Suzuki-Miyaura coupling for general synthesis of dehydrocoelenterazine applicable for 6-position analogs directing toward bioluminescence studies

Article abstract of DOI:10.1016/j.tet.2010.12.001

Synthesis of coelenterazine analogs is in recent demand to supply more luminescent compounds with reasonable stability as substrate for the photoprotein manipulated in a living cells or particular organelle. There are limited methods for the synthesis of

Full text of DOI:10.1016/j.tet.2010.12.001

Tetrahedron 67 (2011) 1150e1157
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
SuzukieMiyaura coupling for general synthesis of dehydrocoelenterazine
applicable for 6-position analogs directing toward bioluminescence studies
*
Wong Phakhodee, Mitsuko Toyoda, Chun-Ming Chou, Nisachon Khunnawutmanotham, Minoru Isobe
Chemistry Department, National Tsing Hua University, 101, 2nd Section, Kuang Fu Road, Hsinchu 30013, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2010
Received in revised form 26 November 2010
Accepted 3 December 2010
Available online 13 December 2010
Synthesis of coelenterazine analogs is in recent demand to supply more luminescent compounds with
reasonable stability as substrate for the photoprotein manipulated in a living cells or particular organelle.
There are limited methods for the synthesis of 6-substituted coelenterazine due to the route and
instability of the compounds under the existing conditions. This paper describes six examples including
SuzukieMiyaura cross coupling reaction with reactive triflate (unstable) and stable tosylate
intermediates of the imidazo[1,2-a]pyrazin-3-one. Five examples of 2-amino-3-benzyl-5-O-Tf-pyrazine
are also discussed. The product coelenterazine analogs are obtained in the form of dehydrocoelenter-
azine, which is the substrate of a squid photoprotein, symplectin.
Keywords:
SuzukieMiyaura coupling
Coelenterazine
Dehydrocoelenterazine
Symplectin
Ó 2010 Elsevier Ltd. All rights reserved.
Pd-mediated cross coupling
1. Introduction
HS-residue of cysteine to form a conjugate adduct 3 as reported by
Isobe’s group (Scheme 1).20e22 In 2008, the active center cysteine
Bioluminescence of marine organisms has widely been recog-
nized among those people, who visit the seashore to observe strong
emission of blue or green light. Coelenterazine 1 has been one of
the most popular substrates for the marine bioluminescent sys-
tems,1 which was found in organisms such as jellyfish Aequoria
victoria,2 sea cactus Cavernularia obesa,3 sea pansy Renilla
reniformis4ꢀ6, deep sea shrimp Oplophorus gracilirostris,7e9 obelin,
Obelia longissima,10 and oceanic squids Symplectoteuthis ouala-
niensis.11,12 Some examples as C. obesa or the tiny squid Watasenia
scintillans uses the corresponding sulfate of 1.3,11e15 Dehy-
drocoelenterazine (DCL) 2, however, is an oxidized chromophore of
coelenterazine 1. But 2 is not luminescent by itself under chem-
iluminescence condition (strong alkaline in dipolar aprotic solvent)
due to the oxidation stage. We have reported that DCL 2 shows
strong bioluminescence as a quite unique chromophore in the
photoprotein, symplectin of the squid, S. oualaniensis (Tobi-Ika,
Japanese name meaning flying squid).16e19 We have elucidated the
chromophore structure as 2 but not 1 due to the fact that only 2
gives the bioluminescence with its apo-symplectin. The molecular
mechanism of the bioluminescent system in symplectin has been
solved in such a way that DCL or its 13C analog binds with an
was determined to be 390-Cys in the 501 amino acids of sym-
plectin.22 Recently, Kuse independently reported that a commer-
cially available photoprotein pholasin, from Pholas dactylus, showed
an increase in the bioluminescence by addition of DCL.23
Synthesis of coelenterazine or dehydrocoelenterazine has been
well established since 1990 until recent 2009.18,20,24e33 This is
largely due to the fact that increase of sample-request has become
much more in order to supply as the substrates for the photo-
proteins, which has been expressed on living cells or organelle
through gene transcription for chemical biology studies.34 A more
stable coelenterazine, a fluorinated-analog, was selected to exhibit
stronger light amount at plant organelle, on which aequorin is often
expressed to monitor increasing amount of Ca2þ ions in the living
cells.35 Besides these applications, fluorinated-analog has been
playing important roles to elucidate the molecular mechanism of
the bioluminescence of symplectin photoprotein.22
Many important syntheses of coelenterazine 1 and its analogs
have been reported by several research groups including ours.
However, most of the synthetic routes are focused on either modi-
fying the aryl group on the 2-position (R1) or introducing only lim-
ited diversified groups at the 6-position (R2) and the 8-position (R3)
starting from 2-aminopyrazine analogs as shown in Scheme 2. This
fact largely depends on the synthetic route, which allows finalizing
the synthesis by condensing aminopyrazine (e.g., G) with ketoal-
dehyde equivalent as shown in Scheme 2. There still exists limitation
* Corresponding author. Tel./fax: þ866 3 573 6494; e-mail address: minoru@
mx.nthu.edu.tw (M. Isobe).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.12.001

W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
1151
Scheme 1. Coelenterazine 1 and dehydrocoelenterazine 2 with its protein bound form substructures with Symplectin.
Scheme 2. General retro-synthetic analysis of coelenterazine analogs.
for synthesizing diversified R2 analogs due to most of the routes,
which starts from introduction of 4-methoxyphenyl group at the R2
from the beginning. Nakamura et al.36 in 2001 and Adamczyk
et al.37,38 in 2003 reported the synthesis of analogs of 1 from 3,
5-dibromo-2-aminopyrazine on the basis of SuzukieMiyaura and
Negishi coupling. In 2009, Knochel reported an elegant synthesis of
1 on the basis of the Pd-mediated multiple cross coupling reactions
starting from 2,5-dichloropyrazine in eight steps.32 These new
synthetic routes donated advantages for synthesizing analogs of R2
and R3 groups, but we still have limitation for diversified synthesis.
Herein we want to report a new route, which enables the easy
synthesis of unstable coelenterazine analogs.
Table 1
Suzuki-Miyaura coupling of 2-aminopyrazin-5-triflate to various aryl analogs
N
NH2
N
NH2
Ar-B(OH)2
Ar
N
TfO
N
boronic acid (2 eq.)
Pd(PPh3)4 (10 mol%)
K3PO4, dioxane, 80 °C
12
9
Entry
eAr
Yield %
In 2004, we reported a new route, which may allow the diversified
synthesisof R2 at the 6-positionof coelenterazines A. Thishadbeenleft
unexplored. Instead of the fact that R2 had to be selectively introduced
at the early stage of the synthesis, the attempted R2 introduction at
a relatively later stage was achieved to provide two kinds of coe-
lenterazine analogs.39 The coupling of 5-O-triflyl-3-benzyl-2-amino-
pyrazine E (X¼OTf) from 4 as a precursor for cross coupling (G from E)
at the corresponding position.39 In this paper, we report more exam-
ples of the crosscouplingreactionwith 2-amino-3-benzyl-5-O-triflate
to form five aromatic compounds as summarized in Table 1.
For the purpose of the long time awaited synthesis route for A
via the direct introduction of the R2 group into imidazopyrazinone
6 (X¼OTf) at the last step of the synthesis for CL, amino-
O-triflylpyrazine E was condensed with the ketoaldehyde to afford
6. The attempted R2 introduction was implemented with six kinds
of aromatic system, and the results are summarized in Table 2. In
addition, the product was not A, but B from C.
