Regio- and stereo-controlled addition reaction of aminoallylic stannanes to aldehydes mediated by germanium dichloride

Article abstract of DOI:10.1246/cl.180291

A GeCl2-mediated addition reaction of α-(N-phthaloylamino) allylic stannane to aldehydes was achieved to give 1, 2-amino alcohol derivatives in high anti-selectivity. Various types of aromatic and aliphatic aldehydes were applicable. The synthesized N-phthaloylamino alcohols were smoothly transformed to aminoalcohols by conventional methods with no loss of stereochemistry.

Full text of DOI:10.1246/cl.180291

Received: April 3, 2018 | Accepted: April 19, 2018 | Web Released: April 27, 2018
CL-180291
Regio- and Stereo-controlled Addition Reaction of Aminoallylic Stannanes to Aldehydes
Mediated by Germanium Dichloride
Kensuke Suzuki,1 Yoshihiro Nishimoto,*2 Hiroshi Yunoki,1 Kensuke Tsuruwa,1 Naoto Esumi,1 and Makoto Yasuda*1
1Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
2Frontier Research Base for Global Young Researchers Center for Open Innovation Research and Education (COiRE),
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
E-mail: nishimoto@chem.eng.osaka-u.ac.jp (Y. Nishimoto), yasuda@chem.eng.osaka-u.ac.jp (M. Yasuda)
Table 1. Investigation of the reaction of phthaloylaminoallylic
stannane 1a with benzaldehyde 2aa
A GeCl2-mediated addition reaction of α-(N-phthaloylami-
no)allylic stannane to aldehydes was achieved to give 1,2-amino
alcohol derivatives in high anti-selectivity. Various types of
aromatic and aliphatic aldehydes were applicable. The synthe-
sized N-phthaloylamino alcohols were smoothly transformed
to aminoalcohols by conventional methods with no loss of
stereochemistry.
NPhth
NPhth
NPhth
SnBu3
H
Ph
additive
MeCN
Ph
Ph
+
+
O
OH
OH
1a
2a
3a
4a
Yield [%]b
3a syn/anti 4a syn/anti
Entry
Additive
Temp.
rt
Keywords: Germanium
| Amino alcohols | Allylic stannanes
1
2
3
4c
5c
none
SnCl2
InCl3
TiCl4
BF3¢OEt2
0
®
1:99
6:94
0
0
0
0
0
®
®
®
®
®
¹10 °C 51
¹10 °C 69
¹78 °C 58 29:71
¹78 °C 20 1:>99
Amino alcohol moieties are very important functional units
that impart pharmacological and biological activity to mole-
cules.1 Compounds that contain such units are also useful
building blocks2 and are versatile ligands for numerous types of
metal complexes.3 Therefore, the development of a synthetic
method for amino alcohols through carbon-carbon bond forma-
tion is one of the most important issues in organic synthesis. An
amino-substituted allylmetal could be a powerful and practical
tool, and the addition of an allylic metal to a carbonyl compound
is part of a highly evolving field of study.4 Actually, addition
reactions to carbonyl compounds using amino-substituted
allylboranes,5 allyllithiums6 or allylsilanes7 have been reported.
Recently, we reported that homochiral carbonylamino-substitut-
ed allylic stannanes reacted with aldehydes in the presence of
SnCl2 or InCl3 to afford 1,4-aminoalcohols in a highly stereo-
selective manner.8-10 While continuing the study, we found that
the regioselectivity of the addition reaction of a carbonylamino-
substituted allylic stannane depends on a used metal salt. Herein,
we describe that GeCl2 mediated the addition of an α-amino-
allylic stannane to an aldehyde, and 1,2-amino alcohols were
obtained in contrast to SnCl2 and InCl3 (Scheme 1).8
6d,e GeCl2¢dioxane
rt
<5
®
60 1:>99f
a1a (1.3 equiv), 2a (1.0 equiv), additive (1.3 equiv). bYields
1
were determined by H NMR using 1,1,2,2-tetrachloroethane
c
or anisole as an internal standard. CH2Cl2 was used instead
d
e
of MeCN. THF was used instead of MeCN. 1a (1 equiv), 2a
f
(5 equiv), additive (1 equiv). E/Z = 67:33.
formation of 1,2-N-phthaloylamino alcohol 4a (Entries 2 and
3).15 This regioselectivity was consistent with our previous
report.8 TiCl4 gave 3a in a moderate yield with a low
diastereoselectivity (syn/anti = 29:71) (Entry 4).16 The use of
BF3¢OEt2 resulted in a low yield (Entry 5).17 Surprisingly,
GeCl2¢dioxane afforded 1,2-N-phthaloylamino alcohol 4a in
high regio- and stereoselectivity (Entry 6).18 These results
suggested that regio- and stereoselectivities were controlled by
the loaded metal halide in the reaction of aminoallylic stannane
1a with carbonyl compounds.
