Micro-flow synthesis of β-amino acid derivatives: Via a rapid dual activation approach

Article abstract of DOI:10.1039/d0cc01403f

Rapid dual activation (≤3.3 s) of both β-amino acid N-carboxy anhydride and alkyl chloroformate for the synthesis of a β-amino acid-derived scaffold was demonstrated. The key to success was the use of rapid mixing enabled by using a micro-flow reactor. Th

Full text of DOI:10.1039/d0cc01403f

ChemComm
View Article Online
View Journal
COMMUNICATION
Micro-flow synthesis of b-amino acid derivatives
via a rapid dual activation approach†
Cite this:DOI: 10.1039/d0cc01403f
ab
a
c
Naoto Sugisawa,
Hiroyuki Nakamura
and Shinichiro Fuse
*
Received 25th February 2020,
Accepted 27th March 2020
DOI: 10.1039/d0cc01403f
rsc.li/chemcomm
Rapid dual activation (r3.3 s) of both b-amino acid N-carboxy
anhydride and alkyl chloroformate for the synthesis of a b-amino
acid-derived scaffold was demonstrated. The key to success was
the use of rapid mixing enabled by using a micro-flow reactor. The
one-flow synthesis of 22 b-amino acid derivatives was achieved.
Dual activation is an attractive synthetic approach (Scheme 1),
because it enables challenging coupling reactions that are impos-
sible or insufficient using other approaches. Elegant bifunctional
and synergistic catalysts have been developed as dual activators of
both substrates A and B.1 One of the important challenges in
developing a dual activation approach is the avoidance of unde-
sired reactions from activated species A0 and B0.2 In particular, if
Scheme 1 Dual activation approach for the coupling of substrates A and
B, and the risk of undesired reactions from activated species A0 and B0.
the lifetime of the active species A0 and B0 is insufficient compared containing both isocyanate and acyl chloride moieties (Scheme 2a)
to the mixing time of the solutions (ca. o1 min),3 it would be and its use in a one-pot, three-component coupling reaction to
difficult to obtain satisfactory results, because undesired reactions synthesize b-amino acid derivatives II.8 Mormann and coworkers
from A0 and B0 occur concomitantly with their generation during also reported derivatization and polymerization based on I.9 Despite
mixing. In order to avoid these undesired reactions, a rapid dual the usefulness of I, its preparation requires treatment with toxic
activation of substrates, and their immediate use for the sub- gases such as phosgene and HCl, the expenditure of time (ca. 1 day),
sequent coupling reactions would be a valuable aid in developing and heating conditions (65–70 1C). This complicates the synthesis of
dual activation approaches.
I containing acid labile R1 and/or R2. The development of a mild and
b-Amino acid derivatives are attractive as drug candidates,4 rapid synthesis of b-amino acid-derived scaffolds containing multi-
asymmetric catalysts in organic transformation,5 and organogels ple reaction centers remains to be an important pursuit.
for surface modification.6 They are usually synthesized by stepwise
Micro-flow technology10 allows precise control for a short
chemical modifications of amino- and/or carboxyl groups in amount of time by rapid mixing (milli-seconds)11 as a result of
b-amino acids.7 b-Amino acid-derived scaffolds containing shortening of the diffusion length. Recently, we reported the one-
multiple-reaction sites that allow selective and sequential step micro-flow synthesis of b-amino acid N-carboxy anhydrides
chemical modifications are useful for the rapid synthesis of (b-NCAs) IV.12 Herein, we report the rapid and mild synthesis of a
b-amino acid derivatives. In 1966, Iwakura and coworkers b-amino acid-derived scaffold VI containing isocyanate and
reported the preparation of b-amino acid-derived scaffold I mixed carbonic anhydride moieties, and its use in the synthesis
of various b-amino acid derivatives. The key to success was the
rapid generation of active species IV0 and V0 and their immediate
use in a subsequent coupling reaction, which avoided the undesired
oligomerization/polymerization of IV0 and the decomposition of V0.
