A synthetic route to enediyne-bridged amino acids

Article abstract of DOI:10.1016/j.tetlet.2007.05.016

A general strategy for the preparation of enediyne-bridged amino acids has been disclosed. A cross-coupling reaction between amino- and carboxyl-modified amino acids under Sonogashira reaction conditions gave a protected enediyne-peptide conjugate. The mo

Full text of DOI:10.1016/j.tetlet.2007.05.016

Tetrahedron Letters 48 (2007) 4687–4690
A synthetic route to enediyne-bridged amino acids
a,
b
*
´
Ivanka Jeric and Hueih-Min Chen
a
-
Division of Organic Chemistry and Biochemistry, Ruder Bosˇkovic´ Institute, PO Box 180, Zagreb, Croatia
bInstitute of Bioagricultural Sciences, No. 128, Academia Road, Section 2 Nankang, Taipei, Taiwan
Received 13 September 2006; revised 26 April 2007; accepted 2 May 2007
Available online 10 May 2007
Abstract—A general strategy for the preparation of enediyne-bridged amino acids has been disclosed. A cross-coupling reaction
between amino- and carboxyl-modified amino acids under Sonogashira reaction conditions gave a protected enediyne–peptide con-
jugate. The model obtained can be used as a template in studies of Bergman cycloaromatization or peptide conformational prefer-
ences induced by the presence of the enediyne moiety.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
ation. On the other hand, rigidity of the enediyne unit is
expected to influence peptide conformation. Although
syntheses of enediyne moieties with amino- and carboxyl
functional groups (enediynyl amino acids) have been
reported,14 we were interested in finding a general
procedure for modification of any amino acid at the N-
and C-terminus in a way suitable for the construction
of an aliphatic or aromatic type of enediyne moiety
(Fig. 1). This Letter reports the first example of C!N
enediyne-bridged dipeptide unit.
Structural diversity in natural products represents an
invaluable source of inspiration for organic and medici-
nal chemists in their attempts to create first-line drug
candidates. Naturally occurring enediyne anticancer
antibiotics possess an exceptional biological profile due
to their unique molecular structure, striking mode of
action and high potency.1,2 They are among the most
effective anticancer drugs acting as DNA cleaving
agents. On-site triggered Bergman cycloaromatization
and subsequent proton abstraction from sugar phos-
phate skeletons leads to scission of the DNA double
helix.3,4 There is a growing body of research, theoretical
and experimental, dedicated to the design and synthesis
of structurally less demanding and more selective mod-
els of enediyne antibiotics, with focus on three main
components: the active ‘warhead’,5 the delivery sys-
tem6,7 and the triggering ‘device’.8 The enediyne moiety
has been embedded within differently sized rings,9,10 and
activated through ligand-to-metal charge transfer11 or
photochemical triggering.12 Also, enediynes equipped
with a polyamine module as the DNA binding group
showed potent DNA damaging ability.13 All these find-
ings prompted us to consider amino acids and peptides
as enediyne carriers. Installing the enediyne functional
group into the peptide framework is expected to intro-
duce interesting properties to novel enediyne–peptide
adducts. Peptides can increase solubility, alter acid–base
properties and provide binding sites for metal complex-
The synthetic approach for amino group modification
was examined on the commercially available lysine
derivative Boc-Lys[Z(2-Cl)]–OH. To avoid undesired
side-reactions, the carboxyl group was converted into
an ester group (compound 1, Scheme 1). The Boc group
was removed and the compound obtained subjected to
the next reaction without purification. To increase the
amino group reactivity, a 2-nitrobenzenesulfonyl (oNbs)
NH
HN
O
R
R
O
HN
*
Corresponding author. Tel.: +385 1 4560 998; fax: +385 1 4680
195; e-mail: ijeric@irb.hr
Figure 1.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.05.016

4688
I. Jeric´, H.-M. Chen / Tetrahedron Letters 48 (2007) 4687–4690
O
O
tants, and found optimal conditions for the construction
of the chloroenyne derivative of lysine 5 (Scheme 2).
