On the formation of seven-membered rings by arene-ynamide cyclization

Article abstract of DOI:10.1007/s00706-018-2320-x

Abstract: A Br?nsted acid-catalyzed selective arene-ynamide cyclization is described. This reaction proceeds via a keteniminium intermediate and enables the preparation of seven-membered ring enamide products. Mechanistic studies uncover an unusual product inhibition behavior. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-018-2320-x

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-018-2320-x
ORIGINAL PAPER
On the formation of seven‑membered rings by arene‑ynamide
cyclization
Bogdan R. Brutiu1 · Wilhelm Andrei Bubeneck1 · Olivera Cvetkovic1 · Jing Li1 · Nuno Maulide1
Received: 3 September 2018 / Accepted: 10 October 2018
© The Author(s) 2018
Abstract
A Brønsted acid-catalyzed selective arene-ynamide cyclization is described. This reaction proceeds via a keteniminium
intermediate and enables the preparation of seven-membered ring enamide products. Mechanistic studies uncover an unusual
product inhibition behavior.
Graphical abstract
Keywords Heterocycles · Strained molecules · Brønsted acid · Catalysis
Introduction
an efcient approach to prepare α,β-disubstituted enamides
via the addition of dialkylzinc reagents to a Bronsted acid-
activated ynamide (in the form of a keteniminium/enam-
ide trifate intermediate) [18]. During this study, we found
that ynamide 1a, carrying a phenyl substituent at the end
of a three-carbon chain, did not aford the desired product
3 (Fig. 1b). In pioneering work on ynamide cyclizations,
Hsung et al. reported an efcient acid-catalyzed process
that delivers fve- or six-membered ring enamide products
as shown in Fig. 1a [6, 19–25]. Surprisingly, the formation
of seven-membered ring enamides was not included in that
report, which motivated us to study this reaction in more
detail. Herein, we wish to report our observations in this
study and our understanding of the reaction mechanism.
Enamides are found as structural elements in biologically
relevant natural compounds and constitute useful intermedi-
ates in organic synthesis [1–4]. Accordingly, the synthesis
of enamides has been the subject of considerable investiga-
tion in the past decades. However, the preparation of cyclic
enamides remains rare [6], in particular when medium-ring
derivatives are targeted [5, 6].
As part of our research, we have made extensive use of
ynamides as versatile reagents for a range of electrophile-
triggered transformations [7–18]. Recently, we developed
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-018-2320-x) contains
supplementary material, which is available to authorized users.
* Nuno Maulide
nuno.maulide@univie.ac.at
1
Institute of Organic Chemistry, University of Vienna,
Währinger Strasse 38, 1090 Vienna, Austria
Vol.:(0123456789)
1 3

B. R. Brutiu et al.
(A)
(B)
Fig. 1 a Previous work on ynamide cationic cyclisations and b planned synthesis of compound 3 according to our prior report and unexpected
observation
Interestingly, the use of an oxygen-tethered substrate 1e
led cleanly to the formation of acryloyl imide 2e in 81%
yield. We believe that this is the result of a fragmentation
process, as outlined in Fig. 2.
Results and discussion
At the outset, we focused on the readily available ynamide
1a to screen a variety of reaction conditions. When com-
pound 1a is treated with a full equivalent (1.0 equiv.) of
TfOH in DCM, the desired cyclization product is formed
in good chemical yield after only 5 min (Table 1, entry 1).
Aiming to employ catalytic amounts of TfOH we found that,
when 5–10 mol% (0.05–0.1 equiv.) TfOH was used, the yield
of product 2a dropped precipitously (entries 2–5). A consist-
ent trend was observed for even slightly larger amounts of
TfOH all the way up to 50 mol% (0.5 equiv.), where no more
than 45% of product could be detected (entries 6–10).
The use of Tf2NH did not lead to better results (entries
11, 12) and we eventually acknowledged the need for a full
equivalent of TfOH to achieve full conversion. Under those
conditions (entry 14), a reaction time of 1 h proved ideal and
allowed the obtention of 81% isolated yield of 1a.
The experiments described herein, in particular those in
Table 1, reinforce the notion that stoichiometric amounts of
TfOH are needed in order to obtain the enamide product.
