Tandem N, N-Dialkylation Reaction of N-Trimethylsilyl α-Iminoesters Utilizing an Umpolung Reaction and Characteristics of the Silyl Substituent: Synthesis of Pyrrolidine, Piperidine, and Iminodiacetate

Article abstract of DOI:10.1021/acs.orglett.9b00654

Umpolung reactions of N-trimethylsilyl α-iminoester with organometallics gave directly N-alkylaminoesters in high yields without the need for removing a protecting group at the nitrogen atom. Efficient syntheses of pyrrolidines, piperidines, and iminodiacetate derivatives were also developed via tandem N,N- or N,C-dialkylation reactions utilizing characteristics of the silyl substituent. Furthermore, under the influence of silica gel, the addition of an enolate to the imino nitrogen proceeded to give an iminodiacetate derivative.

Full text of DOI:10.1021/acs.orglett.9b00654

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tandem N,N‑Dialkylation Reaction of N‑Trimethylsilyl α‑Iminoesters
Utilizing an Umpolung Reaction and Characteristics of the Silyl
Substituent: Synthesis of Pyrrolidine, Piperidine, and Iminodiacetate
Isao Mizota,† Yurie Tadano,† Yusuke Nakamura,† Tomoki Haramiishi,† Miyuki Hotta,†
and Makoto Shimizu*,†,‡
†Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan
‡School of Energy Science and Engineering College, Nanjing Tech University, Nanjing 211800, Jiangsu Province, China
S
* Supporting Information
ABSTRACT: Umpolung reactions of N-trimethylsilyl α-iminoester with organometallics gave directly N-alkylaminoesters in
high yields without the need for removing a protecting group at the nitrogen atom. Efficient syntheses of pyrrolidines,
piperidines, and iminodiacetate derivatives were also developed via tandem N,N- or N,C-dialkylation reactions utilizing
characteristics of the silyl substituent. Furthermore, under the influence of silica gel, the addition of an enolate to the imino
nitrogen proceeded to give an iminodiacetate derivative.
Scheme 1. Previous Work and This Work
itrogen-containing compounds are the most fundamental
Nand important compounds widely existing in various
fields such as pharmaceuticals, agrochemicals, and biological
science. A number of efficient methods for introducing
nitrogen atoms into the target molecules have been developed
so far. Although the generally accepted method involves the
use of imines as electrophiles, imines derived from ammonia
are usually unstable and difficult to handle because they easily
undergo trimerization or hydrolysis back to the starting
materials. Metalloimines have attracted attention as relatively
stable imine equivalents derived from ammonia. The advantage
of metalloimines is that they can easily be transformed into
unprotected N−H derivatives with only hydrolysis.1 In
particular, silylimine is known to exhibit intriguing reactivity
as nucleophiles and electrophiles.2
The α-iminoester is a highly reactive imine due to the
adjacent electron-withdrawing group. In general, C-alkylation
proceeds with a nucleophile,3 while in rare cases, an umpolung
N-alkylation occurs under certain reaction conditions. Our
laboratory has previously developed various tandem C−C
bond formation reactions utilizing umpolung reactivity of α-
iminoesters with organometallic reagents.4,5 We recently
reported the umpolung reaction of α-imino thioesters and
syntheses of β-substituted-α-aminothioester via the unexpected
alkylthio rearrangement.4w However, in our conventional
umpolung reaction, most of substituents of the imino nitrogen
were limited to aryl groups such as phenyl, 4-MeOC6H4, 4-
MeC6H4, and 4-ClC6H4 (Scheme 1a). For conversion into
NH-free amino acids or physiologically active compounds, it is
often necessary to remove the substituent at the nitrogen.4c In
order to develop a more efficient and shortcut transformation
reaction utilizing umpolung of α-iminoester, we devised N-silyl
α-iminoester as a new substrate. Herein, we would report that
the umpolung reaction of N-silyl α-iminoester provides N-
unprotected α-aminoesters directly without further depro-
tection, and the intra- and intermolecular tandem N,N-
Received: February 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00654
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Letter
dialkylation reaction utilizing characteristics of the N−Si bond
also proceeds to afford various pyrrolidines, piperidines, and
iminodiacetate derivatives in high yields. Furthermore, we
found an N-addition of an ester enolate (Scheme 1b).