S
1
2
12a 83
12b 85
MeO
3
4
5
12c 70
12d 76
12e 72
O

1152
W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
Table 2
Suzuki-Miyaura coupling of dihydroimidazopyrazinone-6-triflate to various aryl
analogs
O
O
N
N
N
N
N
OH
OH
ArB(OH)2
Pd(PPh3)4 (10 mol%)
TfO
N
H
Ar
K3PO4, dioxane
40 °C, ~ 1 h
11
14
Entry
1
Ar
Yield %
Scheme 3. Synthesis of the triflate aminopyrazine 9 and imidazopyrazinone 11.
14a 75
14b 61
14c 93
14d 72
H
F
equivalent 10 was subjected for condensation to obtain imidazo-
pyrazinone 6-O-triflate 11 (Scheme 3).
2
3
4
2.2. Pd-mediated cross coupling reactions with benzyl-
aminopyrazine 5-O-triflate 9
MeO
The 2-amino-3-benzyl-5-O-triflylpyrazine was subjected to the
various aromatic boronic acids [Ar-B(OH)2]. This coupling was first
Me2N
S
achieved with the N-tosylamide-triflate
8 and 4-methox-
yphenylboronate to give aryl aminopyrazine in high yield as
reported prevously.39 Here, we first confirmed that the same SeM-
couplings were achievable between the free aminopyrazin-O-tri-
flate 9 and various aryl boronates, such as the thiophyl, naphtyl,
biphenyl, 4-phenoxyphenyl, and stylenyl in good yields as shown in
Table 1. Incidentally, the aminopyrazinetriflate 9 also showed good
reactivity even in the Sonogashira reaction to give the following
cross coupling product 13.
5
6
14e 67
14f 54
These successful couplings were eventually found to be very
inconvenient for extremely low stability of 6, so that one have to
finish the cross coupling reaction of the triflate 6 (X¼O-Tf) within
24 h to have good yields even if 6 was stored in a freezer at ꢀ30 ꢁC.
To solve such a problem, we prepared the corresponding tosylate 6
(X¼O-Ts) with better stability. So we went back to explore the
coupling of 5-imino-O-tosy-3-benzyl-2-aminopyrazine E (X¼O-Ts)
for the SuzukieMiyaura reaction for variety of the R2 with orga-
noboron reagents. This iminotosylate showed good results having
electron donating and electron withdrawing group attached to the
aromatic ring of the boronates. In 2009, Makarasen and Isobe
reported the palladium-mediated cross coupling reaction with
2-amino-3-benzyl-5-O-tosyl-pyrazine E (X¼OTs, R3¼Bn).31 This
product aminopyrazine G has to be further derivatized via the acid-
2.3. SuzukieMiyaura cross coupling with the imidazo[1,2-a]
pyrazine 6-O-triflate 11
The SeM coupling was also examined with the triflate 11 under
the similar condition as aminopyrazinetriflate case. The yield,
however, was lower with phenylboronate to show only 35% after
heating at 80 ꢁC for 2 h, while starting material disappeared.
Heating at lower temperatures at 50 ꢁC and 40 ꢁC for 1 h afforded
64% and 75%, respectively. This difference was due to the instability
of the triflate 11. So the results of the SeM-coupling with various
aryl boronates were carried out right after the triflate 11 was pre-
pared under the condition at 40 ꢁC for 1 h. However, lower yields
were observed after the storage due to partial decomposition. The
results are summarized as shown in Table 2. The products of the
SeM coupling were not the reduced form as A but the oxidized
form 14 as shown below.
mediated condensation with aryl-a-ketoaldehyde in acetal form F
to obtain the DCL analogs. We now envision that the Suzu-
kieMiyaura reaction could be achieved even in much later stage;
thus, SuzukieMiyaura coupling (SeM-coupling) of imidazopyr-
azinone heterocyclic systems (C or D) at the last step of the syn-
thesis to A or B. We describe the details of the results as follows first
with 6 (X¼O-triflyl) and then with 6 (X¼O-tosyl).
2. Results and discussion
The details of the oxidation are not known why the products 14
were always obtained as dehydrocoelenterazine form either during
the reaction in a reductive elimination step of palladium or after the
reaction due to the air-oxidation during work-up. It may be nec-
essary to monitor the color change during the reaction via the
colorimetric analysis. But this result is not bad for the product 14
being the substrate for the symplectin bioluminescence. On the
other hand, the Sonogashira reaction between 11 and trimethylsi-
lylacetylene, for example, was not successful, although the same
reaction with 9 afforded 13 in 77% yield in two steps. This is also
2.1. Preparation of imidazopyrazinone 6-O-triflate 11
As summarized in the retrosynthesis routes in Scheme 2, the
challenging issue is the synthesis of A from C for providing di-
versified analog syntheses. The necessary triflate 9 was prepared in
accordance to our previous paper; thus, 5-pyrazinone 7 was con-
verted to 8 and N-tosyl group was hydrolyzed to amino-benzyl-
pyrazin-O-triflate 9.39 To this aminopyrazine, a ketoaldehyde

W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
1153
due to the instability of the triflate 11 under the homogeneous basic
condition. So we went on the more stable but less reactive tosylate.
Attempted coupling of the tosylate 19 with methoxyphenyl-
boronate, or pinacol aryl boronates was not quite successful under the
various conditions including the coupling employed in the amino-
pyrazine tosylate.31 Though tosylate 19 was more stable than the
corresponding triflate 11, it is still not stable enough to survive the
basic conditions during the cross coupling. We decided to oxidize the
tosylate 19 in order to compare the stabilityand reactivity (Scheme 5).
2.4. Preparation of imidazopyrazinone 6-O-tosylate 19 and
SuzukieMiyaura cross coupling reaction of the imidazo[1,2-a]
pyrazine 6-O-tosylate 24
O
O
The N-tosylpyrazine O-tosylate 16 was synthesized directly from
the N,N0-ditosyl compound 15 in one-pot through intermediate 7.
Namely, stirring 15 with sodium hydride was followed by addition
of p-toluenesulfonyl chloride to give 16 in 61% yield. Treatment of
this product N,O-ditosyl pyrazine 16 with concd sulfuric acid was
only the method to remove the N,O-ditosyl groups to afford
2-amino-3-benzyl-5-hydroxypyrazine, which was selectively
O-tosylated providing 17 in 69% yield. Condensation of this ami-
nopyrazine 17 with ketoacetal 18 was examined in various solvents
such as N,N-dimethylformamide (DMF), n-butanol/water and
dioxane/water. The best solvent was 1,4-dioxane/water (4:1),
which facilitated the condensation under 10% HCl, and the product
was purified by reverse phase Cosmosil open column chromatog-
raphy to provide the imidazopyrazinone O-tosylate 19 in 52% yield.
The ketoacetal 18 was synthesized during the current works via
an alternative route from phenylacetaldehyde 20. The reason of
developing new route was to avoid utilization of diazomethane as
a carbon elongation reagent. It is, however, a potentially explosive
reagent, so that some research groups tried safer preparation
routes. Adamczyk reported a coupling method between benzyl-
magnesium bromide and ethyl diethoxyacetate.37 Knochel reported
an alternative route by DMSO oxidation of the ketonitrate.32 We
have established the third route, which would be applicable for
analogous compounds as shown in Scheme 4. In this case, one
carbon elongation was achieved via the lithiated dithiane 21, which
was formylated by trapping with N,N-dimethylformamide (DMF).
And manipulation of the protective groups of the carbonyl provided
the ketoacetal 18. The aldehyde group of 22 was subsequently
protected to the corresponding acetal 23. Selective hydrolysis of the
thioketal acetal moietie of 23 was achieved under N-chlor-
osuccinimide and silver nitrate in acetonitrile for a very short re-
MnO2
Et2O/EtOH
N
N
N
N
N
OMe
OMe
TsO
N
H
TsO
79%
19
24
Pd(OAc)2,
RPhos Ligand,
Cs2CO3, MeOH, 93%
O
N
N
N
OMe
BF3K
MeO
MeO
25
26
Scheme 5. Oxidation of 19 to the corresponding 6-O-tosylate of dehydroimidazopyr-
azinone 24 and cross coupling reaction.
