Improved and scalable methods for the synthesis of midazolam drug and its analogues using isocyanide reagents

Article abstract of DOI:10.1007/s13738-018-1555-0

Abstract: In this research, two improved and scalable methods for the synthesis of midazolam and its analogues have been described. Midazolam has been synthesized using isocyanide reagents in satisfactory yield. In this methodology, imidazobenzodiazepine intermediates can be easily prepared via an improved process. One-pot condensation of benzodiazepines with mono-anion of tosylmethyl isocyanide or ethyl isocyanoacetate under mild condition led to formation of imidazobenzodiazepine. In the first method, tosylmethyl isocyanide (Tos-MIC) is used and the number of synthetic steps are decreased in comparison to previous report. In the second method, ethyl isocyanoacetate which is commonly used for the synthesis of some imidazobenzodiazepines, is consumed to generate midazolam. The latter, a relatively different method for the synthesis of midazolam analogues has been reported. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-018-1555-0

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1555-0
ORIGINAL PAPER
Improved and scalable methods for the synthesis of midazolam drug
and its analogues using isocyanide reagents
Mohammad Javad Taghizadeh1 · Gholam reza malakpouri1 · Abdollah Javidan1,2
Received: 4 August 2018 / Accepted: 19 November 2018
© Iranian Chemical Society 2018
Abstract
In this research, two improved and scalable methods for the synthesis of midazolam and its analogues have been described.
Midazolam has been synthesized using isocyanide reagents in satisfactory yield. In this methodology, imidazobenzodiazepine
intermediates can be easily prepared via an improved process. One-pot condensation of benzodiazepines with mono-anion
of tosylmethyl isocyanide or ethyl isocyanoacetate under mild condition led to formation of imidazobenzodiazepine. In the
first method, tosylmethyl isocyanide (Tos-MIC) is used and the number of synthetic steps are decreased in comparison to
previous report. In the second method, ethyl isocyanoacetate which is commonly used for the synthesis of some imidazoben-
zodiazepines, is consumed to generate midazolam. The latter, a relatively different method for the synthesis of midazolam
analogues has been reported.
Graphical abstract
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-018-1555-0) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Keywords Midazolam · Tosylmethyl isocyanide · Ethyl isocyanoacetate · Annulation · Imidazo[1,5-a][1,4]benzodiazepines
that imidazobenzodiazepines have been manufactured by the
reaction of isocyanides with an in situ intermediate formed
of the benzodiazepine, preferably the iminophosphate or
iminochloride derivatives [37, 46–48]. But the product has
been obtained in moderate yields (15–30%). Here, a novel
method utilizing two valuable isocyanides for synthesizing
midazolam and novel tricyclic benzodiazepines has been
described. In this methodology, a one-pot annulation process
has been employed using tosylmethyl isocyanide (Tos-MIC)
and ethyl isocyanoacetate reagents. This synthetic sequence
not yet described in literature.
Introduction
Benzodiazepines (BZDs) and their derivatives belong to a
versatile class of biologically active compounds and their
synthesis has been receiving much attention in the field of
medicinal chemistry [1–6]. Among them the members of
1,4-benzodiazepine family have shown sufficient biological
[7, 8] and pharmacological activity [9] to serve as a versa-
tile drug for the treatment of CNS disturbances [10–20].
Moreover, 1,4-benzodiazepines are useful intermediates for
the preparation of other fused ring system such as triazolo,
imidazo, oxazino, or furano-benzodiazepines [21–27]. For
example, imidazobenzodiazepines such as midazolam or cli-
mazolam and triazolobenzodiazepines such as alprazolam or
triazolam possess pharmacological property.
Results and discussion
Midazolam is an imidazobenzodiazepine and excellent
medicinal compound that was first synthesized by Fryer and
Walser in 1978 [28]. This important drug illustrated vari-
ous pharmacological properties such as anesthetic, sedative,
hypnotic, anticonvulsant, muscle relaxant, and anxiolytic
activities. There are several multistep methods for the syn-
thesis of this drug outlined in literature [29–36]. Although,
different groups of researcher synthesized this clinical drug,
none of these syntheses procedures can be scalable or expose
a complex chemistry. The laboratory procedures for synthe-
sis of midazolam have suffered from several problems, such
as long reaction pathway, formation of various by-products,
difficult separation by chromatography. Moreover, the tech-
nical and significant aspect of product purification is not
specified. In fact, many of these procedures are not market-
able and none of these syntheses are scalable commercially.
