A simple, rapid, and efficient N-Boc protection of amines under ultrasound irradiation and catalyst-free conditions

Article abstract of DOI:10.1007/s00706-013-1094-4

A green and simple approach for the N-Boc protection on structurally diverse amines under ultrasound irradiation is described. Selective N-Boc protection was achieved in excellent isolated yield in a short reaction time at room temperature. Mild conditions, inexpensive and an easily available reagent, and absence of any auxiliary substances are the main advantages of this procedure. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-013-1094-4

Monatsh Chem (2014) 145:509–515
DOI 10.1007/s00706-013-1094-4
ORIGINAL PAPER
A simple, rapid, and efficient N-Boc protection of amines
under ultrasound irradiation and catalyst-free conditions
•
•
•
`
A¨ıcha Amira Hacene K’tir Malika Berredjem
Nour-Eddine Aouf
Received: 2 July 2013 / Accepted: 23 September 2013 / Published online: 26 October 2013
Ó Springer-Verlag Wien 2013
Abstract A green and simple approach for the N-Boc
protection on structurally diverse amines under ultrasound
irradiation is described. Selective N-Boc protection was
achieved in excellent isolated yield in a short reaction time
at room temperature. Mild conditions, inexpensive and an
easily available reagent, and absence of any auxiliary
substances are the main advantages of this procedure.
hydrogenolysis and the extreme resistance to basic and
nucleophilic conditions [4, 5], and its labile nature under
several chemical transformations [1].
Several methods have been described to introduce the
Boc protecting group, using di-tert-butyl dicarbonate,
(Boc)2O, under a variety of conditions. N-Boc protection is
frequently reported by base-catalysed reactions using
DMAP [6], aq. NaOH [5], NaHMDS [7], or Lewis acids-
catalysed reactions, such as ZrCl4 [8], LiClO4 [9], HClO4/
SiO2 [10], Cu(BF4)2Á9H2O [11], Zn(ClO4)2Á6H2O [12],
yttria-zirconia [13], La(NO3)3Á6H2O [14], montmorillonite
K-10 [15], amberlyst-15 [16], H3PW12O40 [17], and sul-
famic acid [18]. Many of these methods suffer from
disadvantages such as acidity, high cost, toxicity, corro-
siveness, and requirement of auxiliary substances in the
isolation of the product [19].
Keywords Boc Á Protection Á Ultrasound Á
Green chemistry Á Amines
Introduction
In modern synthetic chemistry, finding a suitable strategy
for protection and deprotection of functional groups con-
stitutes one of the most challenging tasks. Protection of the
amine group is very important due to their presence in
various biologically active compounds [1–3]. The tert-
butoxycarbonyl (Boc) group has a widely useful func-
tionality for the protection of amine among various
protecting groups. The greatest attention is due to the
extreme stability of the N-Boc group toward catalytic
In recent years, the emergence of the sustainable devel-
opment of ‘‘green chemistry’’ has led to new solutions to
existingproblemsinprotectinggroupchemistry.Ionicliquids,
as eco-friendly solvents, catalysts and reagents in green syn-
thesis [20], such as 1-methylimidazolium tetrafluoroborate
[(HMIm)BF4] [21], 1,1,3,3-tetramethylguanidine acetate
[TMG][Ac] [22], 1,3-disulfoimidazolium hydrogen sulfate
[Dsim]HSO4 [23], and 1-alkyl-3-methylimidazolium cation
are employed for N-Boc protection [24]. Furthermore, water
has attracted much attention as a green solvent. An environ-
mentally benign approach is described where the N-Boc
derivatives were prepared chemoselectively in water [25].
In our previous work, eco-sustainable methods for pro-
tection/deprotection of amines and alcohols with t-Boc
group were carried by fusion [26], using heteropolyanion
[27], and in water [28–30]. More recently, Dighe et al. [31]
have reported a green approach for the synthesis of Boc-
protected amines under microwaves without using of sol-
vents and catalysts.
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-013-1094-4) contains supplementary
material, which is available to authorized users.
A. Amira Á H. K’tir Á M. Berredjem Á N.-E. Aouf (&)
LCOA, Bioorganic Chemistry Group, Sciences Faculty,
Chemistry Department, Badji Mokhtar-Annaba University,
Box 12, 23000 Annaba, Algeria
e-mail: noureddineaouf@yahoo.fr;
noureddine.aouf@univ.annaba.dz
A. Amira
e-mail: minaamira75@yahoo.fr
123

510
A. Amira et al.
In the last decade, the sonochemical approach has been
urea [6], and N,N-di-Boc derivatives [36] were detected by
TLC of the crude products (entries 1–8).
widely used in various types of organic transformations
[32]. This method is not only simple and efficient but also
can assist in preserving green chemistry concepts [33].