Oxidation of 19 was carried out with MnO2, providing O-tosyl
dehydroimidazopyrazine 24 in 79% yield. This compound is much
more stable to indicate a single spot on TLC for a few weeks after
storage in a refrigerator at ꢀ30 ꢁC. Interestingly, when 24 was
subjected to the SuzukieMiyaura cross coupling condition, the
reaction was proceeded in the presence of cesium carbonate in
methanol solvent to yield dehydrocoelenterazine 26 in 93% yield.
3. Conclusion
We have established a successful route to synthesize dehy-
drocoelenterazine analogs at the 6-position at the last step of the
synthesis through the SuzukieMiyaura reaction with the 6-O-tri-
flate and 6-O-tosylate with the arylborone compounds. Due to the
unstable nature of the final coelenterazine-equivalent compounds,
the cross coupling with the tosylate was better implemented with
the oxidized heteroaromatic system through the most stable syn-
thetic intermediates. Further synthetic studies are in progress to
prepare analogs with functional groups at the 8-position of dehy-
drocoelenterazine, which would be useful for the biological studies
with the photoprotein, symplectin, and other luciferases.
action period providing a-keto acetal 18 in high yield.
Ts
N
N
NHTs
N
N
NHTs
1) Conc. H2SO4
NHTs
NaH
TsCl
61%
O
N
H
TsO
O
N
H
2) i-Pr2NEt,
TsCl, 69%
4. Experimental
7
16
15
4.1. General procedures
O
80% dioxane in H2O,
10% HCl, 52%
N
N
NH2
N
N
OMe
UVevis spectra were obtained on a JASCO V-570 spectrometer.
Fluorescence spectra and chemiluminescence spectra were mea-
sured with a JASCO FP-777 spectrometer. IR spectra were recorded
on a JASCO FT/IR 6100 spectrometer. Proton NMR spectra were
recorded on a JEOL GSX 270 for 270 MHz and a JEOL A 400 for
TsO
O
TsO
N
H
O
17
19
O
18
OMe
400 MHz. Chemical shift (
d
) are given in parts per million relative to
3.30) as internal standard and cou-
DMSO-d6 2.49) or CD3OD (d
(
d
1) n-BuLi, -20 °C
O
H
S
CHO
S
2) DMF, 6h
pling constants (J) in hertz. Carbon NMR spectra were recorded on
a JEOL A 400 for 400 MHz and a JEOL A 600 for 600 MHz. Chemical
SH SH
S
S
BF3.OEt2 MeO
CH2Cl2, 12h
91%
MeO
87%
MeO
20
shift (
d
) are given in parts per million relative to DMSO-d6 (
d 45.0)
21
22
or CD3OD (
d
77.0) as internal standard. Low-resolution EI mass
OH
OH, pTsOH
O
S
spectra and FAB mass spectra were measured with a JEOL JMS-700.
High-resolution (HR) mass spectra were measured with a JEOL JMS-
700. Light yields of bioluminescence of reconstructed symplectin
O
NCS, AgNO3
S
PhMe, reflux, 12h
62%
18
MeO
MeCN/H2O, 5 mins
91%
were determined with
a ATTO Luminescencer-PSN AB-2200.
23
Dichloromethane (CH2Cl2) was distilled from calcium hydride.
Pyridine was dried over NaOH pellet and used without distillation.
Scheme 4. Preparation of the 6-O-tosylate 19 and new route of ketoaldehyde 18.

1154
W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
The other solvents were of reagent grade. Analytical thin-layer
chromatography (TLC) was conduced on precoated TLC plates: sil-
ica gel 60 F-254 [E. Merk (Art 5715) Darmstadt, Germany], layer
thickness 0.25 nm. Silica gel column chromatography utilized Silica
Gel 60 (spherical) 40e50 mm [KANTO CHEMICAL CO., INC].
(137 mg, 0.691 mmol) and Pd(PPh3)4 (40 mg, 0.0345 mmol) and
K3PO4 (146 mg, 0.691 mmol) in dioxane (3 mL) was heated to 80 ꢁC
for 2 h under argon atmosphere. The mixture was treated with 1 N
NaOH aq (1 mL) and 30% H2O2 (0.5 mL) for 1 h at room temperature
to oxidize the residual borane. To this mixture, 1 N HCl aq (1 mL)
was added for neutralization. The product was extracted with
AcOEt (ꢂ3), washed with brine and, dried over Na2SO4. After
evaporation, the residue was purified by column chromatography
on silica gel with AcOEt/hexane (1:2) to give aminopyrazine (2d)
(81 mg, 70%) as a pale yellow solid. 1H NMR (CDCl3, 400 MHz),
4.1.1. 8-Benzyl-2-(4-hydroxybenzyl)-3-oxo-3,7-dihydroimidazo[1,2-
a]pyrazin-6-yl trifluoromethanesulfonate (11). A solution of amino-
pyrazinetriflate (9) (200 mg, 0.601 mmol) and ketoaldehyde (10)
(247 mg,1.20 mmol) in 15 mL of 30% water/dioxane was degassed. To
this solution was added 5 mL of 10% HCl. This solution was stirred
underargonatmosphere at80 ꢁC for3 h. Aftercooling, tothissolution
was added water at 0 ꢁC. The reaction mixture was extracted with
AcOEt (ꢂ3). The combined organic layer was washed with brine and
dried over Na2SO4, then concentrated under reduced pressure. Puri-
fication of crude oil by silica gel chromatography provided 118 mg of
coelenterazinetriflate (11) as a yellow solid (41% yield). IR (KBr) nmax
d
4.21 (2H, s, CH2Ph), 4.47 (2H, s, NH2), 7.24e7.37 (7H, m, aromatic),
7.45 (1H, t, J¼7.6 Hz, aromatic), 7.65 (2H, dd, J¼8.3, 1.2 Hz, aro-
matic), 7.69 (2H, d, J¼8.5 Hz, aromatic), 8.02 (2H, d, J¼8.5 Hz, aro-
matic), 8.43 (1H, s, CH-6) ppm. 13C NMR (CDCl3, 150 MHz),
d 41.2,
55.3, 105.7, 119.1, 124.3, 124.4, 127.0, 127.3, 128.6, 129.0, 129.1, 129.9,
132.5, 134.3, 136.8, 137.5, 140.6, 142.7, 151.6, 157.9 ppm. FAB-MS
(NBA) m/z 338 (MHþ). HRMS (FAB/NBA) calcd for C23H20N3
338.1657, found 338.1663 (MHþ).
3365,1566,1514,1454 cmꢀ1.1H NMR (CD3OD, 400 MHz),
d 3.96 (2H, s,
CH2Ph), 4.25 (2H, s, CH2Ph), 6.58 (2H, d, J¼8.2 Hz, PhOH), 6.95 (2H, d,
J¼8.2 Hz, PhOH), 7.07e7.25 (5H, m, 6-Ph), 8.02 (1H, s, CH-5) ppm. 13C
4.1.5. 3-Benzyl-5-(4-phenoxyphenyl)pyrazin-2-amine (12d). A mix-
ture of aminopyrazinetriflate (9) (100 mg, 0.300 mmol) and bo-
ronic acid (128 mg, 0.600 mmol) and Pd(PPh3)4 (35 mg,
0.0300 mmol) and K3PO4 (127 mg, 0.600 mmol) in dioxane (3 mL)
was heated to 80 ꢁC for 2 h. The mixture was treated with 1 N NaOH
aq (2 mL) and 30% H2O2 (1 mL) for 1 h at room temperature to
oxidize the residual borane. To this mixture, 1 N HCl aq (2 mL) was
added for neutralization. The product was extracted with AcOEt
(ꢂ3), washed with brine, and dried over Na2SO4. After evaporation,
the residue was purified by column chromatography on silica gel
with AcOEt/hexane (1:2) to give aminopyrazine (2e) (81 mg, 76%)
NMR (CD3OD, 150 MHz),
d 31.7, 39.1, 106.7, 116.3, 116.4, 119.0, 121.1,
125.5, 127.8, 129.4, 130.2, 130.5, 130.9, 131.2, 133.2, 146.0, 157.0,
190.0 ppm. FAB-MS (NBA) m/z 480 (MHþ). HRMS (FAB/NBA) calcd for
C21H17N3O5F3S 480.0841, found 480.0871 (MHþ).
