Acyclic and cyclic thioenamino peptides: solution- and solid-phase synthesis

Article abstract of DOI:10.1016/j.tetlet.2008.12.072

Seven- and 10-membered cyclic thioenamino peptides, that is, 1,4-thiazepinone (11) and cyclic thioenamino peptide 9 (which represents a potential γ-turn mimetic), were synthesized, and the structure of 11 was secured by X-ray diffraction analysis of its T

Full text of DOI:10.1016/j.tetlet.2008.12.072

Tetrahedron Letters 50 (2009) 1048–1050
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Acyclic and cyclic thioenamino peptides: solution- and solid-phase synthesis
*
Lee Goren, Doron Pappo, Israel Goldberg, Yoel Kashman
School of Chemistry, Tel-Aviv University, Ramat Aviv 69978, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven- and 10-membered cyclic thioenamino peptides, that is, 1,4-thiazepinone (11) and cyclic thioena-
Received 28 October 2008
Revised 8 December 2008
Accepted 16 December 2008
Available online 24 December 2008
mino peptide 9 (which represents a potential c-turn mimetic), were synthesized, and the structure of 11
was secured by X-ray diffraction analysis of its TFA salt. The aforementioned compounds were prepared
in solution and by solid-phase synthesis. Additionally, we have prepared thioenamino diketopiperazine
synthon 16.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Cyclic peptides
Solid-phase synthesis
FGly
Endiamine
Thioenamine
Callynormine A,1 a new type of sponge-derived cyclic peptide,
possessing an endiamino functionality, instead of the lactone of
depsipeptides, has prompted the synthesis of cyclic endiamino
and thioenamino peptides. Cyclic endiamino and thioenamino
peptides are expected to be of interest because of their restricted
conformations, and their potential ability to probe the topography
of enzyme active sites and to generate inhibitors devoid of the
typical therapeutic shortcomings of peptides. Moreover, thioena-
mino and endiamino peptides have been reported for their ability
to mimic protein secondary structures.2–4 Cyclization of ‘alpha,
omega’ amino enol-tosylates is a good route for the preparation
of cyclic thioenamino peptides.2–7
Herein, we describe reactions of formylglycine (FGly) in solu-
tion and with the hitherto unknown solid-supported FGly. It is
shown that supported FGly reacts with amines and with nucleo-
philic thiols in the same manner as FGly-OTs in solution.
It is difficult to compare the yields obtained under solution- and
solid-phase conditions, as no optimization of the latter reaction
was attempted and the extent of the resin loading is unknown.
Preparation of linked solid phase FGly, for example, 2, was
carried out in two steps. First Swern oxidation of serine [(COCl)2,
led to endiamine 3,9 and thioenamino peptides 4–7 (Table 1).
The structures of compounds 3–7 were elucidated from 1D and
2D 1H NMR spectra and mass spectrometry.
The Z configuration of endiamine 3,2 and of thioenamino pep-
tides 4–6 was deduced from the NOEs, observed, between the
methoxy group and the vinyl proton. Compound 7,10 with the
Cbz-protecting group, on the other hand, did not show a positive
NOE correlation, and therefore is suggested to be of E configura-
tion. (The NH signal could not be observed in CDCl3 or in DMSO-
d6).
Among the secondary structures of peptides, reverse turns that
include b- and c-turns are known as structural elements that are
involved in biomolecular recognition events.11 In proteins or pep-
tides, b-turns are characterized by a 10-membered ring incorporat-
ing a single hydrogen bond, and are more frequent than c-turns
which form seven-membered rings with a single hydrogen bond
(Fig. 1).12,13 The latter are found mainly in small peptides but are
rare in proteins.
Compounds 9 (Scheme 2) and 11 (Scheme 1) were synthesized
as model compounds. The cyclic seven-membered 1,4-thiazepi-
nones 11 and 14 were synthesized from compound 10 in solution,
or from 13 by solid-phase synthesis, respectively.
