopiperazines
2565
Table 2 Reaction of Amines with Diacetate 7 using DMAPa
Table 3 Deprotection/Oxidation and Synthesis of the ETP Core
O
O
Entry
Amine
Method
Yield (%)
S
R
R
PMB
Ph
N
N
BBr3, CH2Cl2
S
S
1
2
3-chlorobenzylamine
3-chlorobenzylamine
benzylamine
A
B
A
A
B
A
C
A
C
A
A
75
68
68
53
76
70
28
76
46
61
78
N
Ph
N
–78 °C to r.t.
NH4Cl (aq), I2
S
O
13
PMB
O
14
3
Entry
Amine (R1)
Yield (%)
4
allylamine
5
allylamine
1
3
3-chlorobenzylamine
benzylamine
60
85
60
64
75
61
59
6
ethyl 4-aminobutanoate
ethyl 4-aminobutanoate
methyl 6-aminohexanoate
methyl 6-aminohexanoate
ethyl 3-aminopropanoate
methyl 5-aminopentanoate
7
5
allylamine
8
7
ethyl 4-aminobutanoate
methyl 6-aminohexanoate
ethyl 3-aminopropanoate
methyl 5-aminopentanoate
9
9
10
11
10
11
a Reaction conditions: (A) MeCN, p-methoxybenzylmercaptan,
DMAP, reflux; (B) MeCN, p-methoxybenzylmercaptan, DMAP, mi-
crowave (5 min, 150 ºC); (C) MeCN, p-methoxybenzylmercaptan,
DMAP, microwave (30 min, 130 ºC). Yields refer to isolation of the
corresponding protected ETP 12 after purification.
References and Notes
(1) Aliev, A. E.; Hilton, S. T.; Motherwell, W. B.; Selwood, D.
L. Tetrahedron Lett. 2006, 47, 2387.
(2) Cook, K. M.; Hilton, S. T.; Mecinovic, J.; Motherwell, W.
B.; Figg, W. D.; Schofield, C. J. J. Biol. Chem. 2009, 284,
26831.
2, entries 4 and 5) gave a higher yield when microwave
conditions were used. This may be due to the fact that
when the reaction is heated in a sealed vessel in this man-
ner, allylamine cannot be lost by evaporation. The suc-
cessful obtention of the protected ETP core compounds
containing ester moieties (Table 2, entries 6–11) clearly
highlights the importance of DMAP as an alternative cat-
alyst for the cyclisation of ETP intermediates. Using this
approach, we were able to introduce different carbon
length ester groups in the ETP core which had previously
not been possible.
(3) (a) Hilton, S. T.; Motherwell, W. B.; Potier, P.; Pradet, C.;
Selwood, D. L. Bioorg. Med. Chem. Lett. 2005, 15, 2239.
(b) Block, K. M.; Wang, H.; Szabo, L. Z.; Polaske, N. W.;
Henchey, L. K.; Dubey, R.; Kushal, S.; Laszlo, C. F.;
Makhoul, J.; Song, Z.; Meuillet, E. J.; Olenyuk, B. Z. J. Am.
Chem. Soc. 2009, 131, 18078. (c) Waring, P.; Beaver, J.
Gen. Pharmacol. 1996, 27, 1311. (d) Chai, C. L.; Waring, P.
Redox Reports 2000, 5, 257. (e) Waring, P.; Eichner, R. D.;
Müllbacher, A. Med. Res. Rev. 1988, 8, 499. (f) Johnson, J.
R.; Bruce, W. F.; Dutcher, J. D. J. Am. Chem. Soc. 1943, 65,
2005. (g) Gardiner, D. M.; Waring, P.; Howlett, B. J.
Microbiology 2005, 151, 1021.
(4) (a) Isham, C. R.; Tibodeau, J. D.; Bossou, A. R.; Merchan, J.