4.1.2. 3-Benzyl-5-(thiophen-3-yl)pyrazin-2-amine (12a). A mixture
of aminopyrazinetriflate (9) (50 mg, 0.150 mmol) and boronic acid
(40 mg, 0.300 mmol) and Pd(PPh3)4 (17 mg, 0.0150 mmol) and
K3PO4 (64 mg, 0.300 mmol) in dioxane (1 mL) was heated to 80 ꢁC
for 4 h under argon atmosphere. The product was extracted with
AcOEt (ꢂ3), washed with brine, and dried over Na2SO4. After
evaporation, the residue was purified by column chromatography
on silica gel with AcOEt/hexane (1:2) to give aminopyrazine (2b)
(33 mg, 83%) as a pale yellow solid. 1H NMR (CDCl3, 400 MHz),
as a pale yellow solid. 1H NMR (CDCl3, 400 MHz),
d 4.18 (2H, s,
CH2Ph), 4.46 (2H, s, NH2), 7.04e7.14 (3H, m, Ph), 7.10 (2H, d,
J¼8.8 Hz, Ph), 7.28e7.38 (6H, m, Ph), 7.91 (2H, d, J¼8.8 Hz, Ph), 8.36
(1H, s, CH-6) ppm. 13C NMR (CDCl3, 150 MHz),
d 41.2, 118.9, 123.3,
d
4.08 (2H, s, CH2Ph), 4.36 (2H, s, NH2), 7.16e7.26 (5H, m, Ph), 7.31
127.0, 127.2, 128.5, 129.0, 129.7, 132.5, 136.7, 137.0, 140.6, 142.1, 151.5,
157.1, 157.4 ppm. FAB-MS (NBA) m/z 354 (MHþ). HRMS (FAB/NBA)
calcd for C23H20N3O 354.1606, found 354.1636 (MHþ).
(1H, dd, J¼5.0, 3.0 Hz, thiophene-5), 7.51 (1H, dd, J¼5.0, 1.2 Hz,
thiophene-4), 7.68 (1H, dd, J¼5.0, 3.0 Hz, thiophene-2), 8.20 (1H, s,
CH-6) ppm. 13C NMR (CDCl3, 150 MHz),
d 41.1, 121.1, 125.4, 126.3,
127.0, 128.5, 129.0, 136.7, 137.1, 139.5, 139.7, 140.6, 151.4 ppm. FAB-
4.1.6. (E)-3-Benzyl-5-styrylpyrazin-2-amine (12e). A mixture of
aminopyrazinetriflate (9) (100 mg, 0.300 mmol) and boronic acid
(90 mg, 0.600 mmol) and Pd(PPh3)4 (35 mg, 0.0300 mmol) and
K3PO4 (127 mg, 0.600 mmol) in dioxane (2 mL) was heated to 80 ꢁC
for 6 h. The mixture was treated with 1 N NaOH aq (1 mL) and 30%
H2O2 (1 mL) for 1 h at room temperature to oxidize the residual
borane. To this mixture, 1 N HCl aq (1 mL) was added for neutrali-
zation. The product was extracted with AcOEt (ꢂ3), washed with
brine, and dried over Na2SO4. After evaporation, the residue was
purified by column chromatography on silica gel with AcOEt/hex-
ane (1:2) to give aminopyrazine (2f) (63 mg, 72%) as a pale yellow
MS (NBA) m/z 268 (MHþ). HRMS (FAB/NBA) calcd for C15H14N3
S
268.0908, found 268.0858 (MHþ).
4.1.3. 3-Benzyl-5-(6-methoxynaphthalen-2-yl)pyrazin-2-amine
(12b). A mixture of aminopyrazinetriflate (9) (68.8 mg, 0.207 mmol)
and boronic acid (83 mg, 0.413 mmol) and Pd(PPh3)4 (24 mg,
0.0207 mmol) and K3PO4 (88 mg, 0.413 mmol) in dioxane (1.5 mL)
washeatedto80 ꢁCfor2 hunderargonatmosphere. Themixturewas
treated with 1 N NaOH aq (1 mL) and 30% H2O2 (0.5 mL) for 1 h at
room temperature to oxidize the residual borane. To this mixture,1 N
HCl aq (1 mL) was added for neutralization. The product was
extracted with AcOEt (ꢂ3), washed with brine, and dried over
Na2SO4. After evaporation, the residue was purified by column
chromatography on silica gel with AcOEt/hexane (1:2) to give ami-
nopyrazine (2c) (60 mg, 85%) as a pale yellow solid. 1H NMR (CDCl3,
solid. 1H NMR (CDCl3, 400 MHz),
d 4.16 (2H, s, CH2Ph), 4.44 (2H, s,
NH2), 7.10 (1H, d, J¼15.8 Hz, CH-10), 7.24e7.38 (6H, m, Ph), 7.50 (1H,
d, J¼15.8 Hz, CH-20), 7.54e7.56 (4H, m, Ph), 8.02 (1H, s, CH-6) ppm.
13C NMR (CDCl3, 150 MHz),
d 41.3, 124.8, 126.7, 127.1, 127.7, 128.5,
128.7, 129.0, 129.6, 136.6, 137.1, 139.4, 141.0, 141.2, 151.7 ppm. FAB-
MS (NBA) m/z 288 (MHþ). HRMS (FAB/NBA) calcd for C19H18N3
288.1561, found 288.1507 (MHþ).
400 MHz), d 3.93 (3H, s, OCH3), 4.22 (2H, s, CH2Ph), 4.47 (2H, s, NH2),
7.18e7.35 (7H, m, Phþnaphthalene-1,8), 7.82 (1H, d, J¼8.4 Hz,
naphthalene-4), 7.84 (1H, s, naphthalene-3), 8.06 (1H, dd, J¼8.4,
1.6 Hz, naphthalene-5), 8.34 (1H, d, J¼1.6 Hz, naphthalene-7), 8.50
4.1.7. 3-Benzyl-5-ethynylpyrazin-2-amine (13). To
aminopyrazinetriflate (9) (100 mg, 0.300 mmol), Pd(PPh3)4 (35 mg,
0.0300 mmol), CuI (14 mg,0.075 mmol), and TBS-acetylene (67 l,
a mixture of
(1H, s, CH-6) ppm.13C NMR (CDCl3,150 MHz),
d 41.2, 55.3,105.7,119.1,
124.3,124.4,127.0,127.3,128.6,129.0,129.1,129.9,132.5,134.3,136.8,
137.5, 140.6, 142.7, 151.6, 157.9 ppm. FAB-MS (NBA) m/z 342 (MHþ).
HRMS (FAB/NBA) calcd for C22H20N3O 342.1606, found 342.1590
(MHþ).
m
0.36 mmol) in DMF (5 mL) was added NEt3 (1 mL) under argon
atmosphere. The reaction mixture was stirred at 100 ꢁC for 3 h.
