Liquid-phase split-type combinatorial synthesis of tripeptide derivatives encoded by fluorous fmoc reagents

Article abstract of DOI:10.1055/s-0033-1339924

A liquid-phase mixture synthesis of 18 tripeptides, some of which are analogues of ACE inhibitors, was effectively conducted by using fluorous Fmoc reagents as encoding tags. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1339924

LETTER
▌2701
letter
Liquid-Phase Split-Type Combinatorial Synthesis of Tripeptide Derivatives
Encoded by Fluorous Fmoc Reagents
Combinatorial Synthesis of Tripeptide Derivatives
Yuya Sugiyama, Masaki Hirose, Junko Matsui, Natsuki Endo, Hiromi Hamamoto, Takayuki Shioiri, Masato Matsugi*
Department of Applied Biological Chemistry, Faculty of Agriculture, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502,
Japan
E-mail: matsugi@meijo-u.ac.jp
Received: 31.07.2013; Accepted after revision: 10.09.2013
the dipeptide synthesis was accomplished by using the
Abstract: A liquid-phase mixture synthesis of 18 tripeptides, some
HBTU/HOBt method.9
of which are analogues of ACE inhibitors, was effectively conduct-
ed by using fluorous Fmoc reagents as encoding tags.
IC50
(mmol/l)
Key words: combinatorial chemistry, liquid-phase synthesis, mix-
ture synthesis, fluorous tag, peptides
Ala-Ala-Val-OBn
Ala-Ala-Ala-OBn
Ala-Val-Ala-OBn
Phe-Val-Leu-OBn
Phe-Ala-Val-OBn
Phe-Ala-Leu-OBn
Phe-Val-Leu-OBn
Phe-Val-Met-OMe..
Leu-Ala-Val-OBn
Leu-Val-Met-OMe
Leu-Val-Ala-OBn
Leu-Val-Leu-OBn
Leu-Val-Met-OMe
25
93
Ala-Ala-Leu-OBn
Ala-Val-Leu-OBn
Ala-Val-Met-OMe
Phe-Val-Ala-OBn
Leu-Ala-Ala-OBn
7.1
8
Recently, we reported a fluorous mixture synthesis1 of
fluorous-Fmoc reagents (f-Fmoc reagents)2 1a–c through
a one-pot, double fluorous-tagging strategy (Figure 1).3
This method provides an efficient way of obtaining vari-
ous f-Fmoc reagents that are required for fluorous mixture
peptide synthesis. Although we had alluded to the possi-
bility of a liquid-phase combinatorial synthesis of various
peptides by using f-Fmoc reagents in a past report,3 until
now we have not had the opportunity to attempt it. Here,
we would like to describe the first example of a liquid-
phase-mixture synthesis of a variety of tripeptide ACE in-
hibitors encoded by f-Fmoc tags with differing fluorine
content.4 We chose some C-protected tripeptides as the
target compounds for the model experiment of a liquid-
phase combinatorial synthesis of peptides (Figure 2). The
deprotected compounds indicated in bold-type are known
ACE inhibitors,5 and the IC50 value of each compound is
listed in Figure 2.
6
13
Figure 2 Target tripeptides containing ACE inhibitors
1a (C3F7-f-Fmoc)
Ala-OH
f14-Fmoc-Ala-OH
f18-Fmoc-Phe-OH
MeCN, r.t., 2 h, 98%
1b (C4F9-f-Fmoc)
Phe-OH
1,4-dioxane
0 °C to r.t., 2 h, 95%
1c (C6F13-f-Fmoc)
f26-Fmoc-Leu-OH
Leu-OH
1,4-dioxane
0 °C to r.t., 2 h, 89%
Scheme 1 f-Fmoc protection of starting amino acids
One group was reacted with L-alanine benzyl ester in 95%
yield, while the other group was reacted with L-valine
benzyl ester in 98% yield (conditions a and b in Scheme
2).10 Subsequently, after deprotection of the C-terminus
by hydrogenolysis11 of the mixture of the two groups in
quantitative yields (conditions c and d in Scheme 2), each
mixture was divided into three groups. Each group of the
mixture was allowed to react with four different C-pro-
tected amino acids (Ala-OBn, Val-OBn, Leu-OBn, and
Met-OMe) using the same condensation method (condi-
tions e–j in Scheme 2); the yields were satisfactory, being
in the range 86–98%.
