A biomimetic synthesis of coelenterazine analogs

Article abstract of DOI:10.1016/S0040-4039(02)00462-8

N-(Trifluoroacetyl)dehydrodipeptides 2-3 were coupled to aminomethylene dimethylacetal derivatives 4-5. The resulting pseudo-tripeptides 6 were stepwise deprotected (carbonyl function (7) then amine function) and in situ cyclized into imidazolopyrazines 1.

Full text of DOI:10.1016/S0040-4039(02)00462-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3161–3164
A biomimetic synthesis of coelenterazine analogs
Ingrid Devillers, Axelle Arrault, Gilles Olive and Jacqueline Marchand-Brynaert*
Unite´ de Chimie Organique et Me´dicinale, Universite´ catholique de Louvain, Place Louis Pasteur, 1, Baˆtiment Lavoisier,
B-1348 Louvain-la-Neuve, Belgium
Received 1 March 2002; revised 6 March 2002; accepted 7 March 2002
Abstract—N-(Trifluoroacetyl)dehydrodipeptides 2–3 were coupled to aminomethylene dimethylacetal derivatives 4–5. The result-
ing pseudo-tripeptides 6 were stepwise deprotected (carbonyl function (7) then amine function) and in situ cyclized into
imidazolopyrazines 1. © 2002 Elsevier Science Ltd. All rights reserved.
Coelenterazine 1a (luciferin) is an imidazopyrazine
derivative involved in the luminescence reactions of
various marine organisms.1 Recently, we demonstrated
that 1a and related compounds (Scheme 1) show high
antioxidative properties in cells submitted to oxidative
stress induced by reactive oxygen species (ROS).2 This
led us to consider imidazopyrazine bicyclic systems as
potential lead structures in medicinal chemistry for the
discovery of new antioxidants.3
methods reported in the literature were based on the
formation of the imidazole ring by cyclization of ade-
quately substituted pyrazines or by reaction of bifunc-
tional reagents with 2-aminopyrazines.4 This strategy
was very fully applied to the synthesis of coelenterazine
1a and other luciferins.5
A biomimetic approach towards luciferin models had
been proposed 30 years ago,6 but remained unexploited
until now. Recent publications on the biosynthesis of
the GFPs (green fluorescent proteins) chromophore7
stimulated our interest in the preparation of com-
Accordingly, we examined different synthetic strategies
for the construction of such molecules. The major
R1
(A)
(B)
NH2
O
R3
NH
+
+
O
O
R3
R3
N
N
HN
HOOC
R1
O
O
R2
R1
N
H
R2
O
H2N
1
R2
(C)
R1
R2
R3
H2N
NH2
a
PhpOH
CH2Ph PhpOH
CH2Ph Ph
O
b
c
d
e
H
N
CH2Ph Ph
CH2Ph Me
CH2Ph Me
H
N
Scheme 1.
* Corresponding author. Tel.: +32 10 47 27 40; fax: +32 10 47 41 68; e-mail: marchand@chim.ucl.ac.be
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00462-8

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I. De6illers et al. / Tetrahedron Letters 43 (2002) 3161–3164
pounds 1 from dehydropeptide precursors via a double
was performed in the presence of N-(3-dimethyl-
aminopropyl)-N%-ethylcarbodiimide (WSC) and N-
hydroxybenzotriazole (HOBT) to yield 6b12 (N-2,2-
dimethoxyethyl a,b-dehydro-N-[N-(trifluoroacetyl)-1-
phenylalanyl]-phenylalaninamide). The Z-configuration
of the CꢀC double bond was unambiguously confirmed
by X-ray diffraction analysis of a monocrystal of 6b.
The corresponding pyridyl derivative 6c12 (N-2-pyridyl-
2,2-dimethoxyethyl a,b-dehydro-N-[N-(trifluoroacetyl)-
intramolecular dehydration (Scheme 1): the building
blocks of this strategy are b-aminoketone A
tected synthetic equivalent) on the one hand, and pyru-
vic derivative B and a-aminoamide C on the other
6 (or pro-
6
6
hand, which condensation gives dehydrodipeptide
compounds8 (2–3 for instance, see Scheme 2). In apply-
ing this scheme, we were interested in the modification
of substituents R1 and R3 of the natural coelenterazine
(1a), and particularly in the introduction of a pyridyl
substituent in position R1 as a potential bioisosteric
group of phenol.