R.; Bible, K. C. Br. J. Cancer 2012, 106, 314. (b) Staab, A.;
Loeffler, J.; Said, H. M.; Diehlmann, D.; Katzer, A.; Beyer,
M.; Fleischer, M.; Schwab, F.; Baier, K.; Einsele, H.;
Flentje, M.; Vordermark, D. BMC Cancer 2007, 7, 213.
(c) Yano, K.; Horinaka, M.; Yoshida, T.; Yasuda, T.;
Taniguchi, H.; Goda, A. E.; Wakada, M.; Yoshikawa, S.;
Nakamura, T.; Kawauchi, A.; Miki, T.; Sakai, T. Int. J.
Oncol. 2011, 38, 365.
The final step of the reaction sequence involved a com-
bined deprotection/oxidation step for conversion of the
protected derivatives to the ETP core (Table 3).1
All compounds underwent clean conversion to the ETP
core 1 in good yield (Table 3). Pleasingly, protected ETPs
incorporating ester groups (Table 3, entries 7–10) were
readily deprotected and oxidised to the ETP core 1 also in
good yield, clearly highlighting the effectiveness of
DMAP as a catalyst.
(5) Hilton, S. T.; Motherwell, W. B.; Selwood, D. L. Synlett
2004, 2609.
In summary, DMAP proved to be an effective catalyst for
the synthesis of novel ETP derivatives.8 It is clearly supe-
rior to the use of TFA which had proven to be incompati-
ble with a number of functional groups and further
investigations into the scope of the DMAP-catalysed reac-
tion are under active investigation in our laboratories.
(6) (a) Öhler, E.; Tataruch, F.; Schmidt, U. Chem. Ber. 1972,
105, 3658. (b) Öhler, E.; Poisel, H.; Tataruch, F.; Schmidt,
U. Chem. Ber. 1972, 105, 635. (c) Kishi, Y.; Fukuyama, T.;
Nakatsuka, S. J. Am. Chem. Soc. 1973, 95, 6490.
(d) Baumann, E.; Fromm, E. Ber. 1891, 24, 1441. (e) Kim,
J.; Ashenhurst, J. A.; Movassaghi, M. Science 2009, 324,
238. (f) Kim, J.; Movassaghi, M. J. Am. Chem. Soc. 2010,
132, 14376. (g) Iwasa, E.; Hamashima, Y.; Fujishiro, S.;
Hashizume, D.; Sodeoka, M. Tetrahedron 2011, 67, 6587.
(7) (a) Nicolaou, K. C.; Giguere, D.; Totokotsopoulos, S.; Sun,
Y.-P. Angew. Chem. Int. Ed. 2012, 51, 728. (b) Codelli, J.
A.; Puchlopek, A. L. A.; Reisman, S. E. J. Am. Chem. Soc.
2012, 134, 1930.
Acknowledgment
We gratefully acknowledge funding from FCT (grant No.
SFRH/BD/65630/2009) for the provision of a PhD studentship (to
B.C.S.) for this work.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2563–2566

2566
B. C. Sil, S. T. Hilton
LETTER
(8) General Procedures
min. DMAP (0.04 g, 0.31 mmol) was added and the mixture
was heated in the microwave at 130 °C for 30 min, allowed
to cool to r.t. and the solvent was removed under reduced
pressure. Purification via Biotage™ Horizon [petroleum
spirit (40–60 °C)–EtOAc, 3:1; snap 25 g] gave ethyl 4-{4-
benzyl-2,5-bis[(4-methoxybenzyl)thio]-3,6-dioxopiperazin-
1-yl}butanoate (0.11 g, 28%) as a colourless solid; mp 105–
107 °C. IR: 2931, 1730, 1674 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 1.13 (t, J = 7.1 Hz, 3 H, OCH2CH3), 1.31–1.51
(m, 2 H, CH2CO2CH2CH3), 1.93–2.02 (m, 2 H, NCH2CH2),
2.54–2.66 (m, 1 H, NCH2CH2), 3.41–3.54 (m, 2 H,
Method A: n-Butylamine (0.18 mL, 1.82 mmol) was added
to a solution of diacetate 7 (0.31 g, 0.61 mmol) in MeCN
(30.0 mL) followed by addition of p-methoxybenzyl-
mercaptan (0.13 mL, 0.91 mmol) and the resulting mixture
was stirred for 2 min. DMAP (0.04 g, 0.30 mmol) was added
in a single portion and the resulting mixture was heated at
reflux for 16 h, cooled to r.t. and the solvent was removed
under reduced pressure. Purification via Biotage™ Horizon
[petroleum spirit (40–60 °C)–EtOAc, 3:1; snap 25 g] gave
protected ETP 8 (0.28 g, 80%) as a colourless solid (see ref.