The preparation of 1,4-thiazepinone 11 was achieved from
Boc-Cys(Tr)-Ser-OMe,14 which was oxidized and trapped as the
appropriate enol-tosylate 10 (Scheme 1). Deprotection with TFA
(50%) of both the N-Boc and S-Tr-protecting groups, of 10, followed
by base-induced cyclization afforded compound 11,15 and with
excess TFA, its salt 12.
DMSO, DIPEA, DCM, À78 °C] to the unstable
a-formylglycine, fol-
lowed by trapping of its enol tautomer with a sulfonyl chloride
resin,8 gave, after slow warming to room temperature, the desired
polymer-supported FGly 2.
Subsequently, we examined the reactivity of the supported FGly
2 toward reactions with nucleophiles. Overnight stirring of com-
pound 2a or 2b with an amine or thiol in the presence of base
Compound 12 was obtained as crystals which enabled X-ray dif-
fraction analysis to confirm the structure (Scheme 1).16 Selective
deprotection of the thiol group of 10 (Scheme 1) under mild acidic
* Corresponding author. Fax: +972 3 6409293.
E-mail address: kashman@post.tau.ac.il (Y. Kashman).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.12.072

L. Goren et al. / Tetrahedron Letters 50 (2009) 1048–1050
1049
Table 1
O
S
O
CO2CH3
Solid- and solution-phase synthesis of compounds 3–7a,b
1.TIS (10 mmol)
CH Cl -TFA-H O(96:2:2).
O
2
2
2
NH
O
2.Et N in CH CN.
3
3
NHR
NHR
CO2CH3
NHBoc
HO
TsO
(25%)
S
CO2CH3
13
STr
1
Swerno x.
CO2CH3
Nu, Et3N
CH3CN
then sulfonyl
chloride resin
NH
NH
BocHN
1.TIS (7 mmol)
CH Cl -TFA(98:2).
O
14
CO2CH3
NH
O
S
O
R=Boc
R=Cbz
NHR
CO2CH3
NHR
CO2CH3
2
2
Nu,E t3N
dry CH2Cl2
or CH3CN
TsO
2.Et N in CH CN.
3 3
(70%)
(45%)
O
Nu
NHBoc
S
O
a,
1.TIS (7 mmol)
2
3-7
b,
CH Cl -TFA(1:1).
CO2CH3
2
2
2.Et N in CH CN.
STr
3
3
H2N
10
Nu=amine or thiol.
O
+
S
11
Nu
Product
Yield (%)
NHBoc
H
N
CO2CH3
NH2
CO2CH3
CF3COO
NH
H3N
80
90
83
CO2CH3
CO2CH3
O
12
3
X-ray structure of compound 12.16
NHBoc
S
Scheme 1. The synthesis of 1,4-thiazepinones.
CO2CH3
SH
4
14 (Scheme 1). Another seven-membered cyclic thioenamino
1,4-thiazepinone,18 prepared in a different way, has been reported
for its therapeutic potential as an ACP/NEP dual inhibitor.18
The cyclic 10-membered thioenamino tripeptide 9 was synthe-
sized both from compound 15 in solution, and from 16 by solid-
phase synthesis (Scheme 2). The structure of 9 was confirmed by
MS and 1D and 2D NMR including 15NH-HMBC (Fig. 2).19 A 2E con-
figuration is suggested for 9 based on the NOE measured between
H-2 and H-6, which is only possible, according to a model, for the
2E configuration. To the best of our knowledge, this is the first
reported cyclic thioenamino peptide with an E configured double
bond, formed most likely to relieve ring strain.
NHBoc
S
CO2CH3
SH
SH
5
NHBoc
CO2CH3
S
85
O
O
6
O
CO2CH3
NHCbz
O
S
c
O
SH
O
The b-turn of the previously synthesized cyclic thioenamino
7
peptide 83 is compared with the potential
c-turn of 9, as suggested
a
Solution-phase synthesis: 1 equiv of nucleophile, 4–5 equiv of Et3N. The yield
by NMR measurements (Fig. 1).
shown is for the isolated product starting from the protected enol tosylate 1.