This accomplishment was realized via precise control of the reaction
time (r3.3 s) using micro-flow technology.
a Laboratory for Chemistry and Life Science, Institute of Innovative Research,
Tokyo Institute of Technology, Yokohama, 226-8503, Japan
b School of Life Science and Technology, Tokyo Institute of Technology,
Yokohama, 226-8501, Japan
c Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical
Sciences, Nagoya University, Nagoya 464-8601, Japan.
We examined the rapid dual activation of b-NCA (1a, racemic
mixture) and benzyl chloroformate (CbzCl) for the synthesis of
E-mail: fuse@ps.nagoya-u.ac.jp
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cc01403f
Chem. Commun.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Communication
ChemComm
Table 1 Rapid dual activation of 1a and benzyl chloroformate for the
synthesis of 2a
Solvent Solvent
Entry Base
S1
S2
Yieldb (%)
1
2
3
4
5
6
7
8
—
CH2Cl2 CH2Cl2 n.r.c
CH2Cl2 CH2Cl2 n.r.c
CH2Cl2 CH2Cl2 28
CH2Cl2 CH2Cl2 99
CH2Cl2 CH2Cl2 85
Pyridine (pKa = 5.2)13
i-Pr2NEt (pKa = 11.4)14
i-Pr2NEt + pyridinea
Et3N (pKa = 10.7)13
N-Ethylmorphorine (pKa = 7.7)13 CH2Cl2 CH2Cl2 70
N-Methylpiperidine (pKa = 10.1)15 CH2Cl2 CH2Cl2
—
—
—
d
d
d
Scheme 2 (a) Reported preparation of b-amino acid-derived scaffold I
containing isocyanate and acyl chloride moieties. (b) Preparation of
b-amino acid-derived scaffold VI containing isocyanate and mixed carbonic
anhydride moieties via the rapid dual activation approach reported in this
study.
DMAP (pKa = 9.6)14
Me2NBn (pKa = 8.9)15
NMI (pKa = 7.0)16
NMM (pKa = 7.4)13
NMM
CH2Cl2 CH2Cl2
CH2Cl2 CH2Cl2
9
10
11
12
13
14
15
16
CH2Cl2 CH2Cl2 n.r.c
CH2Cl2 CH2Cl2 499g (499e)
THF
THF
CH2Cl2 96
f
NMM
NMM
NMM
NMM
THF
—
MeCN CH2Cl2 73
MeCN MeCN 62
CH2Cl2 CH2Cl2 68–89gh
b-phenylalanine-derived scaffold 2a. Bases, reaction time and
solvents were examined, as shown in Table 1. We connected a
T-shape mixer and Teflons tubing and immersed them in a water
bath (20 1C). Then, b-NCA 1a (1.00 equiv.) and CbzCl (1.00 equiv.) in
solvent S1 and a base (2.00 equiv.) in solvent S2 were independently
injected into the T-shape micromixer. The resultant mixture was
poured into a mixture of 1 M HCl aq. and CH2Cl2. Following a
simple aqueous workup, the yield was determined via 1H NMR
a
b
1.0 + 1.0 equiv. Yields were determined via 1H NMR analysis using
c
1,1,2-tirichloroethane as an internal standard. No reaction of 1a
occurred. A complex mixture was generated. Isolated yields. The
d
e
f
g
reactor was clogged. Three independent experiments were carried out.
h
Batch conditions included 1000 rpm.
