Lanthanum(III) Trifluoromethanesulfonate Catalyzed Direct Synthesis of Ureas from N-Benzyloxycarbonyl-, N -Allyloxycarbonyl-, and N -2,2,2-Trichloroethoxycarbonyl-Protected Amines

Article abstract of DOI:10.1055/s-0040-1707991

A novel lanthanum triflate mediated conversion of N -benzyloxycarbonyl-, N -allyloxycarbonyl-, and N -trichloroethoxycarbonyl-protected amines into nonsymmetric ureas was discovered. In this study, lanthanum triflate was found to be an effective catalyst for preparing various nonsymmetric ureas from protected amines. A variety of protected aromatic and aliphatic carbamates reacted readily with various amines in the presence of lanthanum triflate to generate the desired ureas in high yields. This result demonstrated that this novel lanthanum triflate catalyzed preparation of ureas from Cbz, Alloc, and Troc carbamates can be employed for the formation of various urea structures.

Full text of DOI:10.1055/s-0040-1707991

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–F
letter
A
en
T. T. Bui, H.-K. Kim
Letter
Synlett
Lanthanum(III) Trifluoromethanesulfonate Catalyzed Direct Syn-
thesis of Ureas from N-Benzyloxycarbonyl-, N-Allyloxycarbonyl-,
and N-2,2,2-Trichloroethoxycarbonyl-Protected Amines
Tien Tan Buia,b
Hee-Kwon Kim*a,b
R3
NH2
O
0
0
0
0-0
0
0
1-7
6
1
2-
7
0
4
9
O
R2
La(OTf)3 (15 mol%)
R1
NH
O
N
R1
N
R3
H
H
a Department of Nuclear Medicine, Molecular Imaging and
Therapeutic Medicine Research Center, Jeonbuk National Uni-
versity Medical School and Hospital, Jeonju, 54907, Republic
of Korea
R1 = aromatic, aliphatic amines
2 = Bn, allyl, Cl3CCH2
44 examples
up to 96% yield
R
hkkim717@jbnu.ac.kr
b Research Institute of Clinical Medicine of Jeonbuk National
University-Biomedical Research Institute of Jeonbuk National
University Hospital, Jeonju, 54907, Republic of Korea
Received: 09.01.2020
Accepted after revision: 16.02.2020
Published online: 06.03.2020
the hazardous nature of phosgene. Alternatively, 1,1′-car-
bonyldiimidazole and p-nitrophenyl carbamates are other
useful reagents for preparing the desired urea groups.23–26
Despite the widespread employment of these synthetic
procedures, the target ureas are sometimes generated in
low conversion yields. A reaction of CO2 with amines to pro-
duce several urea structures has also been developed,
which obviates the need for phosgene or phosgene equiva-
lents.27–30 Recently, a new synthetic method has been re-
ported for preparing ureas from aliphatic isocyanates, ob-
tained by direct C–H activation.31
DOI: 10.1055/s-0040-1707991; Art ID: st-2020-l0018-l
Abstract A novel lanthanum triflate mediated conversion of N-benzyl-
oxycarbonyl-, N-allyloxycarbonyl-, and N-trichloroethoxycarbonyl-
protected amines into nonsymmetric ureas was discovered. In this study,
lanthanum triflate was found to be an effective catalyst for preparing
various nonsymmetric ureas from protected amines. A variety of pro-
tected aromatic and aliphatic carbamates reacted readily with various
amines in the presence of lanthanum triflate to generate the desired
ureas in high yields. This result demonstrated that this novel lanthanum
triflate catalyzed preparation of ureas from Cbz, Alloc, and Troc carba-
mates can be employed for the formation of various urea structures.
