Single-Step, Rapid, and Mild Synthesis of β-Amino Acid N-Carboxy Anhydrides Using Micro-Flow Technology

Article abstract of DOI:10.1002/asia.201901429

β-Amino acid N-carboxy anhydrides (β-NCAs) are rarely used in the synthesis of β-peptides, which is due mainly to the poor availability of these potentially useful substrates. Herein, we describe the heretofore challenging synthesis of β-NCAs via a single

Full text of DOI:10.1002/asia.201901429

DOI: 10.1002/asia.201901429
Full Paper
Single-Step, Rapid, and Mild Synthesis of b-Amino Acid N-Carboxy
Anhydrides Using Micro-Flow Technology
Naoto Sugisawa,[a, b] Yuma Otake,[a, b] Hiroyuki Nakamura,[a] and Shinichiro Fuse*[a, c]
Dedicated to the memory of Prof. Jun-ichi Yoshida
Abstract: b-Amino acid N-carboxy anhydrides (b-NCAs) are
rarely used in the synthesis of b-peptides, which is due
mainly to the poor availability of these potentially useful
substrates. Herein, we describe the heretofore challenging
synthesis of b-NCAs via a single-step, rapid, and mild forma-
tion using pH flash switching and flash dilution, which are
aspects of micro-flow technology. We synthesized 15 b-NCAs
in good to excellent yields that included acid-labile b-NCAs
that cannot be readily synthesized using the conventional
Leuchs approach. Scaled-up synthesis using this process can
be readily achieved via continuous operation.
Introduction
b-Peptides have garnered much attention due to properties
that include the formation of stable secondary structures and
good stability against enzymatic degradation as well as their
potential applications for drugs, drug carriers, and catalysts for
organic transformations.[1] a-Amino acid N-carboxy anhydrides
(a-NCAs, Scheme 1, n=0) are important building blocks in the
synthesis of a-peptides[2] and polypeptides,[3] whereas b-NCAs
(Scheme 1, n=1) have not been frequently used for the syn-
thesis of b-peptides despite their potential utility.[4] One of the
prime reasons for their avoidance has been the poor availabili-
ty of b-NCAs due to their slower cyclization rate and lower sta-
bility by comparison with a-NCAs.[4f] Basically both a-NCAs and
b-NCAs readily undergo polymerization under basic conditions.
In particular, even the moisture in ambient air can induce the
spontaneous polymerization of b-NCAs due to their extreme
instability.
[a] N. Sugisawa, Y. Otake, Prof. Dr. H. Nakamura, Dr. S. Fuse
Laboratory for Chemistry and Life Science
Institute of Innovative Research
Tokyo Institute of Technology
Scheme 1. Previously reported synthetic approaches i)–iv) for a-NCAs (n=0)
and b-NCAs (n=1), and the developed approach for b-NCAs (this study).
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
[b] N. Sugisawa, Y. Otake
School of Life Science and Technology
Tokyo Institute of Technology
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
The Fuchs-Farthing approach is the most frequently used for
preparing a-NCAs from a-amino acids A (Scheme 1–i),[5] al-
though this approach cannot produce acid-labile a-NCAs and
often generates undesired ring-opening products.[6] On the
other hand, the Fuchs-Farthing approach does not afford b-
NCAs from b-amino acids B in satisfactory yields (0.3%), but
does afford the undesired isocyanate C (Scheme 1–ii).[4a,f]
Therefore, the Leuchs approach,[7] which involves the PBr3-
mediated cyclization of carbamate-protected b-amino acids
D[4a,f,8] has been used for the synthesis of b-NCA (Scheme 1–
iv).[9] Recently, the gram scale synthesis of a b-NCA from b-phe-
[c] Dr. S. Fuse
Present address:
Department of Basic Medical Sciences
Graduate School of Pharmaceutical Sciences
Nagoya University
Furo-cho, Chikusa-ku Nagoya 464-8601 (Japan)
E-mail: fuse@ps.nagoya-u.ac.jp
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901429.
