C- and N-Selective Grignard Addition Reactions of α-Aldimino Esters in the Presence or Absence of Zinc(II) Chloride: Synthetic Applications to Optically Active Azacycles

Article abstract of DOI:10.1021/acs.orglett.5b00927

Highly practical synthetic methods were developed for the C- and N-selective Grignard addition reactions of N-4-MeOC₆H₄-protected α-aldimino esters in the presence or absence of zinc(II) chloride. Diastereoselective C-alkyl addition, tandem C-alkyl addition-N-alkylation, and some transformations to synthetically useful optically active azacycles were demonstrated. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.orglett.5b00927

Letter
pubs.acs.org/OrgLett
C- and N‑Selective Grignard Addition Reactions of α‑Aldimino Esters
in the Presence or Absence of Zinc(II) Chloride: Synthetic
Applications to Optically Active Azacycles
Manabu Hatano,† Kenji Yamashita,† and Kazuaki Ishihara*,†,‡
†Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
‡Japan Science and Technology Agency (JST), CREST, Furo-cho, Chikusa, Nagoya 464-8603, Japan
S
* Supporting Information
ABSTRACT: Highly practical synthetic methods were developed for
the C- and N-selective Grignard addition reactions of N-4-MeOC6H4-
protected α-aldimino esters in the presence or absence of zinc(II)
chloride. Diastereoselective C-alkyl addition, tandem C-alkyl addition−
N-alkylation, and some transformations to synthetically useful optically
active azacycles were demonstrated.
mpolung α-imino esters, which are versatile α-amino acid
precursors, have two adjacent reactive electrophilic centers
carbon atom, which is reasonably activated by the chelation of
[MgCl]+. As a result, the corresponding optically active α,α-
disubstituted amino acid derivatives (2a) were obtained in high
yields within 5 min. In sharp contrast, α-aldimino esters 4 are
generally much more reactive and less regioselective than α-
ketimino esters 1 due to the relief of steric and electronic factors,
and C-adduct 5 and N-adduct 6 are often competitively obtained
with low regioselectivity (eq 2).8 Moreover, the regioselectivity
on C- or N-alkyl addition generally depends on the α-aldimino
esters, and respective practical methodologies for both the C- and
N-alkyl additions to 4 with Grignard reagents have not yet been
well-established.9−11 In this context, we report here the C- and
N-selective Grignard addition reactions of N-PMP-protected α-
aldimino esters in the presence or absence of zinc(II) chloride.
First, we examined alkyl addition to N-PMP-α-aldimino ester
4a with 1°-, 2°-, and 3°-alkylmagnesium chloride in the presence
or absence of ZnCl2 in tetrahydrofuran (THF) at −78 °C (Table
1). As a result, the reaction of 4a (1 mmol) with alkylzinc(II)ate
(1.1 equiv), which is prepared in situ from RMgCl (3.3 equiv)
and ZnCl2 (1.1 equiv) (method A), proceeded smoothly within
10 min, and the corresponding C-adducts 5a−c were obtained in
92−96% yields selectively (entries 1, 4, and 7).On the other
hand, 1.1 equiv of RMgCl without ZnCl2 (method B) provided a
complex mixture (entries 2, 5, and 8), which might be attributed
to the highly reactive and unstable Mg(II)-enolate intermediates
(7) via N-addition (eq 4). However, very interestingly, N-adduct
6a was obtained in 95% yield when 3.3 equiv of EtMgCl was used
(method C) (entry 3). This result strongly suggests that quick
consumption of 4a would prevent the reaction between
intermediates 7a and 4a (eq 5). Reactions were scalable to 10
mmol of 4a without serious problems (entries 1 and 3). Although
