One-Pot Conversion of Carbohydrates into Pyrrole-2-carbaldehydes as Sustainable Platform Chemicals

Article abstract of DOI:10.1021/acs.joc.5b01349

A practical conversion method of carbohydrates into N-substituted 5-(hydroxymethyl)pyrrole-2-carbaldehydes (pyrralines) was developed by the reaction with primary amines and oxalic acid in DMSO at 90°C. Further cyclization of the highly functionalized pyrralines afforded the pyrrole-fused poly-heterocyclic compounds as potential intermediates for drugs, food flavors, and functional materials. The mild Maillard variant of carbohydrates and amino esters in heated DMSO with oxalic acid expeditiously produced the pyrrole-2-carbaldehyde skeleton, which can be concisely transformed into the pyrrole alkaloid natural products, 2-benzyl- and 2-methylpyrrolo[1,4]oxazin-3-ones 8 and 9, lobechine 10, and (')-hanishin 11 in 23-32% overall yields from each carbohydrate.

Full text of DOI:10.1021/acs.joc.5b01349

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The Journal of Organic Chemistry
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One-Pot Conversion of Carbohydrates into Pyrrole-2-
Carbaldehydes as Sustainable Platform Chemicals
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Nirmal Das Adhikary,1 Sunjeong Kwon,1 Wook-Jin Chung,1 and Sangho Koo*1,2
1 Department of Energy Science and technology, 2 Department of Chemistry, Myong Ji University,
Myongji-Ro 116, Cheoin-Gu, Yongin, Gyeonggi-Do, 449-728, Korea.
*Corresponding Author. Professor Sangho Koo; e-mail) sangkoo@mju.ac.kr; fax) +82-31-335-7248;
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phone) +82-31-330-6185.
TOC Graphic.
Abstract: A practical conversion method of carbohydrates into N-substituted 5-hydroxymethylpyrrole-
2-carbaldehydes (pyrralines) was developed by the reaction with primary amines and oxalic acid in
DMSO at 90 ºC. Further cyclization of the highly functionalized pyrralines afforded the pyrrole-fused
poly heterocyclic compounds as potential intermediates for drugs, food flavors, and functional materials.
The mild Maillard variant of carbohydrates and amino esters in heated DMSO with oxalic acid
expeditiously produced the pyrrole-2-carbaldehyde skeleton, which can be concisely transformed into
the pyrrole alkaloid natural products, 2-benzyl- and 2-methyl-pyrrolo[1,4]oxazin-3-ones 8 and 9,
Lobechine 10, and (–)-Hanishin 11 in 23~32% overall yields from each carbohydrate.
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Introduction
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Searching for an efficient transformation method of biomass into platform chemicals is an imperative
and timely task for securing sustainable chemical and energy materials. The conversion of carbohydrates
into 5-hydroxymethyl-2-furfural (1, 5-HMF) has been extensively studied in this sense (Scheme 1),
because this highly important platform chemical can be transformed into biofuels and useful commodity
chemicals.1 Fructose is most efficiently converted into 5-HMF among all the naturally occurring
carbohydrates because it is the furanoid form that is directly transformed into 5-HMF upon removal of
three water molecules. Glucose should be converted into fructose first by the acid or base-catalyzed
process known as Lobry de Bruyn–Alberda van Ekenstein transformation.2 Because the next dehydration
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step is catalyzed by acids, one-pot conversion of D-glucose into 5-HMF has been practiced under acidic
conditions utilizing metal chloride Lewis acids,3 Brönsted acids,4 solid acids,5 and acidic ionic liquids.6
However, these conversion methods generally suffered from low yields due to the strongly acidic
conditions required for the aldose–ketose transformation. It was envisioned that the carbohydrate
conversion would be facilitated or at least diversified by the presence of a nitrogen substituent at the
anomeric carbon (see glycosylamine 2). Amadori and Maillard reactions are well known carbohydrate
conversion reactions with amines. However, Amadori rearrangement of glycosylamine 2 produced the
equilibrium mixture of furanose 3, pyranose 3-1, and the open chain isomer 3-2 in solution.7
Furthermore, the Maillard reaction, browning of sugars with amines (or amino acids) at 200 ºC or higher,
is impractical producing a complex mixture of poorly characterized molecules presumably due to the
equilibrium among the various reaction pathways and extensive decomposition of the intermediate
compounds.8
We carefully investigated the reaction of D-glucose with N-benzylamine for the synthetic purpose of
stable platform chemicals, and found that 1-benzyl-5-hydroxymethylpyrrole-2-carbaldehyde 4a, named
as benzyl pyrraline (R = benzyl in 4), was a reproducible product under the mild non-aqueous acidic
variant of Maillard reaction. Identification and detection of pyrralines 4, which are formed as the
advanced N-glycosylation end products in vivo, had been studied to understand the correlations with
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aging, diabetes, lipid peroxidation, and Alzheimer disease.9 A couple of chemical syntheses of butyl
-glucose had been reported, but the yield never exceeded 3.4%.9a,10 We were able to
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3
pyrraline 4b from
D
4
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develop a practical conversion method of carbohydrates into diverse alkyl- and aryl-substituted pyrrole-
2-carbaldehydes 4 as platform chemicals, which may lead to various important drugs, food flavors, and
functional materials. Detailed studies on the one-pot conversion of carbohydrates into pyrrole-2-
carbaldehydes 4, its mechanism, and application to the total syntheses of several pyrrole alkaloid natural
products are herein disclosed.
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Scheme 1. Conversion of
pyrraline 4.
D-glucose into 5-HMF 1, N-glycosylamine 2, Amadori products 3, and
Result and Discussion
The variables of acid, solvent, reaction temperature and time were screened to maximize the formation
of benzyl pyrraline 4a (Table 1). First, polar aprotic solvent, DMSO was utilized with oxalic acid
considering its dehydrating efficacy and the polarity of glucose.11 The reaction of
D-glucose and N-
benzylamine was carried out in the presence of 1.5 equiv. of oxalic acid in DMSO at 80 ºC for 1 h to
produce 4a in 19% yield (entry 1). An improved yield (27%) was observed at 100 ºC (entry 2), the
effective temperature for glucose dehydration,3a,3b but a significant amount of humins was also observed
to give a viscous reaction mixture. A further increase of the yield of 4a up to 40% was attained by
decreasing the amount of oxalic acid to 1 equiv. and reducing the reaction time and temperature to 0.5 h
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at 90 ºC, respectively (entry 3-6). Further decreasing the amount of oxalic acid (0.5 equiv.) and changing
the solvent (DMF) were not effective in improving the yield of 4a (entries 7, 8).
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7
8
It seems that oxalic acid (pKa 1.25) provides the optimal acidity for dehydration in the presence of the
basic amino functional groups. Other Brönsted acids tested were not as effective as oxalic acid: acetic
acid (pKa 4.7) allowed the formation of 4a in 8% yield, sulfuric acid (pKa –3.0) in 16% yield, and
trifluoroacetic acid (pKa 0.23) in 28% yield all in DMSO at 90 ºC for 0.5 h (entries 9-11). Even though
Lewis acid CrCl2 in imidazolium ionic liquid was reported to be an excellent condition for glucose
dehydration,3a only 5% yield of 4a was obtained in this condition (entry 12). Additional use of N-
heterocyclic carbene ligand to CrCl2 improved the yield,3b but not higher than 24% (entry 13).
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Table 1. Study on the conversion of D-glucose into benzyl pyrraline 4a.
Entrya
Acid (equiv.)