This manuscript is part of a special collection celebrating the 100th Annual
Meeting of the Chemical Society of Japan (CSJ). Click here to see the Table
of Contents of the special collection.
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nylalanine was reported.[10] The Leuchs approach, however, re-
quires multiple synthetic steps, and a protecting group, but
this approach cannot produce acid-labile b-NCAs. Roberts and
co-workers have reported that a pure sample of b-NCA could
not be prepared via the Leuchs approach.[1e]
zation and/or hydrolysis of b-NCA. The obtained mixture is rap-
idly diluted by injecting ethyl acetate to a T-shape mixer. This
flash dilution avoids the undesired decomposition of acid-
labile functional groups.
b-NCA formation was examined using sodium salts of b-phe-
nylalanine (1a, racemic mixture), N-methyl morpholine (NMM),
and the safely handled triphosgene as a phosgene equivalent
in accordance with our previously reported procedure.[14] The
resultant mixture was collected and was followed by an aque-
ous work-up to remove the hydrochloric salts of NMM and 1a.
As previously described, even moisture in the air induced
spontaneous polymerization of b-NCAs, and, thus, spectrum
measurements were carried out immediately following the syn-
thesis. Initially, the concentration of 1a, and the amount
(equiv) of triphosgene and NMM were examined (Figure 2).
Micro-flow technologies[11] enable the precise control of
both a short reaction time (<1 sec) and the reaction tempera-
ture.[12] The generation of turbulence flow and short diffusion
length in a micro-flow reactor enables control of a short reac-
tion time. The large surface-to-volume ratio of a micro-flow re-
actor enables rapid heat transfer for temperature control. We
have reported efficient micro-flow peptide synthesis.[13] In addi-
tion, we recently reported a rapid, mild, and high-yielding syn-
thesis of a-NCAs using flash pH switching and flash dilution in
a micro-flow reactor (Scheme 1–iii).[14] The developed approach
enabled a single-step synthesis of acid-labile a-NCAs. Herein,
we report the challenging synthesis of b-NCAs via a single-
step, protecting-group-free process using the corresponding b-
amino acids based on our developed flash pH switching and
flash dilution in a micro-flow reactor.
Results and Discussion
The details of a working hypothesis[14] for b-NCA formation
based on micro-flow technologies are shown in Figure 1. b-
Amino acid sodium salts 1[15] and a base in H2O along with a
solution of phosgene in MeCN are independently injected into
a V-shape mixer, which has a mixing efficiency that is higher
than that of a T-shape mixer.[14] We expected the desired cycli-
zation to occur rapidly between 1 (1.0 equiv) and the phos-
gene (>1.0 equiv) under basic conditions and to generate HCl
(1.0 equiv) that would be trapped by the base (>1.0 equiv)
and cause the reaction mixture to remain basic (step 1). The
unreacted phosgene is then decomposed by water to generate
HCl (step 2). After the base is completely protonated by HCl,
the reaction mixture becomes acidic (step 3). This pH flash
switching from basic to acidic avoids the undesired polymeri-
Figure 2. Examination of the concentration of 1a and the amount of tri-
phosgene in the micro-flow synthesis of b-NCA 2a. [a] Yields were deter-
1
mined by H NMR analysis using 1,1,2-trichloroethane as an internal stan-
dard.
The use of a smaller amount of triphosgene (Y=0.33 or
0.50 equiv) resulted in lower yields compared with the use of a
larger amount of triphosgene (Y=0.67 or 1.00 equiv) probably
due to the insufficient conversion of 1a. The use of a higher
concentration solution of 1a (X=0.50m) resulted in lower
yields compared with the use of a lower concentration solu-
tion of 1a (X=0.25m) within the examined range of Y (0.33 to
1.00 equiv). We speculated that the overreaction of an inter-
mediate 3a with phosgene, or its equivalent, occurred under
these higher concentration conditions (X=0.50m) to generate
the undesired 4a (Scheme 2). The combinations of X=0.25m
(1.0 equiv) and Y=0.67 equiv and X=0.10m (1.0 equiv) and
Y=1.0 equiv afforded excellent yields. We used the former
conditions in the following investigations because of their
Figure 1. Working hypothesis for the rapid and mild formation of b-NCAs
based on pH flash switching and flash dilution.