This is at odds with the theoretical mechanism for this cycli-
zation (Fig. 1b), whereby the Friedel–Crafts-like attack of
the aromatic onto the keteniminium presupposes transient
loss of aromaticity which is later regained by loss of a pro-
ton. Reevaluation of the data presented in Table 1 also shows
a direct proportionality between conversion/yield of product
and catalyst loading, clearly suggesting that catalyst inhibi-
tion beyond a single turnover is taking place [19].
1
To obtain further insight on this, H NMR studies were
carried out in Fig. 3. When 1a was treated with TfOH in
DCM, formation of the desired cyclization product 2a was
observed swiftly (within 15 min). However, upon extension
of the reaction time, the product 2a was slowly converted
to another compound. We assigned this new compound as
iminium 4a, the product of protonation of the enamide dou-
ble bond.
With suitable reaction conditions in hand, a number
of ynamides were prepared and screened (Table 2). A
para-Me substrate 1b was also a suitable precursor for
this cyclization. Disappointingly, the higher homologs 1c
and 1d did not lead to the products of cyclization (8- and
9-membered ring products 2c/2d). In these cases, only
starting material was recovered.
As shown in Fig. 4, when pure enamide was treated with
TfOH for 5 min, the formation of compound 4 was also
observed by NMR (spectra 1, 2, and 3). After quenching
1 3

On the formation of seven-membered rings by arene-ynamide cyclization
Table 1 Optimization of reaction conditions
Entry
Time
Temp/°C
Acid
Yield/%
1
2
3
4
5
6
7
8
5 min
2 h
5 h
5 h
5 h
20 h
17 h
1 h
5 h
3 h
2 h
5 h
0.5 h
1 h
0
0
0
r.t.
50
0c
0c
0
0
0
0
0
0
0
1.0 eq TfOH
66a
<5a
10b
13b
14b
<5b
7b
5 mol% TfOH
10 mol% TfOH
10 mol% TfOH
10 mol% TfOH
20 mol% TfOH
30 mol% TfOH
40 mol% TfOH
50 mol% TfOH
50 mol% TfOH
5 mol% Tf2NH
10 mol% Tf2NH
1.0 eq TfOH
23b
38b
45b
6b
9
10
11
12
13
14
7b
70d
81d, e
1.0 eq TfOH
aCrude NMR
bCrude NMR using mesitylene as an internal standard
c0 °C to rt over 16 h
dIsolated yield
eHighest isolated yield
with water, we can observe the formation of ketone (spectra
4 and 5).
Experimental
Taking all this information into account (Fig. 5), it
becomes apparent when catalytic amounts of TfOH were
used, the initially formed 2a further reacts with TfOH to
form compound 4a. This ultimately results in consumption
of all available TfOH. With no addditional acidic catalyst to
promote the cyclization of 1a, conversion of 2a then natu-
rally stopped at a level of actual loading of TfOH.
All glassware was dried before use. All solvents were used
in p.a. quality. All reagents were used as received from
commercial suppliers unless otherwise stated. Reaction
progress was monitored using TLC on aluminum sheets
coated with silica gel 60 with 0.2 mm thickness (Pre-coated
TLC-sheets ALUGRAM® Xtra SIL G/UV254). Visualiza-
tion was achieved by UV light (254 nm and 363 nm) and/
or by treatment with potassium manganite(VII) and heat.
CC was performed using silica gel 60 (230–400 Mesh,
MERCK AND CO.). All H NMR and 13C NMR spectra
1
Conclusion
were recorded on BRUKER AVIII-400 in CDCl3. Chemical
shifts (δ) were given in “parts per million” (ppm), refer-
enced to the peak of TMS (δ = 0.00 ppm), using the sol-
vent as internal standard (1H: δ(CDCl3) = 7.26 ppm; 13C:
δ(CDCl3)=77.16 ppm [26]). Coupling constants (J) were
given in Hz. Spectroscopy splitting patterns were designated
as singlet (s), doublet (d), triplet (t), quartet (q), pentet (p),
We have reported herein a Brønsted acid-catalyzed arene-
ynamide cyclization for the formation of a seven-membered
ring enamide. Through mechanistic studies, an unusual
product inhibition behavior was observed which mandated
the use of stoichiometric amounts of TfOH.