analysis. No reaction of 1a occurred in the absence of a base (entry oligomerization/polymerization of 1a (entries 7–9), whereas the use
1). Insufficient yields occurred with the sole use of either pyridine or of NMI resulted in no reaction (entry 10). We speculated that the
i-Pr2NEt (entries 2 and 3), whereas an excellent yield was observed use of a strong base (pKa 4 8 and 9), with the noted exception of
when both pyridine and i-Pr2NEt were used in combination (entry i-Pr2NEt, induced a very rapid oligomerization/polymerization of
4). It is well known that pyridine reacts with acyl chloride to form a 1a, but a weak base (pKa r 7) could not activate 1a. It seemed that
highly electrophilic acylpyridinium cation, but the nucleophilic the basicity of i-Pr2NEt (pKa = 11.4) was sufficient, but its bulkiness
substitution of i-Pr2NEt against acyl chloride is slow.17 Therefore, avoided the undesired reactions with 1a (entries 3 and 4). To our
it is reasonable that low yields were observed (entries 2 and 3), delight, the sole use of N-methylmorphorine (NMM), which has
because pyridine, which is a nucleophilic and weak base, could not medium levels of both nucleophilicity and basicity, quantitatively
activate 1a, while, i-Pr2NEt, which is a less nucleophilic strong base, afforded 2a (entry 11). The use of either THF or MeCN instead of
could not sufficiently activate CbzCl. However, the combined use of CH2Cl2 resulted in a decrease in yields or clog generation
pyridine and i-Pr2NEt dually activated both substrates, which pro- (entries 12–15). In order to verify the importance of the micro-
duced an excellent yield (entry 4). From a practical point of view, the flow conditions, comparative batch conditions were used. Although
dual activation of both substrates by a sole base would be desirable. the reaction mixture was vigorously stirred during the experiment,
Examination of various bases revealed that the sole use of Et3N or reproducible results were not observed due to batch-to-batch
N-ethylmorpholine afforded 2a in good yields (entries 5 and 6). differences in the mixing efficiency (entry 16). Although base-
It is conceivable that these bases could react with CbzCl to form induced rapid oligomerization/polymerization of 118 and rapid
acylammonium cations due to their decreased steric bulkiness by decarboxylation of acylammonium cation generated from alkyl
comparison with that of i-Pr2NEt. In addition, each was suffi- chloroformate19 are known to occur, we experimentally con-
ciently basic to activate 1a. The use of N-methylpiperidine, firmed the instability of the activated species (Scheme 3). The
N,N-dimethylaminopyridine (DMAP), and Me2NBn resulted in reaction of 1a with NMM in the absence of CbzCl for 1 min
the generation of a complex mixture probably due to an undesired induced oligomerization/polymerization and 69% of 1a was
Chem. Commun.
This journal is © The Royal Society of Chemistry 2020

View Article Online
ChemComm
Communication
secondary amines including a- and b-amino acids could be used
as nucleophiles and the desired derivatives 3g–3k were obtained in
excellent yields, whereas the use of bulky diisopropyl amine as a
nucleophile did not afford the desired product 3l.
In order to expand the structural diversity of the b-amino
acid derivatives obtained by the developed approach, we exam-
ined the sequential introduction of two different nucleophiles
into in situ generated 2b (Fig. 2). b-NCA 1 (1.0 equiv.), isobutyl
chloroformate (1.0 equiv.), and NMM (2.0 equiv.) were reacted
to generate 2b in situ. The first nucleophile (Nu1, 1 equiv.) was
reacted with 2b in the presence of a catalytic amount of DMAP
(0.1 equiv.) that preferentially activated the mixed carbonic
anhydride moiety to generate a highly active acyl pyridinium
cation (for details, see ESI†). The second nucleophile (Nu2,
1 equiv.) was reacted in the presence of a stoichiometric amount
of i-Pr2NEt (1 equiv.)23 in the collection flask to afford the
desired products 4 and 5. The sequential introduction of various
primary and secondary amines including a- and b-amino acids
and subsequent 1-octanethiol afforded the desired products
4a–4e in good yields.24 The use of either a benzyl mercaptan or
a thiophenol instead of 1-octanethiol also afforded the desired
products 4f and 4g in good yields. The sequential introduction of
different amines was examined because b-amino acid derivatives
containing both urea and amide moieties are attractive as drug
candidates and organogels.6,7a,d The desired products 4h and
4i were obtained in good to excellent yields. We carried out
dihydrouracil synthesis because they are frequently found in
bioactive compounds.25 The desired products 5a and 5b were
Scheme 3 Reaction of 1a or CbzCl with NMM in the absence of a
coupling partner in order to confirm the instability of the generated
activated species.