Key words ureas, amines, lanthanum triflate catalysis, benzyloxycar-
bonyl amines, allyloxycarbonyl amines, trichloroethoxycarbonyl amines
Table 1 Screening of Reagents for the Preparation of the Urea Structurea
O
O
catalyst
NH2
+
The urea structure is a component of many biologically
active compounds, natural products, and pharmaceuticals,
including antagonists of natural receptors, enzyme inhibi-
tors, antitumor agents, anti-Parkinson’s agents, plant- and
insect-growth regulators, and antimycobacterial agents.1–9
For example, therapeutic agents such as sorafenib and car-
iprazine contain a urea moiety.10–12 In addition, because of
its unique mode of hydrogen bonding and the polarity of its
structure, the urea framework is found in many organic
materials, and the urea motif is employed in the prepara-
tion of numerous self-assembled molecular tablets and mo-
lecular gels containing macrocycles.13–19
Various synthetic methods have been developed for the
preparation of urea structures, due to their value in many
areas.20,21 The conventional protocol for the synthesis of
ureas involves treatment of amines with isocyanates (gen-
erated from phosgene) or with carbamoyl chlorides.22 Al-
though many urea compounds have been synthesized in
this way, these methods have several drawbacks involving
the instability of the carbamoyl chloride intermediates and
N
N
N
O
PhCF3, 70 °C
H
H
H
1a
2a
3a
Entry Catalyst
Time (h)
Temp (oC)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
Al(OTf)3
Mg(OTf)2
Zn(OTf)2
Sc(OTf)3
Nd(OTf)3
Sm(OTf)3
La(OTf)3
La(OAc)3
LaCI3
12
12
12
12
12
12
12
12
12
12
70
70
70
70
70
70
70
70
70
70
2
1
8
17
51
54
94
2
NRc
NRc
None
a Reaction conditions: N-Cbz-protected amine 1a (1.0 mmol), amine 2a (1.5
mmol), catalyst (0.15 mmol), PhCF3 (2 mL), 12 h.
b Isolated yield after column purification.
c No reaction.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
T. T. Bui, H.-K. Kim
Letter
Synlett
Table 2 Screening of Reaction Conditions for the Preparation of the
Table 3 Scope of Urea Formation from Various N-Cbz-Protected
Urea Structurea
Amines and Butylaminea
O
O
O
O
15 mol% La(OTf)3
PhCF3, 70 °C, 12 h
La(OTf)3
R1
R1
NH2
O
+
NH2
N
R2
N
+
N
R2
N
N
N
O
solvent, 12 h
H
H
H
H
3a
2a
1
2a
3
1a
Yieldb (%)
Entry
N-Cbz-amine
Product
Entry
La(OTf)3 (equiv)
Solvent
Temp (°C)
1
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
1
1,4-dioxane 70
17
18
36
45
81
85
94
95
73
41
95
94
94
53
14
O
O
1
2
CH3CN
DMF
70
70
70
70
70
70
90
50
30
70
70
70
70
70
N
N
N
O
H
H
H
3
3a, 94%
1a
O
O
4
DCE
2
N
N
N
O
5
THF
H
H
H
1b
3b, 92%
6
toluene
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
O
O
7
3
4
N
N
N
O
O
8
H
H
H
1c
3c, 91%
9
H3CO
H3CO
O
10
11
12
13
14
15
N
N
N
H
O
H
H
1d
3d, 92%
0.3
Cl
Cl
Cl
O
O
0.2
5
6
N
N
N
H
O
0.1
H
H
0.03
1e
3e, 91%
Cl
a Reaction conditions: N-Cbz-amine 1a (1.0 mmol), amine 2a (1.5 mmol),
La(OTf)3 (0.15 mmol), solvent (2 mL), 12 h.
O
O
N
N
N
O
b Isolated yield after column purification.