After checking TLC, the reaction mixture was cooled to room
temperature and treated with TBAF (500 ml). The reaction mixture
4.1.4. 3-Benzyl-5-(biphenyl-4-yl)pyrazin-2-amine (12c). A mixture
of aminopyrazinetriflate (9) (115 mg, 0.345 mmol) and boronic acid
was diluted with water and extracted with AcOEt (ꢂ3). The organic
layer was washed with water and brine, dried over Na2SO4. After

W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
1155
evaporation to dryness, purification by column chromatography on
silica gel with AcOEt/hexane (1:2) provided acetylene (13) (58 mg,
77%) as a pale yellow solid. IR (KBr) nmax 3496, 3296, 3175, 1622,
extracted with AcOEt (ꢂ3) and dried over Na2SO4. After evapo-
ration, the residue was purified by preparative TLC to give
dehydrocoelenterazine (14d) (20 mg, 72%) as a purple solid. FAB-
MS (NBA) m/z 449 (MHþ). HRMS (FAB/NBA) calcd for C28H25N4O2
449.1978, found 449.1956 (MHþ).
1522 cmꢀ1
.
1H NMR (CDCl3, 400 MHz),
d
3.20 (1H, s, CeCH), 4.12
(2H, s, CH2Ph), 4.56 (2H, s, NH2), 7.21e7.36 (5H, m, Ph), 8.13 (1H, s,
CH-6) ppm. 13C NMR (CDCl3,150 MHz),
41.2, 77.9, 80.9,127.1,127.3,
d
128.4, 129.1, 135.9, 141.3, 144.6, 152.1 ppm. FAB-MS (NBA) m/z 210
(MHþ). HRMS (FAB/NBA) calcd for C13H12N3 210.1031, found
250.1033 (MHþ).
4.1.12. 8-Benzyl-2-(4-hydroxybenzylidene)-6-(thiophen-3-yl)-imi-
dazo[1,2-a]pyrazin-3(2H)-one (14e). A round bottomed flask was
charged with coelenterazinetriflate (11) (25 mg, 0.0507 mmol),
boronic acid (15 mg, 0.114 mmol), Pd(PPh3)4 (7 mg, 0.00507 mmol),
and K3PO4 (22 mg, 0.114 mmol) and connected to a vacuum/argon
line. The flask was evacuated and then filled with argon, this
evacuation/filling cycle being conducted three times. These re-
agents were dissolved in dioxane (1 mL) and then heated to 40 ꢁC
for 1 h. The mixture was extracted with AcOEt (ꢂ3) and dried over
Na2SO4. After evaporated, the residue was purified by preparative
TLC to give dehydrocoelenterazine (14e) (15 mg, 67%) as a purple
Since all spectroscopic data of 1b, 1c, 1d, 1e were already
reported, just 1H NMR data and MS data were showed.
4.1.8. 8-Benzyl-2-(4-hydroxybenzylidene)-6-phenylimidazo[1,2-a]-
pyrazin-3(2H)-one (14a). A round bottomed flask was charged with
coelenterazinetriflate (11) (25.0 mg, 0.0522 mmol), boronic acid
(14 mg, 0.104 mmol), Pd(PPh3)4 (6 mg, 0.00522 mmol), and K3PO4
(22 mg, 0.104 mmol) and connected to a vacuum/argon line. The
flask was evacuated and then filled with argon, this evacuation/
filling cycle being conducted three times. These reagents were
dissolved in dioxane (1 mL) and then heated to 40 ꢁC for 1 h. The
mixture was extracted with AcOEt (ꢂ3) and dried over Na2SO4.
After evaporation, the residue was purified by preparative TLC to
give dehydrocoelenterazine (14a) (16 mg, 75%) as a purple solid. 1H
solid. IR (KBr) nmax 3604, 1725, 1565 cmꢀ1
.
1H NMR (DMSO-d6,
400 MHz),
d
4.31 (2H, s, CH2Ph), 6.95 (2H, d, J¼8.8 Hz, PheOMe),
7.22e7.50 (5H, m, 6-Ph), 7.51 (1H, s, CHePheOMe), 7.62 (1H, dd,
J¼4.8, 2.8 Hz, thiophene-5), 7.70 (1H, dd, J¼4.8, 0.8 Hz, thiophene-
4), 7.92 (1H, dd, J¼4.8, 2.8 Hz, thiophene-2), 8.10 (1H, s, CH-5), 8.31
(2H, d, J¼8.8 Hz, PheOMe) ppm. 13C NMR (DMSO-d6, 150 MHz),
NMR (CDCl3, 400 MHz),
d
4.20 (2H, s, CH2Ph), 6.93 (2H, d, J¼8.4 Hz,
d 38.9, 110.8, 116.4, 121.9, 125.1, 125.6, 127.3, 128.3, 129.5, 130.5,
PheOH), 7.49e7.25 (10H, m, 6-PhþPh), 7.75 (1H, s, CH-5), 7.94 (2H,
d, J¼8.4 Hz, PheOH), 8.39 (1H, s, CHePheOH) ppm. FAB-MS (NBA)
m/z 406 (MHþ). HRMS (FAB/NBA) calcd for C26H20N3O2 406.1556,
found 406.1561 (MHþ).
133.8, 135.6, 136.2, 136.7, 138.0, 140.2, 157.3, 161.7, 165.9 ppm. FAB-
MS (NBA) m/z 412 (MHþ). HRMS (FAB/NBA) calcd for C24H18N3O2S
412.1120, found 412.1075 (MHþ).
4.1.13. 8-Benzyl-2-(4-hydroxybenzylidene)-6-styrylimidazo[1,2-a]-
pyrazin-3(2H)-one (14f). A round bottomed flask was charged with
coelenterazinetriflate (11) (25 mg, 0.0507 mmol), boronic acid
(15 mg, 0.114 mmol), Pd(PPh3)4 (7 mg, 0.00507 mmol), and K3PO4
(22 mg, 0.114 mmol) and connected to a vacuum/argon line. The
flask was evacuated and then filled with argon, this evacuation/
filling cycle being conducted three times. These reagents were
dissolved in dioxane (1 mL) and then heated to 40 ꢁC for 1 h. The
mixture was extracted with AcOEt (ꢂ3) and dried over Na2SO4.
After evaporated, the residue was purified by preparative TLC to
give dehydrocoelenterazine (14f) (14.0 mg, 54%) as a purple solid.
IR (KBr) nmax 3552, 1710, 1562, 1513, 1405 cmꢀ1. 1H NMR (DMSO-d6,
4.1.9. 8-Benzyl-6-(4-fluorophenyl)-2-(4-hydroxybenzylidene)-imi-
dazo[1,2-a]pyrazin-3(2H)-one (14b). A round bottomed flask was
charged with coelenterazinetriflate (11) (30 mg, 0.0626 mmol),
boronic acid(17 mg, 0.125 mmol), Pd(PPh3)4 (10 mg, 0.00626 mmol),
and K3PO4 (27 mg, 0.125 mmol) and connected to a vacuum/argon
line. The flask was evacuated and then filled with argon, this evacu-
ation/filling cycle being conducted three times. These reagents were
dissolved in dioxane (1 mL) and then heated to 40 ꢁC for 1 h. The
mixture was extracted with AcOEt (ꢂ3) and dried over Na2SO4. After
evaporation, the residue was purified by preparative TLC to give
dehydrocoelenterazine (14b) (16 mg, 61%) as a purple solid. FAB-MS
(NBA) m/z 424 (MHþ). HRMS (FAB/NBA) calcd for C26H19N3O2F1
424.1461, found 424.1490 (MHþ).
