Photochemical oxidation of N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazine derivatives

Article abstract of DOI:10.1515/hc-2014-0202

The photochemical oxidation of N,N-Bis-(tert-butoxycarbonyl)-1,4-dihydropyrazines was investigated by irradiation using a medium-pressure mercury lamp. The main products were isolated, and their structures were determined by spectral methods and single crystal X-ray diffraction analysis. Photooxygenation is suggested to be a [2+2] cycloaddition reaction of oxygen to the double bond.

Full text of DOI:10.1515/hc-2014-0202

Heterocycl. Commun. 2015; 21(2): 83–88
Hong-bo Tan, Xiu-qing Song and Hong Yan*
Photochemical oxidation of N,N-bis-
(
tert-butoxycarbonyl)-1,4-dihydropyrazine
derivatives
Abstract: The photochemical oxidation of N,N-Bis-(tert- [8–11]. The photochemical reactions involving oxygen
butoxycarbonyl)-1,4-dihydropyrazines was investigated were found to result in the oxygenation of nitrogen
by irradiation using a medium-pressure mercury lamp. heterocycles [12–14]. The photochemical oxidation proper-
The main products were isolated, and their structures ties of N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazines
were determined by spectral methods and single crystal 1 were investigated by irradiation with a medium-pres-
X-ray diffraction analysis. Photooxygenation is suggested sure mercury lamp under an oxygen atmosphere. Control
to be a [2+2] cycloaddition reaction of oxygen to the double experiments were designed to study the mechanisms of
bond.
the photooxygenation of compounds 1.
Boc
Keywords: 1,4-dihydropyrazines; cycloaddition; photo-
oxygenation; singlet oxygen.
N
N
R
a: R = H
b: R = Ph
c: R = COOMe
R
Boc
DOI 10.1515/hc-2014-0202
Received December 4, 2014; accepted December 15, 2014
1a-c
Results and discussion
Introduction
The 1,4-dihydropyrazine moiety is a component of flavin
coenzymes and several marine luciferins [1, 2]. The
The photooxygenation of a solution of 1 in acetone in the
presence of oxygen was investigated by irradiation with a
450-W medium-pressure mercury lamp. Acetone is a pho-
tosensitizer that enhances the photochemical reaction
rate of the 1,4-dihydropyrazines [15–18]. After the reaction
was completed, as judged by thin-layer chromatography
1
,4-dihydropyrazine system, with an 8π-electron structure
and two enamine functions, has attracted much interest.
Its antiaromatic character, electron-charge distribution
in the molecule, properties of charge-transfer complexes,
and role in redox-active biological molecules have been
investigated [3–5]. Although the chemical properties of
(
TLC) analysis, the solvent was removed under reduced
pressure, and the residue was subjected to chromatogra-
phy on silica gel using a mixed solvent of petroleum ether
and ethyl acetate (20:3 v/v).
1,4-dihydropyrazines have been studied for many years,
In the photooxygenation of 1a for 6 h, product 2a was
obtained with a yield of 34% (Scheme 1). The formation
of the methylated diol may involve a methyl radical that
is generated by the irradiation of acetone via a Norrish
type I reaction [19]. The initial diol structure is appar-
ently the product derived from an intermediate dioxetane
their photochemical properties have received little atten-
tion. Here, a series of substituted 1,4-dihydropyrazines 1
were synthesized by the reported method [6, 7], and their
photochemical oxidation properties were investigated.
This work is a continuation of our studies of the photo-
chemical properties of heterocyclic compounds, such as
photodimerization and photochemical ring contraction
by reaction with H O as postulated in similar reports by
2
Adam et al. [20, 21]. It can be suggested that H O and
2
methyl radical in the system undergo a reaction with the
dioxetane to yield 2a. This suggestion was substantiated
by control experiments, in which the formation of 2a was
*
Corresponding author: Hong Yan, College of Life Science and
Bio-engineering, Beijing University of Technology, Beijing 100124,
China, e-mail: hongyan@bjut.edu.cn
accelerated with the addition of several drops of H O.
Hong-bo Tan and Xiu-qing Song: College of Life Science and
Bio-engineering, Beijing University of Technology, Beijing 100124,
China
2
When conventional solvents such as CH Cl were used in
2
2
the photooxygenation of 1a, the diol was not found.