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the reaction would become slow. The use of a highly nucleo-
philic and less-basic amine such as pyridine also resulted in a
decreased yield probably due to its insufficient ability to trap
the in situ-generated HCl (entry 6). The conditions shown in
entry 2 were optimal, and the average yield from three inde-
pendent experiments was 93ꢀ1%. The result was highly re-
producible. Gram-scale (2.3 g) preparation of b-NCA 2a was
successfully achieved by continuous running for 21.3 min.
In order to verify the importance of “pH flash switching,”
comparative batch reactions were carried out under the same
reaction conditions with the exception of reaction time (10 s),
because it was impossible to perform the batch reaction in
0.1 sec (Scheme 3b). Average yield from three independent ex-
Scheme 2. Plausible reason for the decreased yield of b-NCA 2a under con-
ditions employing 0.5m solution of 1a.
higher productivity due to the use of a higher concentration
solution of 1a as well as to the decreased amounts of triphos-
gene and NMM that were used.
The reaction time and base were examined (Table 1). The de-
creased yield (86%) was observed when the reaction was car-
ried out in 0.02 sec probably due to insufficient conversion of
Table 1. Examination of time and base.
Entry
t [sec]
Base (pKa of conjugate acid)
Yield[a]
Scheme 3. Verification of the necessity of pH flash switching in the synthesis
of b-NCA 2a.
1
2
3
4
5
6
0.02
0.10
0.20
0.10
0.10
0.10
NMM (7.4[16]
NMM (7.4[16]
NMM (7.4[16]
)
)
)
86
93ꢀ1%
93
73
65
N-methylpiperidine (10.1[16]
)
periments was 71ꢀ3%. The batch conditions afforded re-
duced yields with lower reproducibility. In order to verify the
importance of “pH switching,” the developed micro-flow reac-
tion was carried out with an additional 1.0 equiv of NMM or
NaOH (Scheme 3c and 3d). The medium was kept basic
throughout the reaction under these conditions. Not surpris-
ingly, undesired polymerization occurred and the yields were
reduced (54% and 65%, respectively). These results clearly in-
dicated the importance of the “pH flash switching” that was
enabled via micro-flow, rapid-mixing technology in this reac-
tion. In addition, this reaction involves the generation of toxic
phosgene gas, and, therefore, it would be dangerous to carry
out the batch reaction on a large scale. However, this reaction
can be safely performed using a micro-flow reactor.
N-ethylmorphorine (7.7[16]
pyridine (5.2[17]
)
)
54
[a] Yields were determined by 1H NMR analysis with 1,1,2-trichloroethane
as an internal standard.
1a (entry 1). The 0.10 sec conditions (entry 2) and 0.20 sec con-
ditions (entry 3) afforded the same yield (93%), thus 0.10 sec is
sufficient to complete NCA formation. Triethylamine and N,N-
diisopropylethylamine could not be used as a base due to
their insufficient solubility against water. The use of a stronger
base, N-methylpiperidine, resulted in a decreased yield (entry 3
vs. 4) probably due to a base-mediated side reaction such as
the previously described isocyanate formation. The use of a
more sterically hindered base such as N-ethylmorpholine re-
sulted in a decreased yield (entry 3 vs. 5). Reportedly, NMM can
form corresponding acylammonium cation through its nucleo-
philic acyl substitution.[18] Therefore, it is conceivable that a
less sterically hindered NMM could attack triphosgene to form
a highly active acylammonium cation,[19] whereas a more steri-
cally hindered N-ethylmorpholine could not attack, and, thus,
The substrate scope was examined using the optimized re-
action conditions (Scheme 4). b-Alanine-derived NCA 2b was
obtained in a 57% yield. This is an acceptable result because
the synthesis of NCA 2b is a challenging task due to its ex-
treme instability, as Roumestant and co-workers previously re-
ported.[20] Less lipophilic NCAs such as 2c, 2d, and 2e were
obtained in acceptable to good yields (68, 74, and 69%, re-
spectively). Interestingly, our developed approach tended to
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was our originally developed “pH flash switching” and “flash
dilution” processes that were enabled by micro-flow technolo-
gy. We achieved good to excellent yields in the synthesis of 15
b-NCAs, which included acid-labile NCA that cannot be readily
synthesized using the conventional Leuchs approach. It should
be noted that the developed reaction can be safely and repro-
ducibly performed only by using micro-flow technology.