400 MHz),
d
4.37 (2H, s, CH2Ph), 6.76 (2H, d, J¼8.4 Hz, PheOMe),
7.17e7.54 (10H, m, Ph), 7.33 (1H, d, J¼14.3 Hz, CH-10), 7.44 (1H, d,
J¼14.3 Hz, CH-20), 7.59 (1H, s, CH-5), 7.71 (2H, d, J¼8.4 Hz, PheOMe)
4.1.10. 8-Benzyl-2-(4-hydroxybenzylidene)-6-(4-methoxyphenyl)-
imidazo[1,2-a]pyrazin-3(2H)-one (14c). A round bottomed flask was
charged with coelenterazinetriflate (11) (30 mg, 0.0626 mmol), bo-
ronic acid (19 mg, 0.125 mmol), Pd(PPh3)4 (10 mg, 0.00626 mmol),
and K3PO4 (27 mg, 0.125 mmol) and connected to a vacuum/argon
line. The flask was evacuated and then filled with argon, this evacu-
ation/filling cycle being conducted three times. These reagents were
dissolved in dioxane (1 mL) and then heated to 40 ꢁC for 1 h. The
mixture was extracted with AcOEt (ꢂ3) and dried over Na2SO4. After
evaporation, the residue was purified by preparative TLC to give
dehydrocoelenterazine (14c) (24 mg, 93%) as a purple solid. FAB-MS
(NBA) m/z 436 (MHþ). HRMS (FAB/NBA) calcd for C27H22N3O3
436.1661, found 436.1673 (MHþ).
ppm. 13C NMR (pyridine-d5, 100 MHz),
d 39.9, 113.4, 116.8, 117.4,
123.0,124.0,124.3,126.7,127.0,127.1,127.2,128.1,128.8,129.2,130.3,
130.4, 132.7, 133.4, 137.0, 137.4, 137.7, 158.1, 163.4, 166.0 ppm. FAB-
MS (NBA) m/z 432 (MHþ). HRMS (FAB/NBA) calcd for C28H22N3O2
432.1712, found 432.1666 (MHþ).
4.1.14. 6-Benzyl-5-(4-methylphenylsulfonamido)pyrazin-2-yl 4-meth-
ylbenzenesulfonate (16). To a solution of sodium hydride (31 mg,
0.65 mmol) in tetrahydrofuran (5 mL) was added di-tosylamidine
15 (100 mg, 0.20 mmol) dissolved in tetrahydrofuran (10 mL) at 0 ꢁC
and then continuously stirred for 45 min. The resulting mixture was
slowly added tosyl chloride (41 mg, 0.215 mmol) dissolved in tet-
rahydrofuran (5 mL) at 0 ꢁC and then raised a temperature to
refluxing condition for 2 h. The resulting mixture was quenched
with ammonium chloride solution and extracted with dichloro-
methane (3ꢂmL). Combined organic phases were washed with
water and brined, dried over MgSO4, filtered, concentrated under
reduce pressure to give brown solid, which was purified by column
chromatography using 35% ethyl acetate in hexanes as eluant to
furnish ditosylate pyrazine 16 (0.0610 g) in 61% yield. 1H NMR
4.1.11. 8-Benzyl-6-(4-(dimethylamino)phenyl)-2-(4-hydroxybenzyli-
dene)imidazo[1,2-a]pyrazin-3(2H)-one (14d). A round bottomed
flask was charged with coelenterazinetriflate (11) (30 mg,
0.0626 mmol), boronic acid (21 mg, 0.125 mmol), Pd(PPh3)4
(10 mg, 0.00626 mmol), and K3PO4 (27 mg, 0.125 mmol) and
connected to a vacuum/argon line. The flask was evacuated and
then filled with argon, this evacuation/filling cycle being con-
ducted three times. These reagents were dissolved in dioxane
(1 mL) and then heated to 40 ꢁC for 1 h. The mixture was
(CDCl3, 400 MHz),
d 2.38 (3H, s, CH3eTs), 2.43 (3H, s, CH3eTs), 3.99
(2H, s, CH2Ph), 7.02e7.09 (2H, m, Ph), 7.20 (2H, d, J¼8.0 Hz, Ts, Ph),
7.27e7.29 (3H, m, Ph), 7.27 (2H, d, J¼8.4 Hz, Ts Ph), 7.29 (1H, s,

1156
W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
CH]N), 7.65 (2H, d, J¼8.4 Hz, Ts Ph), 7.75 (2H, d, J¼8.0 Hz, Ts Ph),
7.95 (1H, s, NHTs).
To a solution of 2-(4-methoxybenzyl)-1,3-dithiane-2-carbalde-
hyde 22 (1.000 g, 3.73 mmol) in toluene (20 mL), ethylene glycol
(1.0 mL,17.9 mmol) and p-toluenesulfonic acid (6 mg,1 mol %) were
added. The reaction mixture was refluxed for 12 h under Dean-
eStark conditions. The reaction was cooled to room temperature,
quenched by NaHCO3(aq), and extracted with EA (20 mLꢂ3). The
combined organic layers were dried over MgSO4 and evaporated
under reduced pressure to dryness. The crude was purified by
column chromatography (EA/hexane¼1:8) to give 2-(2-(4-
methoxybenzyl)-1,3-dithian-2-yl)-1,3-dioxolane 23 as a colorless
oil (1.094 g, 94%).
To a solution of 2-(2-(4-methoxybenzyl)-1,3-dithian-2-yl)-1,3-
dioxolane (1.000 g, 3.20 mmol) in the mixed solvent of acetonitrile
(100 mL) and water (35 mL) were added N-chlorosuccimide
(2.128 g, 16.0 mmol) and silver nitrate (2.718 g, 16.0 mmol). The
reaction solution was stirred for 5 min then quenched by Na2SO3(aq)
4.1.15. 5-Amino-6-benzylpyrazin-2-ol. Ditosylated pyrazine 16
(350 mg, 0.69 mmol) was added concd Sulfuric acid at 0 ꢁC and
then continuously stirred for 30 min. The resulting mixture was
quenched with water and neutralized to pH¼6. The neutral solu-
tion was extracted with dichloromethane (10ꢂmL). The combined
organic layers were dried over MgSO4, filtered, concentrated under
reduced pressure to give aminopyrazinol in 85% yield. 1H NMR
(CDCl3, 400 MHz),
(5H, m, Ph), 7.40 (1H, s, CH]N Ph), 10.05 (1H, s, OH). 13C NMR
(CDCl3, 100 MHz), 38.0, 126.1, 126.6, 128.3, 128.8, 135.8, 138.5,
d 3.89 (2H, s, CH2Ph), 5.36 (2H, s, NH2), 7.16e7.28
d
147.8, 152.3. Mass spectrum: (EI) m/z (% relative intensity) 202
(Mþþ1, 12), 201 (Mþ, 100), 172 (10), 91 (16). HREIMS calcd for
C11H1N3O1 [MþH]þ: 201.0902. Found: 201.0901.
(40 mL), NaHCO3 (aq) (40 mL), and NaCl
(40 mL), filtered, and
(aq)
4.1.16. 5-Amino-6-benzylpyrazin-2-yl 4-methylbenzenesulfonate(17). To
extracted with EA (100 mLꢂ3). The combined organic layers were
dried over MgSO4 and evaporated under reduced pressure to dry-
ness. The crude was purified by column chromatography (EA/
hexane¼1:5) to give 1-(1,3-dioxolan-2-yl)-2-(4-methoxyphenyl)
ethanone 18 as a colorless oil (1.094 g, 84%).
a
solution of aminopyrazinol (63 mg, 0.31 mmol) dissolved in
dichloromethane (5 mL) was added di-isopropylehtylamine (0.14 mL,
0.36 mmol) and subsequently added tosyl chloride (72 mg, 0.38 mmol)
dissolved in dichloromethane at 0 . A resulting mixture was stirred at
room temperature for 1.5 h. The reaction was quenched with water and
aqueous layer was extracted with dichloromethane (2ꢂmL). The
combined organic phases were washed with water and brine, dried
over MgSO4, filtered, concentrated under reduced pressure to give
a yellow-brown solid, which was washed with cool ethyl acetate to
give O-tosylate-2-aminopyrazine 17 (90.4 mg) in 81% yield. mp
4.1.18. 8-Benzyl-2-(4-methoxybenzyl)-3-oxo-3,7-dihydroimidazo
[1,2-a]pyrazin-6-yl 4-methylbenzenesulfonate (19). Round bottle
flask, flame-dried, charged with 2-aminopyrazine 17 (80 mg,
0.23 mmol) and a-keto acetal 18 (100 mg, 0.45 mmol) was dissolved
with 20% water in dioxane (water to dioxane (1 mL:4 mL)) and sub-
sequently added 10% HCl (2 mL). A resulting mixture was raised
temperature to 75 ꢁC for 3.5 h. The reaction was quenched with ice
water, and extracted with dichloromethane (4ꢂmL). The combine
organic layers were washed with water and brine, dried over MgSO4,
filtered, and concentrated under reduce pressure to give brown oil,
which waspurified bycolumnchromatography using 2%methanolin
dichloromethane to provide coelenterazine 19 (60 mg) in 52% yield.