(COOH)2 (1.5)
(COOH)2 (1.5)
(COOH)2 (1.0)
(COOH)2 (1.0)
(COOH)2 (1.0)
(COOH)2 (1.0)
(COOH)2 (0.5)
(COOH)2 (1.0)
AcOH (1.0)
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
Temp (ºC)
80
time (h)
1
Yield (%)b
1
2
19
27
32
35
40
39
25
14
8
100
100
100
90
1
3
1
4
0.5
0.5
1
5
6
90
7
90
1
8
90
1
9
DMSO
DMSO
DMSO
90
0.5
0.5
0.5
0.5
0.5
10
11
12
13
CF3CO2H (1.0)
H2SO4 (1.0)
90
28
16
5
90
CrCl2 (1.0)
EMIMClc
BMIMCle
90
NHC/Cr (1.0)d
90
24
a
All reactions were performed with 0.72 g (4.0 mmol) of D-glucose and 0.44 mL (4.0 mmol) of benzylamine in 1.3 M
solution of the designated dry solvent (3 mL) unless otherwise stated. b Isolated yield of 1-benzyl-5-(hydroxymethyl)-1H-
pyrrole-2-carbaldehyde (4a) after column chromatographic purification. c 1-Ethyl-3-methylimidazolium chloride (2.0 g).
d
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Prepared in situ by the reaction of 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride (1.0 equiv.), t-BuOK (1.0 equiv.), and
CrCl2 (1.0 equiv.). e 1-Butyl-3-methylimidazolium chloride (2.0 g).
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6
This reaction was clean (except for some humins), and the organic extract exhibited one major spot in
TLC. Simple filtration through silica gel produced the pure pyrraline. We proved the efficiency of this
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one-pot conversion by a practical scale reaction: heating a solution of 7.21 g (40 mmol) of D-glucose
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and 4.29 g (40 mmol) of N-benzylamine with 3.60 g of oxalic acid (40 mmol) in DMSO (30 mL) at 90
ºC for 0.5 h produced 3.32 g (15.4 mmol) of 4a in 39% yield after purification.
The mechanism of the formation of pyrralines 4 was proposed in Scheme 2. The Maillard reaction of
D
-glucose in aqueous solution was proposed to proceed through 3-deoxyglucosone.9a,12 It would be
reasonable to propose the imine derivative C of 3-deoxyglucosone as a key intermediate for this mild
non-aqueous version of the Maillard reaction. The reaction is undoubtedly initiated from N-
glycosylation of amine to form 2. The ring-opened enamine tautomer B from A pushes electrons to
remove the protonated 3-hydroxyl group, thereby producing the key intermediate C. The enamine D
formed by the addition of another amine may undergo cyclization to E, in which nitrogen pushes
electrons to facilitate the removal of the protonated 4-hydroxyl group and the aromatization to pyrrole.
Hydrolysis of the 2-iminyl group produces pyrraline 4 and regenerates amine.
Scheme 2. Proposed mechanism of the formation of pyrraline 4 from glycosylamine 2.
The configuration of the hydroxyl groups is not important for dehydration due to the participation of
the lone pair electrons on nitrogen (B and E).
D-Galactose and D-mannose also produced benzyl
pyrraline 4a in 37% and 39% yield, respectively under the above optimized condition (entry 5, Table 1).
Similar yields of pyrrole 14 were obtained from
D
-ribose and
D
-xylose (vide infra). It is not necessary to
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rigorously dry the solvent. The original Maillard condition (at 200 °C or higher) was too harsh to give
pyrralines in practical yields, and the equilibrium for the dehydration of carbohydrates in aqueous
condition was not favorable. On the other hand, the oxalic acid-promoted dehydration of carbohydrates
was successful in the presence of the amine functional groups especially in DMSO at 90 °C.
Table 2. Conversion of carbohydrates into diversely N-substituted pyrrole-2-carbaldehydes 4.
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Entry Carbohydrate (X) Amines (R)-NH2
Yield 4 (%)
1
D
-glucose (OH)
b
c
d
e
f
Bu
36
2
Allyl
29
48
21
33
51
21
25
27
35
53
3
Propargyl
HO(CH2)3
C6H5(CH2)2
o-Br-C6H4CH2
Ph
4
5
6
g
h
i
7
8
p-Me-C6H4
p-CN-C6H4
Bu
9
j
10
11
L-rhamnose (H)
k
l
o-Br-C6H4CH2
The generality of the carbohydrate conversion into pyrrole-2-carbaldehydes 4 was demonstrated by the
reaction of -glucose and -rhamnose with various primary amines under the above optimized condition
of using 1 equiv. of oxalic acid in DMSO at 90 ºC for 30 min (Table 2). Aliphatic amines such as n-
butyl, allyl, propargyl, 3-hydroxypropyl, and phenethyl amines reacted with -glucose to produce the
D
L
D
corresponding N-substituted 5-hydroxymethylpyrrole-2-carbaldehydes 4b-4f in 21–48% yield (entries 1-
5). 3-Hydroxypropylamine gave the lowest yield for 4e (21%), which contained terminal hydroxyl
groups susceptible to further reactions (entry 4). Anilines, regardless of the electronic nature of the
substituent, produced the corresponding N-phenyl 5-hydroxymethylpyrrole-2-carbaldehydes 4h-4j in a
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relatively low 21–27% yield range (entries 7-9). L-Rhamnose also reacted with n-butyl amine to give the
1
2
3
4
5
corresponding pyrrole-2-carbaldehyde 4k in 35% yield (entry 10). The highest yields of pyrrole-2-
carbaldehydes 4g (51%) and 4l (53%) from each carbohydrate were obtained for o-bromobenzylamine,
6
7
8
an aliphatic amine with the stabilized electron-deficient aromatic group (entries 6 and 11).
9
1. MsCl, Et3N
CH2Cl2
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O
O
OH
N
N
N
2. NaN3, DMF
60 oC, 8 h
N
4d
4g
5
88%
N
Pd2(dba)3
BINAP
O
O
OH
N
N
O
K2CO3, t-BuOK
toluene,
reflux, 18 h
Br
6
63%
1. BnNH2, CH2Cl2
2. NaBH4, MeOH
O
N
N
3. Pd2(dba)3
BINAP
K2CO3, t-BuOK
toluene
Bn
N
Br
4l
7
67%
reflux, 16 h
Scheme 3. Transformation of selected pyrrole-2-carbaldehydes into poly heterocyclic compounds.
We further demonstrated the utility of pyrrole-2-carbaldehydes 4 as platform chemicals by subsequent
cyclization to poly heterocyclic compounds (Scheme 3). Activation of 5-hydroxymethyl group of 4d by
a methanesulfonyl (Ms) group and then nucleophilic substitution by sodium azide in DMF at 60 ºC
induced the intramolecular azide cycloaddition to form pyrrolo-triazolo-piperazine 5 in 88% yield.13,14
Application of the modified Buchwald’s condition for the palladium-catalyzed amination of aryl halides
[Pd2(dba)3, BINAP, t-BuOK, K2CO3, toluene]15 to 4g produced pyrrolo[1,4]oxazepine 6 in 63% yield
through the intramolecular etherification. Reductive benzyl amination of the 2-formyl group of 4l (78%
yield), followed by the intramolecular Buchwald’s amination with aryl bromide (86% yield) produced
pyrrolo[1,4]diazepine 7. Pyrrole-fused, six- and seven-membered poly heterocyclic compounds were
efficiently prepared in this way.
We then applied this mild variant of the Maillard reaction to the concise total syntheses of the pyrrole
alkaloid natural products, pyrrolo[1,4]oxazin-3-ones 8 (R = Bn) and 9 (R = Me), Lobechine 10, and (–)-
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Hanishin 11. Pyrrole alkaloid natural products exhibit various biological effects such as sedative, anti-
inflammatory, pain-relieving, hepatoprotective, and immunostimulatory activities, which makes their
hosting plants to be used as substances of traditional folk medicines.16 Pyrrolo[1,4]oxazin-3-ones 8 (R =
Bn) and 9 (R = Me), and Lobechine 10 were isolated from the plants of Celastrus orbiculatus,16a
Capparis spinosa,17 and Lobelia chinese,18 respectively (Scheme 4). (–)-Hanishin 11, the brominated
pyrrole alkaloid, was isolated from the marine sponge Acanthella carteri and shows cytotoxicity against
NSCLC-N6 human non-small-cell-lung carcinoma.19
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Scheme 4. Pyrrole alkaloid natural products – Pyrrolo[1,4]oxazin-3-ones 8 (2-benzyl) and 9 (2-
methyl), Lobechine 10, and (–)-Hanishin 11.