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4ꢀꢀꢀꢀꢁꢀH. Tan et al.: Photooxidation N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazines
Boc
N
Boc
N
3b is smoothly transformed to 4b with a yield of 55% upon
further irradiation.
hv
OH
OMe
In the photooxygenation of 1c for 18 h, products 2c
and 5c were obtained with the respective yields of 22%
and 23% (Scheme 3). The formation of 2c can be under-
stood in terms of a similar process as the formation of 2a,
and the formation of 5c is analogous to the formation of
Acetone, air
N
N
Boc
Boc
2a
1a
Scheme 1ꢀ
4
b [26].
Boc
N
Boc
N
Boc
N
These findings form a basis for the suggested unified
C H
C H
hv
Acetone, air
6
O
5
C6H5
6
5
mechanism for the photooxygenation of 1 (Scheme 4). In
the first step, substrate 1 undergoes a reaction with singlet
oxygen by [2+2] cycloaddition that yields dioxetane 3.
Singlet oxygen is generated when triplet oxygen in air
absorbs UV light (wavelengths ꢀ<ꢀ320 nm) [27–31]. The low
energy gap between triplet and singlet oxygen allows for
+
O
O
C H
N
C6H5
N
Boc
C H
5
N CHO
Boc
6
5
6
Boc
1b
3b
4b
Scheme 2ꢀ
In the photooxygenation of 1b for 10 h, products 3b a sensitized excitation that can be achieved by UV light
and 4b were obtained with yields of 22% and 24%, respec- irradiation. The suggestion that singlet oxygen as a criti-
tively (Scheme 2). The dioxetane 3b is apparently the cal intermediate in the photooxygenation reaction of 1
product of the reaction of 1b with O by [2+2] cycloaddi- was verified by flushing the reaction mixture with air or
2
tion, as referred to by Adam et al. [22]. It can be suggested nitrogen. When air was continuously bubbled through the
that the dioxetane is unstable and undergoes decomposi- solution, the formation of 3 was accelerated, and the for-
tion to the carbonyl compound 4b by cleavage of the O-O mation of 3 was abated under a nitrogen atmosphere. The
and C-C bonds. Similar chemistry has been described [23– role of singlet oxygen in this reaction was demonstrated
2
5]. In addition, it was observed that 3b is formed first, by the addition of a quencher, which can capture the
and product 4b is gradually formed as the reaction pro- singlet oxygen, or a photosensitizer, which can convert
gresses. It was also observed that the isolated compound the oxygen molecule to singlet oxygen. The quencher used
Boc
Boc
N
Boc
NH
N
COOMe
COOMe
OH
hv
+
CHO
MeOOC
N
Acetone, air
MeOOC
OMe
MeOOC
N
N
Boc
Boc
Boc
1c
2c
5c
Scheme 3ꢀ
Boc
N
Boc
N
Boc
N
R
O
R
R
hv
OH
O
R
N
R
N
R
N
OMe
Boc
Boc
Boc
1
3
2
O-O
Cleavage
Me
Boc
NH
Boc
Boc
Boc
N
N
R
N
R
O
R
OH
C-N
C-C
Cleavage
H O
2
CHO
Cleavage
O
CHO
O
O
R
N
R
N
R
N
R
N
Boc
Boc
Boc
Boc
5
4
Scheme 4ꢀProposed mechanism for the photooxygenation of 1.
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H. Tan et al.: Photooxidation N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazinesꢂꢁꢀꢀꢀꢂ85
Figure 1:ꢀORTEP diagrams of the crystal structure of 4b.
Figure 2:ꢀORTEP diagrams of the crystal structure of 5c.
was 1,4-diazabicyclo[2.2.2]octane, and the formation of 3 stepwise process involving the homolysis of the perox-
was dramatically suppressed in its presence. By contrast, ide bond to form a biradical intermediate, which then
in the presence of rubrene, a known photosensitizer, the undergoes a reaction with H O to form a hydroxyl com-
2
formation of 3 was considerably accelerated.
pound [32, 33]. The hydroxyl compound is methylated
The formation of 2 from dioxetane 3 is proposed to to give 2. This methylation involves the methyl radical
be an intramolecular electron transfer–mediated cleav- that is generated by irradiation of acetone (Norrish
age of the peroxide. Because the O-O bond is weaker type I) [19]. The formation of 4 is proposed to originate
than the C-C bond, compound 3 is decomposed by a from dioxetane 3 by a stepwise process that involves the
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6ꢀꢀꢀꢀꢁꢀH. Tan et al.: Photooxidation N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazines
homolysis of the peroxide bond to form a biradical inter- N,N-Bis-(tert-butoxycarbonyl)-1,4-dihydropyrazine (1a)ꢁWhite
solid isolated in 43% yield (lit. yield 45% [6]); mp 105–106°C (lit. mp
mediate and a subsequent C-C bond cleavage [32, 34].