Rf
Rf
O
O
N
O
O
O
1a: Rf = C3F7
1b: Rf = C4F9
1c: Rf = C6F13
Figure 1 Fluorous Fmoc reagents
First, as shown in Scheme 1, each amino acid of L-alanine,
L-phenylalanine, and L-leucine was protected with
f-Fmoc reagents bearing different fluorine content.6–8 Af-
ter mixing these protected amino acids, the mixture was
divided into two groups and the condensation reaction for
We then separated each of the pure C-protected tripep-
tides from the mixture of each of the six groups (Group A–
F). Figure 3 shows the chart of the analytical fluorous-
HPLC (FluoroFlash® HPLC column; 4.6 mm i.d.; 150
mm length)12 of the mixture (group A) of the tripeptide
benzyl ester protected f-Fmoc compounds (f14-Fmoc-Ala-
Ala-Ala-OBn, f18-Fmoc-Phe-Ala-Ala-OBn, and f26-Fmoc-
Leu-Ala-Ala-OBn).
SYNLETT 2013, 24, 2701–2704
Advanced online publication: 05.11.2013
DOI: 10.1055/s-0033-1339924; Art ID: ST-2013-U0721-L
© Georg Thieme Verlag Stuttgart · New York
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Y. Sugiyama et al.
LETTER
confirmed that these protected peptides were stable under
aqueous conditions, we used the same conditions used for
analysis also for the preparative f-HPLC (FluoroFlash®
HPLC column; 20 mm i.d.; 250 mm length).2 A 200 mg-
scale fluorous-HPLC purification of the f-Fmoc-protected
peptides f14-Fmoc-Ala-Ala-Ala-OBn, f18-Fmoc-Phe-Ala-
Ala-OBn, and f26-Fmoc-Leu-Ala-Ala-OBn led to their
isolation in 43, 63, and 86 mg, respectively, with minimal
loss.14–16
Similarly, preparative f-HPLC of the other five groups
(groups B–F) were conducted, and each pure tripeptide
compound was effectively separated with almost quanti-
tative recovery.17 In this range of the numbers of fluoride,
the difference in the numbers of fluorine in the Fmoc
groups did not significantly affect the characteristics of
the amino acids or peptides. Therefore, regular organic
synthetic procedures could be used. Finally, deprotection
of the f-Fmoc was conducted. Scheme 3 shows the yield
of each deprotection reaction of f-Fmoc group by using
excess diethylamine in acetonitrile under room tempera-
ture conditions.18
Figure 3 Analytical fluorous HPLC chart of a mixture of f14-Fmoc-
Ala-Ala-Ala-OBn, f18-Fmoc-Phe-Ala-Ala-OBn, and f26-Fmoc-Leu-
Ala-Ala-OBn. Reaction conditions: FluoroFlash® HPLC column;
254nm; flow rate: 1.0 mL/min; 0 to 30 min: 80% MeCN–H2O to
100% MeCN; over 30 min: 100% MeCN.