recovered. The reaction of CbzCl with NMM in the absence of 1a
for 1 min and a subsequent nucleophilic addition using benzyl-
amine20 was performed. The amide was not detected. The
results indicated rapid decarboxylation of the in situ generated
acylammonium cation. Thus, rapid activation of 1a and CbzCl
and their immediate use for the coupling were important in
order to avoid these undesired reactions.
b-Amino acid-derived scaffold 2 has isocyanate and mixed
carbonic anhydride moieties that allow further chemical modifica-
tions. We carried out in situ generation of 221 and the following
reaction with various nucleophiles using the two electrophilic
reaction sites of 2 in a collection flask (Fig. 1). b-NCA 1
(1.0 equiv.), isobutyl chloroformate (1.0 equiv.),22 and NMM
(2.0 equiv.) were reacted to generate 2 in situ, which was reacted
with various nucleophiles (2.0 equiv.) without isolation to obtain
b-amino acid derivatives 3. Desired derivatives 3a–3d containing
an alkyl or a phenyl side chain were obtained in high yields. It
should be noted that 3e and 3f containing acid-labile tert-butyl
ester or silyl ether moieties that cannot be synthesized by Iwakura’s
approach were obtained in high yields. Various primary and
Fig. 1 Introduction of a nucleophile against in situ generated 2. a THF was
used to dissolve 1 and isobutyl chloroformate. b CH2Cl2/MeCN (v/v = 1/1)
was used to dissolve 1 and isobutyl chloroformate. c a-Amino acid tert-
butyl ester hydrochloride was used as Nu. d b-Alanine methyl ester
hydrochloride was used as Nu. e The desired product was not detected.
Fig. 2 Introduction of different nucleophiles against in situ generated 2.
a a-Amino acid tert-butyl ester hydrochloride was used as Nu. b b-Alanine
methyl ester hydrochloride was used as Nu1. c i-Pr2NEt (1.0 equiv.) was not
used. d 2.0 equiv. of diisopropylamine was used as Nu2. e The cyclization
was carried out at 100 1C for 2 h in toluene.
Chem. Commun.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Communication
ChemComm
6 (a) A. Raghavanpillai, V. A. Franco and W. E. Meredith, J. Fluorine
Chem., 2012, 135, 187–194; (b) A. Raghavanpillai and V. A. Franco,
Appl. Sci., 2012, 2, 175–191.
obtained in good yields without using a second nucleophile in
the collection flask.
We demonstrated the mild and rapid synthesis of b-amino
acid-derived scaffold 2 containing isocyanate and mixed carbonic
anhydride moieties. The key to success was a ‘‘rapid dual
activation’’ (r3.3 s) of 1 and alkyl chloroformate in the presence
of NMM and their immediate use for a subsequent coupling
reaction with the avoidance of undesired reactions of the
activated species. This could only be realized by the use of rapid
mixing that was enabled by using a micro-flow reactor. These 22
structurally diverse b-amino acid derivatives are attractive as
drug candidates, and their one-flow synthesis was demon-
strated. Again, we emphasize that the developed reaction can
be safely and reproducibly performed only via the use of micro-
flow technology. The developed methodology should be useful
for developing various dual activation approaches and be valu-
able for accelerating drug, catalyst, and material discovery based
on b-amino acid derivatives.
´
7 (a) R. L. Wolin, A. Santillan, J. T. Barclay, L. Tang, H. Venkatesan,
S. Wilson, D. H. Lee and T. W. Lovenberg, Bioorg. Med. Chem., 2004,
12, 4493–4509; (b) E. Tassoni, R. Conti, G. Gallo, S. Vincenti,
L. Mastrofrancesco, T. Brunetti, W. Cabri and F. Giannessi, Chem-
MedChem, 2011, 6, 1977–1980; (c) M. Durini, E. Russotto,
L. Pignataro, O. Reiser and U. Piarulli, Eur. J. Org. Chem., 2012,
5451–5461; (d) J. Zhang, M. Han, X. Ma, L. Xu, J. Cao, Y. Zhou, J. Li,
T. Liu and Y. Hu, Chem. Biol. Drug Des., 2014, 84, 497–504. Various
synthetic approaches for constructing b-amino acid skeleton have
been reported, see: (e) D. C. Cole, Tetrahedron, 1994, 50, 9517–9582;
( f ) B. Weiner, W. Szymanski, D. B. Janssen, A. J. Minnaard and
B. L. Feringa, Chem. Soc. Rev., 2010, 39, 1656–1691.
8 Y. Iwakura, K. Uno and S. Kang, J. Org. Chem., 1966, 31, 142–146.
9 (a) W. Mormann and E. Hissmann, Tetrahedron Lett., 1987, 28,
3087–3090; (b) W. Mormann, N. Tiemann and E. Turuskan, Polymer,
1989, 30, 1127–1132.
