Photoinduced electron transfer-promoted debenzylation of phenylalanine and tyrosine derivatives using dicyanoarene

Article abstract of DOI:10.1016/j.tet.2017.11.021

Photoinduced debenzylations of phenylalanine and tyrosine derivatives with dicyanoarenes afford glycine derivatives by the generation of radical cations. Despite the limited substrate scope, the radical cation of phenylalanine and tyrosine derivatives bearing both a carbamate (without an aromatic group) at the N-terminal and an amide at the C-terminal could promote the breaking C–C bond at the benzylic position by a photoinduced electron transfer. It is important to understand the chemical behavior of the radical cations of phenylalanine and tyrosine in enzymes involving electron transfer.

Full text of DOI:10.1016/j.tet.2017.11.021

Accepted Manuscript
Photoinduced electron transfer-promoted debenzylation of phenylalanine and tyrosine
derivatives using dicyanoarene
Mugen Yamawaki, Yoshiki Okita, Takashi Yamamoto, Toshio Morita, Yasuharu
Yoshimi
PII:
S0040-4020(17)31157-2
10.1016/j.tet.2017.11.021
TET 29101
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 23 October 2017
Revised Date: 7 November 2017
Accepted Date: 8 November 2017
Please cite this article as: Yamawaki M, Okita Y, Yamamoto T, Morita T, Yoshimi Y, Photoinduced
electron transfer-promoted debenzylation of phenylalanine and tyrosine derivatives using dicyanoarene,
Tetrahedron (2017), doi: 10.1016/j.tet.2017.11.021.
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ACCEPTED MANUSCRIPT
Photoinduced Electron Transfer-promoted Debenzylation of Phenylalanine and Tyrosine
Derivatives Using Dicyanoarene
Mugen Yamawaki, Yoshiki Okita, Takashi Yamamoto, Toshio Morita, Yasuharu Yoshimi*
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan
E-mail of corresponding author: yyoshimi@u-fukui.ac.jp
Graphical Abstract
R1
CN
DCN;
ν
h
DCN or DCB
Boc
NH2
Boc
NH2
N
H
CN
CN
N
H
CH3CN/H2O=9:1
O
O
Phenylalanine and
Tyrosine derivatives
Glycine derivatives
DCB;
CN
OBoc
O
ν
h
H
O
H
Boc
N
N
DCN
Boc
N
OCH3
N
H
OCH3
H
CH3CN/H2O=9:1
O
O
OBoc
OBoc
Selective debenzylation of dipeptide
Keywords
Photoinduced Electron Transfer-promoted Debenzylation, Phenylalanine and Tyrosine
Derivatives, Peptide, Dicyanoarene, Glycine Derivatives
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Abstract
Photoinduced debenzylations of phenylalanine and tyrosine derivatives with dicyanoarenes
afford glycine derivatives by the generation of radical cations. Despite the limited substrate
scope, the radical cation of phenylalanine and tyrosine derivatives bearing both a carbamate
(without an aromatic group) at the N-terminal and an amide at the C-terminal could promote the
breaking C–C bond at the benzylic position by a photoinduced electron transfer. It is important
to understand the chemical behavior of the radical cations of phenylalanine and tyrosine in
enzymes involving electron transfer.
Introduction
Redox-active tyrosine containing a phenol moiety plays an important role in long-distance
electron transfers in several enzymes.1 For example, the tyrosine residue in photosystem II can
aid electron transfer between the primary chlorophyll donor and the manganese cluster.2 In this
case, perturbations of the peptide bond are observed by the oxidation of tyrosine to form
reactive species, such as a radical cation and the resulting radical after deprotonation of the
radical cation in the phenol moiety.3 Thus, the chemical behavior of the radical cations of
tyrosine and its derivatives requires further investigation. Although detailed spectroscopic
studies of these reactive species in the gas phase4, modified system5, and in vitro6 have been
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investigated, studies on chemical reactions involving the reactive species are rare. In particular,
it is important information for modification of peptides by photoinduced electron transfer
(PET).7
Recently, we reported the decarboxylative radical reactions of N-protected amino acids and
peptides initiated by PET.8 When assessing the influence of amino acid side chains on
photoinduced decarboxylation, we observed that photoreactions of the peptide-containing
tyrosine do not take place efficiently and are quenched by the phenol moiety through PET.9 This
finding encouraged us to examine the reactivity of the radical cations of tyrosine and
phenylalanine derivatives using dicyanoarenes as electron acceptors. We thus identified a new
photoinduced debenzylation reaction of these derivatives to form glycine derivatives.