Scheme 2. Scope of Substrates for N-Alkylation of N-Silyl α-
Iminoester
a
As a result of various examinations into the synthesis of N-
silyl α-iminoester as a new substrate, we found that α-
iminoester 1a could be synthesized from ethyl benzoylformate
in a single step (see the Supporting Information). Initially,
when we carried out N-alkylation of N-silyl α-iminoester 1a
with 5 equiv of diethylaluminum chloride in EtCN at room
temperature for 2.5 h, the desired N-ethylated product 2a was
obtained in 28% yield along with a side product of C-ethylated
compound 3 in 17% yield (Table 1, entry 1). This promising
Table 1. Examination of Solvents for N-Alkylation of N-Silyl
a
α-Iminoester
b
b
entry
solvent
EtCN
MeCN
DMSO
DMF
toluene
hexane
CH2Cl2
C2H4Cl2
Et2O
time (h)
2a (%)
3 (%)
1
2
3
4
5
6
7
8
2.5
2.5
3.5
14
9
3
3.5
3
3
9
3
3
28
30
42
59
7
8
3
0
15
30
44
24
26
60
62
55
64
17
22
3
0
22
35
29
17
15
20
13
25
42
1
9
10
11
12
13
14
15
DME
dioxane
CPME
TBME
THF
2-MeTHF
2-MeTHF
2-MeTHF
3
3
4
3
2
4
1
c
16
d
17
3
a
The reaction was carried out according to the general procedure (see
b
c
the Supporting Information). Isolated yield. 2-MeTHF was used as
received. Degassed 2-MeTHF was used.
d
result led us to further examine several reaction parameters
such as solvents, nucleophiles, and the amount of nucleophiles
in this reaction (Table 1 and Tables S2 and S3). Regarding the
reaction solvents shown in Table 1, we found that using polar
solvents such as MeCN, DMSO, and DMF gave the N-
ethylated product 2a in moderate to good yields (entries 2−4),
while in nonpolar solvents such as toluene, hexane, CH2Cl2,
and C2H4Cl2 the reaction provided the C-addition product 3
instead of N-addition counterpart in 17−35% yields (entries
5−8). Although we found that various ethers such as Et2O,
DME, dioxane, CPME, and TBME were not effective for
suppressing C-addition reactions (entries 9−13), THF and 2-
MeTHF worked well to afford the N-alkylated product 2a
selectively in moderate to good yields (entries 14−17).
a
Yield using 5.0 equiv of Grignard reagent is shown in parentheses.
b
nBuLi (5.0 equiv) was used as a nucleophile in 2-MeTHF.
We next investigated the scope of substrates and
nucleophiles under the optimized reaction conditions (see
Supporting Information), and the results are shown in Scheme
2. All of the substrates examined having various ester moieties
yields. Among them, tert-butyl esters gave clean reaction
media, showing that a bulky substituent of the ester moiety
could suppress the side reactions to increase the yields.
Regarding the aryl group (R1), α-aryl iminoesters with both
electron-rich and electron-deficient substitutents reacted read-
i
such as Me, Et, Pr, tBu, and Cy groups underwent N-addition
reaction to afford the desired products 2a−e in good to high
B
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Letter
ily to provide the desired products 2f−p in high yields. We
found that ortho-, meta-, or para- and electron-donating or
electron-withdrawing substituents were applicable (2f−m). It
is noteworthy that even in the presence of a CN group the
desired aminoester 2n was obtained in 82% yield. Heteroaryl
groups such as 2-thienyl and 2-furyl were also tolerated (2o
and 2p). In the case of primary Grignard reagents such as Et,
nPr, nBu, and iBuMgX, the desired N-alkylated products 2a, 2q,
2r, 2s, and 2t were obtained in good yields, while with Me and
tBu Grignard reagents the reaction did not proceed well (2w
and 2x). When Grignard reagents with homoallyl and primary
alkyl groups having an acetal were used, the reaction products
2u and 2v were also obtained in good to high yields. However,
aryl Grignard reagents such as PhMgBr were not effective in
this reaction.
reaction is an intriguing reaction utilizing the characteristics of
the silicon atom in addition to the conventional tandem
umpolung reaction.