The configuration of 9 was confirmed by the temperature coef-
ficients of the amide protons in DMSO-d6.20 Namely, a small coef-
ficient was measured for the thioenamino NH proton (NH-FGly,
À3.8 ppb/K), compared to the NH-Cys proton (À8.4 ppb/K). The
NH-Phe proton exhibited an intermediate temperature coefficient
(À4.5 ppb/K), most likely due to a hydrogen bond with the Boc car-
bonyl). The NH-FGly and NH-Phe coefficients indicated that they
were both involved in intramolecular hydrogen bonds. The NOESY
b
Solid-phase synthesis: 10 equiv of nucleophile, 15 equiv of Et3N. The yield (not
shown) of isolated product is calculated for the two steps (oxidation and nucleo-
philic substitution) based on 100% resin loading, and as the extent of the resin
loading is unknown, it is difficult to compare the yields in solution and solid phase.
For estimated yields, see Supplementary data.
c
Performed only by solid-phase synthesis.
conditions (2% TFA), followed by addition of base, led to compound
14.17
Next, the solid-supported FGly dipeptide 13, obtained from
Boc-Cys(Tr)-Ser-OMe, after selective deprotection of the thiol
group, under mild acidic conditions (2% TFA), gave, as in solution,
after treatment with base, the desired cyclic thioenamino peptide
CO2CH3
NH
TsO
1. TISi n
CH2Cl2-TFA.
2. Et3Ni n
CH3CN.
O
O
(60%)
CO2CH3
Ph
NH
S
BocHN
NH
H6
2H
H
STr
N
H
O
15
O
b
BocHN
a
Ph
1. TISi n
O
N
H
O
Ph
CO2CH3
Ph
O
S
O
CH2Cl2-TFA
H
9
N
-H2O.
2. Et3Ni n
CH3CN.
O
CO2CH3
NH
N
H
H
O
O
H
NH
NH2
O
S
N H
O
N
H
H3CO2C
O
O
S
O
NOE correlation
9
8
NH
STr
Ph
The thioenamino group as a
The thioenamino group as a
BocHN
β
γ
-turn mimetic
-turn mimetic
Figure 1. Cyclic thioenamines as turn-like mimetics: (a) b-turn tetrapeptide 8,3
involving a 10-membered hydrogen bond; (b)
seven-membered hydrogen bond.
16
c-turn compound 9, containing a
Scheme 2. Synthesis of 10-membered cyclic thioenamino tripeptide 9.

1050
L. Goren et al. / Tetrahedron Letters 50 (2009) 1048–1050
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.12.072.
11 12
a
b
CO2CH3
CO2CH3
10
H
H
H
3
δ
N 114.64
H
H
S
H
S 2
O
N
O
N4
H
O
H
H
References and notes
9
5
N
H
H
N
H
H
8
O
N
H
O
O 19
N
18H
7
H
H
δ
N 96.54
1. Berer, N.; Rudi, A.; Goldberg, I.; Benayahu, Y.; Kashman, Y. Org. Lett. 2004, 6,
2543–2545.
2. Pappo, D.; Vartanian, M.; Lang, S.; Kashman, Y. J. Am. Chem. Soc. 2005, 127,
7682–7683.
O
O
H
13
H
δ
N 125.27
9
3. Pappo, D.; Kashman, Y. Org. Lett. 2006, 8, 1177–1179.
4. Marshall, G. R. Tetrahedron 1993, 49, 3547–3558.
5. Chruszez, M.; Laidler, P.; Monkiewicz, M.; Ortlund, E.; Lebioda, L.; Lewinski, K. J.
Inorg. Biochem. 2003, 96, 386–392.
Figure 2. (a) (
and (b) (
compound 9.
) Selected 13CH-HMBC and (
) selected 15NH-HMBC and selected (
) selected HH-COSY correlations
) NOESY correlations for
6. It is suggested that cyclic endiamino peptides are obtained in Nature by the
condensation of FGly (formylglycine) with the amine of an amino acid. FGly,
which is known in Nature only as a hydrate in active sites of enzymes, is
unstable in solution, and has therefore to be protected as, for example, enol-
tosylate (FGly-OTs) 1. The latter moiety was shown to function as the free
aldehyde.