Results and Discussion
Effects of protecting groups of 1, electron acceptor, and solvent in PET-promoted
debenzylation
The initial experiments were carried out using the PET reactions of N-Boc L-phenylalanine
and
L
-tyrosine amide derivatives 1a–f (Boc
=
tertiary-butoxycarbonyl) with
1,4-dicyanonaphthalene (DCN) as an electron acceptor. Irradiation (100 W high-pressure
mercury lamp, Pyrex vessels (λ > 280 nm, 15 × 180 mm), argon atmosphere, 24 h, room
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temperature) of an aqueous acetonitrile solution (CH3CN/H2O = 9:1) containing N-Boc
L
-phenylalanine amide 1a (10 mM) and DCN (10 mM) resulted in the formation of N-Boc
glycine amide 2a in 25% yield via debenzylation (Entry 1, Table 1) along with almost recovery
of DCN. The photoinduced debenzylation of N’-CH3 derivative 1b (R2 = CH3) provided a
similar product yield of 2b (19%, Entry 2), indicating that the presence of an alkyl group at R2
does not affect the photoreaction. In contrast, the photoreactions of N-Boc L-tyrosine amides
1c,d did not proceed, and the starting material was recovered almost completely (Entries 3 and
4). Introduction of Boc-protecting groups on the phenolic hydroxy group of 1c,d giving 1e,f
enabled efficient PET-promoted debenzylation to afford higher yields of 2a,b (Entries 5 and 6).
These results show that the high electron-donating ability of the aromatic group in 1 causes the
high efficiency of the photoinduced debenzylation, even though the presence of free phenolic
hydroxy group disturbed the photoinduced debenzylation. The use of 1,4-dicyanobenzene
(DCB) instead of DCN with a 120 W low-pressure mercury lamp for the photoreaction of 1b in
quartz vessels for 18 h increased the yield of 2b (Entry 7). The efficiency of photoinduced
debenzylation increased due to the high light absorption of both DCB and 1b at 254 nm when
using quartz vessels and
a
low-pressure mercury lamp. However, the use of
1,2,4,5-tetracyanobenzene, 1-cyanonaphthalene, and benzophenone as electron acceptors with
high- and low-pressure mercury lamps in the photoreaction of 1b did not produce 2b. Moreover,
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the use of dry acetonitrile as a solvent in the photoreaction of 1e yielded traces of 2a, and less
polar solvents such as aqueous 1,4-dioxane, THF, and benzene failed to produce 2a, resulting in
near-quantitative recovery of 1e. Thus, it was confirmed that the photoinduced debenzylation
requires dicyanoarenes as electron acceptors and a polar medium in the presence of water.
Table 1 Photoinduced debenzylation of 1a–f using DCN and DCB
R1
ν
h , 24 h
H
H
DCN (10 mM)
Boc
N
Boc
N
N
R2
N
R2
CH3CN/H2O=9:1
H
H
CN
CN
O
O
2
1
(10 mM)
DCN;
Yield of 2 (%)a Recovery of 1 (%)a
Entry
R1
R2
1
1a
1b
1c
1
2
3
H
H
H
CH3
H
25
19
0
62
70
89
CN
DCB;
OH
1d
CH3
4
OH
0
92
32
41
CN
5
1e OBoc
1f OBoc
1b
H
CH3
CH3
53
47
86
6
7b
trace
H
aIsolated yield. bThe photoreaction of 1b (10 mM) and DCB (10 mM) in an aqueous acetonitrile solution
(CH3CN/H2O = 9:1) with 120 W low-pressure mercury lamp using quarts vessel for 18 h was carried out.
Next, the effects of substituents R3 and R4 in
L-tyrosine(OBoc) derivatives 1g–k on the
efficiency of the photoreaction using DCN were investigated (Table 2). The photoreaction of
Ac-protected tyrosine amide 1g at α-NH2 instead of the Boc group did not proceed at all (Entry
1), and 1h with carbamate protection yielded 2h in the similar yield of 2a (Entry 2). However,
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photoinduced debenzylation of other carbamates 1i,j bearing aromatic groups such as Cbz and
Fmoc groups did not occur (Entries 3 and 4), probably because the aryl groups prevented the
photoreaction. In addition, the photoreaction of N-Boc tyrosine ester 1k (R4 = OCH3) did not
produce 2k at all. Thus, the presence of both carbamate (without an aromatic group) and amide
moieties at the N- and C-terminals, respectively, in 1 was essential for photoinduced
debenzylation.