Scaled-up synthesis can be readily achieved via continuous
running. The developed approach will accelerate the use of b-
NCAs in b-peptide synthesis.
Experimental Section
General procedure: Micro-flow b-NCA formation
A solution of amino acid sodium salt (0.250m, 1.00 equiv.), N-
methyl morpholine (0.630m, 2.52 equiv.) in H2O (flow rate:
2.40 mLminꢁ1) and a solution of triphosgene (0.250m, 0.670 equiv.)
in MeCN (flow rate: 4.80 mLminꢁ1) was introduced into a V-shape
mixer at 208C using the syringe pumps. The resultant mixture was
passed through the reaction tube 1 (inner diameter: 0.250 mm,
length: 244 mm, volume: 12.0 mL, reaction time: 0.100 s) at the
same temperature. Then, the resultant mixture and EtOAc (flow
rate: 2.40 mLminꢁ1) were introduced to the T-shape mixer at 208C
with the syringe pumps. The resultant mixture was passed through
the reaction tube 2 (inner diameter: 0.800 mm, length: 298 mm,
volume: 150 mL, reaction time: 0.940 s) at the same temperature.
After being eluted for 20 s to reach a steady state, the resultant
mixture was poured into EtOAc (40 mL) for 100 s at 08C. The aque-
ous layer was extracted with EtOAc. The organic layer was washed
with brine, dried over MgSO4, filtered and concentrated in vacuo at
258C (caution: b-NCAs gradually decomposed).
Scheme 4. Substrate scope of the developed micro-flow b-NCA formation.
Isolated yields appear in parentheses. Boc, tert-butoxycarbonyl; TBS, tert-bu-
tyldimethylsilyl.
b-Phenylalanine-NCA (2a)
afford high yields in the case of lipophilic NCAs containing
sterically bulky side chains, which contrasts with the Leuchs
approach that tends to afford lower yields in the case of NCAs
with sterically bulky side chains.[4f] Our approach afforded
NCAs 2 f–2h containing sterically bulky alkyl side chains in
high to excellent yields (85, 98, and >99%, respectively). NCAs
2i–2m containing substituted phenyl groups were obtained in
high to excellent yields (88%->99%) irrespective of the elec-
tron-donating or -withdrawing nature of the substituents. On
the other hand, an NCA 2n containing a pyridine ring was not
obtained but a complex mixture was generated due to pyri-
dine ring-induced undesired reactions. An NCA 2o containing
an NHBoc side chain was not obtained, which was probably
due to an undesired cyclization that generated the corre-
sponding oxazolone. It should be noted that NCAs 2p and 2q
containing either an acid-labile tert-butyl ester or a TBS ether
were obtained in high to excellent yields. Syntheses of these
acid-labile NCAs are difficult using the Leuchs approach. These
results clearly indicate the mildness of our developed condi-
tions.