These pyrrole alkaloids have interesting structural features in common, namely a carbonyl group at the
2-position and the integrated amino acid portion. It was thus conceived that these pyrrole natural
products might be the enzymatic glycosylation end products of amino acids and that those would be
chemically synthesized by the reaction between carbohydrates and amino acids. The utility and general
applicability of the above practical preparative variant of the Maillard reaction was demonstrated for the
synthesis of various pyrrole-2-carbonyl compounds (Scheme 5), which were further applied to the
concise total syntheses of the pyrrole alkaloid natural products 8, 9, 10, and 11.
The reaction of -glucose with α-amino acid methyl esters under the above optimized condition (1
D
equiv. of oxalic acid in DMSO at 90 ºC for 0.5 h) directly produced the pyrrolo[1,4]oxazin-3-one
skeleton, in which the 5-hydoxymethylpyrrole-2-carbaldehydes underwent intramolecular lactonization
with the amino ester part. Even though roasting -glucose with α-amino acid at 200~250 ºC had been
D
reported to produce the above compound, it came together with various kinds of aroma products, and it
is impractical to obtain pyrrolo[1,4]oxazin-3-ones by the conventional Maillard reaction (maximum
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0.5% yield).20 The natural product 8 possessing sedative and anti-inflammatory activities was
synthesized from -glucose in 27% yield when -phenylalanine methyl ester was used under the above
optimized condition. The methyl ester of -alanine efficiently promoted one-pot conversion of
1
2
3
4
5
6
D
L
L
D-
7
8
9
glucose into 9 in 32% yield, whereas the total synthesis of 9 has been reported in 6 steps with overall
17% yield.16b
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γ-Aminobutyric acid (GABA) methyl ester, which was generated in situ by the reaction of GABA and
thionyl chloride in refluxing MeOH,16c followed by neutralization with Et3N, efficiently produced the
corresponding pyrraline 12 in 34% yield by the reaction with
D-glucose under the above condition (1
equiv. of oxalic acid in DMSO at 90 ºC for 0.5 h). -Ribose also reacted with the amino-ester, derived
D
from N-Boc-protected β-amino-ester 13 under the above condition to give the corresponding pyrrole-2-
carbaldehyde 14 in 48% yield. Tuning the reaction condition allowed the highest 54% yield at 60 ºC for
1 h.
D-Xylose was also effective in producing 14 in 51% yield under this Maillard condition. The
configuration of 3-hydroxyl group is not important for dehydration as is described in the above
mechanism (Scheme 2). It is worth of mentioning that an azido group in 14 and its precursors is
compatible in this Maillard condition, which may undergo further useful transformations by click-
chemistry.
O
H2N
OH
O
OMe
O
HO
HO
OH
OH
N
R
O
R
(COOH)2, DMSO
90 oC, 0.5 h
O
8: R = Bn 27%
9: R = Me 32%
D-glucose
O
H2N
CO2H
O
N
1. SOCl2, MeOH, reflux
2. Et3N, D-glucose, (COOH)2
DMSO, 90 oC, 0.5 h
MeO
12 34%
OH
NHBoc
OH
EtO2C
N3
HO
HO
OH
CHO
N3
N
13
EtO2C
1. CF3CO2H, CH2Cl2, rt, 2 h
2. Et3N, DMSO, (COOH)2
O
14
D-ribose
D-ribose 90 oC, 0.5 h
70 oC, 0.5 h
48%
50%
52%
36%
54%
OH
60 oC, 0.5 h
HO
HO
OH
50 oC, 0.5 h
60 oC, 1.0 h
O
D
-xylose
D-xylose 60 oC, 1.0 h
51%
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Scheme 5. The Maillard reaction of carbohydrates and various amino esters to pyrrole-2-
1
2
3
carbaldehydes.
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5
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7
8
9
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Scheme 6. Preparation of chiral β-aminoester 13 from L-aspartic acid for (–)-Hanishin synthesis.
The required chiral β-amino-ester 13 for pyrrole-2-carbaldehyde 14 was prepared from
in 5 steps, 25% overall yield by the modification of the reported procedure (Scheme 6).21 Compound
G21a which was routinely prepared by esterification (SOCl2 in EtOH) and Boc-protection of
L-aspartic acid
L-aspartic
acid underwent the regioselective reduction of α-aminoester by NaBH4 (1.5 equiv) at 0 ºC for 18 h.21b
The mono-alcohol H was directly protected with a methanesulfonyl group to avoid the lactone
formation,21b which was then replaced by azide in DMF at 70 ºC to produce the β-amino-ester 13.
All of the reactions of carbohydrates and amino-esters under the modified Maillard condition
(stoichiometric oxalic acid in heated DMSO) proceeded smoothly to give the corresponding pyrrole-2-
carbaldehydes in acceptable yields with easy purification. The S-configuration of the amino-ester was
maintained during the reaction, which was evidenced by the almost identical optical rotation value (–
4.7) of (–)-Hanishin synthesized from 14 (vide infra) to the reported one (–4.3).22 The same sign but
even higher optical rotation value (–86.0) was observed in the case of 2-methyl-pyrrolo[1,4]oxazin-3-
one 9, compared to the previously reported one (–33.3).17
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1
2
3
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7
8
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12
Scheme 7. Synthesis of Lobechine 10 from pyrraline 12.
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Scheme 8. Synthesis of (–)-Hanishin 11 from pyrrole-2-carbaldehyde 14.
The concise total syntheses of Lobechine 10 and (–)-Hanishin 11 were completed from the above
pyrrole-2-carbaldehydes 12 and 14, respectively.22,23 Deprotonation of pyrraline 12 by NaH and reaction
with butyl bromide preferentially produced the intramolecular aldol condensation product 15 of the ester
instead of the desired O-alkylation (Scheme 7). This problem was overcome by using silver(I) oxide as a
base and alkylation with butyl iodide at 90 ºC without any solvent (71 % yield).16d Hydrolysis (95%
yield) of the methyl ester by LiOH in aqueous THF produced Lobechine 10 in 67% yield from pyrraline
12 (23% overall yield from
D-glucose). Oxidation of pyrrole-2-carbaldehyde 14 to the corresponding
carboxylic acid by sodium chlorite and hydrogen peroxide in phosphate buffer solution, followed by
esterification (K2CO3, MeI) produced methyl pyrrole-2-carboxylate 17 in 62% yield (Scheme 8). The
azide reduction by catalytic hydrogenation, intramolecular amidation in refluxing toluene, and the
known bromination using NBS (2.0 equiv.)22,24 finally produced (–)-Hanishin 11 in 69% yield (23%
overall yield from D-ribose).
Conclusion
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We developed a practical one-pot conversion method of carbohydrates and primary amines with a
stoichiometric oxalic acid in DMSO at 90 ºC for 30 min, in which N-substituted pyrrole-2-
carbaldehydes 4 were obtained in 21~53% yields. These sustainable platform chemicals from
carbohydrates are highly functionalized and easily converted to the poly heterocyclic compounds such as
piperazine, oxazepine, and diazepine as potential intermediates for drugs, food flavors, and functional
materials. Sedative and anti-inflammatory 2-benzyl- and 2-methyl-pyrrolo[1,4]oxazin-3-one natural
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products 8 and 9 were synthesized in 27% and 32% yields by the mild variant of Maillard reaction of
glucose with methyl esters of -phenylalanine and -alanine, respectively. The Maillard product of
D
D
-
-
L
L
glucose and amino ester derived from GABA was smoothly transformed into hepatoprotective
Lobechine 10, on the other hand, that of -ribose and the β-amino ester derived from -aspartic acid was
efficiently converted into cytotoxic (–)-Hanishin 11.