As shown in Table 1, GeCl2 gave 1,2-N-phthaloylamino
alcohol 4a in the addition reaction of α-(N-phthaloylamino)al-
lylic stannane 1a to benzaldehyde 2a in contrast to SnCl2 and
InCl3. This interesting result prompted us to investigate GeCl2-
mediated reaction toward the selective synthesis of 1,2-amino
alcohols 4. We optimized the reaction conditions in the addition
of 1a to 2a, and the results are summarized in Table 2. When the
addition of GeCl2 was carried out in THF solution, the anti-
isomer 4a was exclusively given in 60% yield (Entry 1). The
ratio of the E/Z isomer was 67:33, and their stereochemistries
were determined by the 3JHH value of vicinal protons. In fact, the
E/Z-mixed anti-4a was hydrogenated by H2/Pd-C to afford a
single product 5a (eq 1). GeBr2 and GeI2 as well as GeCl2
afforded anti-4a in 45% and 47% yields, respectively, although
the yields were slightly lower (Entries 2 and 3). A survey
showed highly polar solvents were effective in this reaction, and
THF afforded the best results (Entries 4-7). The recrystallization
α-(N-Phthaloylamino)allylic stannane 1a was chosen as an
amino-substituted nucleophile because of its facile preparation,11
stability in air,12 and easy deprotection.13 The reaction of 1a with
benzaldehyde 2a under various conditions was investigated, and
the results are summarized in Table 1. No reaction was observed
without using additives (Entry 1). The use of either SnCl2 or
InCl3 as an additive14 preferentially furnished 1,4-N-phthaloyl-
amino alcohol 3a in high diastereoselectivity without the
Previous Work
R1
R2
This Work
N
N
N
H
R2
R2
OH
additive
+
R1
R1
SnBu3
O
OH
1,4-amino alcohol
1,2-amino alcohol
additive
SnCl2, InCl3
GeCl2
Scheme 1. Addition reaction of α-aminoallylic stannane to
aldehyde.
Chem. Lett. 2018, 47, 821–824 | doi:10.1246/cl.180291
© 2018 The Chemical Society of Japan | 821

GeCl2•dioxane
(1.0 eq)
Table 2. Optimization of GeX2-mediated addition reaction of
phthaloylaminoallylic stannane 1a with benzaldehyde 2aa
NPhth
NPhth
SnBu3
R
H
R
+
THF, RT
O
NPhth
Ph
NPhth
Ph
OH
NPhth
SnBu3
Ph
H
1a
2
anti-4
GeX2
+
+
solvent
rt, 8 h
O
Br
NO2
OH
anti-4a
OH
syn-4a
NPhth
NPhth
1a
2a
Amino Alcohol 4a
Yield [%]b anti/syn E/Zc
OH
OH
4c
Entry GeX2
Solvent
4b
79%, >99:1 dr
63%, >99:1 dr
1
2
3
4
5
6
7
GeCl2¢dioxane THF
GeBr2¢dioxane THF
60
45
47
58
21
24
29
>99:1 67:33
>99:1 56:44
>99:1 64:36
>99:1 57:43
>99:1 62:38
>99:1 58:42
>99:1 59:41
OMe
OH
NPhth
NPhth
GeI2
THF
GeCl2¢dioxane DMSO
GeCl2¢dioxane acetone
GeCl2¢dioxane EtOAc
GeCl2¢dioxane dioxane
OH
4d
65%, 90:10 dr
OH
4eb
66%, >99:1 dr
NPhth
NPhth
a1a (0.5 mmol), 2a (2.5 mmol), GeCl2¢dioxane (0.5 mmol)
in solvent (1 mL) at room temperature for 8 h. Yields were
determined by 1H NMR using 1,1,2,2-tetrachloroethane or
b
OH
4f
70%, 98:2 dr
OH
4g
84%, 94:6 dr
c
anisole as an internal standard. E/Z ratio of anti-4a.
