Total synthesis and absolute structure of N55, a positive modulator of GLP-1 signaling

Article abstract of DOI:10.1039/d0ob01722a

Glucagon-like peptide-1 (GLP-1) signaling is an established therapeutic target for type 2 diabetes mellitus (T2DM). We developed a 7-step synthesis of N55, a positive modulator of GLP-1 signaling isolated from fenugreek (Trigonella foenum-graecum) seeds,

Full text of DOI:10.1039/d0ob01722a

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OrPgleanaisce&dBoionmotoaledcjuulsatr mChaermgiinstsry
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DOI: 10.1039/D0OB01722A
ARTICLE
Total Synthesis and Absolute Structure of N55, a Positive
Modulator of GLP-1 Signaling
Nai-Pin Lin and Rong-Jie Chein*
Received 00th January 20xx,
Accepted 00th January 20xx
Glucagon-like peptide-1 (GLP-1) signaling is an established therapeutic target for type 2 diabetes mellitus (T2DM). We
developed a 7-step synthesis of N55, a positive modulator of GLP-1 signaling isolated from fenugreek (Trigonella foenum-
graecum) seeds, with 29% overall yield, and we determined the absolute structure of N55 to be N-((3R,4R,5S)-4,5-dimethyl-
2-oxotetrahydrofur-3-yl)linoleic amide.
DOI: 10.1039/x0xx00000x
We have discovered a positive modulator of GLP-1 signaling,
N55, from the ethanol extract of fenugreek (Trigonella foenum-
graecum) seeds.10 Fenugreek is an edible plant that is utilized as
Introduction
Glucagon-like peptide-1 (GLP-1) is an incretin peptide hormone
secreted after nutrient intake that exhibits various important
functions, such as enhancing glucose-dependent insulin secretion,
protecting against inflammation, decreasing hypertension, and
increasing cognitive function, as well as exhibiting cardio-protection
and neuroprotection activities.1 The physiological response to GLP-1
is mainly mediated through binding and activation of the GLP-1
receptor (GLP-1R).1c,2 GLP-1R is a family B G-protein-coupled
receptor (GPCR) and expressed in a wide array of tissues, including
pancreatic islet β-cells, lungs, heart, kidneys, blood vessels, neurons,
and lymphocytes.1a,1c The most potent actions of GLP-1 are the
insulinotropic and glucagonostatic effects to lower the plasma
glucose level.3 After food intake, the plasma GLP-1 level rapidly rises
3-fold, contributing to normoglycemia.4 GLP-1 is then quickly
eliminated by the degradation of dipeptidyl peptidase-4 (DPP-4) and
renal clearance. The short half-life (1.5-5 minutes) of GLP-1 can
tightly regulate GLP-1 signaling and the insulin secretory response.
The impaired secretion and reduced postprandial concentrations of
GLP-1 observed in type 2 diabetic patients may contribute to their
morbid insulin secretion and blood glucose hemostasis.1e,4,5
food, spice, and traditional medicine worldwide. Extensive
preclinical and clinical research have outlined the therapeutic
utilities of fenugreek due to its anti-diabetic, anti-
hyperlipidemic, anti-obesity, anti-cancer, anti-inflammatory,
and antibiotic effects.11 N55 can specifically enhance GLP-1
potency by more than 40-fold in terms of GLP-1-dependent
3',5'-cyclic adenosine monophosphate (cAMP) production,10
and its ability to lower plasma glucose base on physiological
levels of GLP-1 was later revealed.12 Considering the great anti-
diabetic potential of N55, here we further expatiate its
structure elucidation through the first total synthesis in 7 steps
with an overall yield of 29%. The chirality of synthetic N55 was
created by a nearly enantio- and diastereo-specific D-proline
catalyzed asymmetric anti-Mannich-type reaction to establish
an asymmetric amino group and the vicinal tertiary carbon
center at once.13 Moreover, the chiral centers and substituents
can be altered and replaced by the modification of reactions to
obtain different N55 analogues.
For treatment of type 2 diabetes mellitus (T2DM), several drugs
have already been approved that act through the GLP-1 regulatory
system.6 Apart from the current therapeutic strategies aiming to
constitutively activate GLP-1R by agonists or DPP-4 inhibitors,7,8
positive modulators of GLP-1 signaling are foreseen as a promising
alternative approach in the future.9 The positive modulators are less
likely to exhibit a chronic activation of GLP-1R and are favorable for
the physiological spatiotemporal regulation of GLP-1. Some GLP-1
signaling positive modulators have already been reported that can
enhance the degree of activation according to endogenous GLP-1 but
do not activate the GLP-1R by themselves.9,10
Results and discussion
4-Hydroxylisoleucine is a crucial component in fenugreek
seeds.14 According to the mass spectrometry analysis of the
isolated sample of N55 from fenugreek seeds, the m/z value of
[
N55 + H]+ is 392 with two major MS/MS fragments at m/z of
130 and 263 (Supporting Information, Figure S1). The
compositions of the m/z 263 fragment may contain eighteen
carbons, one oxygen, and three degrees of unsaturation, and
these structural features were clearly observed in the 1H and13
C
NMR spectra (Supporting Information, Figures S2 and S3). These
results highly imply that the fragment should be an 18-carbon
fatty acyl with two degrees of unsaturation. Linoleic acid is the
only natural 18-carbon fatty acid with two degrees of
unsaturation. On the other hand, the m/z value 130 ([M+H]+) is
potentially derived from dehydration of 4-hydroxylisoleucine
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan.
Email: rjchein@chem.sinica.edu.tw
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
(m/z 147), i.e., the lactone form of 4-hydroxylisoleucine. Scheme 1 Synthesis of 4 (PMP: p-methoxyphenyl).View Article Online
OMe
DOI: 10.1039/D0OB01722A
O
Together, we concluded that the structure of N55 is 3-amino-
COOH
4,5-dimethyl-γ-lactone linoleic amide. Possible assignments and
PMP
N
,
H2N
O
N
6
H
1
atomic correlations in the H, 13C, HH-COSY, HSQC, and HMBC
EtO
EtO
Na2SO4, benzene, rt, 0.5 h
DMF, rt, 18 h
NMR spectra are all consistent with this proposed structure.
However, the stereochemistry cannot be determined by these
spectral data. In fenugreek seeds extract, the major 4-
O
O
5
7
PMP
PMP
PMP
PMP
DBN, MTBE
rt, 2 h
NH
O
NH
O
NH
O
NH
O
hydroxylisoleucine
isoform
possesses
a
(2S,3R,4S)
EtO
+
EtO
EtO
+
EtO
configuration.14,15 Therefore, we first aim to synthesize N-
((3S,4R,5S)-4,5-dimethyl-2-oxotetrahydrofur-3-yl)linoleic
O
O
O
O
8
9
4
9
5 : 1
amide (
2
) for the structure elucidation of N55 (Fig. 1).
after recrystallization
40% for 3 steps
O
O
O
The reduction-lactonization tandem reaction from
4
to 10
HO
(PMP-protected lactone) was stereospecifically accomplished
by
-selcectride at -78 °C in 73% yield (Scheme 2).13,16c Although
the yield of the PMP deprotection of 10 was lower than
expected under various conditions,18 compound
was easily
HN
NH2 OH
L
O
2
1
Fig. 1 (2S,3R,4S)-4-Hydroxylisoleucine (
dimethyl-2-oxotetrahydrofur-3-yl)linoleic amide (2).