Experimental Section
D
L
General Experimental. Reactions were performed in a well-dried flask under Argon atmosphere unless
noted otherwise. Solvents used as reaction media were dried over pre-dried molecular sieve (4 Å) by
microwave oven. Solvents for extraction and chromatography were reagent grade and used as received.
The column chromatography was performed with silica gel 60 (70-230 mesh) using a mixture of
1
13
EtOAc/hexane as eluent. H and C NMR spectra were respectively recorded on a 400 MHz and 100
MHz NMR spectrometer in deuterated chloroform (CDCl3) with tetramethylsilane (TMS) as an internal
reference. High resolution mass spectroscopy was performed using magnetic sector analyzer. All the
1
new compounds were identified by H and 13C NMR, IR, and High Resolution Mass Spectroscopy.
1
13
Identity of the known compounds was established by the comparison of their H and C NMR peaks
with the authentic values. The NMR spectra of all the numbered compounds synthesized are included in
the supporting information.
General procedure for the modified Maillard reaction of carbohydrate and amine (amino acid):
To a stirred mixture of
D
-glucose (0.72 g, 4.0 mmol) or
L-rhamnose monohydrate (0.73 g, 4.0 mmol)
and appropriate amines or
L-amino acid methyl esters (4.0 mmol) in dry DMSO (3.0 mL) was added
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oxalic acid (0.36 g, 4.0 mmol). The mixture was heated at 90 ºC for 0.5 h, and cooled to room
temperature. The viscous reaction mixture was diluted with water (10 mL), and the solution was
extracted with EtOAc (3×10 mL). The combined organic layer was washed with brine and water, dried
over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to give the crude products,
which were purified by silica gel flash column chromatography (eluent 1:3 EtOAc:hexane) to give pure
pyrrole-2-carbaldehydes.
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1-Benzyl-5-hydroxymethyl-1H-pyrrole-2-carbaldehyde (4a): clear thick oil, yield 40% (0.34 g, 1.60
mmol); 1H NMR δ = 1.52 (br s, 1H), 4.57 (s, 2H), 5.76 (s, 2H), 6.31 (d, J = 4.0 Hz, 1H), 6.96 (d, J = 4.0
Hz, 1H), 6.97–7.02 (m, 2H), 7.20–7.32 (m, 3H), 9.57 (s, 1H) ppm; 13C NMR δ = 48.5, 56.5, 110.7,
124.3, 126.1, 127.3, 128.7, 132.7, 137.7, 142.0, 179.7 ppm; IR (neat) 3328, 1637, 1484, 1408, 1374,
1295 cm-1; HRMS (CI+) calcd for C13H14NO2 (M+H)+ 216.1025, found. 216.1028.
1-Butyl-5-hydroxymethyl-1H-pyrrole-2-carbaldehyde (4b):10 brown foamy solid, yield 36% (0.26 g,
1
1.44 mmol); mp: 48–49 ºC; H NMR δ = 0.95 (t, J = 7.2 Hz, 3H), 1.33–1.44 (m, 2H), 1.57 (br s, 1H),
1.67–1.76 (m, 2H), 4.36 (t, J = 7.6 Hz, 2H), 4.68 (s, 2H), 6.22 (d, J = 4.0 Hz, 1H), 6.87 (d, J = 4.0 Hz,
1H), 9.52 (s, 1H) ppm; 13C NMR δ = 13.7, 19.9, 33.5, 45.6, 56.4, 110.0, 124.3, 132.3, 141.6, 179.3 ppm;
IR (neat) 3391, 1647, 1469, 1415, 1376, 1191 cm-1; HRMS (CI+) calcd for C10H16NO2 (M+H)+
182.1181, found 182.1178.
1-Allyl-5-hydroxymethyl-1H-pyrrole-2-carbaldehyde (4c): clear thick oil, yield 29% (0.19 g, 1.16
1
mmol); H NMR δ = 1.64 (t, J = 6.0 Hz, 1H), 4.65 (d, J = 6.0 Hz, 2H), 4.83 (d, J = 16.8 Hz, 1H), 5.09–
5.15 (m, 3H), 6.01 (dddd, J = 16.8, 10.0, 5.2, 4.8 Hz, 1H), 6.27 (d, J = 4.0 Hz, 1H), 6.90 (d, J = 4.0 Hz,
1H), 9.53 (s, 1H) ppm; 13C NMR δ = 47.4, 56.2, 110.3, 115.7, 124.2, 132.2, 134.3, 141.9, 179.5 ppm; IR
(neat) 3316, 1643, 1484, 1447, 1293 cm-1; HRMS (CI+) calcd for C9H12NO2 (M+H)+ 166.0868, found
166.0872.
5-Hydroxymethyl-1-prop-2-ynyl-1H-pyrrole-2-carbaldehyde (4d): white foamy solid, yield 48%
1
(0.31 g, 1.92 mmol); mp: 70–71 ºC; H NMR δ = 1.76 (br t, J = 4.8 Hz, 1H), 2.33 (t, J = 2.4 Hz, 1H),
4.79 (d, J = 4.8 Hz, 2H), 5.39 (d, J = 2.4 Hz, 2H), 6.27 (d, J = 4.0 Hz, 1H), 6.90 (d, J = 4.0 Hz, 1H),
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9.56 (s, 1H) ppm; 13C NMR δ = 34.6, 56.4, 72.6, 78.5, 110.9, 124.4, 131.9, 141.4, 179.9 ppm; IR (neat)
3280, 1632, 1462, 1350, 1187 cm-1; HRMS (CI+) calcd for C9H10NO2 (M+H)+ 164.0712, found.
164.0712.
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3
4
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7
8
5-Hydroxymethyl-1-(3-hydroxy-propyl)-1H-pyrrole-2-carbaldehyde (4e): brown oil, yield 21%
9
1
(0.15 g, 0.84 mmol); H NMR δ = 2.03–2.10 (m, 2H), 2.66 (br s, 2H), 3.55 (t, J = 5.6 Hz, 2H), 4.50 (t, J
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= 6.4 Hz, 2H), 4.69 (s, 1H), 6.27 (d, J = 4.0 Hz, 1H), 6.93 (d, J = 4.0 Hz, 1H), 9.49 (s, 1H) ppm; C
NMR δ = 33.6, 41.9, 56.0, 58.6, 111.0, 125.3, 132.5, 142.3, 179.7 ppm; IR (neat) 3221, 1627, 1470,
1402, 1363, 1293 cm-1; HRMS (EI+) calcd for C9H13NO3 183.0895, found 183.0897.
5-Hydroxymethyl-1-phenethyl-1H-pyrrole-2-carbaldehyde (4f): clear oil, yield 33% (0.30 g, 1.32
mmol); 1H NMR δ = 1. 58 (br s, 1H), 3.05 (t, J = 7.2 Hz, 2H), 4.29 (s, 2H), 4.55 (t, J = 7.2 Hz, 2H), 6.17
(d, J = 4.0 Hz, 1H), 6.93 (d, J = 4.0 Hz, 1H), 7.07–7.13 (m, 2H), 7.20–7.30 (m, 3H), 9.58 (s, 1H) ppm;
13C NMR δ = 37.6, 47.5, 56.1, 110.0, 124.8, 126.6, 128.5, 129.0, 132.0, 138.3, 142.0, 179.4 ppm; IR
(neat) 3333, 1646, 1497, 1409, 1376, 1185 cm-1; HRMS (CI+) calcd for C14H16NO2 (M+H)+ 230.1181,
found. 230.1181.