NPhth
NPhth
O
Ph
OH
OH
4i
4h
90%, >99:1 dr
59%, >99:1 dr
O
Scheme 2. Scope of aldehyde 2 in the addition reaction using
α-aminoallylic stannane 1a.a aThe reactions were employed
using 1a (1 equiv), 2 (5 equiv), and GeCl2¢dioxane (1 equiv)
in THF (0.5 M) at room temperature for 8 h. Yields were
determined by 1H NMR using 1,1,2,2-tetrachloroethane or
anisole as an internal standard. A diastereo ratio (dr) shows
anti/syn. In all cases, the geometry of the olefin moiety was
non-selective. b1a (1.0 mmol), GeCl2¢dioxane (1.0 mmol). Slow
addition of 2e (0.5 mmol) in THF (2 mL) for 5 h and additional
mixing for 4 h.
N
O
OH
Figure 1. ORTEP drawing of anti-E-4a with 50% probability
thermal ellipsoids. Some hydrogen atoms are omitted for clarity.
of the E/Z-isomer mixture of 4a gave a single crystal of anti-E-
4a, and then its relative stereochemistry was confirmed by X-ray
diffraction analysis (Figure 1).19
NPhth
NPhth
SnBu3
Ph
H
GeCl2•dioxane
Ph
+
H2
THF
rt, 8 h
NPhth
Ph
NPhth
Ph
O
OH
anti-4j
25% yield
>99:1 (anti/syn)
Pd/C (1 mol%)
ð2Þ
1b
2a
EtOH
rt, 24 h
ð1Þ
OH
anti-4a
E/Z mixture
OH
5a
97%
With the optimized conditions in hand, the scope of
aldehydes 2 was investigated (Scheme 2). Aromatic aldehydes
with electron-withdrawing and electron-donating groups gave
the corresponding 1,2-phthaloylamino alcohols 4 in moderate
to high yields (4b, 4c, 4d, and 4e). Aliphatic aldehydes 2f and
2g were also applicable to this reaction system to afford 4f
and 4g, respectively, in high yields and anti-selectivity. The
α,β-unsaturated aldehyde 2h and heteroaromatic aldehyde 2i
afforded the corresponding 1,2-phthaloylamino alcohols 4h and
4i, respectively.
The addition reaction using (N-phthaloylamino)allylic stan-
nane 1b, which has a longer alkyl group than 1a, also occurred
to give the desired 1,2-amino alcohol anti-4j diastereoselectively
although the yield was low (eq 2). In all cases shown in
Scheme 2 and eq 2, the corresponding 1,2-phthaloylamino
alcohols were exclusively obtained without 1,4-phthaloylamino
alcohols.
We investigated the active species in the GeCl2-mediated
addition reactions of aminoallylic stannane 1. When the mixture
of 1a with GeCl2 was monitored by 119Sn NMR spectroscopy,
the generated Bu3SnCl was confirmed,20,21 which indicated that
transmetalation between 1a and GeCl2 proceeded to give the
corresponding allylic germanium. However, unidentified de-
1
composed species were observed via H NMR due to the short
lifespans of the active species. Plausible reaction mechanisms
based on the experimental results are shown in Scheme 3.
Transmetalation between aminoallylic stannane 1 and GeCl2
through an anti SE2¤ process gives allylic germanium A. The
species A has an E-geometry due to the bulky phthaloylamino
group.22 As the anti-form with E/Z mixed products D and F
were obtained in the experiment, the species A (Ge) reacts with
aldehyde via the cyclic transition states C and E to give the 1,2-
aminoalcohols D and F, respectively.23,24 The steric hindrance
R1 disturbs the transmetalation and the addition reaction via
822 | Chem. Lett. 2018, 47, 821–824 | doi:10.1246/cl.180291
© 2018 The Chemical Society of Japan

Transmetalation
manium A added to an aldehyde without the isomerization of A
to B to give 1,2-amino alcohol derivatives selectively because of
the low Lewis acidity of germanium halide. Various types of
carbonyl compounds such as aliphatic aldehydes and aromatic
aldehydes were applicable. The diastereoselectively synthesized
N-phthaloylaminoalcohol derivatives were successfully trans-
formed to the corresponding stereo-controlled aminoalcohols
via hydrogenation using Pd/C followed by deprotection of the
phthalimido moiety with hydrazine.