The f14-Fmoc-Ala-Ala-Ala-OBn, f18-Fmoc-Phe-Ala-Ala-
OBn, and f26-Fmoc-Leu-Ala-Ala-OBn appeared at 2.7,
3.4, and 12.5 min, respectively. Thus, a liquid-phase mix-
ture synthesis of various peptides by using the f-Fmoc en-
coding method is possible.13 Since we have already
group A
f14-Fmoc-Ala-Ala-Ala-OBn
e
f
f
18-Fmoc-Phe-Ala-Ala-OBn
26-Fmoc-Leu-Ala-Ala-OBn
f14-Fmoc-Ala-Ala-OBn
f18-Fmoc-Phe-Ala-OBn
f26-Fmoc-Leu-Ala-OBn
a
group B
f14-Fmoc-Ala-Ala-Val-OBn
f
split
f
f
f
14-Fmoc-Ala-Ala-OH
18-Fmoc-Phe-Ala-OH
26-Fmoc-Leu-Ala-OH
c
f18-Fmoc-Phe-Ala-Val-OBn
26-Fmoc-Leu-Ala-Val-OBn
f
mix
f14-Fmoc-Ala-OH
f18-Fmoc-Phe-OH
f26-Fmoc-Leu-OH
group C
f14-Fmoc-Ala-Ala-Leu-OBn
split
g
f18-Fmoc-Phe-Ala-Leu-OBn
26-Fmoc-Leu-Ala-Leu-OBn
f
group D
f14-Fmoc-Ala-Val-Ala-OBn
h
f18-Fmoc-Phe-Val-Ala-OBn
26-Fmoc-Leu-Val-Ala-OBn
f14-Fmoc-Ala-Val-OBn
b
f
f18-Fmoc-Phe-Val-OBn
26-Fmoc-Leu-Val-OBn
f
group E
f14-Fmoc-Ala-Val-Leu-OBn
i
f14-Fmoc-Ala-Val-OH
split
d
f18-Fmoc-Phe-Val-Leu-OBn
26-Fmoc-Leu-Val-Leu-OBn
f18-Fmoc-Phe-Val-OH
26-Fmoc-Leu-Val-OH
f
f
group F
f14-Fmoc-Ala-Val-Met-OMe
j
f
f
18-Fmoc-Phe-Val-Met-OMe
26-Fmoc-Leu-Val-Met-OMe
Scheme 2 Fluorous mixture synthesis of C-protected tripeptides. Reagents and conditions: (a) Ala-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU
(1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h, 95%; (b) Val-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv),
DMF, r.t., 24 h, 98%; (c) Pd/C (5 mol%), 1 atm H2, MeOH–THF, r.t., 3 h, quant.; (d) Pd/C (5 mol%), 1 atm H2, MeOH/THF, r.t., 3 h, quant.;
(e) Ala-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h, 97%; (f) Val-OBn (1.2 equiv), HOBt
(1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h, 98%; (g) Leu-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU (1.2 equiv),
DIPEA (2.4 equiv), DMF, r.t., 24 h, 92%; (h) Ala-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h,
95%; (i) Leu-OBn (1.2 equiv), HOBt (1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h, 98%; (j) Met-OMe (1.2 equiv), HOBt
(1.2 equiv), HBTU (1.2 equiv), DIPEA (2.4 equiv), DMF, r.t., 24 h, 86%; HOBt=1-hydroxybenzotriazole, HBTU= O-benzotriazole-N,N,N′,N′-
tetramethyluroniumhexafluorophosphate, DIPEA= N,N-diisopropylethylamine.
Synlett 2013, 24, 2701–2704
© Georg Thieme Verlag Stuttgart · New York

LETTER
Combinatorial Synthesis of Tripeptide Derivatives
2703
Marel, G. A.; Drijfhout, J. W.; van Boom, J. H.; Noort, D.;
Overkleeft, H. S. Tetrahedron Lett. 2003, 44, 9013.
(5) (a) Miyoshi, S.; Ishikawa, H.; Kaneko, T.; Fukui, F.;
Tanaka, H.; Maruyama, S. Agric. Biol. Chem. 1991, 55,
1313. (b) Suetsuna, K. Kiso to Rinsho 1991, 25, 2245.
(c) Matsui, T.; Yukiyoshi, A.; Doi, S.; Sugimoto, H.;
Yamada, H.; Matsumoto, K. J. Nutr. Biochem. 2002, 13, 80.
(6) Analytical Data for f14-Fmoc-Ala-OH: Pale-yellow solid;
mp 95.7–96.5 °C. 1H NMR (270 MHz, CDCl3): δ = 1.49 (d,
J = 7.0 Hz, 3 H), 2.31–2.51 (m, 4 H), 2.96–3.02 (m, 4 H),
4.16–4.21 (m, 1 H), 4.39–4.48 (m, 3 H), 5.28 (d, J = 7.0 Hz,
1 H), 7.25 (d, J = 7.3 Hz, 2 H), 7.42 (s, 2 H), 7.68 (d, J =
8.1 Hz, 2 H). 19F NMR (466 MHz, CDCl3): δ = –80.4 (6F),
–115.4 (4F), –127.6 (4F); HRMS [FAB+]: m/z calcd for
C28H23NO4F14: 704.1482; found: 704.1456.