2°- and 3°-alkylmagnesium chloride (i.e., i-PrMgCl and t-
BuMgCl, respectively) were less selective than EtMgCl as 1°-
U
on the CN double bond. Therefore, the Grignard addition
reaction1 of α-imino esters usually provides a mixture of C- and
N-adducts.2 However, some N-aryl-protected (commonly, 4-
MeOC6H4 (PMP)) α-ketimino esters 1 prefer N-alkyl addition
to C-alkyl addition in the reaction with Grignard reagents, and N-
adduct 3 rather than C-adduct 2 would be obtained as a major
product (eq 1).3,4 We recently developed an unusual C-selective
and diastereoselective alkyl addition to β,γ-alkynyl-α-imino
esters (1a) with alkylzinc(II)ate complexes, which were prepared
in situ from Grignard reagents and zinc(II) chloride (eq 3).5−7
Since the alkylzinc(II)ate consists of a Lewis acidic [MgCl]+ (or
[Mg3Cl5]+) moiety and a nucleophilic [R3Zn]− moiety, the
ionically separated [R3Zn]− can selectively attack the imino
Received: March 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Regioselective Alkyl Addition to α-Aldimino Ester
4a
Scheme 1. C-Phenyl Addition for the Synthesis of β-Lactam 9
and Aspartic Acid Derivative 10
a
yield (%)
entry
RMgCl
method
5/6
5
6
b
b
1
2
EtMgCl
A
B
C
A
B
C
A
B
C
A
B
C
5a/6a
5a/6a
5a/6a
5b/6b
5b/6b
5b/6b
5c/6c
5c/6c
5c/6c
5d/6d
5d/6d
5d/6d
96 (>99)
0 (0)
63
EtMgCl
9
b
b
3
EtMgCl
0 (0)
95 (92)
4
i-PrMgCl
i-PrMgCl
i-PrMgCl
t-BuMgCl
t-BuMgCl
t-BuMgCl
allylMgCl
allylMgCl
allylMgCl
96
0
5
9
19
80
0
6
17
Table 2. Regio- and Diastereoselective Alkyl Addition to
a
7
92
Chiral α-Aldimino Ester 11
8
38
12
54
9
34
c
c
10
11
12
>99 (94)
0 (0)
20
11
0
0
a
The reaction was conducted in THF at −78 °C for 10 min with
method A, B, or C on a 1 mmol scale of 4a unless otherwise noted.
Method A: Grignard reagent (3.3 equiv) and ZnCl2 (1.1 equiv).
Method B: Grignard reagent (1.1 equiv). Method C: Grignard reagent
yield (%)
b
(3.3 equiv). Isolated yields (%) are shown. On a 10 mmol scale of 4a.
entry
RMgX
EtMgCl
method
12/13
12
99
13
c
On a 50 mmol scale of 4a.
1
2
3
4
5
6
A
B
A
B
A
B
A
B
A
B
A
B
D
A
B
D
A
B
A
B
12a/13a
12a/13a
12b/13b
12b/13b
12c/13c
12c/13c
12d/13d
12d/13d
12e/13e
12e/13e
12f/13f
12f/13f
12f/13f
12g/13g
12g/13g
12g/13g
12h/13h
12h/13h
12i/13i
0
85
0
EtMgCl
6
97
12
83
0
i-PrMgCl
i-PrMgCl
78
0
c-PrMgBr
c-PrMgBr
0
b
7
t-BuMgCl
89
21
99
11
99
19
0
0
b
8
t-BuMgCl
74
0
9
PhCH2MgCl
PhCH2MgCl
o-BrC6H4CH2MgBr
o-BrC6H4CH2MgBr
o-BrC6H4CH2MgBr
PhCH2CH2MgCl
PhCH2CH2MgCl
PhCH2CH2MgCl
C8H17MgCl
10
88
0
alkylmagnesium chloride, N-adducts 6b and 6c were obtained as
major products (entries 6 and 9). Moreover, C-allyl addition to
4a smoothly proceeded by method A on a 50 mmol scale, and C-
adduct 5d was obtained quantitatively (12.4 g) (entry 10).
However, the expected N-adduct 6d was not obtained by method
C (entry 12). By methods B (entry 11) and C (entry 12), side
reactions at the activated ester moiety almost consumed 4a,
probably due to a six-membered cyclic transition state of
allylation.
b
11
12
53
87
0
b
13
14
15
>99
0
79
99
0
b
16
0
17
18
19
20
96
0
C8H17MgCl
90
0
3-thienylMgI
3-thienylMgI
73
20
C-Phenyl addition to 4b was also examined with the use of
PhMgBr and ZnCl2 (Scheme 1). In place of routine acidic
workup, treatment with chloroacetyl chloride, followed by 1,8-
diazabicyclo[5.4.0]-7-undecene, was conducted in one pot. As a
result, β-phenyl-β-alkoxycarbonyl-β-lactam (8) was obtained in
87% yield. PMP protection in 8 was readily cleaved by
(NH4)2[Ce(NO3)6] (CAN), and β-lactam 9 was obtained in
92% yield. Ring opening of 8 by NaOMe gave the synthetically
useful α-phenyl aspartic acid derivative12 10 in 47% yield.