M
O
R1
O
O
N
+
MXn
N
O
O
N
R1
(M = In, Sn)
Bu3SnX
O
M
R1
SnBu3
A
B
1
(M = Ge)
RCHO
RCHO
1,2-amino alcohol
1,4-amino alcohol
NPhth
Addition reaction
Ge
R1
H
R2CHO
H
R2
Ge
This work was supported by JSPS KAKENHI Grants
Number JP15H05848 in Middle Molecular Strategy,
JP18H01977 and JP16K05719. Part of the work was supported
by Shorai Foundation for Science and Technology to Y.N. Y.N.
acknowledges support from the Frontier Research Base for
Global Young Researchers, Osaka University, of the MEXT
program. Thanks are due to the Analytical Instrumentation
Facility, Graduate School of Engineering, Osaka University, for
assistance in obtaining the MS spectra. We thank Dr. Nobuko
Kanehisa for valuable advice regarding X-ray crystallography.
R1
PhthN
PhthN
O
O
H
C
OH
D
R2
anti, E
R1
N
R1 NPhth
R2
O
H
R2CHO
H
H
Ge
A
(M = Ge)
O
OH
F
R2
R1
anti, Z
E
Scheme 3. Plausible reaction mechanism.
1) H2, Pd/C, EtOH
2) H2NNH2•H2O
NPhth
NH2
Supporting Information is available on http://dx.doi.org/
10.1246/cl.180291.
SnCl2
EtOH, reflux
NPhth
SnBu3
1a
+
Ph
Ph
Ph
3a
6
OH
OH
80%, >99:1 dr
References and Notes
1) H2, Pd/C, EtOH
Ph
H
1
a) E. J. Corey, J. O. Link, J. Org. Chem. 1991, 56, 442. b) T.
Fujita, M. Yoneta, R. Hirose, S. Sasaki, K. Inoue, M. Kiuchi,
S. Hirose, K. Adachi, M. Arita, K. Chiba, Bioorg. Med.
Chem. Lett. 1995, 5, 847. c) T. Laïb, J. Chastanet, J. Zhu,
J. Org. Chem. 1998, 63, 1709. d) C. W. Johannes, M. S.
Visser, G. S. Weatherhead, A. H. Hoveyda, J. Am. Chem.
Soc. 1998, 120, 8340. e) E. J. Corey, F.-Y. Zhang, Angew.
Chem., Int. Ed. 1999, 38, 1931. f) M. T. Barros, M. A. J.
Charmier, C. D. Maycock, T. Michaud, Tetrahedron 2005,
61, 7960. g) S. C. Bergmeier, Tetrahedron 2000, 56, 2561.
a) G. Li, H.-T. Chang, K. B. Sharpless, Angew. Chem., Int.
Ed. Engl. 1996, 35, 451. b) A. K. Ghosh, P. Mathivanan,
J. Cappiello, Tetrahedron: Asymmetry 1998, 9, 1. c) D.
Rechavi, M. Lemaire, Chem. Rev. 2002, 102, 3467. d) P.
O’Brien, Angew. Chem., Int. Ed. 1999, 38, 326. e) R. S. C.
Kumar, G. V. Reddy, G. Shankaraiah, K. S. Babu, J. M. Rao,
Tetrahedron Lett. 2010, 51, 1114.
2) H2NNH2•H2O
EtOH, reflux
NPhth
NH2
GeCl2
O
Ph
2a
4a
7
OH
OH
78%, >99:1 dr
Scheme 4. Individual synthesis of 1,4- and 1,2-amino alcohols
from 1a and 2a by changing metal salts.
cyclic transition states so that 1b gave lower yield than 1a
(eq 2). On the other hand, InCl3 and SnCl2 gave 1,4-amino
alcohols via the allylic metal species B due to the chelation
between oxygen and the metal center.8 The Lewis acidity of
germanium halide moiety is low so that the isomerization of A
to B allylic metal species does not occur.25
The synthesized N-phthaloylamino alcohols were effective-
ly transformed to the stereo-controlled saturated aminoalcohols
(Scheme 4).2,26 The product 3a, which was produced by SnCl2-
mediated allylation, underwent hydrogenation catalyzed by Pd/
C followed by deprotection of the phthalimido moiety by
H2NNH2¢H2O to give 1,4-amino alcohol 6 without a loss of the
stereochemistry. A similar procedure gave 1,2-aminoalcohol 7
from 4a and maintained the high stereoisomeric ratio. In general,
1,2- and 1,4-amino alcohols represent significant building blocks
in pharmaceutical synthesis. Therefore, this individual synthesis
of the 1,4- and 1,2-amino alcohols from the same starting
materials with a simple change in the metal salts has an
important significance.