1 3

B. R. Brutiu et al.
Yield /%a
Table 2 Scope of ynamide
Starting materials
Products
cyclization
81
89
0
0
81
aTfOH (1 equiv.) was added to ynamide 1 in DCM (0.1 M) at 0 °C
Fig. 2 Proposed mechanism for
the fragmentation of 2e
multiplet (m), or combinations of that. MS were obtained
using a BRUKER maXis spectrometer with ESI and the
main signals were given in m/z units. IR were recorded on
a BRUKER VERTEX FT-IR spectrometer. The following
computer programs were used: MestReNova from Mestrelab
Research and ChemDraw from PerkinElmer.
General procedure of synthesis of ynamides
In a 250 cm3 two-neck round-bottom fask equipped with a
stir-bar, CuCl2 (0.2 equiv.), nitrogen nucleophile (5 equiv.),
and Na2CO3 (2 equiv.) were combined. The reaction
1 3

On the formation of seven-membered rings by arene-ynamide cyclization
O
Ph
N
O
(a)
(b)
(c)
O
N
O
O
O
N
+
TfOH
δ/ppm
Fig. 3 1H NMR (400 MHz, CDCl3) spectra for: a ynamide 1a; b enamide 2a obtained 15 min after the addition of TfOH; and c enamide 2a after
being treated with TfOH for 5 min
fask was purged with oxygen gas. A solution of pyridine
(2 equiv.) in dry toluene (0.06 M) was added to the reac-
tion fask and stirred at 70 °C. After 0.5 h, a solution of
the respective alkyne (1.0 equiv.) in dry toluene (0.033 M)
was added to the fask over 4 h using a syringe pump. After
addition of alkyne/toluene solution, the reaction mixture was
allowed to stir at 70 °C overnight. After cooling to r.t., the
crude mixture was concentrated under reduced pressure. The
residue was purifed by column chromatography on silica
gel.
EE=5:1–1:1] as orange crystals (0.71 g, 2.9 mmol, 33%). 1H
NMR (600 MHz, CDCl3): δ=7.11–7.06 (m, 4H), 4.43–4.39
(m, 2H), 3.89–3.85 (m, 2H), 2.68 (t, 2H), 2.34–2.29 (m,
5H), 1.87–1.80 (m, 2H) ppm; 13C NMR (151 MHz, CDCl3):
δ=156.7, 138.6, 135.5, 129.2, 128.5, 71.0, 70.6, 62.9, 47.2,
34.5, 30.6, 21.1, 18.0 ppm; IR: ̄ꢀ =2923, 2860, 2271, 1767,
1515, 1479, 1415, 1302, 1206, 1112, 1036, 806, 750 cm−1;
HRMS (ESI): m/z calculated for [M+Na]+ 266.1157, found
266.1153.
3‑(5‑Phenylpent‑1‑yn‑1‑yl)oxazolidin‑2‑one (1a,
C14H15NO2) According to the general procedure, the yna-
mide 1a was isolated after purifcation by column chroma-
tography (pentane/EE=5:1–1:1) as a yellow oil (1555 mg,
6.8 mmol, 68%). 1H NMR (400 MHz, CDCl3): δ=7.30 (t,
2H), 7.21 (dd, J=9.9, 4.1 Hz, 3H), 4.43 (t, 2H), 3.89 (t, 1H),
3‑(6‑Phenylhex‑1‑yn‑1‑yl)oxazolidin‑2‑ one (1c,
C15H17NO2) According to the general procedure, 1c was
isolated after purifcation by column chromatography (pen-
tane/EE=5:1–1:1) as a yellow oil (43 mg, 0.19 mmol, 9%).