1
) and N-((3S,4R,5S)-4,5-
2
obtained after an amide bond formation between free amine
and linoleic acid.
3
O
O
Scheme
2
Synthesis of
2
(PyBOP: benzotriazol-1-
HN
oxytripyrrolidinophosphonium hexafluorophosphate, CAN:
ceric ammonium nitrate).
O
2
L-selectride
THF
CAN, NaBH4
MeCN/H2O
PMP
NH
O
O
O
-78 oC, 2 h
O
0 oC to 35 oC, 3 h
EtO
O
O
O
HO
+
32%
73%
O
NH2
O
NHPMP
NH2
3
4
Linoleic acid
10
3
O
linoleic acid, PyBOP, DIPEA
DMF, rt, 18 h
O
O
O
HN
50%
O
NH
O
N
O
2
O
O
+
O
4
O
6
5
Other attempts to improve the synthetic efficiency are
summarized as below: 1) In Scheme 3, the PMP group was
replaced with a tert-butyloxycarbonyl (Boc) group to facilitate
the deprotection. 2) In Scheme 4, linoleic acid was first
conjugated to amino ketone 17 to circumvent the use of
protecting group. However, both of the alternative strategies
resulted in inseparable epimer and were excluded from further
studies.
Fig. 2 The retrosynthesis of 2.
A reterosynthetic analysis was proposed as shown in Fig. 2.
The proposed compound could be obtained from a simple
coupling reaction of amino lactone and commercially available
linoleic acid. Although the synthesis of amino lactone has been
reported by several groups,15,16 the efficiency could be further
improved by developing a shorter and stereoselective synthetic
2
3
3
Scheme 3 Synthesis of
11
3 through Boc-protected aminoketone
route. Therefore, amino lactone
through an asymmetric ketone reduction-lactonization tandem
reaction from amino-keto ester , which can be prepared from
and 2-butanone ( ) through known
3 was planned to be acquired
.
4
PMP-protected imine
procedures.13,15,17
5
6
As shown in Scheme 1, commercially available ethyl 2-
oxoacetate ( ) was first transferred to inmine and then
submitted to an -proline catalyzed asymmetric anti-Mannich-
type reaction to give a 5:1 ratio of amino ketone (> 99% ee)
and regioisomer . The inseparable mixture of and was
further treated with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBN)
in methyl tert-butyl ether (MTBE) to afford amino-keto ester
after recrystallization. The enantiomeric purity (> 99% ee) of
amino ketone was confirmed by HPLC analysis.
7
5
L
8
9
8
9
4
4
2 | J. Name., 2012, 00, 1-3
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ARTICLE
Scheme 4 Conjugation of 17 and linoleic acid (HBTU: 2-(1H-
benzotriazol-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate, LA: linoleic acyl).
View Article Online
DOI: 10.1039/D0OB01722A
Scheme 6 Synthesis of ent-21 (N55).
O
OMe
COOH
LA
LA
N
H
linoleic acid, HBTU
DIPEA, DMF, rt, 18 h
PMP
NH2
O
NH
O
NH
O
,
6
H2N
O
N
EtO
EtO
EtO
Na2SO4, benzene, rt. 0.5 h
+
DMF, rt, 18 h
EtO
EtO
42%
O
O
O
O
O
5 : 1
18
19
5
17
7
inseparable
(NH4)2S2O8
CAN,
MeCN/H2O
PMP
PMP
0 oC to 35 o
3 h
C
NH2
O
NH2
O
However, the 1H NMR spectrum of synthetic compound
2
was
NH
O
NH
O
EtO
EtO
EtO
EtO
+
+
not consistent with that of the isolated N55. We then revised
the configuration of the amino lactone moiety from (3S,4R,5S)
to (3S,4S,5R) 20, which has been observed in other natural
products, (–)-funebral and (–)-funebrine.18c,19 The newly
O
O
O
O
ent-22
ent-23
ent-8
ent-9
5 : 1
L-selectride
H2, Pd(OH)2
EtOAc
BnBr, K2CO3
DMF/H2O
NBn2
O
THF
O
O
60 oC, 18 h
proposed
structure
was
N-((3S,4S,5R)-4,5-dimethyl-2-
EtO
-78 oC, 2 h
rt, 18 h
O
O
oxotetrahydrofur-3-yl)linoleic amide (21) (Fig. 3).
O
94%
99%
NBn2
ent-25
NH2
ent-20
ent-24
33% for 4 steps from 7
O
O
O
O
Linoleic acid, PyBOP
DIPEA, DMF
rt, 18 h
O
O
O
O
O
HO
HO
N
N
N
O
NH2
HN
O
94%
O
ent-21 (N55)
(-)-Funebral
(-)-Funebrine
20
O
1
The H and 13C NMR spectra of 21 are exactly identical with
O
19.8
those of N55, and the specific optical rotation of 21 ([α]D
=
HN
20.7) is equal in magnitude but opposite in sign to that of N55
([α]D = –23.0), which supports the enantiomeric relationship
O
21
21
between 21 and N55. Finally, synthetic N55 (ent-21) was
obtained through the same synthetic route as 21 with only a
Fig. 3 The revised structure of N55
.
replacement of L-proline to D-proline in the asymmetric anti-
Starting from the aforementioned amino ketone
8
and 9, the
Mannich-type reaction (Scheme 6). This time, all the spectra of
ent-21 are consistent with the spectra of isolated N55. A
comparison of 1H NMR between ent-21 and N55 is given in Fig.
4. Thus, the chemical structure of N55 was determined to be N-
((3R,4R,5S)-4,5-dimethyl-2-oxotetrahydrofur-3-yl)linoleic
amide.
PMP protecting groups were removed by ceric ammonium
nitrate. Free amine 22 was later isolated from the mixture after
protected with di-benzyl (Bn) groups to form di-benzyl amino
ketone 24, which was reduced and cyclized after the treatment
of L-selectride to afford lactone 25, exclusively, in 94% yield
(Scheme 5). The following deprotection of the di-benzyl group
by a palladium-catalyzed hydrogenation and PyBOP mediated
amide bond formation with linoleic acid were carried out
successfully
to
produce
N-((3S,4S,5R)-4,5-dimethyl-2-
oxotetrahydrofur-3-yl)linoleic amide 21 in high yields.
Scheme 5 Synthesis of 21
.