The scope of substrates and nucleophiles for the selective
N,N-dialkylation and N,C-dialkylation were investigated under
the optimal three sets of reaction conditions (conditions A−C)
(Scheme 5). Reactions at a lower temperature of conditions A
Scheme 5. Scope of Substrates for Tandem N-Alkylation/
Intramolecular Cyclization
During examination into nucleophiles, when ketene silyl
thioacetal was used as an enolate-type nucleophile in the
presence of silica gel, N-addition product 2y was obtained in
28% yield (Scheme 3). This is a promising result of N-addition
Scheme 3. N-Addition of Enolate for N-Silyl α-Iminoester
reaction of a sterically congested enolate onto the imino
nitrogen utilizing characteristics of N-silyl α-iminoester, despite
the low yields of the desired products.
Next, for the development of a tandem C−C bond
formation reaction utilizing umpolung reaction of the present
N-silyl α-iminoester, the reaction with haloalkyl Grignard
reagents was carried out at room temperature. Not only the 6-
membered ring product 5a but also the unexpected 5-
membered ring product 4a were obtained in 46% and 52%
yields, respectively (Scheme 4). This result indicates that a
Scheme 4. Optimization of Tandem N-Alkylation/
Intramolecular Cyclization
a
nucleophilic addition occurred followed by an electrophilic
alkylation at the amino nitrogen to give the 5-membered ring
product 4a. After more detailed examination of the reaction
conditions, we found that the 6-membered ring product 5a was
obtained in 72% yield at a low temperature (0 °C, conditions
A), while the pyrrolidine 4a was obtained selectively in 84%
yield at a higher reaction temperature (50 °C, conditions B).
Furthermore, pyrrolidine 4a was formed in 90% yield as a sole
product when TMSOTf was used as an additive in this reaction
(conditions C). It is noteworthy that this N,N-dialkylation
The reaction was carried out on a 1 mmol scale.
produced N,C-dialkylated piperidines 5a−g as major products
in good to high yields, which indicates that this procedure
offers a good addition to existing piperidine synthesis.6 Under
the reaction condition B, electronic and steric effects of the
substituents on the aryl ring or the ester moiety of the
iminoester did not exhibit a strong influence on the reactivity
to afford the pyrrolidines 4a−g in moderate to high yields. We
C
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Letter
also found that the yield decreased as the ring size increased
(6, 8). When the substituent α- to the imino group is bulky
such as 1-naphthyl or the ring size of the cyclized compound is
seven or eight, N,C-dialkylated products were not obtained at
all, but instead, the N,N-dialkylated products were obtained
(4d, 6, 8).
due to their steric congestions (10g, 10h). On the other hand,
benzyl bromide and 2-chloroacetyl chloride worked well in this
reaction to give the desired N,N-dialkylated products 10i and
10j in high yields as well. This reaction is a useful one that
makes it possible to synthesize iminodiacetate derivatives, one
of the most important tridentate ligands in organometallic
chemistry.7
Although more detailed examination is needed, at the
present stage we propose a plausible reaction mechanism for
cyclization shown in Scheme 6. First, a Grignard reagent
We next examined an intermolecular version of the present
reaction. Optimization of the intermolecular tandem N,N-
dialkylation reaction was carried out regarding electrophiles,
additives, and the amount of reagents. As the result, we found
that TMSOTf was also the most effective additive in this
reaction, and iodinated compounds were suitable electrophiles
to provide the desired products (see the Supporting
Information). Since iminodiacetate derivatives could be
synthesized by using an α-halocarbonyl compound as an
electrophile, we examined the scope of electrophiles under the