(7) Analytical Data for f18-Fmoc-Phe-OH: White solid; mp
122.2–123.1 °C. 1H NMR (270 MHz, CDCl3): δ = 2.33–2.50
(m, 4 H), 2.94–3.00 (m, 4 H), 3.09–3.25 (m, 2 H), 4.13–4.17
(m, 1 H), 4.33–4.47 (m, 2 H), 4.66–4.73 (m, 1 H), 5.21 (d, J
= 7.8 Hz, 1 H), 7.12 (d, J = 6.5 Hz, 2 H), 7.23–7.26 (m, 5 H),
7.39 (d, J = 3.8 Hz, 2 H), 7.67 (d, J = 8.6 Hz, 2 H). 19F NMR
(466 MHz, CDCl3): δ = –80.8 (6F), –114.7 (4F), –124.2
(4F), –125.9 (4F). HRMS [FAB+]: m/z calcd for
Ala-Ala-Ala-OBn Phe-Ala-Ala-OBn Leu-Ala-Ala-OBn...
95% 100% 100%
Ala-Ala-Val-OBn Phe-Ala-Val-OBn Leu-Ala-Val-OBn
95% 92% 100%
Ala-Ala-Leu-OBn Phe-Ala-Leu-OBn Leu-Ala-Leu-OBn
excess Et2NH
group A
or
group B
or
group C
96%
95%
100%
or
group D
or
group E
or
MeCN
Ala-Val-Ala-OBn Phe-Val-Ala-OBn Leu-Val-Ala-OBn
r.t., 0.5–1.5 h
90%
87%
100%
group F
Ala-Val-Leu-OBn Phe-Val-Leu-OBn Leu-Val-Leu-OBn
91% 95% 82%
Ala-Val-Met-OMe Phe-Val-Met-OMe Leu-Val-Met-OMe.
90% 84% 100%
Scheme 3 Yields for the deprotection of the f-Fmoc group
In summary, we have demonstrated a liquid-phase split-
type combinatorial synthesis of a large variety of tripep-
tides, some of which are ACE inhibitors, by using fluo-
rous Fmoc reagents as an encoding tag. If the tripeptides
were synthesized individually by the same linear synthetic
route as outlined in this work, 90 synthetic steps would
have been required; however, we carried out the syntheses
in a mere 31 steps, which includes f-Fmoc protections.
We believe that the f-Fmoc encoding strategy will be one
of the most useful methods for divergent polypeptide syn-
thesis.
C36H27NO4F18: 880.1731; found: 880.1685.
(8) Analytical Data for f26-Fmoc-Leu-OH: White solid; mp
134.5–135.4 °C. 1H NMR (270 MHz, CDCl3): δ = 0.97 (d,
J = 5.4 Hz, 6 H), 1.59–1.76 (m, 3 H), 2.32–2.45 (m, 4 H),
2.96–3.02 (m, 4 H), 4.17–4.22 (m, 1 H), 4.39–4.44 (m, 3 H),
5.11 (d, J = 8.3 Hz, 1 H), 7.24 (d, J = 7.3 Hz, 2 H), 7.43 (s,
2 H), 7.67 (d, J = 8.1 Hz, 2 H). 19F NMR (466 MHz, CDCl3):
δ = –80.6 (6F), –114.4 (4F), –121.7 (4F), –122.7 (4F),
–123.3 (4F), –125.9 (4F). HRMS [FAB+]: m/z calcd for
C37H29NO4F26: 1046.1760; found: 1046.1747.
(9) (a) Konig, W.; Geiger, R. Chem. Ber. 1970, 103, 788.
(b) Konig, W.; Geiger, R. Chem. Ber. 1970, 103, 2024.
(c) Knorrl, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 30, 1927.