1-phenylalanyl]-phenylalaninamide)
was
similarly
prepared from and 2,2-dimethoxy-2-(4-pyridyl)-
2
ethylamine (5) obtained via the Neber rearrangement of
4-acetylpyridine (E)-oxime tosylate.13
Dehydrodipeptide
2
(a,b-dehydro-N-[N-(trifluoro-
acetyl)-1-phenylalanyl]-phenylalanine; Scheme 2) was
prepared according to known procedures. Briefly, N-
(Boc)-(S)-phenylalaninamide9 was deprotected with
trifluoroacetic acid and then acylated with tri-
fluoroacetic anhydride to furnish N-(trifluoroacetyl)-
phenylalalinamide10 which was condensed with
phenylpyruvic acid.11a The reactivity of the conjugated
acid 2 was first tested with benzylamine: the corre-
sponding benzylamide was isolated in 80% yield by
Deprotection of the carbonyl function was realized, in
moderated yield, by treatment with trimethylsilylbro-
mide (for 6b) or trimethylsilyliodide generated in situ14
(for 6c). The resulting compounds 7b–c15 were next
treated under smooth basic conditions allowing tri-
fluoroacetamide
deprotection
and
subsequent
intramolecular cyclization as controlled by HPLC analy-
sis of the crude mixtures. Imidazopyrazines 1b–c16 were
isolated, in modest yield, after acidification and precipi-
tation of the corresponding hydrochloride salts. Aro-
matic protons, typical of the pyrazine moiety, appeared
using
diisopropylcarbodiimide/hydroxybenzotriazole
(HOBT) as activating agent. Similar reaction with a-
aminoacetophenone hydrochloride failed, whatever the
coupling conditions used: peptide 7 (R1=R3=Ph) was
not formed because dimerization of a-aminoacetophe-
none occurred more rapidly. Therefore, we envisaged
the use of 2-hydroxyphenethylamine as synthetic equiv-
alent of a-aminoacetophenone. The coupling product
with 2 could indeed be obtained (73% yield), but the
subsequent oxidation of the alcohol intermediate into 7
(R1=R3=Ph) failed (tested conditions: MnO2,
KMnO4, CAN, RuCl3, Swern, Dess–Martin). At this
stage, we turned to acetal derivatives 6 as precursors of
7 (Scheme 2). Reaction of 2 with aminoacetaldehyde
dimethyl acetal (4) (commercially available compound)
1
in H NMR at 8.37 l and 8.50 l for 1b and 8.20 l for
1c. The characteristic UV pattern of colenterazine
derivatives was recorded.17
Starting from dehydrodipeptide 311b (a,b-dehydro-N-
[N - (trifluoroacetyl) - 1 - phenylalanyl] - methylalanine;
Scheme 2), we similarly obtained intermediate 6d12 by
coupling with aminoacetaldehyde dimethylacetal (4).
Unfortunately, the same reaction performed with the
corresponding pyridyl derivative 5 led to a very low
yield of precursor 6e, which could not be purified by
column chromatography. Other methods of peptide
coupling were tested without success. Unmasking the
R3
Ph
R3
Ph
i
R1
NHCO
OMe
NHCO
NHCOCF3
HO2C
NHCO
NHCOCF3
MeO
6b-e
H2N
2, R3 = Ph
3, R3 = Me
R1
H
R3
yield
Cmpd
6b
R1
OMe
OMe
Ph
Ph
60 %
73 %
65 %
<10%
N
6c
4, R1 = H
5, R1 =
H
Me
Me
6d
N
N
6e
R3
Ph
ii
iii
R1
1b-d
NHCO
NHCO
NHCOCF3
40-60 %
20-40 %
O
7b-d
Scheme 2. Reagents and conditions: (i) WSC, HOBT, Et3N, THF, 17 h, 20°C; (ii) TMSBr, CHCl3, 20°C, 6 h or TMSI, CHCl3,
50°C, 8 h; (iii) K2CO3, CH3CN–H2O (10:1), 10−3 M solution of precursor, reflux, 4 h then aqueous HCl.