Starting material was recovered as a yellow oil (211 mg,
1
1.3 mmol, 67%). H NMR (400 MHz, CDCl3): δ = 7.28
2.75 (m, 2H), 2.35 (t, J=7.1 Hz, 2H), 1.88 (p, 2H) ppm; 13
C
(ddd, J=7.2, 3.5, 1.4 Hz, 2H), 7.17 (m, 3H), 4.40 (dt, 2H),
3.85 (dt, 2H), 2.63 (t, J= 7.6 Hz, 2H), 2.34 (t, J= 7.1 Hz,
2H), 1.73 (m, 2H), 1.57 (m, 2H) ppm; 13C NMR (100 MHz,
CDCl3): δ= 156.8, 142.4, 128.6, 128.4, 125.9, 71.1, 70.3,
62.9, 60.6, 47.2, 35.5, 31.1, 30.7, 28.4, 18.5 ppm; IR:
̄ꢀ =2925, 2855, 2270, 1602, 1479, 1454, 1414, 1302, 1204,
1112, 1035, 973, 748, 700, 618 cm−1; HRMS (ESI): m/z
calculated for [M]+ 243.1259, found 243.1252.
NMR (100 MHz, CDCl3): δ = 141.6, 128.7, 128.5, 126.0,
70.9, 62.9, 47.2, 34.9, 30.4, 18.0 ppm; IR: ꢀ̄ =3060, 3026,
2924, 2861, 2270, 1766, 1603, 1479, 1454, 1415, 1302,
1207, 1113, 1035, 972, 748, 701 cm−1; HRMS (ESI): m/z
calculated for [M]+ 229.1103, found 229.1093.
3‑(5‑Phenylpent‑1‑yn‑1‑yl)oxazolidin‑2‑one (1b,
C15H17NO2) According to the general procedure, 1b was iso-
lated after purifcation by column chromatography [pentane/
1 3

B. R. Brutiu et al.
O
N
O
c
a
b
(1)
(2)
O
N
O
O
c
c
a
b
TfOH
(0.5 h)
+
A
A
B
B
C
C
O
N
a
b
TfOH
(1.5 h)
+
+
(3)
(4)
O
O
N
TfOH
(20 h)
C
c
B
b
A
a
O
N
O
then H
TfOH
(25 h)
2O
+
'
C
a
c
b
(5)
(6)
O
'
C
δ/ppm
Fig. 4 1H NMR (400 MHz, CDCl3) spectra for: (1) enamide 2a; (2)
enamide 2a was treated with TfOH for 0.5 h; (3) enamide 2a was
treated with TfOH for 1.5 h; (4) enamide 2a was treated with TfOH
for 20 h; (5) enamide 2a was treated with TfOH for 25 h, then add
H2O; and (6) ketone 5
a
A
O
O
O
c
O
b
O
B
TfO
N
N
O
TfOH (1 equiv.)
C'
TfOH
H2O
C
N
DCM, 0 °C, 5 min.
O
5
1a
2a
4
Fig. 5 Rationale for the requirement for stoichiometric amounts of TfOH
3‑(7‑Phenylhept‑1‑yn‑1‑yl)oxazolidin‑2‑one (1d, C16H19NO2)
According to the general procedure, 1d was isolated after
purifcation by column chromatography (pentane/EE=5:1–
1:1) as a yellow oil (197 mg, 0.77 mmol, 44%). Starting
material was recovered as a yellow oil (136 mg, 0.79 mmol,
45%). 1H NMR (400 MHz, CDCl3): δ=7.27 (dd, J=10.6,
5.7 Hz, 3H), 7.17(m, 2H), 4.40 (dd, J=8.6, 7.4 Hz, 2H), 3.84
(dd, J = 8.6, 7.4 Hz, 2H), 2.62 (t, 2H), 2.30 (t, J= 7.2 Hz,
2H), 1.63 (dt, J=15.4, 7.7 Hz, 2H), 1.56 (dt, 2H), 1.43 (m,
2H) ppm; 13C NMR (100 MHz, CDCl3): δ=156.8, 142.7,
1 3

On the formation of seven-membered rings by arene-ynamide cyclization
128.6, 128.4, 125.8, 71.3, 70.3, 62.9, 47.2, 35.9, 31.1, 28.8,
28.6, 18.5 ppm; IR: ̄ꢀ =3060, 3025, 2931, 2857, 2266, 1769,
1603, 1480, 1454, 1415, 1302, 1264, 1206, 1113, 1036, 747,
701 cm−1; HRMS (ESI): m/z calculated for [M]+ 257.1416,
found 257.1412.
by column chromatography (pentane/EE = 3:1–1:1) as a
white solid (25 mg, 0.18 mmol, 89%). 1H NMR (400 MHz,
CDCl3): δ = 7.50 (dd, 1H), 6.56 (dd, J = 17.0, 1.8 Hz,
1H), 5.90 (dd, J= 10.5, 1.8 Hz, 1H), 4.44 (t, 2H), 4.09 (t,
2H) ppm; 13C NMR (100 MHz, CDCl3): δ=165.2, 153.5,
131.9, 127.1, 62.3, 42.8 ppm; IR: ̄ꢀ = 1775, 1717, 1687,
1412, 1389, 1327, 1264, 1118, 1066, 1040, 1014, 732,
702 cm−1; HRMS (ESI): m/z calculated for [M]+ 141.0426,
found 141.0417.