More synthetic approach for midazolam starts from forma-
tion of seven-membered ring. Generally this ring closure
accomplished via an intramolecular reaction from the cor-
responding α-(aminoacetamido)-benzophenone. In addition,
the synthesis of the imidazole ring is a challenging step for
the synthesis of imidazo[1,5-a][1,4]benzodiazepine family,
for achievement to this purpose, several five or six step pro-
cedures have been reported in the literature. This step was
highly affected the overall yield of these ring formation. The
differences in imidazobenzodiazepines synthetic routes lie
basically on how one approaches the formation of imida-
zole nucleus. Notably, the isocyanide reagents have been
reported for synthesize of imidazobenzodiazepine structures
[37–44]. Most of them have been achieved in low yield or
require a multistep method. In some cases, the purification
of the intermediates and target products with chromatogra-
phy was necessary. Certainly, an earlier paper published the
application of Tos-MIC for the preparation of midazolam
and tricyclic benzodiazepines [45]. It is interesting to note
In this research, the possibility of synthesizing midazolam
via two methods has been considered. Our goal is to illus-
trate an optimized process that would yield midazolam and
its analogues within the shortest sequence and maximum
yield. Toward this goal, several patent and literature routes
have been investigated and an efficient and simple process
for midazolam was devised. The method suggested for the
synthesis of midazolam is shown below (Scheme 1).
In the first step, the synthesis of benzodiazepine nucleus
is described. Benzodiazepine nucleus (3a) was synthesized
according to a published literature [49–54]. Starting with
2-amino-5-chloro-2′-fluorobenzophenone (1a), a commer-
cial available compound, was treated with bromoacetyl
bromide in dichloromethane to yield the intermediate,
2-(2-bromoacetamido)-5-chloro-2′-flourobenzophenone (2a)
in good yield. The cyclization of amide (2a) could be carried
out in the presence of liquid ammonia, methanolic ammonia,
ammonia in a mixture of CH2Cl2 and EtOAc, hexamethyl-
enetetramine (HMTA), sodium azide and others. However,
formation of 1,4-benzophenone ring is not a clean reaction
and several by-products formed. Although cyclization of the
aforementioned amide is described in many literatures, the
most reported routes for this goal carried out with metha-
nolic ammonia. The utility of ammonia gas led to signifi-
cant amounts of product. The highest yield was achieved by
employing in situ formation of ammonia gas in methanol.
Benzophenones (1a–c) with different substitution such as Cl,
NO2 in the ring and substitution such as F, Cl in other ring
derivatives to the desired 1,4-benzodiazepin-2-one (3a–c) in
70–87% yields in this process. In the case of benzophenones
with halogen substitution, cyclization of amide intermediates
to benzodiazepines (3a-b) can be performed with ammonia
gas in scale-up amount. However, when this cyclization was
employed for amide intermediate with -NO2 substitution,
no cyclized product was observed. The NO2-substituted
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Journal of the Iranian Chemical Society
Scheme 1 Synthesis of midazolam
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Journal of the Iranian Chemical Society
amide (2c) reacted with HMTA to give the benzodiazepine
(3c) with no difficulty in 87% yield. The para-chloro aniline
also used as starting material in this study. We have tried
to synthesis 2-amino-5-chlorobenzophenone via the acyla-
tion of para-chloro aniline with benzoyl chloride by using
Friedel–Crafts reaction. Incorporation of the acetyl moiety
was accomplished with ZnCl2 as Lewis acid at 195–205 °C.
The corresponding benzoyl chloride functions as protecting
group and reactant. Initially para-chloroaniline reacts with
the benzoyl chloride at low temperature to form an amide,
then the temperature was gradually increased to 195–205 °C
and nucleophilic attack from the ring electrons to the acyl
carbon. In the second step, imidazole ring formation start
from insertion of a carbon nucleophile on position 2 of the
benzodiazepine. Numerous studies and several stepwise
methods for carbon–carbon bond forming on amide group
of benzodiazepines have been investigated. For this purpose
the secondary amide in the 1,4-benzodiazepine ring must be
activated via conversion to an appropriate intermediate such
as thioamides [55, 56], N-nitrosoamidines [26, 57], imidoyl
halides [39, 58, 59] and others, then the carbon–carbon bond
formation of them with carbanion led to generation of target
compound. Certainly, the application of Tos-MIC for this
goal and preparation of midazolam and tricyclic benzodiaz-
epine had been developed in the literature [48]. In previous
research, the carbanion of Tos-MIC has been generated with
butyllithium then condensation with 1,4-benzodiazepine
N-nitrosoamidines gives entry to target imidazobenzodiaz-
epines. However, this process takes some drawbacks, for
instance the separation of the product accomplished with
chromatography which was critical for large-scale reactions.