Purification method: b-NCA 2a was obtained without further pu-
rification. The reaction mixture was collected for 1275 sec. 2.32 g,
12.1 mmol, 95%. White solid; mp 97–998C; IR (ATR): n˜ =3230,
3148. 1791, 1734, 1383, 1332, 1115, 1023, 977 cmꢁ1 1H NMR
;
(500 MHz, CD3CN): d=7.42–7.39 (m, 2H), 7.37–7.33 (m, 3H), 6.94
(brs, 1H), 4.79–4.76 (m, 1H), 3.04 (dd, J=5.5, 16.5 Hz, 1H),
2.88 ppm (dd, J=8.0, 16.5 Hz, 1H); 13C NMR (125 MHz, CD3CN): d=
166.2, 150.4, 139.7, 129.9, 129.5, 127.0, 51.0, 37.2 ppm; HRMS (ESI-
TOF): calcd for [C6H9NO3 +Na]+ 214.0475, found 214.0476.
b-Alanine-NCA (2b)
Purification method: b-NCA 2b was obtained without further pu-
rification. 65.9 mg, 0.57 mmol, 57%. White solid; 84–868C
decomp.; IR (ATR): n˜ =3254, 3150, 2929, 1798, 1705, 1344, 1060,
1
749 cmꢁ1; H NMR (500 MHz, CD3CN): d=6.49 (brs, 1H), 3.33–3.30
(m, 2H), 2.71 ppm (t, J=7.0 Hz, 2H); 13C NMR (125 MHz, CD3CN):
d=167.3, 150.8, 35.6, 29.1 ppm; HRMS (ESI-TOF): calcd for
[C5H7NO3 +Na]+ 138.0162, found 138.0161.
a-Methyl-b-alanine-NCA (2c)
Purification method: b-NCA 2c was obtained without further purifi-
cation. 88.1 mg, 0.68 mmol, 68%. White solid; mp 112–1148C; IR
(ATR): n˜ =3280, 2338, 1792, 1734, 1642, 1354, 1077, 973, 598 cmꢁ1
;
Conclusions
1H NMR (400 MHz, CD3CN): d=6.43 (brs, 1H), 3.36–3.30 (m, 1H),
3.11–3.05 (m, 1H), 2.89–2.80 (m, 1H), 1.19 ppm (d, J=6.8 Hz, 3H);
13C NMR (100 MHz, CD3CN): d=170.4, 150.8, 42.0, 34.5, 12.5 ppm;
A single-step, rapid, and mild formation of synthetically chal-
lenging b-NCAs was successfully achieved. The key to success
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HRMS (ESI-TOF): calcd for [C5H7NO3 +Na]+ 152.0318, found
p-Nitrophenyl-b-alanine-NCA (2i)
152.0317.
Purification method: b-NCA 2i was obtained without further purifi-
cation. 261.0 mg, 1.11 mmol, >99%. Yellow oil; IR (neat): n˜ =3300,
1
a-Dimethyl-b-alanine-NCA (2d)
1801, 1748, 1520, 1350, 1079, 977 cmꢁ1; H NMR (500 MHz, CD3CN):
d=8.19 (d, J=8.5 Hz, 2H), 7.56 (d, J=8.5 Hz, 2H), 7.04 (brs, 1H),
4.95–4.91 (m, 1H), 3.12 (dd, J=5.5, 16.5 Hz, 1H), 2.91 ppm (dd, J=
8.0, 16.5 Hz, 1H); 13C NMR (125 MHz, CD3CN): d=165.5, 150.1,
148.8, 146.9, 128.3, 124.9, 50.6, 36.6 ppm; HRMS (ESI-TOF): calcd for
[C11H11NO4 +Na]+ 259.0325, found 259.0328.
Purification method: b-NCA 2d was obtained without further pu-
rification. 106.2 mg, 0.74 mmol, 74%. White solid; mp 110–1128C;
IR (ATR): n˜ =3258, 3162, 2987, 1794, 1733, 1338, 1051, 961,
1
697 cmꢁ1; H NMR (500 MHz, CDCl3): d=7.06 (brs, 1H), 3.22 (d, J=
2.0 Hz, 2H), 1.36 ppm (s, 6H); 13C NMR (125 MHz, CDCl3): d=170.5,
151.0, 47.7, 37.0, 22.6 ppm; HRMS (ESI-TOF): calcd for [C6H9NO3 +
Na]+ 166.0475, found 166.0474.