Acknowledgment
This research was partially supported by the Ministry of Education,
Science, Sports and Culture, Grant-in-aid for Scientific Research
(C) 23580154 and the Research Institute of Meijo University. We
thank Prof. Dennis P. Curran, University of Pittsburgh, for useful
suggestions for the f-Fmoc synthesis. We also thank Meiji Seika
Pharma Co., Ltd. for funding this work.
(10) Mixture Synthesis for Three-Component Mixture of
f14-Fmoc-Ala-Ala-OBn, f18-Fmoc-Phe-Ala-OBn, and
f26-Fmoc-Leu-Ala-OBn; Typical Procedure: f14-Fmoc-
Ala-OH (560.0 mg, 0.79 mmol), f18-Fmoc-Phe-OH (714.4
mg, 0.79 mmol), and f26-Fmoc-Leu-OH (832.0 mg, 0.79
mmol) were mixed and dissolved in DMF (20 mL). To the
solution were added HOBt·H2O (438.8 mg, 2.86 mmol) and
HBTU (1086 mg, 2.86 mmol), separately. After stirring for
5 min, Ala-OBn (1.01 g, 2.86 mmol) and DIPEA (974 μL,
5.72 mmol) were added to the above reaction mixture
separately. The reaction mixture was stirred for 24 h at room
temperature. After the addition of aq 1.0 M HCl and then
dilution with ethyl acetate, the organic layer was washed
with H2O, sat. aq NaHCO3 and brine, dried over Na2SO4 and
concentrated. The crude residue was purified by silica gel
chromatography (CHCl3–MeOH, 20:1) to give the title
compound (2.4 g, 95% based on the average molecular
weight of the mixture). .
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
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References and Notes
(1) For examples, see: (a) Luo, Z.; Zhang, Q.; Oderaotoshi, Y.;
Curran, D. P. Science 2001, 291, 1766. (b) Manku, S.;
Curran, D. P. J. Org. Chem. 2005, 70, 4470. (c) Kojima, M.;
Nakamura, Y.; Ito, S.; Takeuchi, S. Tetrahedron Lett. 2009,
50, 6143. (d) Reena, B.; Curran, D. P. J. Am. Chem. Soc.
2011, 133, 20435. (e) Yeh, E. A.-H.; Kumli, E.; Damodaran,
K.; Curran, D. P. J. Am. Chem. Soc. 2013, 135, 1577.
(2) Matsugi, M.; Yamanaka, K.; Inomata, I.; Takekoshi, N.;
Hasegawa, M.; Curran, D. P. QSAR Comb. Sci. 2006, 25,
713.
(3) Sugiyama, Y.; Ishihara, K.; Masuda, Y.; Kobayashi, Y.;
Hamamoto, H.; Matsugi, M. Tetrahedron Lett. 2013, 54,
2060.
(4) For examples of peptides synthesis using fluorous tags, see:
(a) Fustero, S.; Sancho, A. G.; Chiva, G.; Sanz-Cervera, J.
F.; del Pozo, C.; Aceña, J. L. J. Org. Chem. 2006, 71, 3299.
(b) Manzoni, L.; Castelli, R. Org. Lett. 2006, 8, 955. (c) de
Visser, P. C.; van Helden, M.; Filippov, D. V.; van der
(11) Wuts, P. G. M.; Greene, T. W. Green’s Protective Groups in
Organic Synthesis; Wuts, P. G. M.; Greene, T. W., Eds.;
Wiley-Interscience: New Jersey, 2007, 3rd ed., 415.
(12) Analytical and preparative f-HPLC were conducted by using
FluoroFlash® columns purchased from Fluorous
Technologies Inc. The corporation is no longer trading;
however, fluorous column (Wakopak® Fluofix-II 120E)
with almost the same separation ability is available from
Wako Pure Chemical Industries, Ltd.
(13) The f-HPLC separation could also be applied to a tripeptide
that was derived from hydrophilic amino acids such as
glycine. The f14-Fmoc-Gly-Gly-Gly-OMe, f18-Fmoc-Gly-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2701–2704

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Y. Sugiyama et al.