1-(2-Bromo-benzyl)-5-hydroxymethyl-1H-pyrrole-2-carbaldehyde (4g): light yellow oil, yield 51%
1
(0.60 g, 2.04 mmol); H NMR δ = 1.56 (br s, 1H), 4.55 (s, 2H), 5.77 (s, 2H), 6.25 (d, J = 7.6 Hz, 1H),
6.37 (d, J = 4.0 Hz, 1H), 7.00 (d, J = 4.0 Hz, 1H), 7.08–7.16 (m, 2H), 7.57 (dd, J = 7.6, 1.6 Hz, 1H),
9.56 (s, 1H) ppm; 13C NMR δ = 49.1, 56.4, 110.8, 121.6, 124.2, 126.1, 127.8, 128.6, 132.6, 132.7,
137.2, 141.9, 179.6 ppm; IR (neat) 3381, 1657, 1463, 1406, 1370, 1218 cm-1; HRMS (CI+) calcd for
C13H13BrNO2 (M+H)+ 294.0130, found 294.0125.
5-Hydroxymethyl-1-phenyl-1H-pyrrole-2-carbaldehyde (4h): brown oil, yield 21% (0.17 g, 0.84
1
mmol); H NMR δ = 1.58 (br s, 1H), 4.45 (s, 2H), 6.44 (d, J = 4.0 Hz, 1H), 7.09 (d, J = 4.0 Hz, 1H),
7.32–7.38 (m, 2H), 7.48–7.54 (m, 3H), 9.41 (s, 1H) ppm; 13C NMR δ = 56.1, 110.6, 120.3, 127.8, 129.0,
129.1, 134.1, 136.5, 141.8, 179.2 ppm; IR (neat) 3340, 1650, 1472, 1409, 1378, 1293, 1183 cm-1;
HRMS (CI+) calcd for C12H12NO2 (M+H)+ 202.0868, found 202.0867.
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5-Hydroxymethyl-1-p-tolyl-1H-pyrrole-2-carbaldehyde (4i): brown gummy oil, yield 25% (0.22 g,
1.0 mmol); 1H NMR δ = 1.59 (br s, 1H), 2.44 (s, 3H), 4.44 (s, 2H), 6.42 (d, J = 4.0 Hz, 1H), 7.08 (d, J =
1
2
3
4
5
6
13
4.0 Hz, 1H), 7.22 (d, J = 8.0 Hz, 2H), 7.29 (d, J = 8.0 Hz, 2H), 9.40 (s, 1H) ppm; C NMR δ = 21.1,
7
8
9
56.0, 110.4, 120.0, 127.5, 129.7, 133.8, 134.0, 139.0, 142.0, 179.3 ppm; IR (neat) 3306, 1647, 1409,
1184, 1034 cm-1; HRMS (CI+) calcd for C13H14NO2 (M+H)+ 216.1025, found 216.1029.
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4-(2-Formyl-5-hydroxymethyl-pyrrol-1-yl)-benzonitrile (4j): colorless gummy oil, yield 27% (0.25 g,
1
1.08 mmol); H NMR δ = 1.56 (br s, 1H), 4.43 (s, 2H), 6.48 (d, J = 4.0 Hz, 1H), 7.09 (d, J = 4.0 Hz,
1H), 7.49 (dd, J = 6.8, 1.6 Hz, 2H), 7.79 (dd, J = 6.8, 1.6 Hz, 2H), 9.50 (s, 1H) ppm; 13C NMR δ = 55.7,
111.6, 112.6, 117.9, 123.6, 128.7, 132.8, 133.6, 141.2, 142.0, 178.7 ppm; IR (neat) 3311, 1645, 1490,
1407, 1374, 1180, 1045 cm-1; HRMS (CI+) calcd for C13H11N2O2 (M+H)+ 227.0821, found. 227.0819.
1
1-Butyl-5-methyl-1H-pyrrole-2-carbaldehyde (4k):10 clear oil, yield 35% (0.23 g, 1.40 mmol); H
NMR δ = 0.94 (t, J = 7.6 Hz, 3H), 1.31–1.41 (m, 2H), 1.62–1.71 (m, 2H), 2.29 (s, 3H), 4.26 (t, J = 7.6
13
Hz, 2H), 6.01 (d, J = 4.0 Hz, 1H), 6.84 (d, J = 4.0 Hz, 1H), 9.40 (s, 1H) ppm; C NMR δ = 12.0, 13.7,
19.8, 33.0, 45.0, 109.7, 124.9, 131.3, 139.9, 178.0 ppm; IR (neat) 1642, 1462, 1411, 1177, 1022 cm-1;
HRMS (CI+) calcd for C10H16NO (M+H)+ 166.1232, found 166.1228.
1-(2-Bromo-benzyl)-5-methyl-1H-pyrrole-2-carbaldehyde (4l): light yellow crystal, yield 53% (0.59
1
g, 2.12 mmol); mp: 89–90 ºC; H NMR δ = 2.15 (s, 3H), 5.66 (s, 2H), 6.15 (d, J = 4.0 Hz, 1H), 6.25
(dd, J = 8.0, 2.0 Hz, 1H), 6.97 (d, J = 4.0 Hz, 1H), 7.07–7.18 (m, 2H), 7.56 (dd, J = 7.6, 1.6 Hz, 1H),
9.46 (s, 1H) ppm; 13C NMR δ = 11.8, 48.7, 110.4, 121.7, 125.0, 126.4, 127.8, 128.6, 131.8, 132.6,
136.9, 140.7, 178.5 ppm; IR (neat) 1653, 1487, 1435, 1369, 1257, 1038 cm-1; HRMS (CI+) calcd for
C13H13BrNO (M+H)+ 278.0181, found 278.0176.
4-Benzyl-3-oxo-3,4-dihydro-1H-pyrrolo[2,1-c][1,4]oxazine-6-carbaldehyde (8):16a yellow solid, yield
23
27% (0.28 g, 1.08 mmol); mp: 106–108 ºC (literature mp = 112–114 ºC);16a [α]D = 186.6 (c 0.96,
MeOH); 1H NMR δ = 3.45 (dd, J = 14.0, 4.8 Hz, 1H), 3.48 (d, J = 14.8 Hz, 1H), 3.54 (dd, J = 14.0, 3.6
Hz, 1H), 4.84 (d, J = 14.8 Hz, 1H), 5.98 (d, J = 4.0 Hz, 1H), 6.04 (dd, J = 4.8, 3.6 Hz, 1H), 6.76–6.82
(m, 2H), 7.06 (d, J = 4.0 Hz, 1H), 7.17–7.23 (m, 2H), 7.24–7.31 (m, 1H), 9.62 (s, 1H) ppm; 13C NMR δ
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= 40.2, 59.1, 63.3, 105.4, 125.5, 128.0, 128.8, 129.5, 130.3, 132.1, 134.4, 167.3, 179.0 ppm; IR (neat)
1749, 1660, 1505, 1467, 1395, 1244 cm-1; HRMS (EI+) calcd for C15H13NO3 255.0895, found 255.0891.
(S)-4-Methyl-3-oxo-3,4-dihydro-1H-pyrrolo[2,1-c][1,4]oxazine-6-carbaldehyde (9):20,16b,17 yellow
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2
3
4
5
6
7
8
23
25
oil, yield 32% (0.23 g, 1.28 mmol); [α]D = -86.0 (c 1.24, MeOH) {literature [α]D = -33.3 (c 0.42,
9
MeOH)};17 1H NMR δ = 1.72 (d, J = 7.2 Hz, 3H), 5.39 (d, J = 15.2 Hz, 1H), 5.47 (d, J = 15.2 Hz, 1H),
5.85 (q, J = 7.2 Hz, 1H), 6.21 (d, J = 4.0 Hz, 1H), 6.98 (d, J = 4.0 Hz, 1H), 9.54 (s, 1H) ppm; 13C NMR
δ = 19.0, 53.7, 63.2, 106.5, 124.5, 130.3, 130.7, 168.1, 179.2 ppm.