Sonochemistry offers a more efficient and facile method
for a large variety of syntheses in comparison to classical
protocols, e.g., an N-sulfonylation reaction using Zn-Al-
hydrotalcites solid base catalyst under ultrasound irradia-
tion in ethanol has been developed [34].
In order to explore the generality of this method, we also
attempted the protection of b-aminoalcohol under the same
reaction conditions (Table 1, entries 12–15). A notable che-
moselectivity of the protection has been established where the
amine functional was only protected without competitive
formation of O-Boc or oxazolidinone derivatives [6].
The mildness of this procedure was next illustrated by a
range of a-aminoesters (entries 16–20), the reactions
worked very well, a methyl ester group was resisted and the
optically pure NH-Boc derivative was confirmed by optical
rotation and comparison with the literature [37, 38], where
the configuration of the chiral center is not affected under
reaction conditions.
In continuation of our interest to improve a facile process
under green conditions not including catalysts and solvents,
we report here the use of ultrasonic irradiation for selective
tert-butoxycarbonylation of various amine derivatives.
Results and discussion
To increase the scope of this reaction, we attempted the
N-tert-butoxycarbonylation to sulfamides synthesized from
a-aminoesters [39–41]. In view of their importance, they
were used as substrates (Table 1, entries 21–25) and were
tested in this method to verify electron-withdrawing effect
of substituents on the formation of pure NH-Boc products.
All N-protected sulfamides were obtained in excellent
yields and the reaction preserves stereochemical integrity
of N-Boc amino acids [42].
Herein we studied the N-tert-butoxycarbonylation of
structurally diverse amines using ultrasound irradiation. In
an initial attempt, we reacted 1 mmol of aniline with
1.1 mmol of di-tert-butyl dicarbonate in the absence of any
solvent or catalyst; after 5 min the reaction was completed
with an excellent yield (Scheme 1).
To find the effect of ultrasound, the same reaction was
carried out under the same conditions in the absence of
ultrasound irradiation. No reaction occurs after 5 h work-
ing time, this shows the essential role of ultrasound
irradiation. This excellent result encourages us to extend
this study to various structurally amines.
The presented results demonstrate the specific ultrasonic
effect on N-tert-butoxycarbonylation giving pure product
with quantitative yields in a few minutes.
The ultrasonic energy applying without any base or
acid catalyst to the reaction generates the acoustic cavi-
tation mechanical effect when sonic waves propagate
through the medium. In solids, both longitudinal and
transverse waves can be transmitted whereas in liquids
only longitudinal waves can be transmitted [43–45].
Vibrations of molecules generate compressions and rar-
efactions which give rise to the phenomenon of bubble
formation and collapse in the reaction mixture [amine
and reactant (Boc)2O] and facilitate the nucleophilic
attack of the amino functional on the carbonyl group.
During cavitation, the chemical bonds break, and carbon
dioxide and the tert-butanol were eliminated to afford the
N-Boc amine (Scheme 3).
To optimize our protocol, we also applied our reaction
conditions to a number of primary and secondary aromatic
and aliphatic (cyclic and acyclic) amines (Scheme 2). In all
cases we obtained the N-Boc products in short reaction times
with quantitative yields (Table 1, entries 1–11). No com-
petitive side reactions lending formation of isocyanate [35],
Scheme 1
NHBoc
NH2
(Boc)2O, )))
In conclusion, the ultrasound irradiation allowed for the
highly chemoselective, simple, efficient, environmentally
benign N-Boc protection of various aliphatic and aromatic
amine derivatives under solvent free conditions, in short
reaction times and excellent isolated yields without for-
mation of isocyanate, urea, N,N-di-t-Boc, or O-t-Boc as
side products. In contrast to conventional energy sources of
traditional methods suffering from harsh conditions, this
easier manipulation delineates the scope of this technique
and offers potential in different applications in organic
transformations.
Solvent-free, r.t.
Scheme 2
(Boc)2O , )))
R1R2NH
R1R2NBoc
Solvent-free, r.t.