Table 2 Photoinduced debenzylation of 1g–k using DCN
OBoc
hν, 24 h
DCN (10 mM)
R3
R4
R3
R4
N
N
CH3CN/H2O=9:1
H
H
O
O
2
1
(10 mM)
Yield of 2 (%)a Recovery of 1 (%)a
Entry
R3
Ac
R4
1
1g
NH2
1
2
3
4
5
0
56
0
90
38
85
90
90
CH3OCO NH2
1h
1i
1j
NH2
NH2
Cbz
Fmoc
Boc
0
OCH3
1k
0
aIsolated yield.
Selective PET-promoted debenzylation of dipeptides 3, 5
In order to demonstrate the utility of this photoreaction for the selective debenzylation of
peptides, the photoreactions of dipeptides 3, 5 having L-phenylalanine and L-tyrosine residues
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were examined. The photoinduced debenzylation of 3, 5 proceeded selectively to provide high
yields of debenzylated peptides 4, 6 containing glycine in the first amino acid residue (Scheme
1). This is because the second amino acid residue without carbamate and amide at the respective
N- and C-terminals did not undergo debenzylation. The obtained photoproducts 4, 6 were
confirmed by comparing their NMR spectra with those of compounds synthesized by a reliable
method (coupling between glycine and phenylalanine or tyrosine derivatives with EDC and
DMAP).
O
ν
h , 24 h
H
O
H
DCN (10 mM)
Boc
N
N
OCH3
Boc
N
N
OCH3
H
CH3CN/H2O=9:1
O
H
O
4
(36%)
3
(10 mM)
OBoc
O
O
ν
h , 24 h
H
N
H
N
DCN (10 mM)
Boc
N
OCH3
Boc
N
OCH3
H
CH3CN/H2O=9:1
O
H
O
6
OBoc
(82%)
5
(10 mM)
OBoc
Scheme 1 Selective photoinduced debenzylation of peptides 3, 5.
Fluorescence quenching of DCN by 1e
As PET from DCN to electron-rich aromatic rings such as toluene has been reported,10 the
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fluorescence quenching of DCN by tyrosine 1e was examined. The fluorescence of DCN in the
aqueous acetonitrile solution was efficiently quenched by 1e and not by glycine 2a, as shown in
Figures 1 and 2. The rate constant for fluorescence quenching (kq) of DCN by 1e was calculated
as 9 × 109 M–1s–1 from the Stern–Volmer plot using the equation I0/I = 1 + kqτ[Q] (I0;
fluorescence intensity of DCN at 385 nm without 1e, I; observed fluorescence intensity of DCN
at 385 nm with 1e, τ = 10.1 ns; fluorescence lifetime of DCN, [Q]; concentration of 1e). This
result indicates that PET from the aromatic part of 1e to the excited state of DCN occurred
smoothly to form the radical cation of 1e and the radical anion of DCN.
8000
7000
[1e] = 0 M
-3
6000
×
×
×
×
0.3 10 M
-3
0.6 10 M
5000
4000
3000
-3
0.9 10 M
-3
1.2 10 M
1.5×10-3 M
-3
×
1.8 10 M
2000
1000
0
350
500
550
400
450
Wavelength [nm]
Figure 1. Fluorescence quenching of DCN by 1e excited at 280 nm in an aqueous acetonitrile
solution (CH3CN/H2O = 9:1).
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9000
8000
2a
[ ] = 0 M
-3
7000
6000
5000
4000
×
×
×
×
×
×
0.3 10 M
-3
0.6 10 M
-3
0.9 10 M
-3
1.2 10 M
-3
3000
2000
1000
0
1.5 10 M
-3
1.8 10 M
350
400
450
500
550
Wavelength [nm]
Figure 2. Fluorescence quenching of DCN by 2a excited at 280 nm in an aqueous acetonitrile
solution (CH3CN/H2O = 9:1).
Deuterium incorporation experiment using D2O for the photoreaction of 1e
To gain further insight into the mechanism of the photoinduced debenzylation reaction, we
carried out a deuterium incorporation experiment employing 1e and D2O (Scheme 2). The
results of irradiation-induced reaction of 1e in CH3CN/D2O = 9:1 showed that deuterated
product 7 was obtained in the similar yield (50%) and with a high d-content (86%). This result
indicates that the protonation after the carbanion formation at the α-carbon occurs via
debenzylation.