(NH4)2S2O8
CAN,
MeCN/H2O
0 oC to 35 o
3 h
C
PMP
PMP
NH2
O
NH2
O
NH
O
NH
O
EtO
EtO
EtO
EtO
+
+
crude: 66%
O
O
O
O
O
8
9
22
23
5 : 1
BnBr, K2CO3
DMF/H2O
NBn2
O
L-selectride, THF
-78 oC, 2 h
H2, Pd(OH)2
EtOAc, rt, 18 h
O
O
60 oC, 18 h
EtO
O
94%
60%
99%
O
NBn2
NH2
24
25
20
O
Linoleic acid, PyBOP
DIPEA, DMF, rt, 18 h
O
1
Fig. 4 Comparative 500 MHz H NMR spectra between isolated
N55 (upper) and synthetic N55 (lower) in d4-methanol (δH = 3.31
ppm).
HN
94%
O
21
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Inspired by the success of N55, we re-examined the synthesis δ 49.0 ppm). The abbreviations were used to interpret NMR peak
View Article Online
of amino lactone
intermediates (Scheme 7). Amino lactone
successfully acquired with high selectivity and yield. The overall on an Applied Biosystems 4800 Proteomics Analyzer equipped with
3
through the di-benzyl protected multiplicities: s = singlet, d = doublet, t = tDriOplIe: 1t0, .q10=39q/Du0aOrtBe0t,17m22A=
was then multiplet, and br = broad. HR MALDI-mass spectra were conducted
3
yield of
3
was 12% over 7 steps.
an Nd/YAG laser (335 mm) operating at a repetition rate of 200 Hz.
HR FAB and HR EI-mass spectra were conducted on a JEOL JMS-700
double-focusing mass spectrometer with a resolution of 8000 (5%
valley definition). HR ESI-mass spectra were conducted on a dual
ionization ESCi® (ESI/APCi) source options, Waters LCT premier XE
(Waters Corp., Manchester, UK). Optical rotations were obtained on
a Jasco P-2000 polarimeter (1-dm cell if not stated). Infrared (IR)
spectra were recorded on a Thermo Nicolet iS-5 FT-IR spectrometer.
Melting points were recorded on a BÜCHI M-565 melting point
apparatus.
Scheme 7 Synthesis of
3
.
O
OMe
COOH
N
PMP
H
,
6
H2N
Na2SO4, benzene, rt. 0.5 h
N
O
DMF, rt, 18 h
EtO
EtO
O
O
5
7
PMP
NH
PMP
PMP
DBN, MTBE
rt, 2 h
O
NH
O
NH
O
EtO
(9Z,12Z)-N-((3R,4R,5S)-4,5-Dimethyl-2-oxotetrahydrofur-3-yl)
octadeca-9,12-dienamide (N55):10
Data of N55 from isolated sample purified by high-performance
EtO
+
EtO
O
O
O
9
4
8
5 : 1
40% for 3 steps
liquid chromatography (HPLC): Rf
=
0.28 [ethyl acetate
BnBr, K2CO3
DMF/H2O
(NH4)2S2O8, CAN
MeCN/H2O
NBn2
O
(EtOAc)/hexane = 1/2, I2]; [α]D21 = –23.0 (c 0.30, CHCl3, 5-cm cell); IR
(film) ṽ = 3303, 3008, 2956, 2926, 2854, 1782, 1657, 1650, 1536, 1461,
1453, 1388, 1183, 1047, 908, 723 cm-1; 1H NMR (500 MHz, CD3OD) δ
5.39–5.29 (m, 4 H, CH2CHCHCH2), 4.38 (d, J =11.8 Hz, 1 H, CHNH), 4.19
(dt, J = 9.8, 6.1 Hz, 1H, COOCH), 2.78 (t, J = 6.5 Hz, 2 H, CHCH2CH),
2.26 (t, J = 7.4 Hz, 2 H, CH2CONH), 2.17–2.04 (m, 5 H, CHCH3,
CH2CH2CH), 1.66–1.60 (m, 2 H, CH2CH2CONH), 1.41 (d, J = 6.1 Hz, 3 H,
COOCHCH3), 1.40–1.28 (m, 14 H, CH2CH2), 1.13 (d, J = 6.6 Hz, 3H,
CHCH3), 0.91 (t, J = 6.8 Hz, 3H, CH2CH3); 13C NMR (125 MHz, CD3OD)
δ 176.5 (s, CONH), 176.2 (s, COOCH), 130.9 (d, CH2CHCHCH2), 130.9
(d, CH2CHCHCH2), 129.1 (d, CH2CHCHCH2), 129.1 (d, CH2CHCHCH2),
81.4 (d, COOCH), 57.5 (d, CHNH), 45.6 (d, CHCH3), 36.9 (t, CH2CONH),
32.7 (t, CH2CH2), 30.7 (t, CH2CH2), 30.5 (t, CH2CH2), 30.3 (t, CH2CH2),
30.3 (t, CH2CH2), 30.2 (t, CH2CH2), 28.2 (t, CH2CH2CH), 26.8 (t,
CH2CH2CONH), 26.5 (t, CHCH2CH), 23.6 (t, CH2CH2), 18.8 (q,
COOCHCH3), 14.4 (q, CH2CH3), 14.0 (q, CHCH3); HRMS (ESI-TOF) m/z
[M + H]+ calcd. for C24H42O3N 392.3165, found 392.3160.
NH2
O
60 oC, 18 h
EtO
EtO
0 oC to 35 oC, 3 h
O
O
57%
crude: 80%
26
17
O
O
L-selectride, THF
-78 oC, 2 h
H2, Pd(OH)2
EtOAc, rt, 18 h
O
O
89%
74%
NBn2
NH2
3
27
Conclusions
We performed the first total synthesis of N55 isolated from
fenugreek seeds with an overall yield of 29% over 7 steps and
elucidated its absolute structure as a N-((3R,4R,5S)-4,5-
dimethyl-2-oxotetrahydrofur-3-yl)linoleic amide. This study
provides a comprehensive platform to further expand the scope
of N55 analogues for pharmaceutical applications.