1).
Method B: 3-Chlorobenzylamine (0.13 mL, 1.07 mmol)
was added to a solution of diacetate 7 (0.18 g, 0.36 mmol) in
MeCN (5.00 mL) followed by addition of p-methoxybenzyl-
mercaptan (0.07 mL, 0.53 mmol) and the resulting mixture
was stirred for 2 min. DMAP (0.02 g, 0.18 mmol) was added
and the mixture was heated in the microwave at 150 °C for 5
min, allowed to cool to r.t. and the solvent was removed
under reduced pressure. Purification via Biotage™ Horizon
[petroleum spirit (40–60 °C)–EtOAc, 3:1; snap 25 g] gave
1-benzyl-4-(3-chlorobenzyl)-3,6-bis[(4-methoxybenzyl)thio]-
piperazine-2,5-dione (0.15 g, 68%) as a colourless solid (see
ref. 1).
NCH2Ph), 3.70 (d, J = 12.2 Hz, 1 H, SCH2), 3.73 (s, 3 H,
OMe), 3.76 (s, 3 H, OMe), 3.78–3.83 (m, 1 H, SCH2), 3.85
(d, J = 13.9 Hz, 1 H, SCH2), 3.90–4.01 (m, 3 H,
CO2CH2CH3, SCH2), 4.39 (s, 1 H, CHS), 4.21 (s, 1 H, CHS),
5.08 (d, J = 14.5 Hz, 1 H, NCH2), 6.61–6.65 (m, 2 H, ArH),
6.77–6.81 (m, 2 H, ArH), 6.80–6.84 (m, 2 H, ArH), 7.03–
7.09 (m, 2 H, ArH), 7.11–7.13 (m, 1 H, ArH), 7.23–7.27 (m,
2 H, ArH), 7.30–7.34 (m, 2 H, ArH). 13C NMR (100 MHz,
CDCl3): δ = 14.04 (CO2Et), 21.95 (CH2), 31.29 (CH2), 36.10
(SCH2), 36.80 (SCH2), 43.13 (CH2), 46.00 (CH2), 55.14
(OMe), 55.15 (OMe), 57.23 (CHS), 59.26 (CHS), 60.24
(CO2Et), 113.96 (ArH), 114.01 (ArH), 127.72 (ArH), 128.46
(ArH), 128.52 (ArH), 128.56 (C), 128.73 (C), 130.60 (ArH),
130.67 (ArH), 134.84 (C), 159.01 (C), 159.03 (C), 164.92
(C), 165.07 (C), 172.26 (C). MS: m/z = 645 (100%) [M +
Na]+. HRMS: m/z [M + Na]+ calcd for C33H38N2O6S2Na:
645.2069; found: 645.2069.
Method C: Ethyl 4-aminobutanoate hydrochloride (0.10 g,
0.61 mmol) was added to a solution of diacetate 7 (0.24 g,
0.47 mmol) in MeCN (3 mL) followed by addition of Et3N
(0.34 mL, 2.45 mmol) and p-methoxybenzylmercaptan (0.10
mL, 0.73 mmol) and the resulting mixture was stirred for 2
Synlett 2013, 24, 2563–2566
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