optimized reaction conditions, and the results are summarized
in Table 2. When various α-iodocarbonyl compounds derived
from esters, amides, thioesters, and ketones were used as
electrophiles, the desired products 10a−f were obtained in
moderate to high yields. In the case of α-di- and trisubstituted
iodocarbonyl compounds, the yield was drastically decreased
Scheme 6. Proposed Reaction Mechanism
Table 2. Tandem N,N-Dialkylation Reaction of N-Silyl α-
Iminoester
coordinates with the imino nitrogen and the carbonyl oxygen
to activate the N-silyl α-iminoester via five-membered
intermediate I. N-Alkylation proceeds to form the enolate
intermediate II, which reacts with haloalkyl moiety of the
Grignard reagent at low temperature to give the six-membered
product 5 (C-alkylation), while at higher temperatures the
formation of the pentavalent silicate intermediate III8 would
occur followed by that of the bromomagnesium amide IV via a
silyl transfer, and the cyclization of the amide proceeds to
provide the five-membered product 4 (N-alkylation). In the
presence of TMSOTf, formation of the bis-silylated
intermediate V may account for the N-alkylation to give the
pyrrolidine 4.9,10
In summary, we have found that N-silyl α-iminoester is a
useful substrate for umpolung reactions to afford, after
hydrolysis, the unprotected amino esters directly. We also
developed selective intra- and intermolecular N,N-dialkylation
reaction utilizing the characteristics of silicon atom to give
pyrrolidines, piperidines, and α-amino dicarbonyls in high
yields. The present method is an attractive one in terms of
controlling the regioselectivity (C vs N-alkylation) only by
choosing an appropriate reaction temperature or adding
TMSOTf. Moreover, an enolate addition to the imino nitrogen
also proceeded to give an iminodiacetate derivative possessing
a bulky substituent, which is promising for the development of
a new amination reaction of enolates, leading to the synthesis
of iminodiacetate of importance as tridentate ligands.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00654.
a
The reaction was carried out at 50 °C.
D
DOI: 10.1021/acs.orglett.9b00654
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Organic Letters
Experimental procedure and compound characterization
Letter
Mizota, I.; Shimizu, M. Pure Appl. Chem. 2014, 86, 755. (q) Shimizu,
M.; Tateishi, M.; Mizota, I. Chem. Lett. 2014, 43, 1752. (r) Mizota, I.;
Maeda, T.; Shimizu, M. Tetrahedron 2015, 71, 5793. (s) Mizota, I.;
Shimizu, M. Chem. Rec. 2016, 16, 688. (t) Tanaka, T.; Mizota, I.;
Umezu, K.; Ito, A.; Shimizu, M. Heterocycles 2017, 95, 830.
(u) Kawanishi, M.; Mizota, I.; Aratake, K.; Tanaka, H.; Nakahama,
K.; Shimizu, M. Bull. Chem. Soc. Jpn. 2017, 90, 395. (v) Mizota, I.;
Nakajima, Y.; Higashino, A.; Shimizu, M. Arabian J. Sci. Eng. 2017, 42,
4249. (w) Mizota, I.; Ueda, C.; Tesong, Y.; Tsujimoto, Y.; Shimizu,
M. Org. Lett. 2018, 20, 2291. (x) Nakahama, K.; Suzuki, M.; Ozako,
M.; Mizota, I.; Shimizu, M. Asian J. Org. Chem. 2018, 7, 910.
(5) For N-alkylation to α-imino esters in other laboratories, see:
(a) Fiaud, J.-C.; Kagan, H. B. Tetrahedron Lett. 1971, 12, 1019.
(b) Yamamoto, Y.; Ito, W. Tetrahedron 1988, 44, 5415. (c) Uneyama,
K.; Yan, F.; Hirama, S.; Katagiri, T. Tetrahedron Lett. 1996, 37, 2045.
(d) Yoo, S.-e.; Gong, Y.-d. Heterocycles 1997, 45, 1251. (e) Bertrand,
M. P.; Feray, L.; Nouguier, R.; Perfetti, P. Synlett 1999, 1999, 1148.
(f) Mae, M.; Amii, H.; Uneyama, K. Tetrahedron Lett. 2000, 41, 7893.
(g) Chiev, K. P.; Roland, S.; Mangeney, P. Tetrahedron: Asymmetry
2002, 13, 2205. (h) Dickstein, J. S.; Fennie, M. W.; Norman, A. L.;
Paulose, B. J.; Kozlowski, M. C. J. Am. Chem. Soc. 2008, 130, 15794.
(i) Dickstein, J. S.; Kozlowski, M. C. Chem. Soc. Rev. 2008, 37, 1166.
(j) Curto, J. M.; Dickstein, J. S.; Berritt, S.; Kozlowski, M. C. Org. Lett.