21
Data of synthetic N55: Rf = 0.28 (EtOAc/hexane = 1/2, I2); [α]D = –
21.8 (c 0.30, CHCl3, 5-cm cell); IR (film) ṽ = 3303, 3009, 2957, 2927,
2855, 1782, 1657, 1650, 1533, 1461, 1454, 1389, 1187, 1047, 908,
723 cm-1; 1H NMR (500 MHz, CD3OD) δ 5.39–5.29 (m, 4 H,
CH2CHCHCH2), 4.38 (d, J =11.8 Hz, 1 H, CHNH), 4.19 (dt, J = 9.8, 6.1
Hz, 1 H, COOCH), 2.78 (t, J = 6.5 Hz, 2 H, CHCH2CH), 2.26 (t, J = 7.5 Hz,
2 H, CH2CONH), 2.17–2.04 (m, 5 H, CHCH3, CH2CH2CH), 1.66–1.60 (m,
2 H, CH2CH2CONH), 1.41 (d, J = 6.1 Hz, 3 H, COOCHCH3), 1.40–1.28 (m,
14 H, CH2CH2), 1.13 (d, J = 6.6 Hz, 3H, CHCH3), 0.91 (t, J = 6.9 Hz, 3H,
CH2CH3); 13C NMR (125 MHz, CD3OD) δ 176.5 (s, CONH), 176.2 (s,
COOCH), 130.9 (d, CH2CHCHCH2), 130.9 (d, CH2CHCHCH2), 129.1 (d,
CH2CHCHCH2), 129.1 (d, CH2CHCHCH2), 81.4 (d, COOCH), 57.5 (d,
CHNH), 45.6 (d, CHCH3), 36.9 (t, CH2CONH), 32.7 (t, CH2CH2), 30.7 (t,
CH2CH2), 30.5 (t, CH2CH2), 30.3 (t, CH2CH2), 30.3 (t, CH2CH2), 30.2 (t,
CH2CH2), 28.2 (t, CH2CH2CH), 26.8 (t, CH2CH2CONH), 26.5 (t,
CHCH2CH), 23.6 (t, CH2CH2), 18.8 (q, COOCHCH3), 14.4 (q, CH2CH3),
14.0 (q, CHCH3); HRMS (MALDI-TOF) m/z [M + Na]+ calcd. for
C24H41O3NNa 414.2979, found 414.2963.
Experimental
General Experimental Methods: All reactions were carried out
under an inert nitrogen atmosphere with dry solvents under
anhydrous conditions unless otherwise stated, and standard
syringe–septa techniques were followed. Solvents were dried by
conventional methods prior to use. Reagents were purchased and
used without further purification. Reactions were monitored by thin-
layer chromatography (TLC) using glass-backed plates pre-coated
with silica gel 60 (Merck, silica gel 60 F254). Ultraviolet (UV) light was
used as the visualizing agent. Ceric ammonium molybdate and heat,
ninhydrin and heat, or iodine were used as developing agents. Flash
silica gel chromatography was performed using silica gel 60 (Merck,
F254 230–400 mesh). Nuclear magnetic resonance (NMR) spectra
were recorded on Bruker AV 400 MHz, AV-III 400 MHz and AV-500
MHz instruments and were calibrated using a residual undeuterated
solvent as an internal reference (d-chloroform, 1H NMR δ 7.26 ppm,
13C NMR δ 77.1 ppm; d4-methanol, 1H NMR δ 3.31 ppm, 13C NMR
Ethyl
(2R,3R)-2-(4-Methoxyphenylamino)-3-methyl-4-
oxopentanoate (ent-8):13,15 Na2SO4 (10.7 g, 75.0 mmol) was added
to a stirred solution of 4-anisidine (3.69 g, 30.0 mmol) in toluene
(PhMe) (30.0 mL), followed by the addition of ethyl glyoxalate (7)
4 | J. Name., 2012, 00, 1-3
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(6.13 mL, 30.0 mmol, 50% in toluene) within 10 to 20 minutes. The
reaction mixture was stirred at room temperature for 30 minutes. Compound 22, the enantiomer of ent-22, was prepared and
After the starting material was consumed, Na2SO4 was filtered out by characterized by the same methods as ent-22 except a replacement
Celite, and the filtrate was concentrated under reduced pressure to of substrate ent-8 to 8 with identical selectivity and yield.
ARTICLE
Ethyl
(2S,3S)-2-Amino-3-methyl-4-oxopentanoate
(22):
View Article Online
DOI: 10.1039/D0OB01722A
give a brown oil containing 5 that was used immediately for the next
Ethyl
(2R,3R)-2-(Dibenzylamino)-3-methyl-4-oxopentanoate
step without further purification. solution of in dry (ent-24): A suspension of the crude ent-22, its regeoisomer (ent-23,
A
5
dimethylformamide (DMF) (15.5 mL) was slowly added to a stirred 5 : 1, 98 mg, ~0.57 mmol) and K2CO3 (235 mg, 1.7 mmol) in DMF/H2O
solution of butanone (6) (59.0 mL, 660 mmol) and D-proline (1.21 g, (1.1 mL/0.11 mL) was stirred at room temperature for 15 minutes,
10.5 mmol) in dry DMF (46.6 mL) over 30 minutes at room followed by the dropwise addition of benzyl bromide (BnBr) (0.34 mL,
temperature, and the resulting mixture was stirred at room 2.8 mmol). After stirring at 60 °C for 18 hours, H2O (5 mL) was added,
temperature for 12 hours. After the starting material was consumed, and the reaction mixture was then extracted with EtOAc (10 mL × 3).
the reaction mixture was filtered through a pad of sieve and The combined organic layers were dried over Na2SO4 and
concentrated under reduced pressure. The resulting yellow oil concentrated under reduced pressure. The residue was purified by
containing ent-8 and its regioisomer (ent-9; 5 : 1) was directly used flash column chromatography on silica gel (EtOAc/hexane = 1/15) to
for the next reaction without further purification. A small amount of give ent-24 as a white solid (120 mg, 0.34 mmol, 60%). Rf = 0.45
23
mixture was applied to column chromatography (silica gel, (EtOAc/hexane = 1/5, UV); mp 58.8–62.3 °C; [α]D = 186 (c 1.03,
EtOAc/hexane = 1/4) and then HPLC for the characterization of ent- CHCl3); IR (film) ṽ = 3062, 3029, 2977, 2929, 2852, 1723, 1602, 1495,
8. Rf = 0.25 (EtOAc/hexane = 1/5, UV); [α]D = 47.3 (c 1.05, CHCl3, 1454, 1370, 1201, 1169, 1026, 961, 748, 699 cm-1; 1H NMR (400 MHz,
23.1
>99% ee); IR (film) ṽ = 3379, 2981, 2936, 1729, 1713, 1514, 1235, CDCl3) δ 7.37–7.22 (m, 10 H, Ar), 4.34–4.16 (m, 2 H, COOCH2), 3.86
1200, 1180, 1035, 823 cm-1; 1H NMR (400 MHz, CDCl3) δ 6.77 (d, J = (d, J = 13.5 Hz, 2 H, PhCH2), 3.49 (d, J = 10.9 Hz, 1 H, CHNBn2), 3.45 (d,
8.8 Hz, 2 H, Ar), 6.66 (d, J = 8.8 Hz, 2 H, Ar), 4.31 (d, J = 5.8 Hz, 1 H, J = 13.4 Hz, 2 H, PhCH2), 3.08 (dt, J = 10.9, 7.2 Hz, 1 H, CHCH3), 2.15
CHNH), 4.19–4.13 (m, 2 H, COOCH2), 3.74 (s, 3 H, OCH3), 3.03 (m, 1 H, (s, 3 H, COCH3), 1.38 (t, J = 7.1 Hz, 3 H, CH2CH3), 1.10 (d, J = 7.3 Hz, 3
CHCH3), 2.23 (s, 3 H, COCH3), 1.25 (d, J = 7.1 Hz, 3 H, CHCH3), 1.22 (t, H, CHCH3); 13C NMR (100 MHz, CDCl3) δ 211.5 (s, COCH3), 171.9 (s,
J = 7.2, 3 H, CH2CH3); 13C NMR (125 MHz, CDCl3) δ 209.3 (s, COCH3), COOCH2), 138.9 (s, Ar), 129.2 (d, Ar), 128.4 (d, Ar), 127.3 (d, Ar), 62.6
172.9 (s, COOCH2), 153.3 (s, Ar), 140.9 (s, Ar), 116.0 (d, Ar), 115.0 (d, (d, CHNBn2), 60.5 (t, COOCH2), 55.2 (t, PhCH2), 46.0 (d, CHCH3), 29.2
Ar), 61.5 (t, COOCH2), 59.8 (d, CHNH), 55.8 (q, OCH3), 49.4 (d, CHCH3), (q, COCH3), 14.7 (q, CH2CH3), 14.5 (q, CHCH3); HRMS (ESI-TOF) m/z [M
28.6 (q, COCH3), 14.3 (q, CH2CH3), 12.4 (q, CHCH3); enantioselectivity + H]+ calcd. for C22H28O3N 354.2069, found 354.2061.