H
H
H
Cl
Cl
1f
3f, 84%
O2N
O2N
The use of functional groups such as amines is frequent-
ly encountered in organic syntheses. To prevent the forma-
tion of undesirable byproducts from reactions of free amine
groups during various multistep synthetic processes, sever-
al amine protecting groups are widely employed. In partic-
ular, benzyloxycarbonyl (Cbz), allyloxycarbonyl (Alloc), and
2,2,2-trichloroethoxycarbonyl (Troc) groups are often used
to protect amines in medicinal chemistry and in total-syn-
thesis sequences32–38 due the ease with which these carba-
mates can be formed from amines, their high stability un-
der both basic and acidic conditions, and their simple
deprotection methods, including reactions with acid or cat-
alytic hydrogenolysis.39 However, to synthesize urea com-
pounds from such carbamates, two independent reaction
steps are required: elimination of the carbamate protecting
group and subsequent amidation through nucleophilic at-
tack of the free amine. Therefore, a thorough study of the
transformation of N-Cbz-, N-Alloc-, and N-Troc-protected
amines into ureas was an attractive challenge. We recently
reported the transformation of these carbamates into non-
symmetric ureas by treatment with bis(trimethylalumi-
num)–1,4-diazabicyclo[2.2.2]octane adduct (DABAL-Me3).40
However, this protocol has some drawbacks, such as its use
of more than one equivalent of DABAL-Me3. We also report-
ed a new synthesis of ureas from N-Troc-protected amines
O
O
7
N
H
O
O
N
N
H
H
1g
3g, 75%
O
8
9
N
N
N
H
O
H
H
3h, 88%
1h
O
O
N
N
N
H
O
H
H
3i, 91%
1i
O
O
10
N
N
N
H
O
H
H
3j, 77%
1j
a Reaction conditions: N-Cbz-amine 1 (1.0 mmol), BuNH2 (2a; 1.5 mmol),
La(OTf)3 (0.15 mmol), PhCF3 (2 mL), 70 °C, 12 h.
b Isolated yield after column purification.
by using DBU, and syntheses of ureas from Alloc, Troc, and
Cbz carbamates by using CaI2.41,42 However, conversion
yields from Troc carbamates to ureas in the presence of DBU
were not high, and the reactions using CaI2 had to be car-
ried out at 80 °C. We were therefore interested in discover-
ing an effective conversion of carbamates into ureas by us-
ing novel metal catalysts, such as metal trifluoromethane-
sulfonates (triflates). To the best of our knowledge, simple
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

C
T. T. Bui, H.-K. Kim
Letter
Synlett
Table 4 Scope of Urea Formation from Reaction of N-Cbz-Amines with
and time-saving formation of ureas from various protected
amines by using a catalytic reagent has rarely been report-
ed. Here, we present a lanthanum(III) triflate mediated
preparation of diverse unsymmetrical ureas from Cbz,
Alloc, and Troc carbamates, which are widely employed
in many organic syntheses.
Various Aminesa
O
O
R4
NH
R5
15 mol% La(OTf)3
PhCF3, 70 °C, 12 h
R1
R4
R1
N
R2
+
N
R5
O
N
R2
3
1
2
In our initial study, we chose N-Cbz-protected aniline
(1a) and butylamine (2a) as substrates to investigate a novel
catalytic reaction for the synthesis of nonsymmetric ureas.
In the urea-preparation procedure, 1.0 equivalent of N-Cbz-
protected aniline (1a) was treated with 1.5 equivalents of
butylamine (2a) and 0.15 equivalent of a catalytic reagent
at 70 °C for 12 hours. A number of triflate reagents have
been used in various synthetic procedures, including
Friedel–Crafts acylations, aldol condensations, cycloadditions,
and syntheses of glycidic esters from diazo esters and car-
bonyl compounds.43–46 Therefore, in the present study, we
tested several triflate reagents in an attempt to discover an
effective synthetic protocol to produce ureas.
First, the triflate salts of aluminum, magnesium, and
zinc were examined (Table 1, entries 1–3). However, with
these catalysts the reaction of N-Cbz-aniline (1a) with bu-
tylamine (2a) gave the urea 3a in low yields of less than
10%. The use of rare-earth metal triflates gave provided bet-
ter yields of urea 3a; the use of Nd(OTf)3 or Sm(OTf)3 gave
3a in yields of 51 and 54%, respectively (entries 5 and 6),
whereas catalysis by La(OTf)3 gave a 94% yield (entry 7). We
also investigated other lanthanum reagents such as
La(OAc)3 and LaCl3 (entries 8 and 9), but these catalysts
gave much poorer results than did La(OTf)3. The reaction
failed to proceed in the absence of a catalyst (entry 10).
Thus, our survey of metal triflates to find suitable catalytic
reagents indicated that an efficient synthesis of ureas from
N-Cbz-protected amines can be achieved by the use of a
catalytic amount of La(OTf)3.