We next examined regio- and diastereoselective alkyl and aryl
addition to 11, which has a bulky chiral 8-phenylmenthyl (R*)
ester13,14 (Table 2). As expected, the desired C-alkyl addition
proceeded smoothly when alkylzinc(II)ate reagents were used
(method A), and the corresponding (S)-C-adducts (12) were
obtained as sole products with perfect diastereoselectivity
(>99:1). Remarkably, alkylzinc(II)ate, method A, was highly
effective with various Grignard reagents, such as ethyl (entry 1),
12i/13i
0
a
The reaction was conducted in THF at −78 °C for 2 h with method
A, B, or D on a 0.5 mmol scale of 11 unless otherwise noted. Method
A: Grignard reagent (3.3 equiv) and ZnCl2 (1.1 equiv). Method B:
Grignard reagent (1.1 equiv). Method D: Grignard reagent (1.5
b
equiv). Reaction time was 5 min.
isopropyl (entry 3), cyclopropyl (entry 5), tert-butyl (entry 7),
benzyl (entries 9 and 11), homobenzyl (entry 14), and octyl
(entry 17) magnesium halides. C-Aryl addition also proceeded
when 3-thienyMgI and ZnCl2 were used, and the corresponding
C-adduct 12i was obtained in 73% yield. Moreover, interestingly,
when we used 1.1 equiv of Grignard reagents for 11 without
ZnCl2 (method B) (unlike 3.3 equiv for 4a without ZnCl2 by
method C in Table 1), N-adducts 13 were selectively obtained in
good to high yields. In some cases, the yields of 13 were
B
DOI: 10.1021/acs.orglett.5b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
improved when 1.5 equiv of Grignard reagents were used by
method D (entries 13 and 16). Unfortunately, N-aryl addition
generally did not proceed well by method B, probably due to a
steric reason between the transferable aryl moiety and PMP or
other unsolved reasons which need further investigations (entry
20).
Based on the results in Table 2, the high N-selectivity with
alkyl Grignard reagents might be inherent in aldimino ester 11. In
particular, unlike aldimino ester 4a, highly sterically hindered
Mg(II)-enolate intermediates 14 via N-alkyl addition to 11
would be stable, and 14 might be unlikely to undergo further
problematic nucleophilic reactions with 11 even when 1.1 equiv
of Grignard reagents is used (Figure 1).14 Therefore, quick
consumption of 11 by an excess amount of Grignard reagent
would not be required in this case.
cleavage of PMP protection by CAN. Moreover, in the case of
compound 12f, Pd(0)/2-dicyclohexylphosphino-2′,6′-
dimethoxybiphenyl (SPhos)-catalyzed N-cyclization16 provided
synthetically useful indoline 21 in 74% yield.
We next focused on the transformation to cyclic α-amino acid
derivatives based on synthetic and medicinal chemistry. In this
context, Kozlowski reported the intramolecular tandem N-alkyl
addition−C-alkylation of achiral ketimino esters, and racemic
piperidine and azepane were obtained in one pot.3a Alternatively,
we examined the intramolecular tandem C-alkyl addition−N-
alkylation of chiral aldimino ester 11 with alkylzinc(II)ate
reagents in one pot (Scheme 4). Fortunately, the desired
Scheme 4. Tandem C-Alkyl Addition−N-Alkylation
Figure 1. Possible stable intermediates via N-alkyl addition.
optically active N-heterocycles 22a and 22b were obtained in
respective yields of 74 and 50% with excellent diastereoselectiv-
ities. The use of tetrabutylammonium iodide was crucial for
promoting cyclization.