was determined by HPLC analysis (Chiralpak-AS, 1.0 mL/minute, 220
nm, hexane/i-PrOH 97/3); retention times were 21.7 (enantiomer) Compound 24, the enantiomer of ent-24, was prepared and
and 30.9 minutes (major). characterized by the same methods as ent-24 except a replacement
Ethyl (2S,3S)-2-(4-Methoxyphenylamino)-3-methyl-4- of substrate ent-22 to 22 with identical selectivity and yield.
oxopentanoate (8): Compound 8, the enantiomer of ent-8, was (3R,4R,5S)-3-(Dibenzylamino)-4,5-dimethyldihydrofuran-2(3H)-
Ethyl (2S,3S)-2-(Dibenzylamino)-3-methyl-4-oxopentanoate (24):
prepared and characterized by the same methods as ent-8 except a one (ent-25): L-selectride [0.54 mL, 1.0 M in tetrahydrofuran (THF),
replacement of D-proline with L-proline with identical selectivity and 0.54 mmol] was added to a stirred solution of ent-24 (173 mg, 0.49
yield.
Ethyl (2R,3R)-2-Amino-3-methyl-4-oxopentanoate (ent-22):
mmol) in dry THF (4.9 mL) at -78 °C under nitrogen. After stirring for
2 hours at -78 °C, the reaction mixture was poured into a vigorously
A
solution of (NH4)2S2O8 (913 mg, 4.0 mmol) and cerium ammonium stirred mixture of EtOAc/1 M HCl aqueous solution (10.0 mL/10.0
nitrate (CAN) (110 mg, 0.2 mmol) in H2O (5.8 mL) was slowly added mL). The aqueous layer was extracted with EtOAc (10 mL × 3). The
to a stirred solution of the mixture of ent-8 and its regeoisomer (ent- combined organic layers were washed with H2O (5 mL × 3–5) until pH
9; 5 : 1, 547 mg, ~2.0 mmol) in MeCN (1.0 mL) at 0 °C. The resulting ~ 6 (pH paper) and then washed with brine, dried over Na2SO4, and
mixture was then heated to 35 °C and stirred for 3 hours. Upon the concentrated under reduced pressure. The residue was purified by
completion of the reaction, the reaction mixture was diluted with flash column chromatography on silica gel (EtOAc/hexane = 1/20,
H2O (5.8 mL) and washed with CH2Cl2 (5 mL × 4). The aqueous layer UV) to give ent-25 as a white solid (143 mg, 0.46 mmol, 94%). Rf =
21
was basified with 1 M Na2CO3 aqueous solution to pH ~ 8 and then 0.33 (EtOAc/hexane = 1/10, UV); mp 66.5–69.7 °C; [α]D = 89.8 (c
extracted with CH2Cl2 (15 mL × 5). The combined organic layers were 1.06, CHCl3); IR (film) ṽ = 3062, 3028, 2972, 2928, 2850, 1769, 1601,
dried over Na2SO4 and concentrated under reduced pressure to give 1493, 1454, 1385, 1325, 1236, 1185, 1171, 1141, 1050, 995, 953, 745,
1
a brown liquid that contains ent-22 and its regioisomer (ent-23, 5 : 1, 699 cm-1; H NMR (400 MHz, CDCl3) δ 7.42–7.40 (m, 4 H, Ar), 7.35–
229 mg). The crude mixture was used immediately for the next 7.31 (m, 4 H, Ar), 7.28–7.23 (m, 2 H, Ar), 4.00 (d, J = 13.8 Hz, 2 H,
reaction without further purification. Data of crude ent-22 from pure PhCH2), 3.91–3.83 (m, 3 H, COOCH, PhCH2), 3.29 (d, J = 11.8 Hz, 1 H,
1
ent-8: Rf = 0.40 (methanol/CH2Cl2 = 1/10, ninhydrin); H NMR (400 CHNBn2), 2.05 (m, 1 H, CHCH3), 1.35 (d, J = 6.1 Hz, 3 H, COOCHCH3),
MHz, CDCl3) δ 4.19–4.13 (m, 2 H, COOCH2), 3.86 (d , J = 4.8 Hz, 1 H, 1.02 (d, J = 6.5 Hz, 3 H, CHCH3); 13C NMR (100 MHz, CDCl3) δ 175.3 (s,
CHNH2), 2.91 (m, 1 H, CHCH3), 2.19 (s, 3 H, COCH3), 1.25 (t, J = 7.1 Hz, COOCH), 139.4 (s, Ar), 128.8 (d, Ar), 128.4 (d, Ar), 127.3 (d, Ar), 79.6
3 H, CH2CH3), 1.11 (d, J = 7.2 Hz, 3 H, CHCH3); 13C NMR (100 MHz, (d, COOCH), 65.4 (d, CHNBn2), 54.9 (t, PhCH2), 42.1 (d, CHCH3), 18.9
CDCl3) δ 209.9 (s, COCH3), 174.3 (s, COOCH2), 61.3 (t, COOCH2), 55.4 (q, COOCHCH3), 14.1 (q, CHCH3); HRMS (ESI-TOF) m/z [M + Na]+ calcd.
(d, CHNH2), 49.8 (d, CHCH3), 28.4 (q, COCH3), 14.2 (q, CH2CH3), 11.0 for C20H23O2NNa 332.1626, found 332.1623.
(q, CHCH3); HRMS (APCI-TOF) m/z [M + H]+ calcd. for C8H16O3N
174.1130, found 174.1127.
(3S,4S,5R)-3-(Dibenzylamino)-4,5-dimethyldihydrofuran-2(3H)-
one (25): Compound 25, the enantiomer of ent-25, was prepared and
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J. Name., 2013, 00, 1-3 | 5
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Organic & Biomolecular Chemistry
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characterized by the same methods as ent-25 except a replacement
of substrate ent-24 to 24 with identical selectivity and yield.