10 Recent selected reviews, see: (a) B. Gutmann, D. Cantillo and C. O.
Kappe, Angew. Chem., Int. Ed., 2015, 54, 6688–6728; (b) M. Baumann
and I. R. Baxendale, Beilstein J. Org. Chem., 2015, 11, 1194–1219;
(c) R. Porta, M. Benaglia and A. Puglisi, Org. Process Res. Dev., 2016,
20, 2–25; (d) I. Rossetti and M. Compagnoni, Chem. Eng. J., 2016,
296, 56–70; (e) S. Kobayashi, Chem. – Asian J., 2016, 11, 425–436;
( f ) C. A. Shukla and A. A. Kulkarni, Beilstein J. Org. Chem., 2017, 13,
960–987; (g) F. Fanelli, G. Parisi, L. Degennaro and R. Luisi, Beilstein
J. Org. Chem., 2017, 13, 520–542; (h) J. Britton and C. L. Raston,
Chem. Soc. Rev., 2017, 46, 1250–1271; (i) M. B. Plutschack, B. Pieber,
K. Gilmore and P. H. Seeberger, Chem. Rev., 2017, 117, 11796–11893;
This work was partially supported by the Scientific Research
on Innovative Areas 2707 Middle molecular strategy (no.
16H01138) from the Japan Society for the Promotion of Science,
and by the Naito Foundation Natural Science Scholarship.
´
( j) R. Gerardy, N. Emmanuel, T. Toupy, V.-E. Kassin, N. N. Tshibalonza,
M. Schmitz and J.-C. M. Monbaliu, Eur. J. Org. Chem., 2018, 2301–2351;
(k) B. T. Ramanjaneyulu, N. K. Vishwakarma, S. Vidyacharan,
P. R. Adiyala and D.-P. Kim, Bull. Korean Chem. Soc., 2018, 39, 757–772.
11 H. Kim, K.-i. Min, K. Inoue, D. J. Im, D.-P. Kim and J.-i. Yoshida,
Science, 2016, 352, 691–694.
Conflicts of interest
There are no conflicts to declare.
12 We reported micro-flow synthesis of b-NCAs, see: (a) Y. Otake,
H. Nakamura and S. Fuse, Angew. Chem., Int. Ed., 2018, 57,
11389–11393; micro-flow synthesis of b-NCAs, see:; (b) N. Sugisawa,
Y. Otake, H. Nakamura and S. Fuse, Chem. – Asian J., 2020, 15, 79–84.
Notes and references
1 Definition of bifunctional catalysis and synergistic catalysis, see:
(a) A. E. Allen and D. W. C. MacMillan, Chem. Sci., 2012, 3, 633–658;
Recent selected reviews, see: (b) Z. Du and Z. Shao, Chem. Soc. Rev., 2013, 13 H. K. Hall, J. Phys. Chem., 1956, 60, 63–70.
´
42, 1337–1378; (c) S. Afewerki and A. Cordova, Chem. Rev., 2016, 116, 14 C. Faltin, E. M. Fleming and S. J. Connon, J. Org. Chem., 2004, 69,
13512–13570; (d) B. Teng, W. C. Lim and C.-H. Tan, Synlett, 2017, 6496–6499.
1272–1277; (e) J. Fu, X. Huo, B. Li and W. Zhang, Org. Biomol. Chem., 15 H. K. Hall, J. Am. Chem. Soc., 1957, 79, 5441–5444.