General procedure of cyclization of ynamide
The respective ynamide (0.20 mmol, 1.0 equiv.) was dis-
solved in dry DCM under inert atmosphere at 0 °C. Trific
acid (0.20 mmol, 1.0 equiv.) was added dropwise using a
microsyringe and stirred for 1 h. The reaction was quenched
by the addition of a sat. NH4Cl solution. The reaction mix-
ture was extracted with DCM and the combined organic
phases were dried over MgSO4 and concentrated under
reduced pressure. The residue was purifed by column chro-
matography on silica gel.
Acknowledgements Open access funding provided by Austrian Sci-
ence Fund (FWF). We are grateful to the ERC (CoG VINCAT) and
the FWF (P30226) for fnancial support. Generous continued support
of our research programs by the University of Vienna is gratefully
acknowledged.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
3‑(6,7‑Dihydro‑5H‑benzo[g]annulen‑9‑yl)oxazolidin‑2‑one
(2a, C14H15NO2) According to the general procedure,
69 mg 1a (0.30 mmol, 1.0 equiv.) and 45 mg trific acid
(0.30 mmol, 1.0 equiv.) were reacted in 3 cm3 DCM and
the corresponding cyclization product 2a was isolated after
purifcation by column chromatography (pentane/EE=3:1–
1:1) as a white solid (56 mg, 0.24 mmol, 81%). The ketone
side product was isolated as white solid (7 mg, 0.03 mmol,
10%). 1H NMR (400 MHz, CDCl3): δ=7.24 (m, 5H), 6.25
(t, J = 7.1 Hz, 1H), 4.39 (ddd, J = 8.1, 7.2, 2.7 Hz, 2H),
3.68 (t, 2H), 2.66 (t, J=6.8 Hz, 2H), 2.15 (p, 2H), 2.00 (p,
2H) ppm; 13C NMR (100 MHz, CDCl3): δ=129.5, 128.2,
126.6, 126.4, 124.3, 61.8, 46.7, 34.7, 32.6, 24.1 ppm; IR:
̄ꢀ =2928, 2857, 1751, 1634, 1481, 1452, 1112, 1086, 1038,
898, 773, 756, 730, 702, 607 cm−1; HRMS (ESI): m/z cal-
culated for: [M]+ 229.1103, found 229.1094.
References
1. Yet L (2003) Chem Rev 103:4283
2. Shen R, Lin CT, Bowman EJ, Bowman BJ, Porco JA (2003) J Am
Chem Soc 125:7889
3. Lin S, Yang ZQ, Kwok BHB, Koldobskiy M, Crews CM, Dan-
ishefsky SJ (2004) J Am Chem Soc 126:6347
4. Martín MJ, Coello L, Fernández R, Reyes F, Rodríguez A, Murcia
C, Garranzo M, Mateo C, Sánchez-Sancho F, Bueno S, de Eguil-
ior C, Francesch A, Munt S, Cuevas C (2013) J Am Chem Soc
135:10164
5. Tian PP, Cai SH, Liang QJ, Zhou XY, Xu YH, Loh TP (2015) Org
Lett 17:1636
6. Zhang YS, Hsung RP, Zhang XJ, Huang J, Slafer BW, Davis A
(2005) Org Lett 7:1047
3‑(3‑Methyl‑6,7‑dihydro‑5H‑benzo [7] annulen‑9‑yl)oxazoli‑
din‑2‑one (2b, C15H17NO2) According to the general proce-
dure, 2b was isolated as a white solid (109 mg, 0.446 mmol,
90%). The ketone side product 8 was isolated as a colorless
oil (0.8 mg, 0.005 mmol, 1%). 1H NMR (600 MHz, CDCl3):
δ=7.14–7.00 (m, 4H), 6.23 (t, J =7.2 Hz, 1H), 4.41–4.38
(m, 2H), 3.70–3.65 (m, 2H), 2), 2.61 (t, J = 6.9 Hz, 2H),
2.33 (s, 3H), 2.11 (p, J=7.2 Hz, 2H), 1.98 (q, J=7.2 Hz,
2H) ppm; 13C NMR (151 MHz, CDCl3): δ=156.9, 139.3,
135.8, 135.4, 134.8, 130.7, 129.4, 129.0, 127.0, 124.4 ppm;
IR: ̄ꢀ = 2923, 2855, 1743, 1632, 1480, 1447, 1397, 1364,
1287, 1258, 1221, 1147, 1119, 1083, 1066, 1036, 976, 923,
825, 794, 757, 732, 683 cm−1; HRMS (ESI): m/z calculated
for [M+H]+ 244.1338, found 244.1335.