166.7e167.3 . IR (CHCl3) nmax 3444, 3312, 1644, 1177 cmꢀ1
.
1H NMR
(CDCl3, 400 MHz), 2.41 (3H, s, CH3eTs), 3.82 (2H, s, CH2Ph), 6.53 (2H,
d
s, NH2), 7.04 (2H, d, J¼8.0 Hz, Ph), 7.17e7.25 (3H, m, Ph), 7.41 (2H, d,
J¼8.4 Hz, Ts Ph), 7.698 (1H, s, CH]N), 7.705 (2H, d, J¼8.4 Hz, Ts Ph). 13C
NMR (CDCl3, 100 MHz),
d 21.2, 37.6, 126.3, 128.1, 128.2, 128.8, 130.0,
132.4, 132.7, 137.0, 138.1, 143.4, 145.4, 153.2. Mass spectrum: (EI) m/z (%
relative intensity) 357 (Mþþ2, 4), 356 (Mþþ1, 13), 355 (Mþ, 55), 200
(96), 172 (61), 130 (90), 91 (100). HREIMS calcd for C18H17N3O3S
[MþH]þ: 355.0991. Found: 355.0991 Anal. Calcd for C18H17N3O3S: C,
60.83; H, 4.82; N, 11.82. Found: C, 60.98; H, 4.98; N, 11.83.
1H NMR (CDCl3, 400 MHz),
d 2.32 (3H, s, CH3eTs), 3.49 (3H, s,
OCH3ePh), 3.69 (2H, s, CH2Ph), 4.04 (2H, s, CH2Ph), 6.32 (2H, d,
J¼8.4 Hz, Ph), 6.58 (2H, d, J¼8.4 Hz, Ph), 6.96e7.00 (5H, m, Ph), 7.13
(2H, d, J¼8.4 Hz, Ts Ph), 7.17 (1H, s, NH), 7.56 (1H, s, C]CHeN), 7.65
4.1.17. 1-(1,3-Dioxolan-2-yl)-2-(4-methoxyphenyl)ethanone
(18). This compound was prepared from methoxyacetaldehyde in
4 sequential steps in total 63% yield as follows: To a solution of 2-
(2H, d, J¼8.4 Hz, Ts Ph). 13C NMR (CDCl3, 100 MHz),
d 21.7, 29.7, 38.0,
55.0, 105.5, 113.6, 126.8, 128.3, 128.6, 128.8, 129.0, 129.6, 129.7, 133.1,
136.1, 143.8, 144.8, 145.3, 146.9, 158.1.
ꢀ
(4-methoxyphenyl)acetaldehyde 20 (1.000 g, 6.66 mmol), 4 A MS
(6.000 g), and 1,3-propanedithiol (0.8 mL, 7.98 mol) in anhydrous
CH2Cl2 (20.0 mL) under Argon at 0 ꢁC, boron trifluoride ethyl
etherate (3.3 mL, 26.6 mmol) was added. The reaction mixture was
stirred for 2 h at 0 ꢁC, and then was allowed to warm to room
temperature and stirred for 12 h. The reaction mixture was
quenched by NaHCO3(aq), filtered, and extracted with CH2Cl2
(10 mLꢂ3). The combined organic layer was dried over MgSO4 and
evaporated under reduced pressure to dryness. The crude was
purified by column chromatography (EA/hexane¼1:8) to give 2-
4.1.19. (Z)-8-Benzyl-2-(4-methoxybenzylidene)-3-oxo-2,3-dihydro-
imidazo[1,2-a]pyrazin-6-yl 4-methylbenzenesulfonate (24). A solu-
tion of coelenterazine 19 (25 mg, 0.048 mmol) dissolved ether and
ethanol (4 mL:1 mL) was added manganese dioxide (105 ng,
1.20 mmol) at 0 , and then stirred at room temperature for 1 h. The
resulting mixture was filtered through pad Celite and concentrated
under reduce pressure to provide red solid, which was purified by
column chromatography using 2% methanol in dichloromethane to
give dehydrocoelenterazine 24 (19.7 mg) in 79% yield as a red solid.
IR (CHCl3) 1715, 1636, 1586, 1509 cmꢀ1. 1H NMR (CDCl3, 400 MHz),
(4-methoxybenzyl)-1,3-dithiane 21 as
a yellow-green solid
(1.487 g, 93%).
d 2.40 (3H, s, CH3eTs), 3.84 (3H, s, PheOCH3), 4.07 (2H, s, CH2Ph),
To a solution of 2-(4-methoxybenzyl)-1,3-dithiane 21 (1.000 g,
4.16 mmol) in anhydrous THF (10 mL) at ꢀ20 ꢁC, n-butyllithium
(2.5 M, 1.7 mL, 4.25 mmol) was added. After 30 min, anhydrous
DMF (1.6 mL, 20.8 mol) was added, and the reaction mixture was
allowed to warm to room temperature and stirred for 6 h. The re-
action was quenched by 10% HCl(aq) (30 mL), then extracted with EA
(20 mLꢂ3). The combined organic layers were dried over MgSO4
and evaporated under reduced pressure to dryness. The crude
product was purified by column chromatography (EA/hexane¼1:8)
to give 2-(4-methoxybenzyl)-1,3-dithiane-2-carbaldehyde 22 as
a colorless oil (0.970 g, 86%).
6.93 (2H, d, J¼8.0 Hz, Ph), 7.18e7.25 (5H, m, Ph), 7.23 (2H, d,
J¼8.0 Hz, Ph), 7.43 (1H, s, C]CHeN), 7.72 (2H, d, J¼8.4 Hz, Ts Ph),
8.18 (2H, d, J¼8.4 Hz, Ts Ph). 13C NMR (CDCl3, 100 MHz),
d 21.8, 39.0,
55.6, 108.6, 114.8, 127.0, 127.2, 128.4, 128.6, 129.6, 129.8, 132.9, 135.5,
136.16, 136.2, 136.7, 141.3, 145.5, 146.5, 157.8, 163.1, 166.0. Mass
spectrum: (EI) m/z (% relative intensity) 513 (Mþ, 13), 359 (Mþþ1,
68), 358 (Mþ, 55), 91 (100). HREIMS calcd for C28H23N3O5S [MþH]þ:
513.1358. Found: 513.1363.
4.1.20. (Z)-8-Benzyl-2-(4-methoxybenzylidene)-6-(4-methoxy-phe-
nyl)imidazo[1,2-a]pyrazin-3(2H)-one (26). Two round bottle flask,

W. Phakhodee et al. / Tetrahedron 67 (2011) 1150e1157
1157
7. Inoue, S.; Kakoi, H.; Goto, T. J. Chem. Soc., Chem. Commun. 1976, 1056e1057.
8. Shimomura, O.; Masugi, T.; Johnson, F. H.; Haneda, Y. Biochemistry 1978, 17,
994e998.
flame-dried, charged with dehydrocoelenterazine 24, potassium
fluoroboronate ester 25, SPhos ligand, Pd(OAc)2, and Cs2CO3 was
dissolved in dry methanol, and subsequently degassed for three
times. The resulting mixture was stirred at 80 ꢁC oil bath for 2 h.