Finally, to demonstrate the versatility of the N-PMP moiety
without deprotection, we used α-aldimino ester 23 with o-
phenylethynyl-substituted PMP (Scheme 5). C-Selective and
Similarly, C-selective alkyl addition to chiral α-oximino
ester13e,15 15 with allylzinc(II)ate was conducted (Scheme 2).
Scheme 2. C-Allyl Addition to Chiral α-Oximino Ester 15
Scheme 5. Transformation to Indoles 25
As expected, the corresponding C-adduct 16 was obtained in
91% yield with high diastereoselectivity (92:8) within 5 min. This
result is comparable to the results with allylZn(II)Br (52% yield,
dr = 87:13).13e Moreover, when Grignard reagent was used in the
absence of ZnCl2, N-adduct 17 could not be obtained and 16 was
obtained in 32% yield with lower diastereoselectivity (75:25).
To demonstrate the synthetic utility of the obtained optically
active C-adducts, some transformations were examined. First, the
chiral auxiliary R* (8-Ph-menthyl) was recovered quantitatively
as R*OH from compound 12e under treatment with LiAlH4
(Scheme 3). Subsequently, the corresponding amino alcohol 18
was transformed to oxazolidinones 19 and then 20 by the
diastereoselective addition of benzyl and vinyl Grignard reagents
was completed within 5 min, and 24a and 24b were obtained in
respective yields of 93 and 92%. Subsequently, 10 mol % of PdCl2
catalyzed the cyclization,17 and synthetically useful optically
active N-indolyl acetic acid derivatives18 (25a and 25b) were
obtained almost quantitatively. X-ray analysis of 25a unambig-
uously showed the structure (see the Supporting Information).
In summary, we have developed a highly practical method for
the C- and N-selective Grignard addition reaction of N-PMP-
protected α-aldimino esters in the presence or absence of
zinc(II) chloride. In particular, with the use of a chiral α-aldimino
ester, diastereoselective C-alkyl addition and tandem C-alkyl
addition−N-alkylation successfully proceeded with the use of
alkylzinc(II)ate.19 Moreover, some transformations of the C-
Scheme 3. Recovery of Chiral Auxiliary and Transformation
to Oxazolidinones 19 and 20 and Indoline 21
C
DOI: 10.1021/acs.orglett.5b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(d) Uchiyama, M.; Nakamura, S.; Ohwada, T.; Nakamura, M.;
Nakamura, E. J. Am. Chem. Soc. 2004, 126, 10897. (e) Murakami, K.;
Yorimitsu, H.; Oshima, K. J. Org. Chem. 2009, 74, 1415. (f) Hevia, E.;
adducts to synthetically useful optically active azacycles, such as
piperidine, azepane, indole, and indoline, were demonstrated.
Overall, this simple procedure with Grignard reagents in the
presence or absence of zinc(II) chloride might be attractive as
one of the most simple methodologies for obtaining α-amino
acid derivatives.
́
Chua, J. Z.; García-Alvarez, P.; Kennedy, A. R.; McCall, M. D. Proc. Natl.
Acad. Sci. U.S.A. 2010, 107, 5294. (g) Armstrong, D. R.; Clegg, W.;
García-Alvarez, P.; McCall, M. D.; Nuttall, L.; Kennedy, A. R.; Russo, L.;
Hevia, E. Chem.Eur. J. 2011, 17, 4470. (h) Armstrong, D. R.; Clegg,
́
W.; García-Alvarez, P.; Kennedy, A. R.; McCall, M. D.; Russo, L.; Hevia,
ASSOCIATED CONTENT
* Supporting Information
́
E. Chem.Eur. J. 2011, 17, 8333. (i) Vidal, C.; García-Alvarez, J.;
■
S
Hernan
́
-Gom
́
ez, A.; Kennedy, A. R.; Hevia, E. Angew. Chem., Int. Ed.
2014, 53, 5969. (j) Bluemke, T. D.; Clegg, W.; García-Alvarez, P.;
Kennedy, A. R.; Koszinowski, K.; McCall, M. D.; Russo, L.; Hevia, E.
Chem. Sci. 2014, 5, 3552.
Experimental procedures and characterization data. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.orglett.5b00927.
(8) (a) Fiaud, J.-C.; Kagan, H. B. Tetrahedron Lett. 1970, 11, 1813.