LETTER
Gly-Gly-OMe and f26-Fmoc-Gly-Gly-Gly-OMe appeared at
2.4, 3.0, and 6.4 min, respectively, in the analytical f-HPLC
trace under the same conditions.
7.8 Hz, 2 H). 19F NMR (466 MHz, CDCl3): δ = –80.6 (6F),
–114.5 (4F), –124.1 (4F), –125.8 (4F); HRMS [FAB+]: m/z
calcd for C52H45N3O5F18: 1112.2943; found: 1112.2969.
(16) Analytical Data for f26-Fmoc-Leu-Ala-Ala-OBn: White
solid; mp 179.9–181.0 °C. 1H NMR (270 MHz, CDCl3): δ =
0.94 (d, J = 5.4 Hz, 6 H), 1.36–1.42 (m, 6 H), 1.50–1.70 (m,
3 H), 2.35–2.48 (m, 4 H), 2.96–3.02 (m, 4 H), 4.15–4.20 (m,
2 H), 4.36–4.59 (m, 3 H), 4.45–4.59 (m, 1 H), 5.16–5.22 (m,
3 H), 6.48 (d, J = 6.5 Hz, 2 H), 7.32–7.43 (m, 9 H), 7.68 (d,
J = 7.2 Hz, 2 H). 19F NMR (466 MHz, CDCl3): δ = –80.6
(6F), –114.4 (4F), –121.6 (4F), –122.6 (4F), –123.2 (4F),
–125.9 (4F). HRMS [FAB+]: m/z calcd for C52H46N3O6F26:
1278.2971; found: 1278.3009.
(14) Analytical Data for f14-Fmoc-Ala-Ala-Ala-OBn: White
solid; mp 153.8–154.6 °C. 1H NMR (270 MHz, CDCl3): δ =
1.36–1.42 (m, 9 H), 2.31–2.44 (m, 4 H), 2.96–3.02 (m, 4 H),
4.16–4.19 (m, 1 H), 4.22–4.28 (m, 1 H), 4.39–4.50 (m, 3 H),
4.56–4.61 (m, 1 H), 5.13 (d, J = 11.9 Hz, 1 H), 5.19 (d, J =
12.7 Hz, 1 H), 5.44 (d, J = 7.3 Hz, 1 H), 6.55–6.62 (m, 2 H),
7.24 (d, J = 9.9 Hz, 2 H), 7.31–7.36 (m, 5 H), 7.42 (s, 2 H),
7.68 (d, J = 8.1 Hz, 2 H). 19F NMR (466 MHz, CDCl3): δ =
–80.3 (6F), –115.4 (4F), –127.6 (4F). HRMS [FAB+]: m/z
calcd for C41H40N3O6F14: 936.2694; found: 936.2704.
(15) Analytical Data for f18-Fmoc-Phe-Ala-Ala-OBn: White
solid; mp 171.5–172.4 °C. 1H NMR (270 MHz, CDCl3): δ =
1.30 (d, J = 6.8 Hz, 3 H), 1.40 (d, J = 7.3 Hz, 3 H), 2.34–2.44
(m, 4 H), 2.95–3.02 (m, 4 H), 3.07–3.11 (m, 2 H), 4.10–4.15
(m, 1 H), 4.30–4.46 (m, 4 H), 4.52–4.58 (m, 1 H), 5.13 (d, J
= 11.9 Hz, 1 H), 5.19 (d, J = 12.7 Hz, 1 H), 5.35 (d, J =
6.8 Hz, 1 H), 6.38 (d, J = 5.9 Hz, 1 H), 6.45 (d, J = 7.0 Hz,
1 H), 7.13–7.16 (m, 2 H), 7.23–7.40 (m, 12 H), 7.68 (d, J =
(17) Analytical data for these tripeptides are provided in the
Supporting Information
(18) (a) Perich, J. W.; Johns, R. B. Tetrahedron Lett. 1988, 29,
2369. (b) Wuts, P. G. M.; Greene, T. W. Green’s Protective
Groups in Organic Synthesis; Wuts, P. G. M.; Greene, T.
W., Eds.; Wiley-Interscience: New Jersey, 2007, 3rd ed.,
506.
Synlett 2013, 24, 2701–2704
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