Generally, cis-dichloroethene, CuI and Pd catalyst were
dissolved in diethylamine and stirred at room tempera-
ture under argon for 15 min. A solution of acetylene 4
in diethylamine was added dropwise and the reaction
mixture stirred for further 3 h. The chloroenyne deriva-
tive of lysine 5 was obtained in 47% yield after the col-
umn chromatography. The second part of an enediyne
bridge was prepared from Boc-Phe–OH by converting
the carboxyl group into a propargylamide group
(Scheme 2), amide 6 was obtained in 78% yield using
the mixed anhydride method. The intermolecular Pd–
Cu catalyzed cross-coupling of terminal acetylene 6
BocHN
BocHN
BOP, HOBt in DMF
OH
O
NMM, EtOH (93%)
NHZ(2-Cl)
NHZ(2-Cl)
Boc-Lys[Z(2Cl)]-OH
1
1. 90% TFA
2. oNbs-Cl, TEA in DCM (65%)
TMS
O
O
oNbs
Br
HN
oNbs
N
O
O
TMS
O
K2CO3 in DMF (90%)
O
BocHN
OH
oNbs
N
NHZ(2-Cl)
O
NHZ(2-Cl)
3
2
Br
K2CO3 in DMF (87%)
Boc-Phe-OH
TBAF in THF (50%)
NHZ(2-Cl)
4
cis ClCH=CHCl
Cl
NH2
O
(PPh3)3PdCl2,
CuI in DEA (47%)
NMM in THF (78%)
oNbs
N
O
O
O
oNbs
N
O
BocHN
N
H
NHZ(2-Cl)
4
Scheme 1.
NHZ(2-Cl)
(PPh3)3PdCl2,
CuI in DEA (25%)
6
5
group was introduced by reaction with oNbs–Cl at
room temperature in the presence of TEA to afford 2
in 65% yield. N-Alkylation of 2 with trimethylsilylprop-
argyl bromide in the presence of K2CO3 gave 3 in 90%
yield. Attempts to remove the TMS group in 3 with a
1:1 mixture of THF and methanol containing a few
drops of 1 M NaOH15 revealed that this deprotection
step proceeds slowly and was incomplete; hydrolysis of
the ester bond also took place. An alternative procedure
involved tetrabutylammonium fluoride (TBAF) in THF,
which gave secondary amine 4 in an improved 50%
yield. To simplify the reaction scheme, N-terminal acti-
vated lysine derivative 2 was treated with propargyl bro-
mide under the same conditions as described for the
preparation of 3, and the N-alkylated derivative 4 was
obtained in 87% yield.
oNbs
NH
N
O
O
NHBoc
O
7
HNZ(2-Cl)
1. HSCH2CH2OH, DBU in DMF
2. 90% TFA
3. NaOH in MeOH (30%)
Syntheses of enediyne moieties are well described in the
literature. Generally, the cross-coupling reaction of sp2-
C halides and terminal acetylenes occurs through Pd–Cu
catalyzed Sonogashira reaction.16 However, a number
of examples show that the actual reaction conditions
strongly depend on characteristics of the aryl or alkenyl
halide and acetylene compound involved in the cross-
coupling reaction.17 We have examined a number of dif-
ferent approaches, varying the type and quantity of
base, solvent, catalyst and order of addition of reac-
NH
HN
O
O
NH2
OH
8
H2N
Scheme 2.