(b) Fiaud, J.-C.; Kagan, H. B. Tetrahedron Lett. 1971, 12, 1019. (c) Yoo,
S.-E.; Gong, Y.-D. Heterocycles 1997, 45, 1251. (d) Chiev, K. P.; Roland,
S.; Mangeney, P. Tetrahedron: Asymmetry 2002, 13, 2205.
(9) C-alkyl addition to N,N-dibenzyl α-aldiminium salts in situ:
Shimizu, M.; Itou, H.; Miura, M. J. Am. Chem. Soc. 2005, 127, 3296.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ishihara@cc.nagoya-u.ac.jp.
Notes
(10) N-Alkyl addition to N-Boc α-aldimino esters: (a) Munster, P.;
̈
Steglich, W. Synthesis 1987, 223. (b) Ermert, P.; Meyer, J.; Stucki, C.;
Schneebeli, J.; Obrecht, J.-P. Tetrahedron Lett. 1988, 29, 1265.
(11) With organozinc(II) reagents for α-aldimino esters: (a) van Vliet,
M. R. P.; Jastrzebski, J. T. B. H.; Klaver, W. J.; Goubitz, K.; van Koten, G.
Recl. Trav. Chim. Pays-Bas 1987, 106, 132. (b) Courtois, G.; Miginiac, L.
J. Organomet. Chem. 1989, 376, 235. (c) Bertrand, M. P.; Feray, L.;
Nouguier, R.; Perfetti, P. Synlett 1999, 1148. (d) Basra, S.; Fennie, M.
W.; Kozlowski, M. C. Org. Lett. 2006, 8, 2659. (e) Lin, L.; Fu, X.; Ma, X.;
Zhang, J.; Wang, R. Synlett 2012, 2559.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was partially provided by MEXT, KAKENHI
(26288046), Program for Leading Graduate Schools “IGER
program in Green Natural Sciences”, MEXT, Japan. We thank
Dr. Tomokazu Mizuno in Nagoya University for helpful
discussions.
(12) Martin, T. A.; Comer, W. T.; Combs, C. M.; Corrigan, J. R. J. Org.
Chem. 1970, 35, 3814.
REFERENCES
■
(13) (a) Corey, E. J.; Ensley, H. E. J. Am. Chem. Soc. 1975, 97, 6908.
(b) Oppolzer, W.; Kurth, M.; Reichlin, D.; Chapuis, C.; Mohnhaupt, M.;
Moffatt, F. Helv. Chim. Acta 1981, 64, 2802. (c) Yamamoto, Y.; Ito, W.;
Maruyama, K. J. Chem. Soc., Chem. Commun. 1985, 1131. (d) Yamamoto,
Y.; Nishii, S.; Maruyama, K.; Komatsu, T.; Ito, W. J. Am. Chem. Soc.
1986, 108, 7778. (e) Yamamoto, Y.; Ito, W. Tetrahedron 1988, 44, 5415.
(14) To understand the correlation between the bulkiness of the ester
moieties and regio- and diastereoselectivity, we examined different
auxiliaries for a model reaction, and poor results are reported in the
Supporting Information (eqs S1−S3).
(1) Reviews for Grignard reagents: (a) Wakefield, B. J. Organo-
magnesium Methods in Organic Chemistry; Academic Press: San Diego,
CA, 1995. (b) Silverman, G. S.; Rakita, P. E. Handbook of Grignard
Reagents; Marcel Dekker: New York, 1996. (c) Richey, H. G., Jr.
Grignard Reagents: New Development; Wiley: Chichester, UK, 2000.
(d) Knochel, P. Handbook of Functionalized Organometallics; Wiley-
VCH: Weinheim, Germany, 2005. (e) Yorimitsu, H. Aryl Grignard
Reagents. Science of Synthesis, Knowledge Updates (2011); George
Thieme: Stuttgart, 2010; Chapter 7.6.5.6, pp 1−9. (f) Yorimitsu, H.
Alkyl Grignard Reagents. In Science of Synthesis, Knowledge Updates
(2011); George Thieme: Stuttgart, 2010; Chapter 7.6.10.9, pp 11−19.