2014, 16, 1948.
(6) For the synthesis of pyrrolidines and piperidines, see: (a) Nanda,
K. K.; Trotter, B. W. Tetrahedron Lett. 2005, 46, 2025. (b) Deng, Q.-
H.; Xu, H.-W.; Yuen, A. W.-H.; Xu, Z.-J.; Che, C.-M. Org. Lett. 2008,
10, 1529. (c) Moriyama, K.; Sakai, H.; Kawabata, T. Org. Lett. 2008,
10, 3883. (d) Kano, T.; Sakamoto, R.; Mii, H.; Wang, Y.-G.; Maruoka,
K. Tetrahedron 2010, 66, 4900. (e) Frauenlob, R.; García, C.;
Bradshaw, G. A.; Burke, H. M.; Bergin, E. J. Org. Chem. 2012, 77,
4445. (f) Fan, W.; Ma, S. Angew. Chem., Int. Ed. 2014, 53, 14542.
(g) Huo, C.; Yuan, Y.; Chen, F.; Tang, J.; Wang, Y. Org. Lett. 2015,
17, 4208.
(7) (a) Martin, R. P.; Petit-Ramel, M. M.; Scarff, J. P. Mixed Ligand
Metal Ion Complexes in Amino Acids and Peptides. In Metal Ions in
Biological Systems; Sigel, H., Ed.; M. Dekker: New York, 1973; Vol. 2,
p 3. (b) Prokhorov, A. M.; Kozhevnikov, V. N.; Kopchuk, D. S.;
Bernard, H.; Le Bris, N.; Tripier, R.; Handel, H.; Koenig, B.;
Kozhevnikov, D. N. Tetrahedron 2011, 67, 597.
(8) For hypervalent silicates, see: (a) Colvin, E. W. Silicon in Organic
Synthesis; Butterworths: London, 1981. (b) Magnusson, E. J. Am.
Chem. Soc. 1990, 112, 7940. (c) Gillespie, R. J. Chem. Soc. Rev. 1992,
21, 59. (d) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem.
Rev. 1993, 93, 1371. (e) Denmark, S. E.; Stavenger, R. A. Acc. Chem.
Res. 2000, 33, 432. (f) Rendler, S.; Oestreich, M. Synthesis 2005,
2005, 1727. (g) Chen, J.; Captain, B.; Takenaka, N. Org. Lett. 2011,
13, 1654.
data (PDF)
1H and 13C NMR spectra for all new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mshimizu@chem.mie-u.ac.jp.
ORCID
Makoto Shimizu: 0000-0003-0467-9594
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (B) and on Innovative Areas “Organic Synthesis
Based on Reaction Integration. Development of New Methods
and Creation of New Substances” from JSPS and MEXT and
the Sumitomo and Okasan-Kato Foundations.
REFERENCES
■
(1) (a) Cainelli, G.; Panunzio, M.; Andreoli, P.; Martelli, G.; Spunta,
G.; Giacomini, D.; Bandini, E. Pure Appl. Chem. 1990, 62, 605.
(b) Cainelli, G.; Giacomini, D.; Mezzina, E.; Panunzio, M.;
Zarantonello, P. Tetrahedron Lett. 1991, 32, 2967. (c) Qin, W.;
Long, S.; Panunzio, M.; Biondi, S. Molecules 2013, 18, 12264. Also
see references cited therein.
(2) (a) Kruger, C.; Rochow, E. G.; Wannagat, U. Chem. Ber. 1963,
96, 2132. (b) Chan, L.-H.; Roschow, E. G. J. Organomet. Chem. 1967,
9, 231. (c) Hart, D. J.; Kanai, K.; Thomas, D. G.; Yang, T. K. J. Org.
Chem. 1983, 48, 289. (d) Panunzio, M.; Zarantonello, P. Org. Process
Res. Dev. 1998, 2, 49. (e) Gyenes, F.; Bergmann, K. E.; Welch, J. T. J.
Org. Chem. 1998, 63, 2824. (f) Black, D. A.; Arndtsen, B. A.
Tetrahedron 2005, 61, 11317. (g) Candito, D. A.; Lautens, M. Angew.
Chem., Int. Ed. 2009, 48, 6713. (h) Hong, K.; Morken, J. P. J. Am.