Next, we tested several solvents in an attempt to im-
prove the reaction conditions (Table 2). The use of 1,4-diox-
ane or CH3CN led to low yields of 3a (Table 2, entries 1 and
2). Reactions in DCE and DMF gave moderate yields of urea
3a (entries 3 and 4). When THF or toluene was used as the
reaction solvent, the yield of 3a increased to more than 80%
(entries 5 and 6). Finally, PhCF3 was tested as a solvent and
the desired urea was obtained in significantly enhanced
yield of 94% (entry 7). It is noteworthy that PhCF3 gave a
higher yield than toluene. The reason might be that PhCF3
has a more polar structure than toluene, so that La(OTf)3 is
more soluble in PhCF3 than in toluene, resulting in a higher
yield in PhCF3. These results suggests that PhCF3 was the
most efficient solvent for La(OTf)3-mediated synthesis of
urea from N-Cbz-protected amines. The effect of tempera-
ture on the reaction was also investigated (entries 8–10).
Increasing the temperature to 90 °C gave a 95% yield of urea
3a compared with 73% at 50 °C. As the synthetic yields at
90 and 70 °C were similar, we chose the lower temperature
for the next phase of our study.
Entry
N-Cbz-amine
Amine
Productb
O
O
1
NH2
N
O
O
O
N
N
H
H
H
1a
2a
3a, 94%
O
O
NH2
2
3
4
5
N
N
H
N
H
H
1a
2b
3k, 96%
O
O
NH2
N
H
N
N
H
H
1a
2c
3l, 93%
O
O
NH
N
N
N
O
H
H
1a
2d
3m, 87%
O
O
NH
N
N
N
H
O
O
H
1a
2e
3n, 82%
O
O
N
H
6
7
N
N
NH2
H
H
1k
2a
3o, 88%
O
O
N
N
H
NH2
N
H
O
H
1k
3p, 91%
2b
2c
O
O
NH2
8
N
H
O
O
O
N
N
H
H
3q, 90%
1k
O
O
NH
9
N
N
N
H
H
1k
2f
3r, 83%
O
O
10
NH
N
N
N
H
O
H
O
1k
2g
2a
2b
3s, 78%
O
O
11
12
N
H
O
N
N
H
NH2
H
1l
3t, 85%
O
O
NH2
N
N
H
N
H
O
H
1l
3u, 81%
a Reaction conditions: N-Cbz-amine 1 (1.0 mmol), amine 2 (1.5 mmol),
La(OTf)3 (0.15 mmol), PhCF3 (2 mL), 70 °C, 12 h.
b Isolated yield after column purification.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

D
T. T. Bui, H.-K. Kim
Letter
Synlett
Table 5 Scope of Urea Formation from N-Alloc- or N-Troc-Protected Aminesa
O
3
O
R4
15 mol% La(OTf)3
PhCF3, 70 °C, 12 h
R1
R4
R1
R3
NH
N
R2
+
N
R5
O
N
R2
R5
4
2
Entry
N-Alloc-Amine
Amine
Productb
Entry
N-Troc-Amine
Amine
Productb
O
O
O
O
NH2
N
H
O
O
O
CCl3
NH2
1
12
N
N
N
N
N
O
H
H
H
H
H
4e
2a
2b
3a, 96%
4a
2a
3a, 94%
O
O
O
O
NH2
13
14
15
NH2
N
N
2
3
N
H
CCl3
CCl3
N
N
H
H
N
O
O
H
H
H
4a
2h
2f
3k, 96%
4e
3v, 92%
O
O
O
O
NH
NH
N
H
N
N
H
N
N
H
N
H
3w, 88%
3m, 93%
4a
4e
2d
O
O
O
O
NH
NH
N
N
4
5
N
H
N
N
H
O
O
O
H
N
O
CCl3
O
H
4a
2g
3x, 78%
4e
2f
3w, 92%
O
O
O
O
N
N
NH2
N
H
N
N
16
17
NH2
H
H
N
H
O
CCl3
H
H
4b
2a
3o, 93%
4f
3o, 94%
2a
O
O
O
O
NH
N
H
N
N
NH2
N
H
O
O
N
N
N
O
CCl3
6
H
H
H
4b
2d
3y, 88%
2b
4f
3p, 92%
O
O
O
O
NH
N
H
NH
N
N
N
H
7
8
N
H
O
O
CCl3
18
19
H
O
O
3s, 90%
3r, 87%
4f
2g
2f
4b
O
O
O
O
NH2
NH2
N
H
O
N
N
N
N
N
H
CCl3
H
H
H
H
3z, 89%
2a
4c
4g
2a
3z, 92%
O
O
O
O
NH2
NH
9
20
N
H
N
N
H
O
N
N
N
H
O
CCl3
H
H
4c
2f
2b
4g
3ab, 88%
3aa, 84%
O
O
O
O
NH
NH
O
10
11
21
22
N
H
N
N
N
N
O
N
O
CCl3
H
H
H
O
4c
4g
2g
2a
3ab, 78%
2f
3ac, 87%
O
O
O
O
NH2
N
O
NH2
N
N
N
O