(2) For selected reviews and accounts: (a) Dickstein, J. S.; Kozlowski,
M. C. Chem. Soc. Rev. 2008, 37, 1166. (b) Shimizu, M.; Hachiya, I.;
Mizota, I. Chem. Commun. 2009, 874. (c) Shimizu, M. Pure Appl. Chem.
2006, 78, 1867. (d) Hata, S.; Shimizu, M. J. Synth. Org. Chem., Jpn. 2011,
69, 1134.
(15) Mizutani, Y.; Tanimoto, H.; Morimoto, T.; Nishiyama, Y.;
Kakiuchi, K. Tetrahedron Lett. 2012, 53, 5903.
(16) Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem. Soc. 2003, 125,
5139.
(17) (a) Iritani, K.; Matsubara, S.; Utimoto, K. Tetrahedron Lett. 1988,
29, 1799. (b) Rudisill, D. E.; Stille, J. K. J. Org. Chem. 1989, 54, 5856.
(18) N-Indolyl acetic acids are bioactive compounds in intracellular
signaling pathways such as apoptosis. (a) van Zandt, M. C.; Jones, M. L.;
Gunn, D. E.; Geraci, L. S.; Jones, J. H.; Sawicki, D. R.; Sredy, J.; Jacot, J.
L.; DiCioccio, A. T.; Petrova, T.; Mitschler, A.; Podjarny, A. D. J. Med.
Chem. 2005, 48, 3141. (b) Wu, L.; Lu, K.; Packiarajan, M.; Jubian, V.;
Chandrasena, G.; Wolinsky, T. C.; Walker, M. W. Bioorg. Med. Chem.
Lett. 2012, 22, 2167.
(3) (a) Dickstein, J. S.; Fennie, M. W.; Norman, A. L.; Paulose, B. J.;
Kozlowski, M. C. J. Am. Chem. Soc. 2008, 130, 15794. (b) Curto, J. M.;
Dickstein, J. S.; Berritt, S.; Kozlowski, M. C. Org. Lett. 2014, 16, 1948.
(c) Curto, J. M.; Kozlowski, M. C. J. Org. Chem. 2014, 79, 5359.
(4) (a) Niwa, Y.; Takayama, K.; Shimizu, M. Tetrahedron Lett. 2001,
42, 5473. (b) Mizota, I.; Matsuda, Y.; Kamimura, S.; Tanaka, H.;
Shimizu, M. Org. Lett. 2013, 15, 4206. (c) Tanaka, H.; Mizota, I.;
Shimizu, M. Org. Lett. 2014, 16, 2276.
(19) Some alkylzinc(II)ates are commercially available from TCI and
Wako as named ‘Alkyl Z reagents’.
(5) Hatano, M.; Yamashita, K.; Mizuno, M.; Ito, O.; Ishihara, K. Angew.
Chem., Int. Ed. 2015, 54, 2707.
(6) We have reported the catalytic and stoichiometric use of ZnCl2
with Grignard reagents for alkyl addition to ketones. (a) Hatano, M.;
Suzuki, S.; Ishihara, K. J. Am. Chem. Soc. 2006, 128, 9998. (b) Hatano,
M.; Suzuki, S.; Ishihara, K. Synlett 2010, 321. (c) Hatano, M.; Ito, O.;
Suzuki, S.; Ishihara, K. Chem. Commun. 2010, 46, 2674. (d) Hatano, M.;
Ito, O.; Suzuki, S.; Ishihara, K. J. Org. Chem. 2010, 75, 5008.
(7) Zinc(II)ate reagents in recent organic synthesis: (a) Harada, T.;
Katsuhira, T.; Osada, A.; Iwazaki, K.; Maejima, K.; Oku, A. J. Am. Chem.
Soc. 1996, 118, 11377. (b) Uchiyama, M.; Furumoto, S.; Saito, M.;
Kondo, Y.; Sakamoto, T. J. Am. Chem. Soc. 1997, 119, 11425.
(c) Uchiyama, M.; Kameda, M.; Mishima, O.; Yokoyama, N.; Koike,
M.; Kondo, Y.; Sakamoto, T. J. Am. Chem. Soc. 1998, 120, 4934.
D
DOI: 10.1021/acs.orglett.5b00927
Org. Lett. XXXX, XXX, XXX−XXX