2017, 15, 9747–9759; ( f ) K. T. Mahmudov, A. V. Gurbanov, F. I. Guseinov 16 M. L. Bender and B. W. Turnquest, J. Am. Chem. Soc., 1957, 79, 1656–1662.
and M. F. C. Guedes da Silva, Coord. Chem. Rev., 2019, 387, 32–46.
2 (a) B. M. Trost, X. Luan and Y. Miller, J. Am. Chem. Soc., 2011, 133,
17 G. W. Anderson, J. E. Zimmerman and F. M. Callahan, J. Am. Chem.
Soc., 1967, 89, 5012–5017.
´
12824–12833; (b) F. Nahra, Y. Mace, D. Lambin and O. Riant, Angew. 18 Polymerization rate of b-NCA in the presence of organic base was
Chem., Int. Ed., 2013, 52, 3208–3212.
3 Jensen and coworkers reported required time (seconds to minutes)
reported, see: H. R. Kricheldorf and R. Mu¨lhaupt, J. Macromol. Sci.,
Part A: Pure Appl. Chem., 1980, 14, 349–377.
for complete mixing of solutions using batch reactors, see: 19 A. R. Choppin and J. W. Rogers, J. Am. Chem. Soc., 1948, 70, 2967.
R. L. Hartman, J. P. McMullen and K. F. Jensen, Angew. Chem., Int. 20 Both CbzCl and acylammonium cation are unstable, therefore, we
Ed., 2011, 50, 7502–7519.
intended to convert them to a stable amide using benzyl amine.
4 Recent selected reviews, see: (a) R. P. Cheng, S. H. Gellman and 21 We examined substrate scope with the reaction time of 3.3 s.
W. F. DeGrado, Chem. Rev., 2001, 101, 3219–3232; (b) D. Seebach,
A. K. Beck and D. J. Bierbaum, Chem. Biodiversity, 2004, 1, 1111–1239;
Because yield was not changed by reducing the reaction time from
10 s to 3.3 s in our preliminary examination. Details, see ESI†.
¨
´
(c) F. Fu¨lop, T. A. Martinek and G. K. Toth, Chem. Soc. Rev., 2006, 35, 22 Inexpensive, readily available, and stable isobutyl chloroformate
323–334; (d) F. Kudo, A. Miyanaga and T. Eguchi, Nat. Prod. Rep., 2014,
31, 1056–1073. Recent selected publications, see: (e) M.-R. Lee, N. Raman,
S. H. Gellman, D. M. Lynn and S. P. Palecek, ACS Chem. Biol., 2017, 12,
was used instead of CbzCl for investigation of substrate scope. We
experimentally confirmed instability of acylammonium cation gen-
erated from isobutyl chloroformate. Details, see ESI†.
´
´
2975–2980; ( f ) E. Bartus, Z. Hegredu¨s, E. Weber, B. Csipak, G. Szakonyi 23 E. Dyer, J. F. Glenn and E. Lendrat, J. Org. Chem., 1961, 26, 2919–2925.
and T. A. Martinek, ChemistryOpen, 2017, 6, 236–241; (g) J. W. Checco 24 Synthesis of 4a under batch conditions afforded decreased yield.
and S. H. Gellman, ChemBioChem, 2017, 18, 291–299.
Details, see ESI†.
5 Recent selected publications, see: (a) P. E. Coffey, K.-H. Drauz, 25 (a) J. Guillon, M. Daoust, D. Radulovic, M. Boulouard, P. Dallemagne,
S. M. Roberts, J. Skidmore and J. A. Smith, Chem. Commun., 2001,
2330–2331; (b) P. S.-P. Wang, J. B. Nguyen and A. Schepartz, J. Am.
Chem. Soc., 2014, 136, 6810–6813.
E. Legrand, S. Rault, M. Quermonne and M. Robba, Eur. J. Med. Chem.,
1996, 31, 335–339; (b) L. G. Nair, S. Bogen, R. J. Doll, N.-Y. Shih and
F. G. Njoroge, Tetrahedron Lett., 2010, 51, 1276–1279.
Chem. Commun.
This journal is © The Royal Society of Chemistry 2020