ν
D
h , 24 h
DCN (10 mM)
Boc
NH2
1e
(10 mM)
N
H
CH3CN/D2O=9:1
O
7
(50%)
d-content: 86%
Scheme 2 Deuterium incorporation experiment using CH3CN/D2O for the photoreaction of 1e.
9

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Plausible mechanism of PET-promoted debenzylation of 1 using DCN
On the basis of the aforementioned results, a plausible mechanism for the photoreaction of 1
is shown in Scheme 3. First, PET between the aromatic part of 1 and the excited state of DCN
by light absorption (>280 nm) generates the radical cations of 1 and the radical anion of DCN.9
The PET-imposed disturbing by aromatic carbamates of 1i,j did not produce 2. Subsequently,
homolysis of the C–C bond in the radical cation of 1 at the benzylic position, assisted by water,
leads to the formation of benzyl alcohol and radical 8 in the cases of 1a,b,e,f,h. Breaking of the
C–C bond of the radical cation of 1 requires both carbamate without an aromatic group and
amide moieties at the respective N- and C-terminals, probably because of the stability of radical
8. Even though a similar homolysis of the radical cation of 1 in the gas phase has been reported
by mass spectrometry analysis,4 a detailed study of the water assisted C–C bond breaking at the
benzylic position is in progress in our laboratory. Attempt to trap radicals such as benzyl radical
and 8 in the photoreaction of 1e (10 mM) and DCN (10 mM) using TEMPO
(2,2,6,6-tetramethylpiperidine 1-oxyl, 20 mM) was unsuccessful and recovered 1e (83%) and
DCN (84%), because TEMPO prevented the photoreaction. The generation of benzyl alcohol
1
was not observed by GC-mass spectrometry, H NMR analysis, and isolation, because the
benzyl alcohol decomposed upon exposure to these photochemical conditions for 0.5 h. This
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indicates that as benzyl alcohol is more reactive than 1, it is not detected under these conditions.
The second electron transfer from the radical anion of DCN to radical 8 generates the carbanion,
as mentioned in the deuterium incorporation experiment, followed by protonation to yield
glycine derivatives 2. DCN can be used as a catalyst, and most of it can be recovered after the
photoreaction. When DCB and a low-pressure mercury lamp are employed as the catalyst and
light source, respectively, efficient absorption of both 1 and DCB results in a high yield of 2 by
a similar mechanism as that underlying the formation of the radical cation of 1.
In the case of 1g,k bearing an amide at the N-terminal or an ester at the C-terminal, the low
stability of radical 8 did not promote homolysis of the C–C bond; hence, back electron transfer
from the radical anion of DCN to the radical cation of 1 occurs. The photoreactions of 1c,d
having a free phenolic hydroxy group also produce the radical cation of 1, which releases a
proton to form phenoxide radical 9, because deprotonation of the radical cation is the faster than
homolysis of the C–C bond. Subsequent electron transfer from the radical anion of DCN and
protonation yield the starting material 1.
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1a,b,e,f,h
R1 = H, OBoc, R3 = Boc, CH3OCO
R4 = NH2, NHCH3
+ DCN
R1
+ H+
R4
R3
R4
R3
2
N
N
H
H
O
O
8
+
R1
H2O
R3
OH
ν
h , DCN
+ DCN
1
PET
R4
N
H
O
O
1c,d
R1 = OH, R3 = Boc
R4 = NH2, NHCH3
Back electron transfer
+ H+
+ DCN
1g,k
1
R3
R4
R1 = OBoc, R3 = Ac, Boc
R4 = NH2, OCH3
N
H
O
9
Scheme 3 Plausible mechanism of PET-promoted debenzylation of 1
Conclusion
This study reveals the chemical behavior of the radical cations of phenylalanine and tyrosine,
not only in peptides and enzymes involving electron transfer, but also in the PET-promoted
modification of peptides. The breaking of the C–C bond of the radical cations at the benzylic
position was strongly dependent on the substituent and required both a carbamate without an
aromatic group at the N-terminal and an amide at the C-terminal, probably due to the stability of
the generated radical. The unprotected phenolic hydroxyl group in tyrosine disturbed the
debenzylation. These results confirmed the selective debenzylation of dipeptides. Further
detailed investigation on the mechanism underlying the C–C bond breaking of the radical
cations is in progress.