7. DeKorver KA, Li H, Lohse AG, Hayashi R, Lu Z, Zhang Y, Hsung
RP (2010) Chem Rev 110:5064
8. Evano G, Blanchard N, Compain G, Coste A, Demmer CS, Gati
W, Guissart C, Heimburger J, Henry N, Jouvin K, Karthikeyan G,
Laouiti A, Lecomte M, Martin-Mingot A, Métayer B, Michelet B,
Nitelet A, Theunissen C, Thibaudeau S, Wang J, Zarca M, Zhang
C (2016) Chem Lett 45:574
9. Evano G, Theunissen C, Lecomte M (2015) Aldrichim Acta 48:59
10. Evano G, Jouvin K, Coste A (2013) Synthesis 45:17
11. Peng B, Huang XL, Xie LG, Maulide N (2014) Angew Chem Int
Ed 53:8718
12. Xie LG, Niyomchon S, Mota AJ, Gonzalez L, Maulide N (2016)
Nat Comm 7:10914
13. Kaldre D, Klose I, Maulide N (2018) Science 361:664
14. Maryasin B, Kaldre D, Galaverna R, Klose I, Ruider S, Drescher
M, Kählig H, González L, Eberlin M, Jurberg I, Maulide N (2018)
Chem Sci 9:4124
15. Pinto A, Kaiser D, Maryasin B, Di Mauro G, González L, Maulide
N (2018) Chem Eur J 24:2515
3‑Acryloyloxazolidin‑2‑one (2e, C6H7NO3) According to
the general procedure, 46 mg 1e (0.2 mmol, 1.0 equiv.) and
30 mg trific acid (0.2 mmol, 1.0 equiv.) were reacted in
2 cm3 DCM and the amide 2e was isolated after purifcation
16. Kaldre D, Maryasin B, Kaiser D, Gajsek O, González L, Maulide
N (2017) Angew Chem Int Ed 56:2212
1 3

B. R. Brutiu et al.
17. Xie LG, Shaaban S, Chen X, Maulide N (2016) Angew Chem Int
Ed 55:12864
23. Theunissen C, Métayer B, Henry N, Compain G, Marrot J, Mar-
tin-Mingot A, Thibaudeau S, Evano G (2014) J Am Chem Soc
136:12528
18. Baldassari LL, de la Torre A, Li J, Lüdtke DS, Maulide N (2017)
Angew Chem Int Ed 56:15723
24. Sabbasani VR, Lee H, Xie P, Xia Y, Lee D (2017) Chem Eur J
16:8161
19. Terada M, Sorimachi K (2007) J Am Chem Soc 129:292
20. Li L, Chen XM, Wang ZS, Zhou B, Liu X, Lu X, Ye LW (2017)
ACS Catal 7:4004
25. Lecomte M, Evano G (2016) Angew Chem Int Ed 55:4547
26. Gottlieb HE, Kotlyar V, Nudelman A (1997) J Org Chem 62:7512
21. Nayak S, Ghosh N, Prabagar B, Sahoo AK (2015) Org Lett
17:5662
22. Yamaoka Y, Yoshida T, Shinozaki M, Yamada KI, Takasu K
(2015) J Org Chem 80:957
1 3