In conclusion, the GeCl2-mediated stereoselective allylation
of aldehydes with α-(N-phthaloylamino)allylic stannane 1 was
achieved. In this reaction, a γ-(N-phthaloylamino)allylic germa-
nium, which is generated by transmetalation between GeCl2
and allylic stannane 1, reacted with aldehydes to produce the
1,2-amino alcohol derivatives with high diastereoselectivity.
In contrast to our previously reported SnCl2- or InCl3-mediated
allylation affording 1,4-amino alcohol derivatives, allylic ger-
2
3
4
a) E. J. Corey, F. J. Hannon, Tetrahedron Lett. 1987, 28,
5237. b) D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev.
1996, 96, 835. c) C. Tanyeli, S. Odabaş, M. Erdem, E. Çakır,
E. Keskin, Tetrahedron: Asymmetry 2007, 18, 2349.
See, selected reviews: a) Main Group Metals in Organic
Synthesis, ed. by H. Yamamoto, K. Oshima, Wiley-VCH,
Weinheim, 2004. doi:10.1002/3527602607. b) Comprehen-
sive Organometallic Chemistry, 3rd ed., R. H. Crabtree,
D. M. P. Mingos, Elsevier, Amsterdam, 2007.
5
a) A. Schlapbach, R. W. Hoffmann, Eur. J. Org. Chem. 2001,
323. b) F. Berrée, N. Gernigon, A. Hercouet, C. H. Lin, B.
Carboni, Eur. J. Org. Chem. 2009, 329.
6
7
8
D. J. Pippel, G. A. Weisenburger, S. R. Wilson, P. Beak,
Angew. Chem., Int. Ed. 1998, 37, 2522.
R. J. P. Corriu, V. Huynh, J. J. E. Moreau, J. Organomet.
Chem. 1983, 259, 283.
M. Yasuda, Y. Nagano, H. Yunoki, K. Tsuruwa, A. Baba,
Organometallics 2014, 33, 3924.
Chem. Lett. 2018, 47, 821–824 | doi:10.1246/cl.180291
© 2018 The Chemical Society of Japan | 823

9
See, selected reviews for allylic stannanes in organic
synthesis: a) Y. Yamamoto, N. Asao, Chem. Rev. 1993, 93,
2207. b) A. Baba, I. Shibata, M. Yasuda, in Comprehensive
Organometallic Chemistry, 3rd ed., ed. by R. H. Crabtree,
D. M. P. Mingos, Elsevier, Amsterdam, 2007, Vol. 9,
pp. 341-379.
17 a) A. Hosomi, H. Iguchi, M. Endo, H. Sakurai, Chem. Lett.
1979, 977. b) Y. Yamamoto, H. Yatagai, Y. Naruta, K.
Maruyama, J. Am. Chem. Soc. 1980, 102, 7107. c) S. E.
Denmark, E. J. Weber, T. M. Wilson, T. M. Willson,
Tetrahedron 1989, 45, 1053. d) A. Takuwa, Y. Nishigaichi,
K. Yamashita, H. Iwamoto, Chem. Lett. 1990, 1761. e) T. N.
Mitchell, U. Schneider, K. Heesche-Wagner, J. Organomet.
Chem. 1991, 411, 107. f) B. Ghosh, M. D. R. I. Amado-
Sierra, D. Holmes, R. E. Maleczka, Jr., Org. Lett. 2014, 16,
2318. g) S. Raghavan, S. Rajendar, Org. Biomol. Chem.
2015, 13, 5044.
10 See, reports for reactions using heteroatom-substituted
allylic stannanes: a) A. J. Pratt, E. J. Thomas, J. Chem.
Soc., Chem. Commun. 1982, 1115. b) M. Shibasaki, H.