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Experimental Section
All reagents and solvents were used as received from commercial suppliers. IR spectra were
1
recorded on an FT-IR spectrometer. H NMR spectra were recorded in CDCl3 or d6-DMSO or
CD3OD containing tetramethylsilane as an internal standard, and were acquired on either a 300
or a 500 MHz spectrometer. 13C NMR spectra were acquired on either a 75 or a 125 MHz
spectrometer. High-resolution mass spectra were obtained using FAB mass spectrometer. The
light source was a high-pressure (100 W) or low-pressure (120 W) mercury arcs.
General procedures for the synthesis of N-protected phenylalanine and tyrosine amide 1
and dipeptides 3, 5
Chloroformate (benzyl chloroformate) or acid anhydride ((Boc)2O) (1.2 equiv.) was added
dropwise to a solution of phenylalanine or tyrosine in 0.5 M aq. NaOH:1,4-dioxane = 1:1 at 0 °C.
The mixture was stirred overnight at room temperature, and evaporated to remove 1,4-dioxane.
The aqueous solution was acidified by the slow addition of 1M HCl until the pH decreased to 3
and then extracted with EtOAc. The organic layer was dried over Na2SO4 and concentrated in
vacuo. The crude product was recrystallized by hexane/EtOAc to give the corresponding
N-protected phenylalanine and tyrosine (70–80%).
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EDC hydrochloride (1.5 equiv.), DMAP (1.5 equiv.), and i-Pr2EtN (3 equiv.) were added to a
solution containing N-protected phenylalanine or tyrosine, and ammonium hydrochloride or
°
°
methylammonium hydrochloride (1.1 equiv.) in DMF at 0 C. The mixture was stirred at 0 C
for 2 h and overnight at room temperature, and then concentrated in vacuo. The residue was
dissolved in CHCl3 and sequentially washed with 1 M HCl, 4% NaHCO3, and water. The
organic layer was dried over Na2SO4 and concentrated in vacuo. The crude product was purified
by silica-gel column chromatography using hexane/EtOAc as the eluent to give the
corresponding N-protected phenylalanine and tyrosine amides 1a–d (45–60%).
Further Boc-O-protection of the phenolic hydroxy group in N-protected tyrosine amides such
as 1c,d by the aforementioned method using (Boc)2O (1.2 equiv.) and silica-gel column
chromatography purification using hexane/EtOAc as the eluent yielded 1e–j (80–95%).
Dipeptides 3, 5 were synthesized by a previously reported method (coupling between
phenylalanine and tyrosine derivatives with EDC and DMAP).8
N-BocPheNH2 (1a). Compound 1a has been previously reported.11 1H NMR (300 MHz,
CDCl3) δ 7.32–7.21 (m, 5H), 6.20 (bs, 1H), 6.00 (bs, 1H), 5.28 (bs, 1H), 4.44–4.42 (m, 1H),
3.07–3.05 (m, 2H), 1.39 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 174.1, 155.5, 136.6, 129.3,
128.6, 126.9, 80.1, 55.3, 38.5, 28.2.
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N-BocPheNHCH3 (1b). Compound 1b has been previously reported.12 1H NMR (300
MHz,CDCl3) δ 7.33–7.21 (m, 5H), 5.73 (bs, 0.7H), 5.50 (bs, 0.4H), 4.30–4.28 (m, 1H),
3.07–3.05 (m, 2H), 2.73–2.71 (m, 3H), 1.39 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 171.9,
155.5, 136.6, 129.3, 128.6, 126.9, 80.1, 56.0, 38.5, 28.2, 26.2.
N-BocTyrNH2 (1c). Compound 1c has been previously reported.13 1H NMR (300 MHz,
d6-DMSO) δ 9.17 (s, 1H, OH), 7.32 (s, 1H, NH), 7.03 (d, J=8.6 Hz, 2H), 6.79 (s, 1H, NH), 6.71
(s, 1H, NH), 6.63 (d, J=8.6 Hz, 2H), 4.02–3.96 (m, 1H), 2.85–2.78 (m, 1H), 2.67–2.58 (m, 1H),
13
1.31 (s, 9H); C NMR (75 MHz, d6-DMSO) δ 173.8, 155.7, 155.2, 130.1, 128.3, 114.8, 77.9,
55.9, 36.7, 28.2.