When reaction mixture was become to room temperature, it was
filtered through pad Celite. The resulting solution was dilute with
ethyl acetate (mL) and washed with water followed by brine. The
organic phase was dried over MgSO4, filtered, and concentrated
under reduce pressure to give red oil, which purified by column
chromatography using 2% methanol in dichloromethane to provide
dehydrocoelenterazine 26 in 93% yield. IR (CHCl3) 1713, 1637, 1587,
9. Shimomura, O. J. Biol. Chem. 1995, 10, 91e101.
10. Eremeeva, E. V.; Markova, S. V.; Westphal, A. H.; Visser, A. J. W. G.; van Berkel,
W. J. H.; Vysotski, E. S. FEBS Lett. 2009, 583, 1939e1944.
11. Goto, T.; Iio, H.; Inoue, S.; Kakoi, H. Tetrahedron Lett. 1974, 2321e2324.
12. Inoue, S.; Kakoi, H.; Goto, T. Tetrahedron Lett. 1976, 2971e2974.
13. Tsuji, F. I. Biochim. Biophys. Acta, Biomembr. 2002, 1564, 189e197.
14. Tsuji, F.I. Bioluminescence & Chemiluminescence Progress and Perspectives,
[International Symposium on Bioluminescence & Chemiluminescence], 13th,
Yokohama, Japan, Aug. 2e6, 2004, 2005; pp 35e38.
15. Teranishi, K.; Shimomura, O. Biochim. Biophys. Acta, Gen. Subj. 2008, 1780,
784e792.
16. Takahashi, H.; Isobe, M. Bioorg. Med. Chem. Lett. 1993, 3, 2647e2652.
17. Takahashi, H.; Isobe, M. Chem. Lett. 1994, 843e846.
18. Isobe, M.; Kuse, M.; Yasuda, Y.; Takahashi, H. Bioorg. Med. Chem. Lett. 1998, 8,
2919e2924.
19. Isobe, M.; Kuse, M.; Fujii, T.; Takahashi, H.; Ohshima, K.; Mori, H.; Ahn, J. Y.;
Tsukasa, M. Tennen Yuki Kagobutsu Toronkai Koen Yoshishu 2000, 42nd, 97e102.
20. Kuse, M.; Isobe, M. Tetrahedron 2000, 56, 2629e2639.
21. Fujii, T.; Ahn, J.-Y.; Kuse, M.; Mori, H.; Matsuda, T.; Isobe, M. Biochem. Biophys.
Res. Commun. 2002, 293, 874e879.
22. Isobe, M.; Kuse, M.; Tani, N.; Fujii, T.; Matsuda, T. Proc. Jpn. Acad. Ser. B 2008, 84,
386e392.
23. Kuse, M.; Tanaka, E.; Nishikawa, T. Bioorg. Med. Chem. Lett. 2008, 18,
5657e5659.
1508 cmꢀ1 1H NMR (CDCl3, 400 MHz),
. d 3.82 (3H, s, OCH3), 3.89
(3H, s, OCH3), 4.36 (2H, s, CH2Ph), 6.93 (2H, d, J¼8.8 Hz, Ph), 6.99
(2H, d, J¼8.8 Hz, Ph), 7.23e7.34 (3H, m, Ph), 7.46 (1H, s, C]CHeN,
Ph), 7.56 (2H, d, J¼8.0 Hz, Ph), 7.62 (1H, s, NeC]CH), 7.74 (2H, d,
J¼8.8 Hz, Ph), 8.27 (2H, d, J¼8.8 Hz, Ph). 13C NMR (CDCl3, 100 MHz),
d
39.8, 55.3, 55.5, 109.1, 114.2, 114.7, 121.0, 126.6, 126.8, 127.5, 128.2,
128.4, 127.7,134.4, 134.5, 135.9, 136.7, 137.0, 146.9, 157.7, 159.9, 162.7,
166.8. Mass spectrum: (EI) m/z (% relative intensity) 450 (Mþþ1,
34), 449 (Mþ, 100), 421 (85), 379 (37), 91 (90). HREIMS calcd for
C28H23N3O5S [MþH]þ: 449.1739. Found: 449.1730.
24. Teranishi, K.; Goto, T. Bull. Chem. Soc. Jpn. 1990, 63, 3132e3140.
25. Chen, F.-Q.; Hirano, T.; Hashizume, Y.; Ohmiya, Y.; Ohashi, M. J. Chem. Soc.,
Chem. Commun. 1994, 2405e2406.
Acknowledgements
26. Teranishi, K.; Isobe, M.; Yamada, T. Tetrahedron Lett. 1994, 35, 2565e2568.
27. Thomson, C. M.; Herring, P. J.; Campbell, A. K. J. Mar. Biol. Assoc. U.K. 1995, 75,
165e171.
28. Zheng, J. L.; Chen, F. Q.; Hirano, T.; Ohmiya, Y.; Maki, S.; Niwa, H.; Ohashi, M.
Bull. Chem. Soc. Jpn. 2000, 73, 465e469.
29. Nakamura, H.; Wu, C.; Inouye, S.; Murai, A. J. Am. Chem. Soc. 2001, 123,
1523e1524.
30. Pan, Y.; Adamczyk, M.; Johnson, D.; Mattingly, P.G. and Reddy, R.E. Abstracts of
Papers, 222nd ACS National Meeting, Chicago, IL, United States, August 26e30,
2001, 2001; ORGN-370.
Authors are indebted to National Science Foundation and
National Tsing Hua University for the generous financial supports to
the current studies. Part of the works was contributed for the
earlier stage by the ex-Isobe’s Laboratory in Nagoya University, to
whom thanks are due.
Supplementary data
31. Makarasen, A.; Kuse, M.; Nishikawa, T.; Isobe, M. Bull. Chem. Soc. Jpn. 2009, 82,
870e878.
32. Mosrin, M.; Bresser, T.; Knochel, P. Org. Lett. 2009, 11, 3406e3409.
33. Oba, Y.; Kato, S.-I.; Ojika, M.; Inouye, S. Biochem. Biophys. Res. Commun. 2009,
390, 684e688.
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2010.12.001.
34. Furuichi, T.; Mori, I. C.; Takahashi, K.; Muto, S. Plant Cell Physiol. 2001, 42,
1149e1155.
References and notes
35. Kagenishi, T.; Yokawa, K.; Kuse, M.; Isobe, M.; Bouteau, F.; Kawano, T.; Zeits-
chrift fur Naturforschung, C. J. Biosci. 2009, 64, 411e417.
36. Wu, C.; Nakamura, H.; Murai, A.; Shimomura, O. Tetrahedron Lett. 2001, 42,
2997e3000.
37. Adamczyk, M.; Johnson, D. D.; Mattingly, P. G.; Pan, Y.; Reddy, R. E. Synth.
Commun. 2002, 32, 3199e3205.
38. Adamczyk, M.; Akireddy, S. R.; Johnson, D. D.; Mattingly, P. G.; Pan, Y.; Reddy, R.
E. Tetrahedron 2003, 59, 8129e8142.
39. Kuse, M.; Kondo, N.; Ohyabu, Y.; Isobe, M. Tetrahedron 2004, 60, 835e840.
1. Shimomura, O.; Inoue, S.; Johnson, F. H.; Haneda, Y. Comp. Biochem. Physiol., Part
B: Biochem. Mol. Biol. 1980, 65B, 435e437.
2. Shimomura, O.; Johnson, F. H. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 2611e2615.
3. Shimomura, O.; Inoue, S.; Goto, T. Chem. Lett. 1975, 247e248.
4. Hori, K.; Cormier, M. J. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 120e123.
5. Hori, K.; Wampler, J. E.; Cormier, M. J. J. Chem. Soc., Chem. Commun. 1973,
492e493.
6. Awad, E. W.; Anctil, M. J. Exp. Biol. 1993, 177, 181e200.