I. De6illers et al. / Tetrahedron Letters 43 (2002) 3161–3164
3163
carbonyl function of 6d as usual (compound 7d15)
followed by basic treatment and acidification gave imi-
dazopyrazine 1d16 in modest yield (Scheme 2). The
Rzeszotarska, B.; Pietrzynski, G.; Kubica, Z. Liebigs
Ann. Chem. 1988, 485.
12. Compounds 6b–d: General procedure:
1
typical pattern of the ethyl substituent was visible in H
To a solution of a,b-dehydro-N-[N-(trifluoroacetyl)-1-
phenylalanyl]-phenylalanine (1.00 g, 2.46 mmol, 1 equiv.)
in THF (20 mL) were added: triethylamine (342 mL, 2.5
mmol, 1.01 equiv.), aminoacetaldehyde dimethylacetal
(273 mL, 2.5 mmol, 1.01 equiv.) and hydroxybenzotria-
zole (339 mg, 2.5 mmol, 1.01 equiv.). The mixture was
stirred at 0°C and N-(3-dimethylaminopropyl)-N%-ethyl-
carbodiimide (472 mg, 2.46 mmol, 1 equiv.) was added.
Then, the mixture was stirred at room temperature dur-
ing the night. The precipitate was filtered and the filtrate
was evaporated under vacuo. The residue was diluted in
CH2Cl2 (50 mL), washed with a solution of 0.1N HCl
(3×15 mL), brine (15 mL) and then dried over MgSO4
and concentrated under vacuo. Column chromatography
on silica gel (CH2Cl2/AcOEt: 75/25) gave compound 6b
(0.70 g, 59% yield).
NMR (t at 1.18 l and q at 3.61 l). The required mass
was observed in APCI (atmospheric pressure chemical
ionization).
We could not improve the synthesis of compounds 1 by
varying the final cyclization conditions. Changing the
sequence of reactions, i.e. amine deprotection and sub-
sequent cyclization in acidic medium, furnished
untractable mixtures. Thus, our attempt to develop a
convergent biomimetic synthesis of pyridyl-substituted
luciferin derivatives was somewhat disappointing: only
compound 1c has been prepared on a small scale. In
our opinion, the classical approach towards imidazo-
pyrazines making use of pre-formed 1,4-pyrazine hete-
rocycles remains the best route.18
Compound 6b: white solid; mp 61.5–62.5°C; Rf 0.25
(SiO2; CH2Cl2–EtOAc, 75:25); 1H NMR (CDCl3, 500
MHz) l 2.94 (dd, J=13.7 Hz, 5.6 Hz, 1H), 3.18 (dd,
J=13.7 Hz, 8.2 Hz, 1H), 3.30 (s, 6H), 3.34 (dd, J=5.8
Hz, 5.2 Hz, 2H), 4.37 (t, J=5.2 Hz, 1H), 4.82 (ddd,
J=8.2 Hz, 8.0 Hz, 5.6 Hz, 1H), 6.69 (t, J=5.8 Hz, NH),
6.88 (s, 1H), 7.12–7.20 (m, 10H), 7.85 (d, J=8.0 Hz,
NH), 8.76 (s, NH); 13C NMR (CDCl3, 125 MHz) l 36.9,
41.2, 53.8, 53.9, 54.8, 102.0, 115.4, 127.1, 127.8, 128.5,
128.6, 129.0, 129.1, 129.3, 132.8, 135.2, 157.2, 165.9,
169.3. Anal. calcd for C24H26F3N3O5 (493.48): C, 58.41;
H, 5.31; N, 8.52%. Found: C, 57.99; H, 5.29; N, 8.4%.