I. Jeric´, H.-M. Chen / Tetrahedron Letters 48 (2007) 4687–4690
4689
and chloroenyne 5 was carried out under the same con-
ditions as described for 5, and the enediyne-bridged Phe-
Lys motif was obtained in 25% yield after the column
chromatography. Compound 7 can be further used as
a template in peptide chain elongation reactions or for
studying the Bergman cyclization. We ran a deprotec-
tion scheme to test the susceptibility of various groups
toward certain deprotection methods. The procedure
was performed in a stepwise manner without isolation
of intermediate products, however, the success of each
step was checked by NMR analysis. Cleavage of the
oNbs group was achieved with DBU/2-sulfanylethanol
in DMF, through a Meisenheimer complex with subse-
quent loss of SO2 and liberation of the amine. The next
step was the cleavage of the acid labile Boc and Z(2-Cl)
groups by treatment with 90% TFA. Finally, base-cata-
lyzed ester hydrolysis gave the deprotected Phe-Lys
bridged enediyne 8 in 30% overall yield (Scheme 2).
the initial compounds, the H110 protons adjacent to the
phenylalanine amide nitrogen appeared at 4.13 ppm,
while the diastereotopic H880 protons attached to the
lysine sulfonamide nitrogen occurred at 3.98 and
4.10 ppm. The vinyl protons H4 and H5 were apparent
at 6.15 ppm. In deprotected compound 8 only the H880
protons proximal to the free amino group were shifted
up-field due to the absence of the deshielding effect of
the oNbs group.
In summary, we have reported general route for the syn-
thesis of enediyne-bridged amino acid units. The proce-
dure presented here will be used for the synthesis of
peptide-based enediynes for the study of Bergman cyclo-
aromatization and additionally for studying peptide
conformational preferences induced by the presence of
the enediyne moiety. The results of these studies will
be published separately, however a preliminary data
on the reactivity of compound 7 in the Bergman reaction
is provided here. Treatment of 7, dissolved in methanol,
with 100 equiv of 1,4-cyclohexadiene (1,4-CHD) under
anaerobic conditions in a sealed ampoule at 50 °C
showed the disappearance of 50% of the starting mate-
rial after 24 h (followed by NMR analysis), associated
with the formation of the Bergman product. The reac-
tion conducted under the same conditions at 100 °C
showed complete conversion after 24 h. These initial
data imply that peptide-based enediynes undergo Berg-
man cycloaromatization.
Characterization of all the compounds relied predomi-
nantly on 1D and 2D NMR analysis. Propargylation
of sulfonamide nitrogen in 4 was evident by the appear-
ance of a terminal acetylene proton at 2.26 ppm and dia-
stereotopic CH2 protons at 4.14 and 4.37 ppm (Table 1).
The structure of compound 5 was confirmed by the loss
of the terminal acetylene proton and the appearance of
two signals at 5.80 and 6.35 Hz assigned to the cis-chlo-
roethene moiety. Also, the signal at 5.80 ppm, assigned
to H5 (Table 1) appeared as a weakly resolved td, due
to coupling with the H880 protons achieved through five
bonds. NMR spectra of fully protected enediyne-