Chem. Soc. 2013, 135, 9252. (i) Makley, D. M.; Johnston, J. N. Org.
Lett. 2014, 16, 3146.
(3) For C-alkylation to α-imino esters, see: (a) Fiaud, J.-C.; Kagan,
H. B. Tetrahedron Lett. 1970, 11, 1813. (b) Harwood, L. M.; Vines, K.
J.; Drew, M. G. B. Synlett 1996, 1996, 1051. (c) Ferraris, D.; Young,
B.; Dudding, T.; Lectka, T. J. Am. Chem. Soc. 1998, 120, 4548.
́
(d) Cordova, A.; Notz, W.; Zhong, G.; Betancort, J. M.; Barbas, C. F.,
III J. Am. Chem. Soc. 2002, 124, 1842. (e) Mitani, M.; Tanaka, Y.;
Sawada, A.; Misu, A.; Matsumoto, Y. Eur. J. Org. Chem. 2008, 2008,
1383. (f) Fu, P.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc.
(9) To elucidate the reaction mechanism, we carried out a series of
experiments, e.g., detection of the reaction intermediates by measuring
29Si NMR spectra, isolation of ketene silyl acetal intermediates as well
as Ac derivatives by adding AcCl at each temperature, and so on.
However, all attempts were unsuccessful.
(10) Regarding the pathways from the intermediate II to the
compound 5 and from the intermediates IV and V to compound 4,
one of the explanations may involve the difference of the bond
dissociation energies. See, for example: Luo, Y.-R.; Cheng, J.-P. CRC
Handbook of Chemistry and Physics, 96th ed.; Haynes, W. M., Ed.;
CRC Press: Boca Raton, FL, 2015; pp 9-80−9-85.
2008, 130, 5530. (g) Fustero, S.; Mateu, N.; Albert, L.; Acena, J. L. J.
̃
Org. Chem. 2009, 74, 4429. (h) Hatano, M.; Yamashita, K.; Mizuno,
M.; Ito, O.; Ishihara, K. Angew. Chem., Int. Ed. 2015, 54, 2707.
(i) Hatano, M.; Yamashita, K.; Ishihara, K. Org. Lett. 2015, 17, 2412.
(4) For N-alkylation to α-imino esters in our laboratory, see:
(a) Shimizu, M.; Niwa, Y. Tetrahedron Lett. 2001, 42, 2829. (b) Niwa,
Y.; Takayama, K.; Shimizu, M. Tetrahedron Lett. 2001, 42, 5473.
(c) Niwa, Y.; Takayama, K.; Shimizu, M. Bull. Chem. Soc. Jpn. 2002,
75, 1819. (d) Niwa, Y.; Shimizu, M. J. Am. Chem. Soc. 2003, 125,
3720. (e) Shimizu, M.; Itou, H.; Miura, M. J. Am. Chem. Soc. 2005,
127, 3296. (f) Shimizu, M. Pure Appl. Chem. 2006, 78, 1867.
(g) Shimizu, M.; Hachiya, I.; Mizota, I. Chem. Commun. 2009, 874.
(h) Mizota, I.; Tanaka, K.; Shimizu, M. Tetrahedron Lett. 2012, 53,
1847. (i) Nishi, T.; Mizota, I.; Shimizu, M. Pure Appl. Chem. 2012, 84,
2609. (j) Hata, S.; Maeda, T.; Shimizu, M. Bull. Chem. Soc. Jpn. 2012,
85, 1203. (k) Shimizu, M.; Takao, Y.; Katsurayama, H.; Mizota, I.
Asian J. Org. Chem. 2013, 2, 130. (l) Shimizu, M.; Kurita, D.; Mizota,
I. Asian J. Org. Chem. 2013, 2, 208. (m) Sano, T.; Mizota, I.; Shimizu,
M. Chem. Lett. 2013, 42, 995. (n) Mizota, I.; Matsuda, Y.; Kamimura,
S.; Tanaka, H.; Shimizu, M. Org. Lett. 2013, 15, 4206. (o) Tanaka, H.;
Mizota, I.; Shimizu, M. Org. Lett. 2014, 16, 2276. (p) Koyama, K.;
E
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