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4,9-Dihydropyrrolo[1,2-a][1,2,3]triazolo[1,5-d]pyrazine-6-carbaldehyde (5): To stirred solution of
N-propargylic pyrrole 4d (0.60 g, 3.68 mmol) in dry CH2Cl2 (10 mL) at 0 ºC were added Et3N (1.02 mL,
7.36 mmol) and MsCl (0.34 mL, 4.42 mmol). The reaction mixture was stirred at that temperature for 30
min, and allowed to warm to room temperature. Stirring for 30 min, it was poured into crushed ice, and
then extracted with CH2Cl2 (3×10 mL). The combined organic layer was washed with water, dried over
Na2SO4, filtered, and concentrated under reduced pressure to give the crude mesylation product as sticky
oil. Without further purification, the crude product was dissolved in dry DMF (10 mL), and NaN3 (0.72
g, 11.04 mmol) was added. The mixture was heated at 60 ºC for 8 h, cooled to room temperature, diluted
with H2O (15 mL), and extracted with EtOAc (3×10 mL). The organic layer was concentrated under
reduced pressure, and the crude product was purified by silica-gel column chromatography (eluent 2:1
EtOAc:hexane) to give pyrrolo-triazolo-piperazine 5 (0.61 g, 3.23 mmol) in 88% yield as white solid.
Data for 5: mp: 184 ºC; 1H NMR δ = 5.71 (s, 2H), 5.80 (s, 2H), 6.36 (d, J = 4.0 Hz, 1H), 7.09 (d, J = 4.0
Hz, 1H), 7.74 (s, 1H), 9.61 (s, 1H) ppm; 13C NMR δ = 41.6, 43.8, 107.4, 125.0, 128.6, 129.7, 130.9,
131.2, 179.4 ppm; IR (neat) 1648, 1486, 1327, 1174, 1035 cm-1; HRMS (CI+) calcd for C9H9N4O
(M+H)+ 189.0776, found 189.0778.
Reductive amination of 4l with benzylamine: Benzyl-[1-(2-bromo-benzyl)-5-methyl-1H-pyrrol-2-
ylmethyl]-amine (4m): N-(2-bromobenzyl)pyrrole 4l (0.55 g, 2.0 mmol) was dissolved in dry CH2Cl2
(5 mL). The resulting solution was treated with MgSO4 and then with benzylamine
H
N
Ph
N
4m
Br
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(0.26 g, 2.4 mmol) at 0 ºC. The mixture was stirred at room temperature for 12 h under argon
atmosphere, and then filtered. The filtrate was concentrated under reduced pressure, and the crude
product was dissolved in dry MeOH (10 mL). Small portions of NaBH4 (151 mg, 4.0 mmol) was added
over a period of 1 h at 0 ºC. Stirring was continued for another 2 h at room temperature. The solvent was
evaporated under reduced pressure, and the residue was extracted with CH2Cl2 (3×10 mL). The
combined organic layers were washed with water, dried over anhydrous Na2SO4, filtered, and
concentrated. Flash chromatography over silica gel (eluent 1:9 EtOAc:hexane) afforded amine 4m (0.58
g, 1.56 mmol) in 78% yield as clear thick oil. Data for 4m: 1H NMR δ = 2.09 (s, 3H), 3.60 (s, 2H), 3.73
(s, 2H), 5.20 (s, 2H), 5.90 (dd, J = 3.6, 0.8 Hz, 1H), 6.04 (d, J = 3.6 Hz, 1H), 6.21 (dd, J = 7.6, 1.6 Hz,
1H), 7.06–7.29 (m, 7H), 7.55 (dd, J = 7.6, 1.2 Hz, 1H) ppm; 13C NMR δ = 12.0, 45.3, 47.4, 53.1, 105.7,
107.6, 121.4, 126.7, 126.8, 127.9, 127.9, 128.3, 128.4, 129.5, 130.3, 132.3, 138.2, 140.3 ppm; IR (neat)
3332, 1449, 1360, 1310, 1225, 1035 cm-1; HRMS (EI+) calcd for C20H21BrN2 368.0888, found
368.0891.
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General procedure for palladium catalyzed cyclization: To a stirred solution of alcohol 4g or amine
4m (1.0 mmol) in dry toluene (10 mL) were added t-BuOK (224 mg, 2.0 equiv.), K2CO3 (276 mg, 2.0
equiv.), Pd2(dba)3 (5 mol% for 4g, 3 mol% for 4m), and (+)-BINAP (10 mol% for 4g, 6 mol% for 4m).
The reaction mixtures were heated at reflux (18 h for 4g, 16 h for 4m) under argon atmosphere. After
completion of reaction (monitored by TLC), the crude mixtures were passed through a bed of silica gel.
The solvents were evaporated, and the residues were extracted with CH2Cl2 (3×10 mL). The organic
layer was washed with water, dried over anhydrous Na2SO4, and filtered. The filtrate was concentrated
under reduced pressure to give the crude product which was purified by flash column chromatography
over silica gel (eluent 1:19 EtOAc:hexane) to furnish the corresponding cyclized product 6 or 7 in a pure
form.
5,11-Dihydrobenzo[f]pyrrolo[2,1-c][1,4]oxazepine-3-carbaldehyde (6): brown foamy solid, yield
1
63% (0.13 g, 0.63 mmol); mp: 110 ºC; H NMR δ = 5.28 (s, 2H), 5.97 (s, 2H), 6.28 (d, J = 4.0 Hz, 1H),
13
6.86 (d, J = 4.0 Hz, 1H), 6.88–6.94 (m, 2H), 7.18–7.25 (m, 2H) 9.56 (s, 1H) ppm; C NMR δ = 47.2,
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62.7, 109.8, 120.0, 121.8, 121.9, 123.7, 129.7, 130.5, 131.6, 138.7, 156.9, 180.3 ppm; IR (neat) 1662,
1485, 1452, 1211, 1036 cm-1; HRMS (CI+) calcd for C13H12NO2 (M+H)+ 214.0868, found 214.0864.
10-Benzyl-3-methyl-10,11-dihydro-5H-benzo[e]pyrrolo[1,2-a][1,4]diazepine (7): colorless gummy
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oil, yield 86% (0.25 g, 0.86 mmol); H NMR δ = 2.31 (s, 3H), 4.37 (s, 2H), 4.45 (s, 2H), 5.14 (s, 2H),
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5.78 (d, J = 2.8 Hz, 1H), 5.88 (d, J = 2.8Hz, 1H), 6.71 (t, J = 7.2 Hz, 1H), 6.77 (d, J = 8.0 Hz, 1H),
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7.04–7.12 (m, 2H), 7.26–7.32 (m, 2H), 7.33–7.38 (m, 3H) ppm; C NMR δ = 12.1, 47.8, 48.1, 58.8,
105.0, 105.2, 118.8, 118.8, 124.3, 127.0, 127.6, 127.8, 128.1, 128.6, 129.0, 129.5, 139.0, 149.9 ppm; IR
(neat) 1661, 1607 cm-1; HRMS (EI+) calcd for C20H20N2 288.1626, found 288.1629.
Methyl 4-(2-Formyl-5-hydroxymethyl-pyrrol-1-yl)-butyrate (12):16d,16e The mixture of gamma-
aminobutyric acid methyl ester, hydrochloride salt (0.61 g, 4.0 mmol), Et3N (0.40 g, 4.0 mmol),
D-
glucose (0.72 g, 4.0 mmol) and oxalic acid (0.36 g, 4.0 mmol) in dry DMSO (3.0 mL) was heated at 90
°C for 0.5 h. The resulting black viscous reaction mixture was cooled to room temperature and quenched
with water (10 mL). The mixture was extracted with EtOAc (3×10 mL). The combined extract was
washed with brine and water, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the
crude product which was purified by SiO2 flash column chromatography (EtOAc:hexane 1:2) to give 12
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(0.31 g, 1.36 mmol) in 34% yield as colorless thick oil; Data for 12: H NMR δ = 2.10 (heptet, J = 7.2
Hz, 2H), 2.44 (t, J = 7.2 Hz, 2H), 2.69 (br s, 1H), 3.68 (s, 3H), 4.41 (t, J = 7.2 Hz, 2H), 4.69 (s, 2H),
6.24 (d, J = 3.6 Hz, 1H), 6.88 (d, J = 3.6 Hz, 1H), 9.50 (s, 1H) ppm; 13C NMR δ = 25.9, 30.5, 44.6, 51.8,
56.0, 110.6, 124.5, 132.3, 141.9, 174.1, 179.4 ppm; IR (neat) 3423, 1740, 1657, 1373, 1178, 1026 cm-1;
HRMS (EI+) calcd for C11H15NO4 225.1001, found 225.0998.