N-BocTyrNHCH3 (1d). White solid, mp 143–144 °C; IR (KBr, cm-1) 3356, 2984, 2936, 1753,
1683, 1660, 1H NMR (500 MHz, d6-DMSO) δ 9.24 (s, 1H, OH), 7.82 (s, 1H, NH), 7.03–7.00 (d,
J=8.6 Hz, 2H), 6.84 (s, 1H, NH), 6.66–6.63 (d, J=8.6, 2H), 4.06–3.92 (m, 1H), 2.83–2.73 (m,
1H), 2.64–2.50 (m, 4H), 1.31 (s, 9H); 13C NMR (125 MHz, d6-DMSO) δ 172.2, 155.7, 155.2,
130.1, 128.3, 114.8, 77.9, 56.2, 36.9, 28.2, 25.9; HRMS (FAB, m/z) calcd for (M+H)+
C15H23N2O4: 295.1658, found: 295.1657.
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N-BocTyr(O-Boc)NH2 (1e). Compound 1e has been previously reported.14 1H NMR (300 MHz,
CDCl3) δ 7.27–7.22 (m, 2H), 7.13–7.08 (m, 2H), 6.05 (bs, 1H, NH), 5.69 (bs, 1H, NH), 5.16 (bs,
1H, NH), 4.39–4.37 (m, 1H), 3.07–3.05 (m, 2H), 1.55 (s, 9H), 1.41 (s, 9H); 13C NMR (75 MHz,
CDCl3) δ 173.6, 155.4, 151.9, 150.0, 134.1, 130.3, 121.4, 83.6, 80.3, 55.2, 37.6, 28.2, 27.7.
N-BocTyr(O-Boc)NHCH3 (1f). White solid, mp 143–144 °C; IR (KBr, cm-1) 3356, 2984, 2936,
1753, 1660, 1H NMR (300 MHz, CDCl3) δ 7.20 (d, J=8.6 Hz, 2H), 7.10 (d, J=8.6 Hz, 2H), 5.78
(bs, 0.8H, NH), 5.02 (bs, 0.4H, NH), 4.29–4.26 (m, 1H), 3.08–3.02 (m, 2H), 2.74–2.72 (m, 3H),
1.56 (s, 9H), 1.41 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 171.5, 155.3, 151.8, 150.0, 134.2,
130.2, 121.4, 83.6, 80.3, 55.9, 37.9, 28.3, 27.7, 26.1; HRMS (FAB, m/z) calcd for (M+H)+
C20H31N2O6: 395.2182, found: 395.2179.
N-AcTyr(OBoc)NH2 (1g). White solid, mp 162–163 °C; IR (KBr, cm-1) 3398, 3302, 2982, 1759,
1679, 1627; 1H NMR (300 MHz, CDCl3) δ 7.24 (d, J=8.6 Hz, 2H), 7.11 (d, J=8.6 Hz, 2H), 6.37
(br, 1H, NH), 6.06 (br, 1H, NH), 5.53 (bs, 1H, NH), 4.68–4.64 (m, 1H), 3.10–3.01 (m, 2H), 2.00
(s, 3H), 1.69 (s, 3H), 1.55 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 172.4, 170.1, 151.9, 150.1,
138.4, 134.0, 130.2, 83.7, 54.0, 37.6, 27.7; HRMS (FAB, m/z) calcd for (M+H)+ C16H23N2O5:
16

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323.1607, found: 323.1607.
N-CH3OCOTyr(O-Boc)NH2 (1h). White solid, mp 161–162 °C; IR (KBr, cm-1) 3383, 3336, 2985,
1
1752, 1687, 1647; H NMR (500 MHz, CD3OD) δ 7.26 (d, J=8.6 Hz, 2H), 7.05 (d, J=8.6 Hz,
2H), 4.33–4.31 (m, 1H), 3.57 (s, 3H), 3.13–3.09 (m, 1H), 2.85–2.81 (m, 1H), 1.54 (s, 9H); 13C
NMR (125 MHz, CD3OD) δ 176.7, 158.9, 153.5, 151.4, 136.3, 131.3, 122.2, 84.2, 58.0, 52.7,
38.5, 27.9; HRMS (FAB, m/z) calcd for (M+H)+ C16H23N2O6: 339.1556, found: 339.1555.