CCl3
N
N
H
H
H
H
H
H
4d
2a
3t, 87%
4h
3t, 90%
a Reaction conditions: N-Alloc- or N-Troc-amine (1.0 mmol), amine (1.5 mmol), La(OTf)3 (0.15 mmol), PhCF3 (2 mL), 70 °C, 12 h.
b Isolated yield after column purification.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

E
T. T. Bui, H.-K. Kim
Letter
Synlett
In addition, we examined the effect of the amount of
La(OTf)3 in the direct urea synthesis. The amount of
La(OTf)3 significantly affected the conversion yield (Table 2,
entries 10–15). An increased amount of La(OTf)3 in the re-
action led to a higher yield of urea 3a, but the use of more
than 0.2 equivalents of La(OTf)3 gave a similar result to that
with 0.15 equivalents [95% for 1.0 equiv and 94% for 0.3
equiv of La(OTf)3]. Thus, 15 mol% of La(OTf)3 was employed
in further investigations of our urea synthesis.
With these initial promising reaction results in hand,
we examined the scope of this preparation of ureas (Table
3). First, several N-Cbz-protected aromatic amines 1a–g
were treated with butylamine (2a) to produce the corre-
sponding ureas. Reactions of N-Cbz-protected anilines bear-
ing electron-donating (methyl or methoxy) or electron-
withdrawing (halo or nitro) groups at 70 °C successfully
gave ureas 1a–g in yields of 75–94% (Table 3, entries 1–7).
N-Cbz-protected aliphatic amines, including N-Cbz-propar-
gylamine, also reacted with butylamine to give ureas 1h–j
in high yields (entries 8–10).
our novel La(OTf)3-catalyzed reaction is effective as a syn-
thetic method for producing various ureas from protected
amines with high conversions.
Next, a scaled-up reaction using a catalytic amount of
La(OTf)3 was investigated to evaluate the practical prepara-
tion of ureas from N-Cbz-protected amines (Scheme 1). A
gram-scale reaction of N-Cbz-aniline (1a, 5.00 g) with bu-
tylamine (2a) in the presence of 0.15 equivalents of
La(OTf)3 under the optimized reaction conditions success-
fully gave the corresponding urea 3a in 90% yield. This re-
sult suggests that our La(OTf)3-catalyzed method is practi-
cal and scalable for the preparation of unsymmetrical ureas.
O
O
15 mol% La(OTf)3
+
N
N
N
O
NH2
PhCF3
70 °C, 12 h
H
H
H
3a
2a
1a
3.74 g, 90%
22.0 mmol, 5.00 g
Scheme 1 A gram-scale reaction of N-Cbz-aniline with butylamine
Next, various amines were tested in the La(OTf)3-cata-
lyzed synthesis of ureas (Table 4). N-Cbz-protected aniline
(1a), isobutylamine (1k), and tert-butylamine (1l) reacted
successfully with butylamine (2a), benzylamine (2b), and
cyclohexylamine (2c) to yield the corresponding nonsym-
metric ureas 3 in high yields (Table 4, entries 1–3, 6–8, 11,
and 12). Furthermore, N-Cbz-aniline 1a reacted with the
secondary amines piperidine (2d) and dipropylamine (2e)
to give ureas 3m and 3n in yields of 87 and 82%, respective-
ly (entries 4 and 5), and N-Cbz-isobutylamine reacted with
pyrrolidine (2f) and morpholine (2g) to give ureas 3r and 3s
in yields of 83 and 78%, respectively (entries 9 and 10).