(2S,3R)-Ethyl
2-((4-Methoxyphenyl)amino)-3-methyl-4-
View Article Online
oxopentanoate (4):15,17 DBN (0.15 mL, 1.2 DmOmI: o10l).1w03a9s/Da0dOdBe0d17t2o2Aa
(3R,4R,5S)-3-Amino-4,5-dimethyldihydrofuran-2(3H)-one (ent- solution of the mixture of 8 and its regeoisomer (9; 5 : 1, 30.0 mmol)
20):18c,19 Pd(OH)2/C (6.2 mg, 20%) was added to a stirred solution of in MTBE (1.62 mL, 18.5 M) at room temperature under nitrogen.
ent-25 (61.9 mg, 0.20 mmol) in EtOAc (4.0 mL) under nitrogen and After the reaction mixture was stirred for 2 hours, the MTBE was
then purged with hydrogen (1 atm, balloon) for an hour at room evaporated slowly for 18 hours at room temperature. A solid cake
temperature. After stirring at room temperature under hydrogen for was obtained and recrystallized from a layered EtOAc/Hexane
18 hours, the reaction mixture was filtered through Celite and solution to give a white needle crystal 4 (overall yield from p-
concentrated under reduced pressure to give ent-20 as a colorless oil anisidine: up to 40%). Rf = 0.25 (EtOAc/hexane = 1/5, UV); [α]D22.7 = -
(25.8 mg, 0.20 mmol, >99%) without further purification. Rf = 0.4 34.6 (c = 0.99, CHCl3); mp 98.7–99.3 °C; IR (neat, NaCl plate) ṽ = 3341,
(methanol/CH2Cl2 = 1/10, ninhydrin); [α]D24 = 29.0 (c 0.98, CHCl3); IR 2978, 2937, 1731, 1707, 1514, 1235, 1162, 1034, 819 cm-1; 1H NMR
(film) ṽ = 3374, 3310, 2973, 2927, 2878, 2852, 1771, 1456, 1389, 1330, (400 MHz, CDCl3) δ 6.76 (m, 2 H, Ar), 6.66 (m, 2 H, Ar), 4.21–4.09 (m,
1
1192, 1145, 1044, 983, 946, 918, 735, 702 cm-1; H NMR (400 MHz, 4 H, CHNH, COOCH2, CHNH), 3.74 (s, 3 H, OCH3), 3.02 (m, 1 H, CHCH3),
CDCl3) δ 4.05 (dq, J = 9.9, 6.1 Hz, 1 H, COOCH), 3.25 (d, J = 5.2 Hz, 1 H, 2.23 (s, 3 H, COCH3), 1.23–1.18 (m, 6 H, CHCH3, CH2CH3); 13C NMR
CHNH2), 1.80 (m, 1 H, CHCH3), 1.42 (d, J = 6.2 Hz, 3 H, COOCHCH3), (100 MHz, CDCl3) δ 209.6 (s, COCH3), 172.7 (s, COOCH2), 153.2 (s, Ar),
1.21 (d, J = 6.6 Hz, 3 H, CHCH3); 13C NMR (100 MHz, CDCl3) δ 178.2 (s, 140.8 (s, Ar), 115.9 (d, Ar), 115.0 (d, Ar), 61.4 (t, COOCH2), 60.6 (d,
COOCH), 79.8 (d, COOCH), 58.9 (d, CHNH2), 47.5 (d, CHCH3), 18.6 (q, CHNH), 55.8 (q, OCH3), 49.5 (d, CHCH3), 28.7 (q, COCH3), 14.3 (q,
COOCHCH3), 14.2 (q, CHCH3); HRMS (EI+) m/z M+ calcd. for C6H11O2N CH2CH3), 13.1 (q, CHCH3); HPLC (Daicel Chiralpak AS-H, hexane/i-
129.0790, found 129.0791.
PrOH = 96:4, flow rate 1.0 mL/minute, λ= 254 nm): tR = 12.7 minutes.
(3S,4R,5S)-3-(4-Methoxyphenylamino)-4,5-
(3S,4S,5R)-3-Amino-4,5-dimethyldihydrofuran-2(3H)-one (20):
Compound 20, the enantiomer of ent-20, was prepared and dimethyldihydrofuran-2(3H)-one (10): L-selectride (1.10 mL, 1.0 M
characterized by the same methods as ent-20 except a replacement in THF, 1.10 mmol) was added to a stirred solution of 4 (279 mg, 1.00
of substrate ent-25 to 25 with identical selectivity and yield.
(9Z,12Z)-N-((3R,4R,5S)-4,5-Dimethyl-2-oxotetrahydrofur-3-
mmol, 1.00 equiv.) in dry THF (10.0 mL, 0.10 M) at -78 °C under
nitrogen. After stirred for 2 hours at -78 °C and the starting material
yl)octadeca-9,12-dienamide (ent-21, N55): PyBOP (62.5 mg, 0.12 was consumed, the reaction mixture was poured into a vigorously
mmol) was added to a stirred solution of ent-20 (12.9 mg, 0.10 mmol) stirred mixture of EtOAc/1 M HCl aqueous solution (10.0 mL/10.0
and linoleic acid (31 μL, 0.10 mmol) in dry DMF (1.0 mL), followed by mL). The aqueous layer was extracted with EtOAc (10 mL × 3). The
freshly distilled N,N-diisopropylethylamine (DIPEA) (21 μL, 0.12 combined organic layers were washed with H2O (5 mL) until pH ~ 6
mmol) at room temperature under nitrogen. The reaction mixture (pH paper) and then washed with brine, dried over Na2SO4, and
was stirred for 18 hours. After the starting material was consumed, concentrated under reduced pressure. The residue was purified by
the reaction mixture was diluted with EtOAc (10 mL) and washed flash column chromatography on silica gel (Hexanes/EtOAc = 1/8,
with H2O (5 mL) and brine (5 mL). The organic layer was dried over UV) to give pale yellow solid 10 (172 mg, 0.73 mmol, 73%). Rf = 0.50
19.5
Na2SO4 and concentrated under reduced pressure. The residue was (EtOAc/hexane = 1/2); mp 50–52 °C; [α]D
= +105.7 (c = 0.50,
purified by flash column chromatography on silica gel (EtOAc/hexane CHCl3); IR (neat, NaCl plate) ṽ = 3378, 2976, 2934, 2833, 1769, 1515,
= 1/4) to give ent-21 (N55) as a colorless oil (36.9 mg, 0.094 mmol, 1313, 1179, 1146, 1036, 1024, 822 cm-1; 1H NMR (400 MHz, CDCl3) δ
94%). Rf = 0.28 (EtOAc/hexane = 1/2, I2); [α]D21 = –21.8 (c 0.30, CHCl3, 6.81 (d, J = 8.8 Hz, 2 H, Ar), 6.61 (d, J = 8.8 Hz, 2 H, Ar), 4.42 (q, J = 6.7
5-cm cell); IR (film) ṽ = 3303, 3009, 2957, 2927, 2855, 1782, 1657, Hz, 1 H, COOCH), 4.24 (d, J = 7.5 Hz, 1 H, CHNH), 3.76 (s, 3 H, OCH3),
1
1650, 1533, 1461, 1454, 1389, 1187, 1047, 908, 723 cm-1; H NMR 2.66 (dt, J = 7.3, 7.3 Hz, 1 H, CHCH3), 1.50 (d, J = 6.7 Hz, 3 H,
(500 MHz, CD3OD) δ 5.39–5.29 (m, 4 H, CH2CHCHCH2), 4.38 (d, J =11.8 COOCHCH3), 1.00 (d, J = 7.3 Hz, 3 H, CHCH3); 13C NMR (125 MHz,
Hz, 1 H, CHNH), 4.19 (dt, J = 9.8, 6.1 Hz, 1 H, COOCH), 2.78 (t, J = 6.5 CDCl3) δ 175.8 (s, COOCH), 152.9 (s, Ar), 140.9 (s, Ar), 115.0 (d, Ar),
Hz, 2 H, CHCH2CH), 2.26 (t, J = 7.5 Hz, 2 H, CH2CONH), 2.17–2.04 (m, 114.3 (d, Ar), 81.9 (d, COOCH), 56.1 (d, CHNH), 55.7 (q, OCH3), 40.4
5 H, CHCH3, CH2CH2CH), 1.66–1.60 (m, 2 H, CH2CH2CONH), 1.41 (d, J (d, CHCH3), 20.4 (q, COOCHCH3), 13.5 (q, CHCH3); HRMS (ESI-TOF)
= 6.1 Hz, 3 H, COOCHCH3), 1.40–1.28 (m, 14 H, CH2CH2), 1.13 (d, J = calcd. for C13H17O3NNa+ [M+Na+] 258.1106, found 258.1107.