Compound 6c: white solid; mp 96–97.5°C; Rf 0.21 (SiO2;
CH2Cl2–EtOAc, 60:40); 1H NMR (CDCl3, 300 MHz) l
2.87 (dd, J=13.9 Hz, 8.4 Hz, 1H), 3.06 (dd, J=13.9 Hz,
7.7 Hz, 1H), 3.21 (s, 6H), 3.68 (dd, J=14.1 Hz, 5.4 Hz,
1H), 3.81 (dd, J=14.1 Hz, 6.3 Hz, 1H), 4.74 (ddd, J=8.4
Hz, 7.7 Hz, 7.0 Hz, 1H), 6.42 (dd, J=6.3 Hz, 5.4 Hz,
NH), 6.67 (s, 1H), 6.97–7.19 (m, 10H), 7.45 (d, J=6.1
Hz, 2H), 8.06 (d, J=7.0 Hz, NH), 8.39 (d, J=6.1 Hz,
2H), 9.05 (s, NH); 13C NMR (CDCl3, 75 MHz) l 37.1,
43.7, 49.3, 49.4, 55.1, 101.0, 114.3, 122.8, 127.4, 128.0,
128.1, 128.7, 128.8, 129.1, 129.6, 132.8, 135.1, 148.4,
149.6, 157.6, 165.0, 169.3. Anal calcd for C29H29F3N4O5
(570.56): C, 61.05; H, 5.12; N, 9.82%. Found: C, 61.46;
H, 5.46; N, 9.56%.
Compound 6d: colorless oil; Rf 0.21 (SiO2; CH2Cl2–
EtOAc, 60:40); 1H NMR (acetone-d3, 500 MHz) l 1.53
(d, J=7.2 Hz, 3H), 3.14 (dd, J=13.7 Hz, 5.3 Hz, 1H),
3.28 (dd, J=5.5 Hz, 2.4 Hz, 2H), 3.30 (s, 6H), 3.34 (dd,
J=13.7 Hz, 6.4 Hz, 1H), 4.39 (t, J=5.5 Hz, 1H), 4.88
(ddd, J=6.8 Hz, 6.4 Hz, 5.3 Hz, 1H), 6.54 (q, J=7.2 Hz,
1H), 6.99 (t, J=2.4 Hz, NH), 7.24 (m, 1H), 7.31 (m, 2H),
7.35 (m, 2H), 8.75 (s, NH), 8.75 (d, J=6.8 Hz, NH); 13C
NMR (acetone-d3, 500 MHz) l 13.3, 38.0, 42.0, 53.7,
54.0, 56.4, 103.2, 116.8, 127.7, 129.3, 130.2, 130.3, 131.1,
137.6, 157.8, 164.8, 169.5; MS (APCI) m/z=431.2 (M,
15%), 430.2 (M−1, 100%), (C19H24F3N3O5).
References
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3164
I. De6illers et al. / Tetrahedron Letters 43 (2002) 3161–3164
15. Compounds 7b–d: General procedure:
127.7, 128.4, 129.1 (×2), 129.4 (×2), 131.8, 135.1, 164.7,
164.8, 168.8, 196.8; MS (APCI) m/z=386.0 (M, 100%),
387.0 (M+1, 15%) (C17H18F3N3O4).
To a solution of compound 6b (100 mg, 0.2 mmol, 1
equiv.) in CHCl3 (1 mL) was added trimethylsilyl bro-
mide (160 mg, 1.01 mmol, 5 equiv.). The mixture was
stirred for 6 h at room temperature and then hydrolyzed
with H2O (0.5 mL) and diluted with CHCl3 (10 mL). The
organic phase was washed with H2O (3×2 mL), dried
over MgSO4 and concentrated under vacuo. Column
chromatography on silica gel (CH2Cl2–AcOEt, 60:40)
gave compound 7b (51 mg, 55% yield).
16. Compounds 1b,c: General procedure:
To a solution of K2CO3 (15 mg, 0.11 mmol, 1 equiv.) in
H2O (1 mL) and acetonitrile (40 mL) was added a
solution of pseudotripeptide 7b (50 mg, 0.11 mmol, 1
equiv.) in acetonitrile (50 mL). The mixture was stirred
for 4 h under reflux, cooled to room temperature and
acidified with a solution of 1N HCl. White precipitate
was filtered and the filtrate was concentrated under vacuo
to furnish crude compound 1b (15 mg, 40% yield).