The scope of the La(OTf)3-catalyzed reaction was then
extended to the conversion of N-Alloc-protected amines
into ureas, because N-Alloc groups have been widely em-
ployed to protect amines in many organic syntheses. Under
the same reaction conditions, N-Alloc-protected aniline
(4a), an aromatic carbamate, was treated with various pri-
mary and secondary amines 2 to give ureas 3a and 3v–x, re-
spectively, in high yields (Table 5, entries 1–4). In addition,
the reaction of several amines with N-Alloc-protected ali-
phatic carbamates derived from isobutylamine, cyclohexyl-
amine, or tert-butylamine resulted in successful production
of the desired ureas in yields of 78–93% (entries 5–11).
Because of the widespread use of the Troc group for
amine protection in numerous organic syntheses, we ap-
plied our new synthetic protocol to synthesis of ureas from
several N-Troc-protected amines (Table 5, entries 12–22).
The La(OTf)3-catalyzed reaction of N-Troc-aniline, N-Troc-
isobutylamine, N-Troc-cyclohexylamine, or N-Troc-tert-bu-
tylamine with the primary amines butylamine and benzyl-
amine and the secondary amines piperidine, pyrrolidine,
and morpholine successfully gave the desired nonsymmet-
ric ureas 3 in yields of 87–96%. These syntheses suggest that
In conclusion, we have developed a novel and efficient
synthesis of ureas from N-Cbz-, N-Alloc-, or N-Troc-protect-
ed amines by using catalytic La(OTf)3. In this study, various
catalytic metal triflates were examined as catalysts for pre-
paring ureas, and La(OTf)3 was found to catalyze the syn-
thesis of ureas from N-Cbz-protected amines effectively and
in high yields. Furthermore, the synthetic procedure was
successfully applied to the conversion of N-Alloc- and N-
Troc-protected amines into ureas. These results demon-
strate that our novel La(OTf)3-catalyzed transformation of
N-Cbz-, N-Alloc-, and N-Troc-protected amines is a practical
method for producing diverse ureas.
Funding Information
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by
the Ministry of Education (2018R1D1A1B07047572).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1707992.
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References and Notes
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(47) N-Butyl-N′-phenylurea (3a); Typical Procedure
Butylamine (2a; 0.110 g, 1.5 mmol) was added dropwise to a
solution of N-Cbz-aniline (1a; 0.177 g, 1.00 mmol) and La(OTf)3
(0.088 g, 0.15 mmol) in PhCF3 (2 mL) at r.t., and the mixture was
stirred and heated to 70 °C for 12 h under N2. The mixture was
then cooled to r.t. and 1 M aq HCl (10 mL) was added. The
resulting mixture was extracted with CH2Cl2 (2 × 20 mL), and
the combined organic layers were dried (Na2SO4) and concen-
trated under reduced pressure. The resulting residue was puri-
fied by flash column chromatography (silica gel, 0–50% hexane–
EtOAc) to give a white solid; yield: 0.180 g (94%); mp 129–
131 °C.1H NMR (400 MHz, DMSO-d6):  = 8.35 (s, 1 H), 7.39–
7.36 (m, 2 H), 7.22–7.18 (m, 2 H), 6.89 (t, J = 7.2 Hz, 1 H), 6.08 (t,
J = 5.2 Hz, 1 H), 3.08 (q, J = 6.0 Hz, 2 H), 1.45–1.38 (m, 2 H),
1.34–1.24 (m, 2 H), 0.89 (t, J = 7.2 Hz, 3 H). 13C NMR (100 MHz,
DMSO-d6):  = 155.64, 141.06, 129.05 (2 C), 112.31, 111.7 (2 C),
39.13, 32.35, 19.99, 14.16. HRMS (ESI): m/z [M + H]+ calcd for
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127, 4182.
C
11H17N2O: 193.1341; found: 193.1336.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F