6.6 Hz, 3H, CHCH3), 0.91 (t, J = 6.9 Hz, 3H, CH2CH3); 13C NMR (125 MHz,
(3S,4R,5S)-3-Amino-4,5-dimethyldihydrofuran-2(3H)-one (3):15
A
CD3OD) δ 176.5 (s, CONH), 176.2 (s, COOCH), 130.9 (d, CH2CHCHCH2), solution of CAN (959 mg, 1.75 mmol) in H2O (3.50 mL, 0.50 M) was
130.9 (d, CH2CHCHCH2), 129.1 (d, CH2CHCHCH2), 129.1 (d, slowly added to a stirred solution of 10 (165 mg, 0.70 mmol) in MeCN
CH2CHCHCH2), 81.4 (d, COOCH), 57.5 (d, CHNH), 45.6 (d, CHCH3), 36.9 (3.50 mL, 0.20 M) followed by NaBH4 (53.0 mg, 1.40 mmol)
(t, CH2CONH), 32.7 (t, CH2CH2), 30.7 (t, CH2CH2), 30.5 (t, CH2CH2), 30.3 portionwise at 0 °C. After stirred for an hour and the starting material
(t, CH2CH2), 30.3 (t, CH2CH2), 30.2 (t, CH2CH2), 28.2 (t, CH2CH2CH), was consumed, the reaction mixture was diluted with H2O (3.50 mL)
26.8 (t, CH2CH2CONH), 26.5 (t, CHCH2CH), 23.6 (t, CH2CH2), 18.8 (q, and washed with CH2Cl2 (3.50 mL × 4). The aqueous layer was basified
COOCHCH3), 14.4 (q, CH2CH3), 14.0 (q, CHCH3); HRMS (MALDI-TOF) with 1 M Na2CO3 to pH ~ 8 (pH paper) and then extracted with CH2Cl2
m/z [M + Na]+ calcd. for C24H41O3NNa 414.2979, found 414.2963.
(9Z,12Z)-N-((3S,4S,5R)-4,5-Dimethyl-2-oxotetrahydrofur-3-
(20 mL × 5). The combined organic layers were dried over Na2SO4 and
concentrated under reduced pressure. The brown liquid crude
yl)octadeca-9,12-dienamide (21): Compound 21, the enantiomer of containing 3 (29 mg, 32%) was directly used without purification.
ent-21, was prepared and characterized by the same methods as ent- Compound 3 can also be prepared by the same method as ent-20
21 except a replacement of substrate ent-20 to 20 with identical except a replacement of substrate ent-25 to 27 to give colorless oil 3
selectivity and yield.
(89%). Rf = 0.50 (MeOH/CH2Cl2 = 1/20, ninhydrin); 1H NMR (400 MHz,
6 | J. Name., 2012, 00, 1-3
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CDCl3) δ 4.31 (dq, J = 3.8, 6.4 Hz, 1 H, COOCH), 3.78 (d, J = 7.5 Hz, 1 H, (s, 3 H, COCH3), 1.40 (t, J = 7.1 Hz, 3 H, CH2CH3), 0.99 (Vdie,wJ =Art6ic.l8e OHnzli,n3e
13
DOI: 10.1039/D0OB01722A
CHNH2), 2.28 (ddq, J = 3.7, 7.2, 7.2 Hz, 1 H, CHCH3), 1.39 (d, J = 6.5 Hz, H, CHCH3); C NMR (100 MHz, CDCl3) δ 209.7 (s, COCH3), 169.8 (s,
3 H, COOCHCH3), 1.08 (d, J = 7.2 Hz, 3 H, CHCH3).
COOCH2), 138.7 (s, Ar), 129.5 (d, Ar), 128.3 (d, Ar), 127.3 (d, Ar), 64.1
(d, CHNBn2), 60.5 (t, COOCH2), 54.7 (t, PhCH2), 47.6 (d, CHCH3), 25.8
(9Z,12Z)-N-((3S,4R,5S)-4,5-Dimethyl-2-oxotetrahydrofur-3-
yl)octadeca-9,12-dienamide (2): PyBOP (102 mg, 0.195 mmol) was (q, COCH3), 14.8 (q, CH2CH3), 14.3 (q, CHCH3); HRMS (ESI-TOF) calcd.
added to a stirred solution of crude 3 (21 mg, 0.163 mmol,) and for C22H28O3+ [M+H+] 354.2069, found 354.2061.