bridged Phe-Lys compound
7
were recorded in
2. Synthesis of oNbs-(cis-ClCH@CH–C„C–CH2)-Lys-
[Z(2-Cl)]–OEt (5)
DMSO-d6 and CDCl3. Although the 1H NMR spectrum
in DMSO was better resolved, for comparison purposes
the chemical shifts in CDCl3 are listed in Table 1. As for
cis-Dichloroethene (70 ll, 0.94 mmol), (PPh)3PdCl2
(32 mg, 0.047 mmol) and CuI (10 mg, 0.047 mmol) were
dissolved in diethylamine (1 ml) and stirred for 15 min
under argon. Compound 4 (260 mg, 0.47 mmol) was dis-
solved in diethylamine (3 ml) and added dropwise via
syringe. After 3 h the solvent was evaporated and prod-
uct extracted with ethyl acetate–water. The organic layer
was washed with brine and water and dried over
Na2CO3. After solvent evaporation the residue was puri-
fied by flash column chromatography on silica gel with
petrol ether–ethyl acetate (3:2) as eluent to yield com-
pound 5 as a yellow-brown oil (150 mg, 47%). 1H
Table 1. NMR chemical shifts of the enediyne bridge protons in
compounds 4–8a
4
5
3
6
2
7
1
8
NH
R
N
3
NMR (CDCl3): 1.13 (t, 3H, OEt, JH,H = 7.14 Hz),
O
1.50 (m, 1H, c Lys), 1.54 (m, 1H, d Lys), 1.60 (m, 2H,
c0, d0 Lys), 1.98, 2.04 (m, 2H, bb0 Lys), 3.19 (m, 2H, e
3
Lys), 4.05 (q, 2H, OEt, JH,H = 7.10 Hz), 4.35, 4.58
=
=
Boc-Phe- (7)
H-Phe- (8)
R = oNbs (7)
= H (8)
=
=
-LysZ(2-Cl)-OEt (7)
-Lys-OH (8)
(dd, 2H, H110 chloroenyne, 2JH3,H = 19.00 Hz
5JH,H = 1.35 Hz), 4.68 (dd, 1H, a Lys, Ja,b 9.97 Hz,
3
H110
H3
H4
H5
H6
H880
0
Ja;b 5.10 Hz), 4.86 (br s, 1H, NHe Lys), 5.21 (s, 2H,
3
CH2 Z(2-Cl)), 5.80 (td, 1H, H4 chloroenyne, JH,H
4
5
2.26
4.14
4.37
4.35
4.68
5
7.45, JH,H = 1.35 Hz), 6.35 (d, 1H, H5 chloroenyne,
3JH,H 7.45 Hz), 7.27 (m, 2H, H4,5 Z(2-Cl)), 7.38 (d,
6.35
5.80
3
1H, H5 Z(2-Cl), JH,H = 6.79 Hz), 7.43 (d, 1H, H3
Z(2-Cl), JH,H = 6.69 Hz), 7.62 (m, 1H, H5 oNbs),
3
6
7
3.98
4.13
2.18
6.15
6.15
6.15
6.15
3.98
4.10
3.12
3.30
7.70 (m, 2H, H4,6 oNbs), 8.16 (d, 1H, H3 oNbs,
3JH,H = 7.40 Hz).
Elemental
Anal.
Calcd
for
8
4.10
C27H29Cl2N3 O8S: C, 51.75; H, 4.67; N, 6.71. Found:
C, 51.42; H, 4.81; N, 6.73. Rf = 0.40 (petrol ether–ethyl
acetate 3:2). [a]D 32.5 (c 1.04, MeOH).
a In CDCl3 at room temperature.

4690
I. Jeric´, H.-M. Chen / Tetrahedron Letters 48 (2007) 4687–4690
ˇ
´
3. Synthesis of cis-Boc-Phe–NH(CH2–C„C–CH@CH–
C„C–CH2)(oNbs)-Lys[Z(2-Cl)]–OEt (7)
Mr. Z. Marinic for useful discussions regarding NMR
analysis.
oNbs-(cis-ClCH@CH–C„C–CH2)-Lys[Z(2-Cl)]–OEt (5)
(196 mg, 0.31 mmol) was dissolved in 3 ml of diethyl-
amine together with Pd(PPh3)4 (35.8 mg, 0.031 mmol)
and CuI (5.9 mg, 0.031 mmol) under argon. After
15 min acetylene 6 (100 mg, 0.31 mmol) in 3 ml of dieth-
ylamine was added dropwise. The reaction mixture was
stirred for 5 h at room temperature and then extracted
with ethyl acetate and water. After purification by flash
column chromatography on silica gel by petrol ether–
References and notes
1. Nicolaou, K. C.; Smith, A. L.; Yue, E. W. Proc. Natl.
Acad. Sci. U.S.A. 1993, 90, 5881–5888.
2. Smith, A. L.; Nicolaou, K. C. J. Med. Chem. 1996, 39,
2103–2117.
3. Yang, X.-L.; Wang, A. H.-J. Pharmacol. Ther. 1999, 83,
181–215.