N-CbzTyr(O-Boc)NH2 (1i). White solid, mp 135–136 °C; IR (KBr, cm-1) 3385, 3328, 2981, 1756,
1662; 1H NMR (300 MHz, CDCl3) δ 7.38–7.31 (m, 5H), 7.21–7.18 (m, 2H), 7.10–7.08 (m, 2H),
5.74 (bs, 0.6H, NH), 5.36 (bs, 2H, NH), 5.09 (s, 2H), 4.41–4.39 (m, 1H), 3.16–3.00 (m, 2H),
1.55 (s, 9H); 13C NMR (125 MHz, CD3Cl) δ 172.8, 155.8, 151.8, 150.0, 135.9, 133.7, 128.4,
128.2, 128.0, 121.5, 83.6, 67.1, 55.6, 37.6, 27.6; HRMS (FAB, m/z) calcd for (M+H)+
C22H27N2O6: 415.1869, found: 415.1865.
°
N-FmocTyr(O-Boc)NH2 (1j). White solid, mp 148–149 C; IR (KBr, cm-1) 3318, 2980, 1757,
1
1663, 1617; H NMR (500 MHz, CD3OD) δ 7.85 (d, J=7.4 Hz, 1H), 7.75–7.70 (m, 3H),
7.59–7.52 (m, 2H), 7.49–7.44 (m, 1H), 7.36–7.32 (m, 2H), 7.28–7.22 (m, 2H), 7.00 (d, J=8.0
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Hz, 2H), 4.34–4.31 (m, 2H), 4.20–4.18 (m, 1H), 4.12–4.08 (m, 1H), 3.13–3.10 (m, 1H),
2.88–2.81 (m, 1H), 1.46 (s, 9H); 13C NMR (125 MHz, CD3OD) δ 176.6, 158.2, 153.5, 151.4,
145.3, 145.2, 142.6, 136.7, 131.4, 129.6, 128.7, 128.4, 128.1, 127.2, 126.3, 126.2, 122.2, 120.9,
111.4, 84.2, 67.9, 57.5, 38.5, 27.9; HRMS (FAB, m/z) calcd for (M+H)+ C29H31N2O6: 503.2182,
found: 503.2181.
N-BocTyr(O-Boc)OCH3 (1k). Compound 1k has been previously reported.15 1H NMR (300
MHz, CDCl3) δ 7.15–7.08 (m, 4H), 5.07 (br, 0.8H, NH), 4.61–4.54 (m, 1H), 3.71 (s,3H),
3.13–3.02 (m, 2H), 1.55 (s, 9H), 1.42 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 172.0, 154.9, 151.7,
150.0, 133.4, 130.1, 121.2, 83.4, 79.8, 54.2, 52.1, 37.4, 28.1, 27.5.
N-BocPhePheOCH3 (3). Compound 3 has been previously reported.16 1H NMR (300 MHz,
CDCl3) δ 7.31–7.20 (m, 8H), 7.01–6.96 (m, 2H), 6.33 (bs, 1H, NH), 4.98 (bs, 0.9H, NH),
4.81–4.75 (m, 1H), 4.35–4.33 (m, 1H), 3.66 (s, 3H), 3.11–2.98 (m, 4H), 1.39 (s, 9H); 13C NMR
(75 MHz, CDCl3) δ 171.3, 170.7, 155.2, 136.4, 135.6, 129.6, 129.3, 129.2, 129.0, 128.9, 128.6,
128.5, 127.0, 126.9, 80.1, 55.6, 53.2, 52.2, 38.2, 37.9, 28.2.
N-BocTyr(O-Boc)Tyr(O-Boc)OCH3 (5). White solid, mp 140–141 °C; IR (KBr, cm-1) 3362, 2981,
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1
1758, 1695, 1664; H NMR (300 MHz, CDCl3) δ 7.21–7.19 (m, 2H), 7.18–7.01 (m, 6H), 6.37
(bs, 1H, NH), 4.93 (bs, 0.8H, NH), 4.79–4.77 (m, 1H), 4.32–4.30 (m, 1H), 3.67 (s, 3H),
3.11–2.98 (m, 4H), 1.60 (s, 18 H), 1.47 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 171.2, 170.7,
151.8, 150.1, 150.0, 134.0, 133.1, 130.3, 130.2, 121.4, 121.3, 83.5, 55.6, 53.1, 52.4, 37.2, 28.2,
27.6, 27.3; HRMS (FAB, m/z) calcd for (M+H)+ C34H47N2O11: 659.3180, found: 659.3151.
