Synthesis of 2-amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine

Article abstract of DOI:10.1248/cpb.50.301

2-Amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine (2), a precursor of Watasenia preluciferin (coelenterazine) (1), is widely distributed in marine bioluminescent animals. It was prepared from p-hydroxyphenylglyoxal aldoxime (5) in two steps; by condensation wi

Full text of DOI:10.1248/cpb.50.301

February 2002
Notes
Chem. Pharm. Bull. 50(2) 301—302 (2002)
301
Synthesis of 2-Amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine
Hisae KAKOI
Faculty of Pharmacy, Meijo University, 150 Yagotoyama, Tempaku-ku, Nagoya 468–8503, Japan.
Received September 17, 2001; accepted October 30, 2001
2-Amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine (2), a precursor of Watasenia preluciferin (coelenterazine)
(1), is widely distributed in marine bioluminescent animals. It was prepared from p-hydroxyphenylglyoxal al-
doxime (5) in two steps; by condensation with a-aminophenylpropiononitrile in the presence of TiCl4 in pyridine,
followed by reduction of the resulting N-oxide (6) with Zn–AcOH in CH2Cl2 and produced 2, with an 89% overall
yield. This procedure was linked with the facile one-step preluciferin synthesis reported in the previous paper.
Thus, Watasenia preluciferin (1), frequently required for various chemiluminescent and bioluminescent studies,
was coveniently synthesized in three steps from 5, with a 56% overall yield, overcoming the difficulty of obtaining
it from natural sources.
Key words 2-aminopyrazine derivative; precursor; Watasenia preluciferin; bioluminescent compound
JEOL A-600 (600 MHz) spectrometer with trimethylsilane (TMS) as inter-
nal standard.
8-Benzyl-2-(p-hydroxybenzyl)-6-(p-hydroxyphenyl)imi-
dazo[1,2-a]pyrazin-3(7H)-one (1) from the liver of the squid,
Watasenia scintillans (Japanese name: Hotaruika),1) was first
isolated in 1975 and termed Watasenia preluciferin. It plays a
2-Amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine-1-oxide (6) To a cold
solution (Ϫ5 °C) of p-hydroxyphenylglyoxal aldoxime (5)6) (330 mg, 2
mmol) and a-aminophenylpropiononitrile hydrochloride7) (436 mg, 2.4
key role in the light emission systems of various marine lu- mmol) in pyridine (8 ml) was added TiCl4 (1.89 g, 10 mmol) drop-wise with
minescent organisms2a) such as squids,2b) shrimps,2c) coelen-
vigorous stirring. The mixture was warmed gradually to 20 °C. On comple-
terates2d) and fishes.2e) All specimens contained Watasenia
tion of the reaction (ca. 30 min, judging from TLC, MeOH–CH2Cl2ϭ1 : 10),
the mixture was poured into ice-water, 10% HCl (40 ml) was added to the
mixture and the separated precipitate was taken up in AcOEt. The AcOEt
preluciferin (coelenterazine) (1) or its derivatives together
with 2-amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine (2) and/
or its derivatives as the precursor or the light emitter of their
bioluminescent compounds. Compound 2 was first isolated
as the single chromophor from the jellyfish, Aequorea, by
Shimomura and Johnson3) in 1972 and it was subsequently
synthesized by Goto and colleagues4) from p-methoxyphenyl-
glyoxal aldoxime (3) via N-oxide (4) in three steps. Watase-
nia preluciferin (1) was prepared from its precursor (2)
by cyclization with p-hydroxybenzylglyoxal in an acidic
medium and was converted to various bioluminescent com-
pounds.2b,d,e) Recently, the authors found that p-hydroxy-
phenylpyruvic acid reacted with 2, without any reductive
treatment, to give 1 directly in a one batch reaction process.5)
As part of the work on the simple step preparation of 1, a
more compact synthesis of 2 was examined. Thus, when p-
layer was washed with water and dried over Na2SO4. Removal of the solvent
left a crystalline solid which was washed with ether to afford 6 (551 mg,
94%), mp 230 °C (decomp). This product was sufficiently pure for further
use. A small sample was recrystallized from MeOH to give pale yellow nee-
dles, mp 230—233 °C (decomp). 1H-NMR (DMSO-d6) d: 4.17 (2H, s), 6.80
(2H, d, Jϭ8.4 Hz), 6.95 (2H, s), 7.20 (1H, t, Jϭ7.6 Hz), 7.30 (2H, t,
Jϭ7.6 Hz), 7.35 (2H, d, Jϭ8.0 Hz), 7.76 (2H, d, Jϭ8.4 Hz), 8.56 (1H, s),
9.69 (1H, br s). Anal. Calcd for C17H15N3O2: C, 69.61; H, 5.15; N, 14.33.
Found: C, 69.68; H, 5.23; N, 14.36.
2-Amino-3-benzyl-5-(p-hydroxyphenyl)pyrazine (2)9) To a solution of
6 (293 mg, 1 mmol) in MeOH–CH2Cl2ϭ1 : 30 (60 ml) was added Zn powder
(586 mg) and AcOH (0.18 ml, 3 mmol). After being refluxed for 20 min with
vigorous stirring, the mixture was filtered and the filtrate was diluted with
AcOEt. The system was then washed with H2O and 10% NaHCO3, dried
over Na2SO4 and evaporated to dryness under reduced pressure. Silica gel
column chromatography (MeOH–CH2Cl2ϭ1 : 15) of the residue gave a crys-
talline solid which was crystallized from ether-hexane to give 2-aminopy-
razine (2) (263 mg, 95%) as pale yellow prisms, mp 217—219 °C.2c,4) 1H-
hydroxyphenylglyoxal aldoxime (5),6) in place of 3, was re- NMR (DMSO-d6) d: 4.05 (2H, s), 6.19 (2H, s), 6.79 (2H, d, Jϭ8.8 Hz), 7.18
acted with a-aminophenylpropiononitrile7) in the presence of
TiCl4 in pyridine at Ϫ5 to 20 °C, it gave N-oxide (6) in 94%
yield. On treating 6 with Zn–AcOH in CH2Cl2, N-oxide
(1H, t, Jϭ7.7 Hz), 7.27 (2H, t, Jϭ7.7 Hz), 7.33 (2H, d, Jϭ8.0 Hz), 7.72 (2H,
d, Jϭ8.8 Hz), 8.28 (1H, s), 9.51 (1H, br s).4) Anal. Calcd for C17H15N3O: C,
73.63; H, 5.45; N, 15.15. Found: C, 73.77; H, 5.51; N, 15.20.
was readily removed to afford the desired precursor (2) in
95% yield. Subsequently, 2 was condensed with p-hydroxy-
phenylpyruvic acid, to give 1 (63%) according to the im-
proved method.5) With this, the overall yield of the total syn-
thesis of 1 starting from 5 in three steps was 56.26%. Com-
pound 1 and related luminous analogues, frequently required
for chemiluminescent and bioluminescent studies, are diffi-
cult to obtain from natural sources. The short step synthesis
of 1 described herein has made it possible to easily obtain a
considerable amount of each compound. In addition, this
compact preparative method is generally applicable to the
synthesis of a wide range of luminous imidazopyrazinone
analogues.8a—d)
Experimental
Melting points are uncorrected. 1H-NMR spectra were recorded on a
Fig. 1
e-mail: kakoi@ccmfs.meijo-u.ac.jp
© 2002 Pharmaceutical Society of Japan