Suzuki, Y. Torisawa, S. Ikegami, Chem. Lett. 1983, 1303.
c) J. A. Marshall, K. W. Hinkle, J. Org. Chem. 1996, 61,
105. d) J. A. Marshall, J. A. Jablonowski, G. S. Welmaker,
J. Org. Chem. 1996, 61, 2904. e) G. A. Weisenburger, P.
Beak, J. Am. Chem. Soc. 1996, 118, 12218. f) C. Serino, N.
Stehle, Y. S. Park, S. Florio, P. Beak, J. Org. Chem. 1999,
64, 1160. g) W. H. Pearson, Y. Ren, J. Org. Chem. 1999, 64,
688. h) M. Yasuda, T. Azuma, K. Tsuruwa, S. A. Babu, A.
Babu, Tetrahedron Lett. 2009, 50, 3209.
18 a) J. Yamaguchi, Y. Tamada, T. Takeda, Bull. Chem. Soc.
Jpn. 1993, 66, 607. b) Y. Hashimoto, H. Kagoshima, K.
Saigo, Tetrahedron Lett. 1994, 35, 4805.
19 CCDC number of anti-E-4a: 1567194.
20 See Supporting Information about the observation of
Bu3SnCl by 119Sn NMR.
21 We synthesized, isolated, and characterized some allylic
indium compounds: a) M. Yasuda, M. Haga, A. Baba,
Organometallics 2009, 28, 1998. b) M. Yasuda, M. Haga, A.
Baba, Eur. J. Org. Chem. 2009, 5513. c) M. Yasuda, M.
Haga, Y. Nagaoka, A. Baba, Eur. J. Org. Chem. 2010, 5359.
22 When transmetalation between hydroxyallylic stannane and
SnCl2, Z-form is generated because of coordination of
oxygen to metal center. See ref 10h.
11 W. H. Pearson, N. S. Barta, J. W. Kampf, Tetrahedron Lett.
1997, 38, 3369.
12 The allylic stannane 1a can be isolated by column
chromatography and stored in air for months.
13 P. G. M. Wuts, T. W. Greene, Protection for the Amino
Group. in Greene’s Protective Groups in Organic Synthesis,
4th ed., John Wiley & Sons, Inc., Hoboken, 2015, p. 696.
doi:10.1002/9780470053485.ch7.
14 a) M. Yasuda, M. Tsuchida, A. Baba, Chem. Commun. 1998,
563. b) M. Yasuda, Y. Matsukawa, K. Okamoto, T. Sako,
N. Kitahara, A. Baba, Chem. Commun. 2000, 2149. c) M.
Yasuda, K. Okamoto, T. Sako, A. Baba, Chem. Commun.
2001, 157. d) M. Yasuda, K. Hirata, M. Nishino, A.
Yamamoto, A. Baba, J. Am. Chem. Soc. 2002, 124, 13442.
15 M. Yasuda, T. Miyai, I. Shibata, A. Baba, R. Nomura, H.
Matsuda, Tetrahedron Lett. 1995, 36, 9497.
16 a) Y. Yamamoto, T. Komatsu, K. Maruyama, J. Organomet.
Chem. 1985, 285, 31. b) J. Fujiwara, M. Watanabe, T. Sato,
J. Chem. Soc., Chem. Commun. 1994, 349. c) M. Nakada, Y.
Urano, S. Kobayashi, M. Ohno, Tetrahedron Lett. 1994, 35,
741.
23 H. E. Zimmerman, M. D. Traxler, J. Am. Chem. Soc. 1957,
79, 1920.
24 In general, the reaction of allylic stannanes with electro-
philes, which includes the transmetalation between allylic
stannanes and metal halides, proceeds via SE2¤ like other
allylic metals. In fact, we previously reported that an α-
aminoallylic stannane transmetalates with InBr3 or SnCl2 via
SE2¤. See ref 8, 9, and 10.
25 a) M. Yasuda, S. Tanaka, A. Baba, Org. Lett. 2005, 7, 1845.
b) S. Tanaka, N. Tagashira, K. Chiba, M. Yasuda, A. Baba,
J. Org. Chem. 2008, 73, 6312.
26 E. A. Hallinan, T. J. Hagen, R. K. Husa, S. Tsymbalov, S. N.
Rao, J. P. vanHoeck, M. F. Rafferty, A. Stapelfeld, M. A.
Savage, M. Reichman, J. Med. Chem. 1993, 36, 3293.
824 | Chem. Lett. 2018, 47, 821–824 | doi:10.1246/cl.180291
© 2018 The Chemical Society of Japan