Methyl 3-Hydroxymethyl-5,6-dihydro-indolizine-7-carboxylate (15): To a stirred solution of 12
(0.40 g, 1.78 mmol) in dry THF (10 mL) at 0 °C was added 60% NaH (0.085g, 2.14 mmol, 1.2 equiv.)
in several portions. The mixture was stirred for 5 min, and 1-bromobutane (0.29 g, 2.14 mmol) was
added drop wise. The mixture was then stirred at room temperature for 3 h, and cooled to 0 °C again.
Excess NaH was discharged by a drop wise addition of saturated NaHCO3 solution in H2O. The
resulting mixture was evaporated to dryness under reduced pressure and extracted with EtOAc (5×3 mL).
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The combined organic extract was washed with water, dried over anhydrous Na2SO4, filtered and
evaporated to give crude 15, which was purified by SiO2 flash column chromatography (EtOAc:hexane
1:4) as a colorless viscous liquid in 82% yield (0.30 g, 1.46 mmol). Data for 15: 1H NMR δ = 1.46 (br s,
1H), 2.81 (dt. Jd = 1.2, Jt = 7.2 Hz, 2H), 3.80 (s, 3H), 4.07 (t, J = 7.2 Hz, 2H), 4.63 (d, J = 3.6 Hz, 2H),
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6.17 (d, J = 3.6 Hz, 1H), 6.28 (d, J = 3.6 Hz, 1H), 7.45 (d, J = 1.2 Hz, 1H) ppm; C NMR δ = 23.3,
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41.0, 51.7, 56.5, 110.2, 111.2, 119.3, 128.2, 129.5, 134.7, 167.5 ppm; IR (neat) 3464, 1705, 1614, 1440,
1187, 1031 cm-1; HRMS (CI+) calcd for C11H14NO3 (M+H)+ 208.0974, found 208.0969.
Methyl 4-(2-Butoxymethyl-5-formyl-pyrrol-1-yl)butyrate (16):16c A mixture of 12 (0.23 g, 1.0
mmol), Ag2O (0.35 g, 1.5 mmol) and 1-iodobutane (1.84 g, 10 mmol) was heated at 90 °C for 4 h. The
mixture was then cool to room temperature, diluted with CH2Cl2 (10 mL) and filtered through a short
pad of celite. The solvent was evaporated under reduced pressure and the remaining mass was subjected
to SiO2 flash column chromatography (EtOAc:hexane 1:6) to give 16 (0.20 g, 0.71 mmol) in 71% yield
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as colorless oil. Data for 16: H NMR δ = 0.91 (t, J = 7.2 Hz, 3H), 1.36 (sextet, J = 7.2 Hz, 2H), 1.57
(quintet, J = 7.2 Hz, 2H), 2.06 (quintet, J = 7.2 Hz, 2H), 2.38 (t, J = 7.2 Hz, 2H), 3.47 (t, J = 6.4 Hz, 2H),
3.67 (s, 3H), 4.39 (t, J = 7.6 Hz, 2H), 4.48 (s, 2H), 6.22 (d, J = 4.0 Hz, 1H), 6.87 (d, J = 4.0 Hz, 1H),
9.50 (s, 1H) ppm; 13C NMR δ = 13.8, 19.3, 26.3, 30.9, 31.6, 44.8, 51.6, 63.9, 70.4, 111.4, 124.2, 132.5,
139.3, 173.3, 179.3 ppm; IR (neat) 1742, 1662, 1370, 1189, 1105 cm-1; HRMS (EI+) calcd for
C15H23NO4 281.1627, found 281.1621.
Lobechine 10:16c To a stirred solution of the above methyl ester 16 (0.10 g, 0.36 mmol) in THF and
H2O (1:1 v:v, 10 mL) at 0 °C was added LiOH·H2O (0.045 g, 1.08 mmol). The mixture was warmed to
room temperature and stirring was continued for 2 h. The reaction mixture was then acidified with
saturated KHSO4 solution and extracted with EtOAc (5×3 mL). The combined organic extract was
washed with water, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to
give analytically pure Lobechine 10 (0.091 g, 0.34 mmol) as colorless viscous oil in 95% yield. Data for
Lobechine 10: 1H NMR δ = 0.90 (t, J = 7.2 Hz, 3H), 1.36 (sextet, J = 7.2 Hz, 2H), 1.57 (quintet, J = 7.2
Hz, 2H), 2.08 (quintet, J = 7.2 Hz, 2H), 2.44 (t, J = 7.2 Hz, 2H), 3.46 (t, J = 6.4 Hz, 2H), 4.41 (t, J = 7.6
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Hz, 2H), 4.48 (s, 2H), 6.23 (d, J = 4.0 Hz, 1H), 6.88 (d, J = 4.0 Hz, 1H), 9.49 (s, 1H) ppm; 13C NMR δ =
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13.8, 19.3, 26.1, 30.8, 31.6, 44.7, 63.9, 70.4, 111.6, 124.5, 132.4, 139.4, 178.4, 179.5 ppm.
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Ethyl 4-Azido-3-tert-butoxycarbonylaminobutyrate (13): L-Aspartic acid (1.00 g, 7.51 mmol) was
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suspended in dry EtOH (10 mL) and SOCl2 (0.6 mL) was added drop wise at 0 °C. The mixture was
then heated at reflux for 16 h. Upon cooling to room temperature, solvent was evaporated under reduced
pressure and the resulting solid was dissolved in saturated NaHCO3 solution (10 mL) and 1,4-dioxane
(10 mL). Boc2O (1.639 g, 7.51 mmol) was added and the mixture was stirred at room temperature for 12
h. The mixture was then extracted with EtOAc (10×3 mL). Combined organic extract was washed with
brine and H2O, dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure to give N-
Boc diester G (1.79 g, 6.2 mmol crude).21
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The crude N-Boc diester G (1.79 g, 6.2 mmol) was dissolved in absolute EtOH and treated with
NaBH4 (0.36 g, 9.5 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 18 h, after
which it was concentrated and extracted with CH2Cl2 (10×3 mL). Combined organic extract was washed
with H2O, dried over anhydrous Na2SO4, and evaporated to give mono-alcohol H (0.60 g, 2.43 mmol
crude).
To a stirred solution of the above mono-alcohol H (0.60 g, 2.43 mmol) in dry CH2Cl2 (12 mL) at 0 °C
were added Et3N (0.61 g, 6.07 mmol) and MsCl (0.33 g, 2.92 mmol). The mixture was stirred at room
temperature for 1 h and crushed ice was added. The mixture was extracted with CH2Cl2 (10×3 mL).
Combined organic extract was washed with H2O, dried over anhydrous Na2SO4, filtered and
concentrated to give the corresponding mesylate. The crude mesylate was dissolved in dry DMF (10
mL) and NaN3 (0.47 g, 7.30 mmol) was added. The mixture was heated at 70 °C for 8 h and cooled to
room temperature. The mixture was treated with H2O (10 mL) and extracted with Et2O (10×3 mL).