R1 and R2 = H, alkyl, or aryl
123

A simple, rapid, and efficient N-Boc protection of amines
511
Table 1 N-tert-Butoxycarbonylation of amines under ultrasound irradiation
Entry
1
Substrate
Product
Time /min
5
M.p. /°C
132
(132 [14])
92-94
2
3
3
3
(94-96 [14])
34-36
146
4
5
3
5
(146 [14])
56-58
(52-54 [14])
6
4
80-82
64-66
7
8
2
2
(65-67 [42])
30-32
9
6
4
29-30
71-73
10
(70-71 [42])
11
12
4
2
29-30
32-33
13
2
Oil
123

512
A. Amira et al.
Table 1 continued
Entry
Substrate
Product
Time /min
2
M.p. /°C
104-106
14
96-98
15
2
2
(96-97 [42])
40-41
16
17
18
19
(39-41 [43])
2
2
2
Oil [43]
33-35
29-30
109
20
21
2
6
89-90
[27]
67-68
[27]
22
23
6
6
75-77
131-132
[27]
24
25
7
7
78-80
123

A simple, rapid, and efficient N-Boc protection of amines
513
Scheme 3
H
O
O
O
O
O
O
)))
R1
+ t-BuOH
O
O
O
O
O
O
N
R2
N
CO2
R1
R1R2-NH
R2
N-(tert-Butoxycarbonyl)propylamine (8, C8H17NO2)
Experimental
White solid, m.p.: 30–32 °C; 1H NMR (300 MHz, CDCl3):
d = 0.80 (t, J = 6.0 Hz, 3H), 1.47 (s, 9H), 1.59 (m, 2H),
2.96 (t, J = 5.8 Hz, 2H, CH2-NH), 4.55 (s, 1H, NH) ppm;
13C NMR (74 MHz, CDCl3): d = 11.5 (CH3), 24.8 (CH2),
27.7 (3 CH3), 46.3 (CH2), 80.1 (C), 156.9 (C = O) ppm.
All commercial chemicals were used without further
purification. Sonication was performed in a FUNGILAB
ultrasonic bath with a frequency of 40 kHz and a power of
250 W. All reactions were monitored by thin layer chro-
matography (TLC) on silica Merck 60 F254 percolated
N-(tert-Butoxycarbonyl)diphenylamine (9, C17H19NO2)
White solid, m.p.: 29–30 °C; 1H NMR (300 MHz, CDCl3)
d = 1.49 (s, 9H), 6.82–7.31 (m, 10H, 2 Ph) ppm; 13C NMR
(74 MHz, CDCl3): d = 27.2 (3 CH3), 83.0 (C), 117.5 (4
CH), 120.6 (2 CH), 129.1 (4 CH), 143.1 (2C), 146.7
(C = O) ppm.
1
aluminum plates. H and 13C NMR spectra were recorded
in a 250 or 300 MHz Bruker spectrometer. Chemical shifts
are reported in d units (ppm) with tetramethylsilane (TMS)
as a reference. All coupling constants (J) are reported in
Hertz. Multiplicity is indicated as s (singlet), d (doublet), t
(triplet), q (quartet), m (multiplet). Optical rotations were
measured on a JUSCO DIP-370 digital polarimeter.
N-(tert-Butoxycarbonyl)-1,2,3,4-tetrahydroisoquinoline
(11, C14H19NO2)
White solid, m.p.: 29–30 °C; 1H NMR (300 MHz, CDCl3):
d = 1.54 (s, 9H), 2.85 (t, J = 7.8 Hz, 2H), 3.66(t,
J = 7.8 Hz, 2H), 4.59 (s, 2H), 7.12-7.28 (m, 4H, Ar)
ppm; 13C NMR (74 MHz, CDCl3): d = 28.4 (3 CH3), 31.7
(CH2), 49.9 (CH2), 53.9 (CH2), 81.0 (C), 128.3 (CH), 129.1
(CH), 131.1 (CH), 134.7 (CH), 145.6 (C), 147.9 (C), 156.0
(C = O) ppm.
General procedure for tert-butoxycarbonylation
of amines
Amine (1 mmol) and di-tert-butyl dicarbonate [(Boc)2O,
1.1 mmol] were placed in a glass tube under neat condi-
tions and were sonicated for a suitable time (as indicated in
Table 1). All reactions were performed in a water bath at
room temperature. After completion of the reaction (as
indicated by TLC), 5 cm3 of diethyl ether was added to the
mixture, the resulting tert-butanol was freely soluble in
diethyl ether and the N-Boc product was crystallized.
Purification of the product was accomplished by recrys-
tallization from diethyl ether.