Photoreaction of 1 with DCN
An aqueous CH3CN solution (CH3CN 90 mL, H2O 10 mL) of N-Boc phenylalanine or tyrosine
amides 1 (10 mM), and DCN (10 mM) in Pyrex vessels (18 mm x 180 mm) was purged with Ar
for 10 min. The mixture was irradiated with 100 W high-pressure mercury lamp for 24 h, and
then the solvent was removed under reduced pressure. The crude product was purified by
silica-gel column chromatography using hexane/EtOAc as the eluents to give N-Boc glycine
derivatives 2. The photoreactions of dipeptides 3, 5 were also carried out similarly.
N-BocGlyNH2 (2a). Compound 2a has been previously reported.17 1H NMR (300 MHz, CDCl3)
δ 6.71 (bs, 0.9H, NH), 6.54 (bs, 0.9H, NH), 5.27 (bs, 0.9H, NH), 3.83 (s, 2H), 1.46 (s, 9H); 13C
NMR (125 MHz, CDCl3) δ 172.0, 156.0, 80.3, 43.9, 28.2.
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N-BocGlyNHCH3 (2b). White solid, mp 75–76 °C; IR (KBr, cm-1) 3347, 3301, 2989, 1671;
1H NMR (300 MHz, CDCl3) δ 6.26 (bs, 0.7H, NH), 5.22 (bs, 0.8H, NH), 3.80–3.79 (m, 2H),
2.85–2.83 (m, 3H), 1.46 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 170.0, 156.1, 80.4, 44.5, 28.3,
26.2; HRMS (FAB, m/z) calcd for (M+H)+ C8H17N2O3: 189.1239, found: 189.1236.
1
N-CH3COOGlyNH2 (2h). Colorless liquid, IR (neat, cm-1) 3335, 2961, 1713, 1677; H NMR
(500 MHz, CD3OD) δ 3.73 (s, 2H), 3.65 (s, 3H); 13C NMR (125 MHz, CD3OD) δ 175.2, 159.7,
53.0, 44.5; HRMS (FAB, m/z) calcd for (M+H)+ C4H9N2O3: 133.0613, found: 133.0613.
N-BocGlyPheOCH3 (4). Compound 4 has been previously reported.18 1H NMR (300 MHz,
CDCl3) δ 7.31–7.20 (m, 3H), 7.12–7.09 (m, 2H), 6.63 (bs, 0.9H, NH), 5.20 (bs,0.9H, 1H),
4.92–4.85 (m, 1H), 3.81–3.75 (m, 2H), 3.71 (s, 3H), 3.18–3.05 (m, 2H), 1.44 (s, 9H); 13C NMR
(125 MHz, CDCl3) δ 171.7, 169.0, 155.9, 135.6, 129.2, 128.6, 127.1, 80.2, 53.0, 52.3, 44.1, 37.8,
28.2.
°
N-BocGlyTyr(O-Boc)OCH3 (6); White solid, mp 82–83 C; IR (KBr, cm-1) 3415, 2981, 1758,
1
1718, 1675; H NMR (300 MHz, CDCl3) δ 7.13–7.09 (m, 4H), 6.50 (bs, 0.6H, NH), 5.06 (bs,
0.5H, NH), 4.90–4.85 (m, 1H), 3.85–3.77 (m, 2H), 3.71 (s, 3H), 3.13–3.11 (m, 2H), 1.55 (s, 9H),
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1.44 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 171.6, 169.3, 156.0, 151.8, 150.2, 133.2, 130.2,
121.4, 83.6, 80.2, 53.0, 52.4, 44.1, 37.2, 28.2, 27.7; HRMS (FAB, m/z) calcd for (M+H)+
C22H33N2O8: 453.2237, found: 453.2248.
1
N-BocGly(D)NH2 (7). H NMR (300 MHz, CDCl3) δ 6.23 (bs, 0.9H, NH), 5.84 (bs, 1H, NH),
5.29 (bs, 1H, NH), 3.82–3.79 (m, 1.1H), 1.45 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 172.2,
156.1, 80.4, 43.9 (t, J=19.1 Hz), 28.3; GC-MS m/z 175 (M+).
Acknowledgments
This work was partially supported by the Japan Society for the Promotion of Science (JSPS),
Grant-in-Aid no. 17K05779, for scientific research.
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Supplementary Material
1H and 13C NMR spectra data for all compounds.
23