linoleic acid (50.8 μL, 0.163 mmol) in dry DMF (1.63 mL, 0.1 M) at
(3S,4R,5S)-3-(Dibenzylamino)-4,5-dimethyldihydrofuran-
room temperature under nitrogen followed by freshly distilled DIPEA 2(3H)-one (27): Compound 27 was prepared by the same
(34.0 μL, 0.195 mmol,) and stirred for 18 hours. After the starting method as ent-25 except a replacement of substrate ent-24 to
material was consumed, the reaction mixture was diluted with EtOAc 26 to give colorless oil 27 (74%). Rf = 0.45 (EtOAc/hexane = 1/5,
19.7
(10 mL) and washed with H2O (5 mL) and then brine (5 mL). The UV); [α]D = -84.6 (c = 0.98, CHCl3); IR (neat, NaCl plate) ṽ =
organic layer was dried over Na2SO4 and concentrated under 3085, 3062, 3028, 2973, 2930, 2849,1763, 1603, 1492, 1454,
reduced pressure. The residue was purified by flash column 1383, 1298, 1194, 1144, 1055, 1028, 990, 952, 746, 699 cm-1; 1H
chromatography on silica gel (EtOAc/hexane = 1/4) to give white low NMR (400 MHz, CDCl3) δ 7.45 (d, J = 7.4 Hz, 4 H, Ar), 7.33 (t, J =
melting point or hygroscopic solid 2 (32 mg, 0.082 mmol, 50%). Rf = 7.4 Hz, 4 H, Ar), 7.26 (t, J = 7.4 Hz, 2 H, Ar), 4.25 (dq, J = 6.1, 8.7
0.3 (EtOAc/hexane = 1/2, I2); [α]D20.9 = 38.5 (c = 1.03, CHCl3), [α]D
Hz, 1 H, COOCH), 3.78 (s, 4 H, PhCH2), 3.59 (d, J = 10.1 Hz, 1 H,
24.1
= 38.8 (c = 0.50, CHCl3); IR (neat, NaCl plate) ṽ = 3304, 3008, 2926, CHNBn2), 2.11 (m, 1 H, CHCH3), 1.34 (d, J = 6.1 Hz, 3 H,
2854, 1781, 1655, 1649, 1543, 1535, 1458, 1383, 1205, 1144 cm-1; 1H COOCHCH3), 1.23 (d, J = 7.1 Hz, 3 H, CHCH3); 13C NMR (100 MHz,
NMR (400 MHz, CDCl3) δ 5.79 (s, 1 H, NH), 5.42–5.29 (m, 4 H, CDCl3) δ 175.7 (s, COOCH), 138.8 (s, Ar), 128.8 (d, Ar), 128.7 (d,
CH2CHCHCH2), 4.73 (dd, J = 4.7, 5.5 Hz, 1 H, CHNH), 4.40 (dt, J = 6.6, Ar), 128.6 (d, Ar), 128.4 (d, Ar), 127.4 (d, Ar), 83.2 (d, COOCH),
6.6, Hz, 1 H, COOCH), 2.77 (t, J = 6.4 Hz, 2 H, CHCH2CH), 2.69 (ddt, J = 59.7 (d, CHNBn2), 55.9 (t, PhCH2), 41.4 (d, CHCH3), 20.6 (q,
7.3, 7.3, 7.3 Hz, 1 H, CHCH3), 2.27 (dd, J = 7.1, 8.3 Hz, 2 H, CH2CONH), COOCHCH3), 11.6 (q, CHCH3); HRMS (ESI-TOF) calcd. for
2.05 (dt, J = 6.8, 6.8 Hz, 4 H, CH2CH2CH), 1.65 (m, 2 H, CH2CH2CONH), C20H24O2N+ [M+H+] 310.1807, found 310.1808.
1.45 (d, J = 6.7 Hz, 3 H, COOCHCH3), 1.38–1.25 (m, 14 H, CH2CH2), 0.94
(d, J = 7.2 Hz, 3 H, CHCH3), 0.88 (t, J = 6.7 Hz, 3 H, CH2CH3); 13C NMR
(125 MHz, CDCl3) δ 175.3 (s, CONH), 173.9 (s, COOCH), 130.3 (d,
Conflicts of interest
CH2CHCHCH2), 130.1 (d, CH2CHCHCH2), 128.2 (d, CH2CHCHCH2), 128.0
(d, CH2CHCHCH2), 82.7 (d, COOCH), 52.2 (d, CHNH), 39.2 (d, CHCH3),
36.2 (t, CH2CONH), 31.6 (t, CH2CH2), 29.7 (t, CH2CH2), 29.4 (t, CH2CH2),
29.3 (t, CH2CH2), 29.2 (t, CH2CH2), 27.3 (t, CH2CH2), 25.7 (t,
CH2CH2CONH), 25.7 (t, CHCH2CH), 22.7 (t, CH2CH2), 20.2 (q,
COOCHCH3), 14.2 (q, CH2CH3), 13.7 (q, CHCH3); HRMS (ESI-TOF) calcd.
for C24H41O2NNa+ [M+Na+] 414.2984, found 414.2980.
There are no conflicts to declare.
Acknowledgements
We thank Academia Sinica [AS-SUMMIT-108] and the Ministry
of Science and Technology (Taiwan) (MOST 108-3114-Y-001-
002 and MOST 107-0210-01-19-01) for their financial support
and Dr. Mei-Chun Tseng of the MS laboratory of the Institute of
Chemistry, Academia Sinica, for the assistance with mass
spectrometric analyses.
Ethyl
(2S,3R)-2-Amino-3-methyl-4-oxopentanoate
(17):
(NH4)2S2O8 (456 mg, 2.00 mmol) and CAN (54.8 mg, 0.10 mmol) in
H2O (2.90 mL, 0.69 M) was dropwise added to a stirred solution of 4
(279 mg, 1.00 mmol) in MeCN (1.00 mL, 0.1 M) at 0 °C and then the
reaction mixture was allowed to warm to 35 °C and stirred for 3 hours.
After the starting material was consumed, the reaction mixture was
diluted with H2O (2.9 mL) and washed with CH2Cl2 (3 mL × 4). The
aqueous layer was basified with 1 M Na2CO3 to pH ~ 8 (pH paper) and
then extracted with CH2Cl2 (10 mL × 5). The combined organic layers
were dried over Na2SO4 and concentrated under reduced pressure.
The brown liquid crude containing 17 (139 mg, 80%) was directly
used without purification. Rf = 0.40 (MeOH/CH2Cl2 = 1/10, ninhydrin);
1H NMR (400 MHz, CDCl3) δ 4.16 (m, 2H, COOCH2), 3.54 (d, J = 6.4 Hz,
1 H, CHNH2), 2.94 (dt, J = 7.0, 7.0 Hz, 1 H, CHCH3), 2.18 (s, 3 H, COCH3),
1.25 (t, J = 7.1 Hz, 3 H, CH2CH3), 1.18 (d, J = 7.2 Hz, 3 H, CHCH3).
Ethyl (2S,3R)-2-(Dibenzylamino)-3-methyl-4-oxopentanoate (26):
Compound 26 was prepared by the same method as ent-24 except a
replacement of substrate ent-22 to 17 to give colorless oil 26 (57%).
Rf = 0.43 (EtOAc/hexane = 1/5, UV); [α]D20.5 = -99.2 (c = 1.00, CHCl3);
IR (neat, NaCl plate) ṽ = 3086, 3062, 3029, 2977, 2936, 2848, 1727,
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