N-(tert-Butoxycarbonyl)-2-hydroxyethylamine
(12, C7H15NO3)
White solid, m.p.: 32–33 °C; 1H NMR (250 MHz, CDCl3):
d = 1.45 (s, 9H, N-t-Bu), 3.25 (q, J = 5.2 Hz, 2H, CH2-
NH), 3.65 (t, J = 5.1 Hz, 2H, CH2-OH), 5.25 (s, 1H, NH)
ppm; 13C NMR (62 MHz, CDCl3): d = 28.2 (3 CH3), 43.6
(CH2-NH), 62.5 (CH2-OH), 80.0 (C), 157.3 (C = O) ppm.
N-(tert-Butoxycarbonyl)-2-methoxyaniline
(3, C12H17NO3)
White solid, m.p.: 34–36 °C; 1H NMR (300 MHz, CDCl3):
d = 1.50 (s, 9H), 3.75 (s, 3H), 6.56 (s, 1H, NH), 6.81–7.26
(m, 4H, Ar) ppm; 13C NMR (74 MHz, CDCl3): d = 28.4 (3
CH3), 55.4 (CH3), 80.1 (C), 114.1 (CH), 116.1 (CH), 120.6
(CH), 131.5 (CH), 139.8 (C), 153.4 (C), 155.6 (C = O).
(S)(-)-N-(tert-Butoxycarbonyl)valinol (13, C10H21NO3)
Oil; 1H NMR (300 MHz, CDCl3): d = 0.92 (d,
J = 8.5 Hz, 3H), 0.94 (d, J = 8.5 Hz, 3H), 1.44 (s, 9H),
1.86 (m, 1H, CH), 3.39 (m, 2H), 4.96 (m, 1H, *CH), 5.35
(s, 1H, NH) ppm; 13C NMR (CDCl3,74 MHz): d = 18.3
(CH3), 19.36 (CH3), 28.2 (3 CH3), 57.8 (CH), 60.2 (CH2),
63.4 (*CH), 79.1(C), 156.6 (C = O) ppm.
N-(tert-Butoxycarbonyl)-2-phenylethylamine
(6, C13H18NO2)
White solid, m.p.: 80–82 °C; 1H NMR (300 MHz, CDCl3):
d = 1.44 (d, J = 6.1 Hz, 3H), 1.53 (s, 9H), 2.18 (m, 1H),
4.79 (s, 1H, NH), 7.11–7.53 (m, 5H, Ar) ppm; 13C NMR
(74 MHz, CDCl3): d = 23.5 (CH3), 28.7 (3 CH3), 47.5
(*CH), 80.1 (C), 126.5 (CH), 127.9 (2 CH), 128.7 (2 CH),
142.4 (C), 157.6 (C = O) ppm.
(S)(-)-N-(tert-Butoxycarbonyl)leucinol (14, C11H23NO3)
White solid, m.p.: 104–106 °C; 1H NMR (250 MHz,
CDCl3): d = 0.92 (d, J = 7.5 Hz, 6H), 1.27 (m, 2H), 1.5
(s, 9H), 1.65 (dd, J = 5.7, 7.0 Hz, 2H, CH2-OH), 1.90 (m,
1H), 4.25 (m, 1H, *CH), 4.80 (d, J = 8.3 Hz, 1H, NH)
ppm; 13C NMR (62 MHz, CDCl3): d = 21.4 (2 CH3), 23.7
123

514
A. Amira et al.
The authors are thankful to students of Master II: Rachida Mansouri
and Louar Radia for their help in this work.
(CH), 27.9 (3 CH3), 42.0 (CH2), 45.3 (*CH), 64.2 (CH2),
83.7 (C), 151.8 (C = O) ppm.
(S)(-)-N-(tert-Butoxycarbonyl)serine methyl ester
(18, C9H16NO5)
References
White solid, m.p.: 33–35 °C; 1H NMR (250 MHz, CDCl3):
d = 1.47 (s, 9H), 3.68 (s, 3H, OCH3), 4.43 (dd, J = 7.8,
6.0 Hz, 2H, CH2), 4.61 (m, 1H, *CH), 5.10 (d, J = 8.0 Hz,
1H, NH) ppm; 13C NMR (62 MHz, CDCl3): d = 28.8 (3
CH3), 54.0 (CH3), 61.0 (CH2), 69.3 (*CH), 79.5 (C), 157.0
(C = O), 172.0 (C = O) ppm.