Combined organic extract was washed with brine and H2O, dried over anhydrous Na2SO4, filtered,
concentrated and SiO2 column chromatographed (EtOAc:hexane 1:9) to produce the corresponding
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azide 13 (0.51 g, 1.88 mmol) as colorless oil in overall 25% yield. Data for 13: H NMR δ = 1.27 (t, J =
7.2 Hz, 3H), 1.45 (s, 9H), 2.58 (d, J = 6 .0 Hz, 2H), 3.46 (dd, J = 12.0, 5.6 Hz, 1H), 3.54 (dd, J = 12.0,
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4.8 Hz, 1H), 4.10 (br s, 1H), 4.16 (q, J = 7.2 Hz, 2H), 5.10 (br s, 1H) ppm; C NMR δ = 14.1, 28.3,
36.2, 47.1, 53.6, 60.9, 79.9, 154.9, 171.0 ppm; IR (neat) 2112, 1714, 1505, 1376, 1166, 1037 cm-1;
HRMS (CI+) calcd for C11H21N4O4 (M+H)+ 273.1563, found 273.1565.
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Ethyl 4-Azido-3-(2-formylpyrrol-1-yl)butyrate (14): To a stirred solution of 13 (1.52 g, 5.58 mmol)
in dry CH2Cl2 (12 mL) at 0 °C was slowly added trifluoroacetic acid (12 mL). Stirring was continued for
1 h at room temperature. The reaction mixture was evaporated to give the amine salt, which was
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neutralized with Et3N (0.56 g, 5.58 mmol). The above mixture was then added to a solution of D-ribose
(0.84 g, 5.58 mmol) and oxalic acid (0.50 g, 5.58 mmol) in dry DMSO (5 mL). The mixture was heated
at 60 °C for 1 h and cooled to room temperature. The resulting black viscous mixture was quenched
with water (10 mL) and extracted with EtOAc (3×10 mL). The combined extract was washed with brine
and water, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to give the
crude pyrrole 14, which was purified by SiO2 flash column chromatography (EtOAc:hexane 1:5) to give
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the pure product 14 (0.75 g, 3.01 mmol) in 54% yield as colorless oil. Data for 14: H NMR δ = 1.19 (t,
J = 7.2 Hz, 3H), 2.94 (dd, J = 16.4, 6.4 Hz, 1H), 3.03 (dd, J = 16.4, 8.0 Hz, 1H), 3.66 (dd, J = 12.8, 4.8
Hz, 1H), 3.87 (dd, J = 12.8, 6.4 Hz, 1H), 4.03–4.16 (m, 2H), 5.53 (br s, 1H), 6.30 (dd, J = 4.0, 2.4 Hz,
1H), 7.02 (dd, J = 4.0, 1.6 Hz, 1H), 7.16 (dd, J = 2.4, 1.6 Hz, 1H), 9.52 (s, 1H) pm; 13C NMR δ = 14.0,
36.5, 54.2, 54.4, 61.0, 110.4, 127.1, 130.1, 131.4, 169.8, 179.3 ppm; IR (neat) 2108, 1736, 1669, 1223,
1034 cm-1; HRMS (CI+) calcd for C11H15N4O3 (M+H)+ 251.1144, found 251.1141.
Methyl 1-(2-Azido-1-ethoxycarbonylmethylethyl)-1H-pyrrole-2-carboxylate (17): To a stirred
solution of 14 (0.56 g, 2.24 mmol) in CH3CN (5 mL) at 0 °C were added 80% NaClO2 (0.34 g, 2.97
mmol) in H2O (2 mL), 96% NaH2PO4 (0.56 g, 4.48 mmol) in H2O (2 mL) and 28% aqueous solution of
H2O2 (0.63 g, 5.2 mmol). The reaction mixture was stirred at room temperature for 20 h. The solvent
was evaporated to half of the initial volume and saturated NaHCO3 solution (10 mL) was added. The
mixture was extracted with CH2Cl2 (3×10 mL). The aqueous layer was acidified with saturated KHSO4
solution and was extracted with EtOAc (3×10 mL). The combined extract was washed with brine and
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H2O, and dried over anhydrous Na2SO4. Filtration and evaporation of the filtrate gave the crude pyrrole
carboxylic acid (0.43 g, 1.60 mmol).
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To a solution of the above pyrrole carboxylic acid in dry DMF (5 mL) were added K2CO3 (0.88 g, 6.4
mmol) and CH3I (1.14 g, 8.0 mmol). The mixture was stirred at room temperature for 12 h and
quenched with H2O (10 mL). The resulting mixture was extracted with Et2O (10×3 mL). Combined
organic extract was washed with brine and H2O, dried over anhydrous Na2SO4, filtered, concentrated,
and SiO2 column chromatographed (EtOAc:hexane 1:9) to give the azido methyl ester 17 (0.39 g, 1.39
mmol) as colorless oil in 62% overall yield. Data for 17: 1H NMR δ = 1.19 (t, J = 6.8 Hz, 3H), 2.95 (d, J
= 7.2 Hz, 2H), 3.67 (dd, J = 12.4, 4.8 Hz, 1H), 3.80 (dd, J = 12.4, 6.4 Hz, 1H), 3.82 (s, 3H), 4.10 (q, J =
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6.8 Hz, 2H), 5.82 (br s, 1H), 6.20 (br s, 1H), 7.01 (br s, 2H) ppm; C NMR δ = 13.9, 37.0, 51.1, 52.9,
54.8, 60.9, 109.0, 119.0, 121.9, 125.9, 161.5, 169.7 ppm; IR (neat) 2116, 1734, 1702, 1228, 1114 cm-1;
HRMS (CI+) calcd for C12H17N4O4 (M+H)+ 281.1250, found 281.1253.
(–)-Hanishin (11):19,22,25 To a stirred solution of azido methyl ester 17 (0.30 g, 1.07 mmol) in dry
MeOH (20 mL) was added activated 10% Pd on carbon (0.053 g, 0.05 mmol Pd). The mixture was
stirred under 1 atm pressure of H2 gas for 2 h. The mixture was then filtered through a short pad of celite
and solvent was evaporated under reduced pressure. The concentrate was dissolved in dry toluene (10
mL) and the resulting solution was gently refluxed for 1 h. Solvent was removed under reduced pressure
and the solid mass was dissolved in dry THF (10 mL). To this solution was added NBS (0.38 g, 2.14
mmol) and the resulting mixture was stirred at room temperature for 3 h. The mixture was concentrated
under reduced pressure and the resulting mass was SiO2 chromatographed (1:1 EtOAc:hexane) to
produce (–)-Hanishin (0.28 g, 0.74 mmol) as white solid in 69% overall yield. Data for 11: mp = 150 °C
(literature mp = 148-150 °C);25 [α]D23 = –4.7 (c 0.09, MeOH) {literature [α]D20 = –4.3 (c 1, MeOH) };22
1H NMR δ = 1.28 (t, J = 7.2 Hz, 3H), 2.58 (ddd, J = 16.4, 2.8, 1.2 Hz, 1H), 2.95 (dd, J = 16.4, 10.8 Hz,
1H), 3.69 (ddd, J = 13.2, 5.2, 1.2 Hz, 1H), 3.93 (ddd, J = 13.2, 4.0, 1.6 Hz, 1H), 4.19 (dq, Jd = 1.2, Jq =
7.2 Hz, 2H), 4.76 (m, 1H), 6.42 (br s, 1H), 7.00 (s, 1H) ppm; 13C NMR δ = 14.1, 35.5, 43.1, 50.4, 61.3,
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101.2, 106.5, 113.5, 116.2, 159.1, 169.7 ppm; HRMS (CI+) calcd for C11H13Br2N2O3 (M+H)+ 378.9293,
found 378.9286.
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Acknowledgments. This research was supported by Priority Research Center Program (Grant No.
2009-0093816) and by Basic Science Research Program (Grant No. NRF-2012R1A1A2000702; NRF-
2012R1A2A2A01045344) through the National Research Foundation of Korea.
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Supporting Information. H and C NMR spectra for compounds 4a–4m, 8, 9, 5, 6, 7, 12, 15, 16,
10, 13, 14, 17, and 11. This material is available free of charge via the Internet at http://pubs.acs.org.
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