302
Acknowledgment The encouragement and support of Professor K.
Vol. 50, No. 2
5) Kakoi H., Inoue S., Heterocycles, 48, 1669—1672 (1998).
Okada in this work are gratefully acknowledged.
6) Karg E., Arch. Pharm., 282, 49—56 (1944).
7) Freifelder M., Hasbrouck R. B., J. Am. Chem. Soc., 82, 696—698
(1960).
References and Notes
1) Inoue S., Sugiura S., Kakoi H., Hashizume K., Goto T., Iio H., Chem.
Lett., 1975, 141—144.
8) a) Keenan M., Jones K., Hibbert F., Chem. Comm., 1997, 323—324;
b) Chen F. Q., Gomi Y., Hirano T., Ohashi M., Ohmiya Y., Tsuji F. I.,
J. Chem. Soc. Perkin Trans. 1, 1992, 1607—1611; c) Usami K., Isobe
M., Tetrahedron, 52, 12061—12090 (1996); d) Saito R., Hirano T.,
Niwa H., Ohashi M., J. Chem. Soc. Perkin Trans. 2, 1997, 1711—
1716.
9) This compound was submitted to the facile one-step cyclization re-
ported in the previous paper.5) That is, a mixture of 2 and excess p-hy-
droxyphenylpyruvic acid (7 equiv.) was heated in dioxane at 130 °C
under gentle refluxing. After 30 min, the solvent was then allowed to
evaporate almost to dryness at 140 °C. The resultant dark resinous
residue was purified by chromatographic work to give 1 in 63% yield.
2) a) Shimomura O., Inoue S., Johnson F. H., Haneda Y., Comp.
Biochem. Physiol., 65B, 435—437 (1980), and references therein; b)
Inoue S., Kakoi H., Okada K., Tanino H., Goto T., Agric. Biol. Chem.,
47, 635—636 (1983), and references therein; c) Inoue S., Kakoi H.,
Goto T., Chem. Comm., 1976, 1056—1057; d) Inoue S., Kakoi H.,
Murata M., Goto T., Shimomura O., Chem. Lett., 1979, 249—252, and
references therein; e) Inoue S., Okada K., Tanino H., Kakoi H., Chem.
Lett., 1987, 417—418.
3) Shimomura O., Johnson F. H., Biochemistry, 11, 1602—1608 (1972).
4) Kishi Y., Tanino H., Goto T., Tetrahedron Lett., 27, 2747—2748
(1972). Overall yield of 2 from 3 in three steps was 57.6%.