1. Wuts PGM, Greene TW (2007) Greene’s protective groups in
organic synthesis, 4th edn. John Wiley & Sons, New York
2. Kocienski PJ (2000) Protecting groups, 3rd edn. Georg Thieme
Verlag, Stuttgart
3. Jarowinski K, Kocienski P (2001) J Chem Soc Perkin Trans
1:2109
4. Agami C, Couty F (2000) Tetrahedron 58:2701
5. Lutz C, Lutz V, Knochel P (1998) Tetrahedron 54:6385
6. Basel Y, Hassner A (2000) J Org Chem 65:6368
7. Kelly TA, McNeil DW (1994) Tetrahedron Lett 35:9003
8. Sharma GVS, Reddy JJ, Lakshmi PS, Krishna PR (2004) Tetra-
hedron Lett 45:6963
9. Heydari A, Hosseini SE (2005) Adv Synth Cat 347:1929
10. Chakraborti AK, Chankeshwara SV (2006) Org Biomol Chem
4:2769
(S)(-)-N-(tert-Butoxycarbonyl)cysteine methyl ester
(19, C9H16NO5S)
White solid, m.p.: 29–30 °C; 1H NMR (250 MHz, CDCl3):
d = 1.47 (s, 9H), 3.23 (dd, J = 8.0, 6.8 Hz, 2H, CH2), 3.71
(s, 3H, OCH3), 4.81 (m, 1H, *CH), 5.50 (d, J = 8.0 Hz,
1H, NH) ppm; 13C NMR (62 MHz, CDCl3): d = 28.8 (3
CH3), 40.0 (CH2), 54.0 (CH3), 60.3 (*CH), 80.5 (C), 157.2
(C = O), 171.8 (C = O) ppm.
11. Chankeshwara SV, Chakraborti AK (2006) Tetrahedron Lett
47:1087
12. Bartoli G, Bosco M, Locatelli M, Marcantoni E, Massaccesi M,
Melchiorre P, Sambri L (2004) Synlett 10:1794
13. Pandey RK, Dagade SP, Upadhyay RK, Bongare MK, Kumar P
(2002) Arkivoc 7:28
14. Suryakiran N, Prabhakar P, Reddy ST, Rajesh K, Venkateswarlu
Y (2006) Tetrahedron Lett 47:8039
15. Chakraborti AK, Chankeshwara SV (2006) J Mol Cat A 253:198
16. Jahani F, Tajbakhsh M, Khaksar S, Reza Azil M (2011) Mon
Chem 142:1035
17. Heydari A, Kazem Shiroodi R, Esfandyari M, Pourayoubi M
(2007) Tetrahedron Lett 48:5865
N-(tert-Butoxycarbonyl)glycine methyl ester
(20, C8H15NO4)
1
White solid, m.p.: 109 °C; H NMR (250 MHz, CDCl3):
d = 1.49 (s, 9H), 3.75 (d, J = 6.0 Hz, 2H, CH2), 3.71 (s,
3H, OCH3), 6.10 (t, J = 8.0 Hz, 1H, NH) ppm; 13C NMR
(62 MHz, CDCl3): d = 28.5 (3 CH3), 49.0 (CH2), 52.9
(CH3), 80.5 (C), 157.2 (C = O), 171.8 (C = O) ppm.
(S)(-)-N-(tert-Butoxycarbonylaminosulfonyl)isoleucine
methyl ester (23, C12H24N2O6S)
18. Upadhyaya DJ, Barge A, Stefania R, Gravotto G (2007) Tetra-
hedron Lett 48:5865
White solid, m.p.: 75–77 °C; 1H NMR (250 MHz, CDCl3):
d = 0.90 (t, J = 6.2 Hz, 3H, CH3), 0.96 (d, J = 5.8 Hz,
3H, CH3), 1.55 (s, 9H), 1.71 (m, 2H, CH2), 1.87 (m, 1H,
CH), 3.70 (s, 3H, OCH3), 4,25 (m, 1H, *CH), 5.80 (d,
J = 8.9 Hz, 1H, NH-*C), 7.60 (s,1H, NH-Boc) ppm; 13C
NMR (62 MHz, CDCl3): d = 20.4 (CH3), 21.7 (CH3), 24.6
(CH2), 28.1 (3 CH3), 42.8 (CH), 53.1 (CH3), 55.0 (*CH),
80.7 (C), 148.8 (C = O), 171.7 (C = O) ppm.