4. Wenk, H. H.; Winkler, M.; Sander, W. Angew. Chem., Int.
Ed. 2003, 42, 502–528.
5. Jones, G. B.; Fouad, F. S. Curr. Pharm. Des. 2002, 8,
2415–2440.
6. Damle, N. K. Expert Opin. Biol. Ther. 2004, 4, 1445–1452.
7. Sinha, S. C.; Li, L.-S.; Miller, G. P.; Dutta, S.; Rader, C.;
Lerner, R. A. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
3095–3099.
8. Suzuki, I.; Uno, S.; Tsuchiya, Y.; Shigenaga, A.; Nemoto,
H.; Shibuya, M. Bioorg. Med. Chem. Lett. 2004, 14, 2959–
2962.
9. Basak, A.; Kar, M.; Mandal, S. Bioorg. Med. Chem. Lett.
2005, 15, 2061–2064.
10. Kar, M.; Basak, A.; Bhattacharjee, M. Tetrahedron Lett.
2006, 47, 117–120.
ethyl acetate 3:2, 70 mg (25%) of product 7 was isolated.
3
1H NMR (DMSO-d6): 0.98 (t, 3H, OEt, JH,H
=
7.03 Hz), 1.28 (s, 9H, CH3 Boc), 1.32 (m, 2H, c, d
Lys), 1.34 (m, 2H, c, d Lys), 1.61, 1.67 (m, 2H, b Lys),
3
2
0
2.72, (dd, 1H,
b
Phe, Jb;b ¼ 13:20 Hz, Ja,b
=
10.31 Hz), 2.92 (m, 5H, b Phe, e Lys, H8 bridge), 3.88
(m, 3H, OEt, a Lys), 4.13 (br s, 2H, H1 bridge), 4.12
(m, 1H, a Phe), 5.08 (s, 2H, CH2 Z(2-Cl)), 6.96 (d, 1H
NHa Phe, Ja,NH = 8.42 Hz), 7.18, 7.25 (d, 2H, H4,5
bridge, JH,H = 6.30 Hz), 7.26 (m, 5H Phe aromatic),
3
3
3
7.33 (t, 1H, NHe Lys, JH,NH = 5.24 Hz), 7.35, 7.46
(m, 4H, Z(2-Cl) aromatic), 7.84 (m, 2H, C4,5 oNbs),
3
7.94 (d, 1H, H6 oNbs, JH,H = 7.42 Hz), 7.99 (d, 1H,
H3 oNbs, 3JH,H = 7.39 Hz), 8.50 (t, 1H, NH Phe amide,
3JH,NH = 4.70 Hz). Elemental Analy. Calcd for
C44H50ClN5O12S: C, 58.18; H, 5.55; N, 7.71. Found:
C, 57.82; H, 5.77; N, 7.43. Rf = 0.20 (petrol ether–ethyl
acetate 3:2). [a]D 28 (c 0.95, MeOH).
11. Bhattacharyya, S.; Pink, M.; Baik, M.-H.; Zaleski, J. M.
Angew. Chem., Int. Ed. 2005, 44, 592–595.
12. Kovalenko, S. V.; Alabugin, I. V. Chem. Commun. 2005,
1444–1446.
13. Suzuki, I.; Shigenaga, A.; Nemoto, H.; Shibuya, M.
Tetrahedron Lett. 2004, 45, 1955–1959.
14. Basak, A.; Bag, S. S.; Bdour, H. M. M. Chem. Commun.
2003, 2614–2615.
15. Boldi, A. M.; Diederich, F. Angew. Chem., Int. Ed. 1994,
33, 468–471.
Acknowledgements
16. Sonogashira, K. J. Organomet. Chem. 2002, 653, 46–49.
17. Alami, M.; Gueugnot, S.; Domingues, E.; Linstrumelle,
G. Tetrahedron 1995, 51, 1209–1220.
This research was financially supported by the Ministry
of Science, Education and Sports (Croatia). We thank