O
S
O
CO2CH3
NH
O
NHBoc
O
1. TFA:CH2Cl2 (1:10)
2. Et3N, dry DMF
7. Nakazawa, T.; Suzuki, T.; Ishii, M. Tetrahedron Lett. 1997, 38, 8951–8954.
8. Sulfonyl chloride resin, Sigma–Aldrich, # 498211.
17
9. Compound 3. Amorphous solid, HRESMS (m/z 345.2020 (M+)).
10. The structure of compound 7, obtained as an amorphous solid, was established
from 1D and 2D NMR and HRESMS (m/z 376.0825 (M+Na+)) data.
11. Smith, J. A.; Pease, L. G. CRC Crit. Rev. Biochem. 1980, 8, 315–399.
H
N
O
O
S
O
O
N
H
O
O
12. Examples of peptides forming c-turns: (a) Levian-Teitlbaum, D.; Kolodny, N.;
Chorev, M.; Selinger, Z.; Gilon, C. Biopolymers 1989, 28, 51–64; (b) Raghothama,
S.; Ramakrishnan, C.; Balasubramanian, D.; Balaram, P. Biopolymers 1989, 28,
573–588; (c) Milon, A.; Miyazawa, T.; Higashijima, T. Biochemistry 1990, 29,
65–75; (d) Walse, B.; Kihlberg, J.; Drakenberg, T. Eur. J. Biochem. 1998, 252,
428–440.
18
4-methylbenzenethiol
Et3N, CH2Cl2
H
O
N
S
13. For c-turn mimetics, see: (a) Schmidt, B.; Lindman, S.; Tong, W.; Lindeberg, G.;
N
H
Gogoll, A.; Lai, Z.; Thörnwall, M.; Synnergren, B.; Nilsson, A.; Welch, C. J.;
Sohtell, M.; Westerlund, C.; Nyberg, F.; Karlén, A.; Hallberg, A. J. Med. Chem.
1997, 40, 903–919; (b) Brickmann, K.; Yuan, Z.; Sethson, I.; Somfai, P.; Kihlberg,
J. Chem. Eur. J. 1999, 5, 2241–2253; (c) Jiménez, A. I.; Cativiela, C.; Marraud, M.
Tetrahedron Lett. 2000, 41, 5353–5356.
19
Scheme 3. The solid-phase synthesis of
thioenamino compound 19.
a diketopiperazine synthon, and a
14. Boc-Cys(Tr)-Phe-Ser-OMe was prepared by standard peptide synthesis
procedures. Its structure was established by 1D and 2D NMR and HRESMS
(m/z 734.2878 (M+Na+)) data.
15. The structure of compound 11, a yellow oil, was established from 1D and 2D
NMR and HRESMS (m/z 203.0484 (M+)) data. Inter alia, the HMBC experiment
showed a correlation between H-2 and C-7 in the ring. A characteristic vinylic
thioenamino proton resonance at d 7.12 (s) ppm, and the corresponding carbon
at d 121.1 ppm were present.
transannular cross peak between the
vinylic thioenamino proton (H-2, Fig. 2) further supported the pos-
sible -turn configuration.
Another application of the solid-phase methodology is demon-
strated by the synthesis of diketopiperazine 19,18 which is a poten-
tially interesting bioactive21 synthon (Scheme 3).22
The solution synthesis of diketopiperazines via enol-tosylates
was reported earlier by us.3 The solid-phase synthesis requires
two synthetic steps on the resin, that is, deprotection of the N-
Boc group of compound 17 (synthesized from Val-Ser), with TFA,
followed by base treatment to afford synthon 18, which after thiol
substitution afforded compound 19.23
In summary, we have demonstrated two synthetic approaches,
one in solution and a new solid-phase route, for the preparation of
thioenamino and endiamino compounds. Inter alia, the synthesis of
1,4-thiazepinone11andofthe10-memberedthioenaminocyclictri-
peptide9wasdemonstrated.Thelattercompoundmostlikelyprefers
a-Phe proton (H-6) and the
c
16. The structure of compound 12 was confirmed by single crystal X-ray
diffraction analysis (Scheme 1). The measurements were carried out on a
Nonius KappaCCD diffractometer at low temperature (ca. 110 K) in order to
optimize the precision of the crystallographic determination, with Mo K
radiation. Crystal data: C7H11N2O3S, M = 316.26, triclinic, space group P1,
a = 4.9238(6), b = 5.9807(7), c = 11.4838(8) Å, = 73.489(7)°, b = 89.571(7)°,
= 75.771(5)°, V = 313.57(6) Å3, Z = 1, T = 110(2) K, Dc = 1.675 g cmÀ3
(Mo
) = 0.32 mmÀ1 2014 unique reflections to 2hmax = 53.0°, 184 refined
(I), R1 = 0.056
a
a
c
, l
K
a
parameters, R1 = 0.046 for 1772 observations with I > 2
r
(wR2 = 0.103) for all unique data. Crystal data for the structural analysis have
been deposited with the Cambridge Crystallographic Data Centre, CCDC
709744. The supplementary crystallographic data for this Letter can be
obtained free of charge from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44 1233 336033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk/).