p-Fluorophenyl-b-alanine-NCA (2j)
Purification method: b-NCA 2j was obtained without further purifi-
cation. 199.3 mg, 0.95 mmol, 95%. White solid; mp 85–878C; IR
b-Homoalanine-NCA (2e)
(ATR): n˜ =3243, 3155, 1792, 1737, 1511, 1323, 1121, 834, 517 cmꢁ1
;
1H NMR (500 MHz, CD3CN): d=7.37–7.34 (m, 2H), 7.15–7.11 (m,
2H), 6.96 (brs, 1H), 4.79–4.76 (m, 1H), 3.05–3.00 (m, 1H), 2.87 ppm
(dd, J=8.5, 16.5 Hz, 1H); 13C NMR (125 MHz, CD3CN): d=166.1,
163.4 (d, JC-F =243.9 Hz), 150.3, 135.8 (d, JC-F =2.8 Hz), 129.2 (d, JC-
F =8.5 Hz), 116.6 (d, JC-F =21.8 Hz), 50.5, 37.2 ppm; HRMS (ESI-TOF):
calcd for [C10H8FNO3 +Na]+ 232.0380, found 232.0382.
Purification method: b-NCA 2e was obtained without further pu-
rification. The reaction mixture was collected for 63 sec. 56.4 mg,
0.44 mmol, 69%. White solid; 92–958C decomp.; IR (ATR): n˜ =3154,
2975, 1807, 1714, 1386, 1331, 1106, 995, 595 cmꢁ1 1H NMR
;
(500 MHz, CDCl3): d=6.95 (brs, 1H), 3.79–3.78 (m, 1H), 2.87 (dd,
J=4.0, 16.0 Hz, 1H), 2.53 (dd, J=9.5, 16.0 Hz, 1H), 1.34 ppm (d,
3H); 13C NMR (125 MHz, CDCl3): d=164.5, 150.4, 42.9, 36.2,
1
20.7 ppm. H NMR and 13C NMR spectra of DL-b-homoalanine-NCA
p-Chlorophenyl-b-alanine-NCA (2k)
were identical to the previously reported spectra of (S)-b-homoala-
nine-NCA.[4f]
Purification method: b-NCA 2k was obtained without further pu-
rification. 231.1 mg, 1.02 mmol, >99%. White solid, mp 117–
1208C, IR (ATR): n˜ =3240, 3150, 2258, 1793, 1739, 1371, 1326, 1119,
b-Homovaline-NCA (2 f)
972 cmꢁ1 1H NMR (500 MHz, CD3CN): d=7.39 (d, J=8.5 Hz, 2H),
;
7.32 (d, J=8.5 Hz, 2H), 6.96 (brs, 1H), 4.79–4.76 (m, 1H), 3.04 (dd,
J=5.5, 16.5 Hz, 1H), 2.86 ppm (dd, J=8.5, 16.5 Hz, 1H); 13C NMR
(125 MHz, CD3CN): d=165.9, 150.3, 138.6, 134.7, 129.9, 128.9, 50.5,
37.0 ppm; HRMS (ESI-TOF): calcd for [C10H8ClNO3 +Na]+ 248.0085,
found 248.0087.
Purification method: b-NCA 2 f was obtained without further purifi-
cation. 133.6 mg, 0.85 mmol, 85%. White solid, mp 69–728C; IR
(ATR): n˜ =3224, 2972, 2941, 1791, 1723, 1396, 1336, 1069,
966 cmꢁ1 1H NMR (500 MHz, CDCl3): d=7.19 (brs, 1H), 3.45–
;
3.41(m, 1H), 2.82 (dd, J=5.0, 16.0 Hz, 1H), 2.64 (dd, J=8.0, 16.0 Hz,
1H), 1.87–1.81 (m, 1H), 1.00 ppm (t, J=7.5 Hz, 6H); 13C NMR
(125 MHz, CDCl3): d=165.1, 150.9, 52.4, 32.1, 31.6, 17.9, 17.8 ppm.