1
1
05–106°C[6]); HNMR(CDCl ):δ1.45(s,18H),5.77(s,2H),5.89(brs,2H),
3
Compound 4 is directly converted to product 5 via a C-N
bond cleavage, which is similar to the cleavage of N-boc
oxamates, giving N-boc carbamates [26, 35]. Note that
5.94 (br s, 2H), 6.04 (s, 2H).
N,N-Bis-(tert-butoxycarbonyl)-2,5-diphenyl-1,4-dihydropyra-
compound 4c is unstable and is converted to the final zine (1b)ꢁWhite solid isolated in 69% yield (lit. yield 72% [6]); mp
1
1
88–189°C (lit. mp 188–189°C [6]); H NMR (CDCl ): δ 1.07 (s, 18H), 6.46
product 5c via a C-N bond cleavage. The three-dimen-
sional structures of 4b as E-isomer and 5c as Z-isomer
reflect the stability of 4 by the steric hindrance effect of
the groups on the nitrogen atom. The increased steric
3
(
s, 2H), 7.20–7.35 (m, 10H).
N,N-Bis-(tert-butoxycarbonyl)-2,5-bismethoxycarbonyl-
,4-dihydropyrazine (1c)ꢁWhite solid isolated in 85% yield (lit.
1
interactions between the benzoyl and the phenyl groups yield 89% [7]); mp 155–156°C (lit. mp 155–156°C [7]);
lead to the E-isomer of 4b being much more stable than δ 1.50 (s, 18H), 3.80 (s, 6H), 7.09 (s, 2H).
1
H NMR (CDCl ):
3
the Z-isomer of 4b.
General procedure for the photooxygenation of N,N-Bis-
tert-butoxycarbonyl)-1,4-dihydropyrazines 1
(
Conclusions
Products of the photooxygenation of N,N-bis-(tert-
A solution of 1 (0.32 mmol) in 200 mL of acetone in a quartz vessel
was irradiated using a 450-W medium-pressure mercury lamp. The
butoxycarbonyl)-1,4-dihydropyrazines were isolated, progress of the reaction was monitored by TLC. Af er completion, the
1
13
solvent was removed under reduced pressure, and the residue was
subjected to chromatography on silica gel using a mixed solvent of
petroleum ether and ethyl acetate (20:3 v/v) as an eluent to provide
products 2–5.
and their structures were determined by H NMR,
C
NMR, high-resolution mass spectral (HRMS), and single
crystal X-ray diffraction analysis. The photooxygenation
is suggested to proceed via the intermediary 1,2-dioxetane
derived by [2+2] cycloaddition of singlet oxygen to the Di-tert-butyl 2-hydroxy-3-methoxy-2,3-dihydropyrazine-1,4-di-
1
double bond.
carboxylate (2a)ꢁColorless needles; mp 147–148°C; H NMR (DMSO-
d ): δ 6.31–6.46 (m, 1H, -OH), 5.94–6.14 (m, 2H, ꢀ=ꢀCH), 5.17–5.55
6
(
m, 2H, -CH), 3.20 (d, 3H, -OCH ), 1.46 (s, 9H, -OC(CH ) ), 1.45 (s, 9H,-
3 3 3
OC(CH ) ); C NMR (DMSO-d ): δ 174.5, 152.5, 151.4, 73.0, 71.5, 55.1,
1
3
3
3
6
Experimental
28.3, 28.2. HRMS (ESI). Calcd for C H N O (M+Na ): m/z 353.36651.
Found: m/z 353.36619.