17. The structure of compound 14, an amorphous solid, was established from 1H,
13C NMR, and HRESMS (m/z 325.0828 (M+Na+)) data. The characteristic vinylic
thioenamino proton resonance at d 7.20 (s) ppm, and the corresponding carbon
at d 120.9 ppm were present.
a
c-turn-like conformation, hence being a potential
c-turn motif.
Reverse turn mimetics as prepared herewith are considered to
be promising candidates for drug discovery due to their ability to
function as agonists of important biological processes.24
*
*
18. [S-(R ,S )]-6-[(2-Mercapto-1-oxo-3-phenylpropyl)amino]-4,5,6,7-tetrahydro-
2-methyl-5-oxo-1,4-thiazepine-3-carboxylic acid has been reported for its
therapeutic potential as an ACP/NEP dual inhibitor. Crescenza, A.; Botta, M.;
Corelli, F.; Santini, A.; Tafi, A. J. Org. Chem. 1999, 64, 3019–3025.
19. Compound 9, amorphous solid, HRESMS (m/z 472.1515 (M+Na+)). The
characteristic vinylic thioenamino proton resonance at d 7.39 (s) ppm, and
carbon resonance at d 144.5 ppm were present.
Acknowledgments
We thank the Israeli Science Foundation for financial support,
Grant # 180/05, Dr. Amira Rudi for her help with NMR measure-
ments, Dr. Moshe Portnoy for helpful discussions, Ms. Yael Kogon
for her contribution to this project, and Dr. Ayelet Sacher (of the
Maiman Institute for Proteome Research, Tel Aviv University) for
performing the electrospray mass measurements.
20. (a) Well, R. M. V.; Marinelli, L.; Altona, C.; Erkelens, K.; Siegal, G.; Raaij, M. V.;
Liamas- Saiz, A. L.; Kessler, H.; Novellino, E.; Lavecchia, A.; Van Boom, J. H.;
Overhand, M. J. Am. Chem. Soc. 2003, 125, 10822–10829; (b) Imperiali, B.;
Fisher, S. L.; Moats, R. A.; Prins, T. J. J. Am. Chem. Soc. 1992, 114, 3182–3188; (c)
Jeannotte, G.; Lubell, W. D. J. Am. Chem. Soc. 2004, 126, 14334–14335; (d) Xiao,
J.; Weisblum, B.; Wipf, P. J. Am. Chem. Soc. 2005, 127, 5742–5743.
21. Martins, M. B.; Carvalho, I. Tetrahedron 2007, 63, 9923–9932.
22. Fischer, P. M. J. Peptide Sci. 2003, 9, 9–35.
23. The structure of compound 19 (obtained as a white solid) was established from
1H, 13C NMR, and HRESMS (m/z 313.1009 (M+Na+)) data. The characteristic
vinylic thioenamino proton resonance at d 6.64 (s) ppm, and carbon resonance
at d 122.9 ppm were present.
24. Disclosure of reverse-turn regions of biologically active peptides and proteins;
Moon, S. H. U.S. Pat. Appl. Publ. 826, 972, 2007; Chem. Abst. 146, 274, 627.
Supplementary data
Supplementary data (general procedures and spectral data (1D
and 2D NMR) for selected compounds are provided) associated