1H NMR and 13C NMR spectra of DL-b-homovaline-NCA were identi-
cal to the previously reported spectra of (S)-b-homovaline-NCA.[4f]
p-Methoxyphenyl-b-alanine-NCA (2l)
Purification method: b-NCA 2l was obtained without further purifi-
cation. 193.6 mg, 0.99 mmol, 88%. White solid; mp 117–1198C; IR
(ATR): n˜ =3221, 3146, 1799, 1739, 1614, 1378, 1174, 823, 598 cmꢁ1
;
1H NMR (500 MHz, CD3CN): d=7.26–7.23 (m, 2H), 6.94–6.91 (m,
2H), 6.87 (brs, 1H), 4.72–4.69 (m, 1H), 3.75 (s, 3H), 2.98 (dd, J=5.0,
16.0 Hz, 1H), 2.86 ppm (dd, J=8.5, 16.0 Hz, 1H); 13C NMR
(125 MHz, CD3CN): d=166.4, 160.7, 150.4, 131.5, 128.4, 115.1, 55.8,
50.6, 37.3 ppm; HRMS (ESI-TOF): calcd for [C11H11NO4 +Na]+
244.0580, found 244.0579.
b-Homoleucine-NCA (2g)
Purification method: b-NCA 2g was obtained without further pu-
rification. The reaction mixture was collected for 76 sec. 127.5 mg,
0.74 mmol, 98%. White solid; mp 63–658C; IR (ATR): n˜ =3270,
2959, 1797, 1721, 1389, 1329, 1120, 1012, 977 cmꢁ1 1H NMR
;
(500 MHz, CDCl3): d=7.29 (s, 1H), 3.69 (brs, 1H), 2.88 (dd, J=4.5,
16.0 Hz, 1H), 2.55 (dd, J=8.0, 16.0 Hz, 1H), 1.78–1.70 (m, 1H),
1.57–1.52 (m, 1H), 1.39–1.35 (m, 1H), 0.95 ppm (d, J=4.5 Hz, 6H);
13C NMR (125 MHz, CDCl3): d=164.8, 150.7, 45.7, 43.8, 34.7, 24.2,
p-Tolyl-b-alanine-NCA (2m)
Purification method: b-NCA 2m was obtained without further pu-
rification. 202.6 mg, 0.99 mmol, 99%. White solid; mp 119–1208C;
1
22.5, 22.0 ppm. H NMR and 13C NMR spectra of DL-b-homoleucine-
IR (ATR): n˜ =3253, 3165, 2933, 1792, 1741, 1340, 1079 cmꢁ1
;
NCA were identical to the previously reported spectra of (S)-b-ho-
moleucine-NCA.[4f]
1H NMR (500 MHz, CD3CN): d=7.21 (s, 4H), 6.92 (brs, 1H), 4.74–
4.70 (m, 1H), 3.00 (dd, J=5.5, 16.5 Hz, 1H), 2.85 (dd, J=8.5,
16.5 Hz, 1H), 2.31 ppm (s, 3H); 13C NMR (125 MHz, CD3CN): d=
166.3, 150.5, 139.4, 136.7, 130.5, 126.9, 50.8, 37.3, 21.0 ppm; HRMS
(ESI-TOF): calcd for [C6H9NO3 +Na]+ 228.0631, found 228.0632.