+
1
5
26
2
6
All chemicals were purchased from commercial sources and used
without further purification. TLC was conducted on silica gel 60 1,4-Di-tert-butyl 2,5-dimethyl-2-hydroxy-3-methoxy-2,3-dihydr-
F254 plates (Merck KGaA). Melting points were determined on a opyrazine-1,2,4,5-tetracarboxylate (2c)ꢁColorless needles; mp
1
1
XT-5A digital melting point apparatus and are uncorrected. H NMR 155–156°C; H NMR (CDCl ): δ 7.41 (s, 1H, ꢀ=ꢀCH), 5.42 (s, 1H, -CH),
3
1
3
spectra and C NMR spectra were recorded on a Bruker Avance 4.48 (s, 1H, -OH), 3.84 (s, 3H, -OCH ), 3.80 (s, 3H, -OCH ), 3.37 (s, 3H,
3
3
1
3
4
00 spectrometer at 400 and 100 MHz, respectively, in CDCl or -OCH ), 1.50 (s, 9H, -OC(CH ) ), 1.47 (s, 9H,-OC(CH ) ); C NMR (CDCl ):
3
3
3
3
3
3
3
DMSO-d . HRMS analyses were conducted using a VG 70SE mass δ169.3, 164.3, 152.8, 150.7, 122.2, 109.5, 55.3, 53.7, 51.8, 27.9. HRMS (ESI).
6
+
spectrometer operating in electrospray ionization mode. Irradiation Calcd for C H N O (M+Na ): m/z 469.43867. Found: m/z 469.43429.
1
9
30
2
10
for photochemical reactions was conducted using an Osram HBO
4
50W medium-pressure mercury lamp. The samples were irradiated Di-tert-butyl 1,4-diphenyl-7,8-dioxa-2,5-diazabicyclo[4.2.0]oct-
1
in quartz cuvettes.
3-ene-2,5-dicarboxylate (3b)ꢁWhite needles; mp 153–155°C; H NMR
(
CDCl ): δ 9.13 (s, 1H, ꢀ=ꢀCH), 7.28–7.75 (m, 10H, Ar-H), 6.96 (s, 1H, -CH),
3
13
1
.31 (s, 9H,-OC(CH ) ), 1.23 (s, 9H,-OC(CH ) ); C NMR (CDCl ): δ 170.5,
3
3
3
3
3
1
1
61.0, 151.6, 151.2, 136.1, 135.5, 132.5, 129.1, 128.5, 128.4, 128.2, 125.3,
Preparation of N,N-bis-(tert-butoxycarbonyl)-1,4-dihy-
+
25.2, 123.7, 84.3, 27.5, 27.4. HRMS (ESI). Calcd for C H N O (M+Na ):
2
6
30
2
6
dropyrazine derivatives 1
m/z 489.20016. Found: m/z 489.19871.
Compound 1a was synthesized by the treatment of 1,4-dihydropyra- tert-Butyl (E)-benzoyl(2-(N-(tert-butoxycarbonyl)formamido)-
zine bis(vinyl phosphate) with triethylammonium formate, palla- 2-phenylvinyl)carbamate (4b)ꢁColorless needles; mp 135–136°C;
1
dium acetate, and triphenylphosphine in THF [6]. Compound 1b was
H NMR (CDCl ): δ 9.44 (s, 1H, -CHO), 7.24–7.53 (m, 10H, Ar-H), 6.52
3
1
3
synthesized by a Pd-catalyzed Suzuki-Miyaura coupling reaction of (s, 1H, ꢀ=ꢀCH), 1.36 (s, 9H,-OC(CH ) ), 1.10 (s, 9H,-OC(CH ) ); C NMR
3
3
3 3
1
,4-dihydropyrazine bis(vinyl phosphate) [6]. Compound 1c was syn- (CDCl ): δ 171.3, 162.8, 152.1, 151.3, 135.4, 134.7, 131.8, 130.1, 128.7, 128.5,
3
thesized by the treatment of methyl ester of N-(4-toluenesulfonyl)- 128.3, 128.0, 127.9, 127.1, 27.7, 27.2. HRMS (ESI). Calcd for C H N O
2
6
30
2
6
+
N-(tert-butoxycarbonyl)-α,β-didehydroalanine with DMAP and K CO
(M+Na ): m/z 489.20039. Found: m/z 489.19871. Single crystal X-ray
diffraction of 4b: deposition CCDC-995232 (see next section).
2
3
[
7].
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H. Tan et al.: Photooxidation N,N-bis-(tert-butoxycarbonyl)-1,4-dihydropyrazinesꢂꢁꢀꢀꢀꢂ87
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13
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