19. Sartori G, Maggi R (2004) Chem Rev 104:199
20. Wasserscheid P, Welton T (2003) Ionic liquids in synthesis.
Wiley-VCH, Weinheim
21. Sunitha S, Kanjilal S, Reddy PS, Prasad RBN (2008) Tetrahedron
Lett 49:2527
22. Akbari J, Heydari A, Ma’mani L, Hassan Hosseini S (2010) C R
Chimie 13:544
23. Zolfigol MA, Khakyzadeha V, Moosavi- Zarea AR, Chehardoli
G, Derakhshan-Panaha F, Zare A, Khalediana O (2012) Sci Iran
Trans C 19:1584
24. Shirini F, Nader Ghafari K (2013) J Mol Liq 177:386
25. Chankeshwara SV, Chakraborti AK (2006) Org Lett 8:3259
26. Klai N, Berredjem M, Khettache N, Belghit MY, Regainia Z,
Aouf N (2004) J Heterocycl Chem 41:57
27. Belghiche R, Cheraiet Z, Berredjem M, Abbessi M, Aouf N
(2012) Eur J Chem 3:305
28. Cheraiet Z, Ouarna S, Jamel Z, Berredjem M, Aouf N (2012) Int J
Chem 4:3
29. Cheraiet Z, Ouarna S, Hessainia S, Berredjem M, Aouf N (2012)
ISRN Org Chem 2012:8
30. Cheraiet Z, Hessainia S, Ouarna S, Berredjem M, Aouf N (2013)
Green Chem Lett Rev 6:211
31. Dighe SN, Jadhav HR (2012) Tetrahedron Lett 53:5803
32. Gravotto G, Cintas P (2006) Chem Soc Rev 35:180
33. Dilbeck GA, Field L, Gallo AA, Gargiulo RJ (1978) J Org Chem
43:4593
34. Mokhtar M, Saleh TS, Ahmed NS, Al-Thabaiti SA, Al-Shareef
RA (2011) Ultra Sonochem 18:172
35. Knoelker HJ, Braxmeier T (1996) Tetrahedron Lett 37:5861
(S)(-)-N-(tert-Butoxycarbonylaminosulfonyl)methionine
methyl ester (25, C11H22N2O6S2)
White solid, m.p.: 78–80 °C; 1H NMR (250 MHz, CDCl3):
d = 1.48 (s, 9H), 2.18 (s, 3H, CH3), 2.27 (m, 2H, CH2),
2.67 (t, J = 6.5 Hz, 2H, CH2), 3.38 (m, 1H, *CH), 3.70 (s,
3H, OCH3), 6.30 (d, J = 8.4 Hz, 1H, NH-*C), 7.80 (s, 1H,
NH-Boc) ppm; 13C NMR (62 MHz, CDCl3): d = 23.8
(CH3), 28.7 (3 CH3), 29.6 (CH2), 30.1 (CH2), 53.2
(CH3), 51.8 (*CH), 80.1 (C), 158.2 (C = O), 171.5
(C = O) ppm.
Acknowledgments This work was generously supported by the
´
´
Direction Generale de la Recherche Scientifique et du Developpement
Technologique, DGRS-DT, Algerian Ministry of Scientific Research
(FNR), and fruitful discussions with Dr. Malika Ibrahim-Ouali,
´
Universite d’Aix Marseille II France, and Dr. Zinelaabidine Cheraiet.
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A simple, rapid, and efficient N-Boc protection of amines
515
36. Darnbrough S, Mervic M, Condon SM, Burns CJ (2001) Synth
Comm 31:3273
41. Bouchouk D, Colacino GE, Toupet L, Dewynter G, Aouf N,
Martinez J (2009) Tetrahedron Lett 50:1100
37. Marcantoni E, Massaccesi M, Torregiani E (2001) J Org Chem
66:4430
42. Kishore PS, Viswanathan B, Varadarajan TK (2006) Tetrahedron
Lett 47:429
38. Anderson GW, McGregor AC (1957) J Am Chem Soc 79:6180
39. Dewynter G, Aouf N, Regainia Z, Montero JL (1996) Tetrahe-
dron 52:993
40. Baronetti GT, Briand L, Sedran U, Thomas H (1998) Appl Cat A
172:265
43. Nasir Baig RB, Varma RS (2012) Chem Soc Rev 41:1559
44. Vilaivan T (2006) Tetrahedron Lett 47:6739
45. Jahani F, Tajbakhsh M, Golchoubian H, Khaksar S (2011) Tet-
rahedron Lett 52:1260
123