b-Homophenylalanine-NCA (2h)
Purification method: b-NCA 2h was obtained without further pu-
rification. 208.6 mg, 1.02 mmol, >99%. White solid; mp 97–998C;
IR (ATR): n˜ =3306, 1794, 1721, 1387, 1341, 1087, 982, 745,
;
695 cmꢁ1 1H NMR (500 MHz, CDCl3): d=7.35–7.17 (m, 5H), 6.77
a-O-tertButyl-l-b-aspartate-NCA (2p)
(brs, 1H), 3.86–3.83 (m, 1H), 2.91–2.79 (m, 3H), 2.61 ppm (dd, J=
8.0, 16.5 Hz, 1H); 13C NMR (125 MHz, CDCl3): d=164.4, 150.1, 134.8,
129.3, 129.2, 127.7, 48.1, 41.2, 33.7 ppm. 1H NMR and 13C NMR spec-
tra of DL-b-homophenylalanine-NCA were identical to the previ-
ously reported spectra of (S)-b-homophenylalanine-NCA.[4f]
Purification method: b-NCA 2p was obtained without further pu-
rification. 225.1 mg, 1.05 mmol, >99%. White solid; mp 84–868C;
IR (ATR): n˜ =3231, 3167, 2985, 1801, 1759, 1720, 1366, 1090 cmꢁ1
;
1H NMR (500 MHz, CD3CN): d=6.82 (brs, 1H), 4.10–4.07 (m, 1H),
3.02 (dd, J=6.5, 16.5 Hz, 1H), 2.86 (dd, J=4.5, 16.5 Hz, 1H),
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1.42 ppm (s, 9H); 13C NMR (125 MHz, CD3CN): d=169.7, 165.4,
149.8, 84.2, 50.3, 31.8, 27.9 ppm; HRMS (ESI-TOF): calcd for
[C9H13NO5 +Na]+ 238.0686, found 238.0683; ½aꢂ2D9 = +40.12 (c=
1.00, CH3CN).
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O-tertButyldimethylsilyl-l-b-homoserine-NCA (2q)
Purification method: b-NCA 2q was obtained without further pu-
rification. The reaction mixture was collected for 25 sec. 59.9 mg,
0.23 mmol, 92%. White amorphous solid, IR (neat): n˜ =3380, 2938,
1798, 1746, 1651, 1404, 1110, 1038 cmꢁ1
;
1H NMR (400 MHz,
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CD3CN): d=6.40 (brs, 1H), 3.69 (dd, J=2.8, 10.8 Hz, 1H), 3.62 (dd,
J=2.8, 8.8 Hz, 1H), 2.88 (dd, J=6.8, 16.4 Hz, 1H), 2.60 (m, 1H), 0.87
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d=167.0, 151.0, 67.2, 49.1, 31.5, 26.0, 18.8, ꢁ5.6 ppm; HRMS (ESI-
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Acknowledgements
This work was partially supported by a Scientific Research on
Innovative Areas 2707 Middle molecular strategy (no.
16H01138) from the Japan Society for the Promotion of Sci-
ence, and by The Naito Foundation Natural Science Scholar-
ship. Y.O. gratefully acknowledges financial support from the
Japan Society for the Promotion of Science (JSPS, ID No.
19J14624).
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Conflict of interest
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855; Angew. Chem. 2014, 126, 870–874; b) S. Fuse, Y. Mifune, H. Naka-
mura, H. Tanaka, Nat. Commun. 2016, 7, 13491.
The authors declare no conflict of interest.
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Keywords: acylation · anhydrides · flow chemistry · b-amino
acids
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Manuscript received: October 11, 2019
Revised manuscript received: November 6, 2019
Version of record online: && &&, 0000
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FULL PAPER
A challenging synthesis of b-amino
acid N-carboxy anhydrides (b-NCAs) via
a single-step, rapid, and mild formation
using pH flash switching and flash dilu-
tion, in a micro-flow reactor has been
achieved. Fifteen b-NCAs were synthe-
sized in good to excellent yields that in-
cluded acid-labile b-NCAs that cannot
be readily synthesized using the con-
ventional Leuchs approach. Scaled-up
synthesis can be readily achieved
Naoto Sugisawa, Yuma Otake,
Hiroyuki Nakamura, Shinichiro Fuse*
&& – &&
Single-Step, Rapid, and Mild Synthesis
of b-Amino Acid N-Carboxy
Anhydrides Using Micro-Flow
Technology
through continuous operation.
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These are not the final page numbers! ÞÞ