Compound 1b (hydrochloride): brown solid; Rf 0.25
(SiO2; hexane–iPrOH, 20:80); 1H NMR (CDCl3, 300
MHz) l 3.84 (s, 2H), 4.23 (s, 2H), 7.20–7.60 (m, 10H),
8.37 (d, J=2.1 Hz, 1H), 8.50 (d, J=2.1 Hz, 1H); UV
(10−4 M; H2O–CH3CN) umax (A) 433 (0.28), 269 (0.47),
207 (0.66).
Compound 7b: yellow solid; mp 83.4–85.2°C; Rf 0.22
(SiO2; CH2Cl2–EtOAc, 60:40); 1H NMR (CDCl3, 500
MHz) l 3.06 (dd, J=13.4 Hz, 5.6 Hz, 1H), 3.20 (dd,
J=13.4 Hz, 8.2 Hz, 1H), 4.13 (d, J=5.8 Hz, 2H), 4.82
(ddd, J=8.2 Hz, 8.0 Hz, 5.6 Hz, 1H), 6.74 (t, J=5.8 Hz,
NH), 7.12 (s, 1H), 7.16–7.33 (m, 10H), 7.40 (d, J=8.0
Hz, NH), 7.99 (s, NH), 9.54 (s, 1H); 13C NMR (CDCl3,
125 MHz) l 37.2, 50.3, 54.8, 115.4, 127.4, 128.7, 128.8,
129.1, 129.2, 129.4, 131.1, 132.7, 135.0, 157.5, 165.6,
169.3, 196.9; MS (FAB+) m/z 448 (C22H20F3N3O4).
Compound 7c: yellow foam; Rf 0.27 (SiO2; CH2Cl2–
Compound 1c (hydrochloride): brown solid; 1H NMR
(CD3OD, 200 MHz) l 3.49 (s, 2H), 4.23 (s, 2H), 7.20–
7.60 (m, 10H), 7.7–7.85 (m, 4H), 8.20 (s, 1H); UV (10−4
M; H2O–CH3CN) umax (A) 398 (0.17), 282 (0.59), 208
(0.72).
1
EtOAc, 30:70); H NMR (CDCl3, 300 MHz) l 2.95–3.30
(m, 2H), 4.72 (d, J=5.2 Hz, 2H), 4.92 (dd, J=8.0 Hz, 7.0
Hz, 1H), 7.0–7.3 (m, 13H), 7.64 (d, J=5.9 Hz, 2H), 8.49
(s, NH), 8.78 (d, J=5.9 Hz, 2H); 13C NMR (CDCl3, 75
MHz) l 37.4, 47.1, 54.8, 115.9, 120.7, 127.5, 128.8, 128.9,
129.1, 129.2, 129.4, 132.7, 135.1, 140.1, 150.8, 151.1,
165.4, 169.3, 194.1; MS (FAB+) m/z 525 (C27H23F3N4O4).
Compound 7d: yellow foam; Rf 0.25 (SiO2; CH2Cl2–
EtOAc, 30:70); 1H NMR (CDCl3, 300 MHz) l 1.70 (d,
J=7.2 Hz, 3H), 3.30–3.50 (m, 2H), 4.31 (d, J=5.4 Hz,
2H), 5.00 (m, 1H), 6.72 (q, J=7.2 Hz, 1H), 6.82 (m, 5H),
7.58 (d, J=7.2 Hz, NH), 7.73 (s, NH), 9.76 (s 1H); 13C
NMR (CDCl3, 75 MHz) l 13.7, 38.0, 50.4, 55.4, 113.7,
Compound 1d (hydrochloride): brown solid; 1H NMR
(CD3OD, 300 MHz) l 1.18 (t, J=7.2 Hz, 3H), 3.32 (s,
2H), 3.61 (q, J=7.2 Hz, 2H), 7.16–7.36 (m, 4H), 7.81 (d,
J=7.2 Hz, 1H), 7.91 (dd, J=7.2 Hz, 1.8 Hz, 1H); MS
(APCI in positive mode) m/z=254.1 (M+1),
(C15H13N3O).
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