In addition, the formation of tricyclic benzodiazepine pro-
ceeds after four steps from initial benzodiazepine. Moreo-
ver, butyllithium is very sensitive and difficult-to-handle
compared to other bases, its application involved the low-
temperature for prevented side reactions. Furthermore the
N-nitrosoamidines derivatives are stable intermediates
which were prepared after three steps reaction from initial
1, 4-benzodiazepin-2-one. The one-pot imidazo-annulation
of benzodiazepine with diethyl chlorophosphate had been
reported in the literature [60–62]. All these process accom-
plished via the reaction of enol phosphonate with ethyl iso-
cyanoacetate in the presence of different bases (NaH, LDA
or potassium tert-butoxide). We have focused on improving
this one-pot annulation reaction. In these synthetic methods,
the condensation of in situ-formed iminophosphate deriva-
tives with carbanion of Tos-MIC or ethyl isocyanoacetate
gives entry to imidazobenzodiazepines. For this purpose,
the intermediate enol phosphonate generated by reaction of
1,4-benzodiazepin-2-one with diethyl chlorophosphate in the
presence of potassium tert-butoxide in dry THF at −20 °C.
Then, isocyanides were added portionwise to a solution of
enol phosphonate in aforementioned reaction condition. This
procedure is carried out more easily compared to previous
work [48]. This improved one-pot method required neither
the application of the sensitive base butyllithium nor does
it require the purification of the products with chromatog-
raphy. The initial benzodiazepines (3a–d) were converted
in one-step into the midazolam intermediates and its ana-
logues (5a–d). The mechanism of this reaction is shown
in Scheme 2. Moreover, some of the target products were
successfully precipitated from diethyl ether (Table 1).
This is the first time that an enol phosphonate and
Tos-MIC reagent are used for preparation of the imida-
zobenzodiazepines family. In route I, at first the mono-
anion of Tos-MIC is generated by reaction with potas-
sium tert-butoxide in moderate condition. The reaction
temperature was kept in − 20 °C and lower temperature
was not needed for this process. In addition, potassium
tert-butoxide is a mild and safer base that is not used
with Tos-MIC for this ring closing. In the next step for
synthesizing of midazolam, methylation of compound
5a carried out with one equivalent of butyl lithium, fol-
lowed by the addition of methyl iodide. Elimination of
the tosyl group on imidazobenzodiazepine 6a was per-
formed by reaction with Na–Hg amalgam. In the second
Scheme 2 Plausible mechanism for the formation of 5a-d
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Table 1 Preparation of target imidazo[1,5-a][1,4]benzodiazepines
Experimental section
Entry
Product
R1
R2
Time (h)
Yield (%)a
General
1
5a
6a
7a
5e
6e
7e
8e
7a
5b
5c
5d
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
F
F
F
F
F
F
F
F
H
F
Cl
5
24
4
6
4
4
2
24
5
5
60
90
72b
60
98
83
70
90c
80
62
85
2
All the chemicals were used as purchased (Merck) for the
reactions without further purification. All the organic sol-
vents were purchased from commercial suppliers and were
purified according to standard procedures. Infrared spectra
were obtained using a Perkin-Elmer Spectrum-100 FT-IR
spectrometer. IR spectra of liquids were recorded as thin
3
4
5
6
7
1
films on NaCl plates. The HNMR and 13CNMR spectra
8
9
Cl
NO2
NO2
were determined using TMS as an internal reference with
an Avance FT NMR spectrometer operating at 250 and
60 MHz, respectively. Mass spectra were recorded on an
Agilent Technologies, Model: 5975C VL MSD by EI mass
spectrometry on a Q-TOF instrument. Thin layer chromatog-
raphy was carried out on silica gel 60 F-254 TLC plates of
20 cm×20 cm. Column chromatography was accomplished
using Merck Silica gel 60 (0.063–0.200 mm). Elemental
analysis on C, H and N were accomplished using a Perkin-
Elmer 2400 Elemental Analyzer.
10
11
5
aIsolated yield
bYield of 7a by desulfonylation of 6a with 10% Na–Hg amalgam
cYield of 7a after reaction of 8e with MeI
method (route II), midazolam synthesized using the ethyl
isocyanoacetate which is commonly used for the synthesis
of some imidazobenzodiazepines. It is a relatively dif-
ferent method in which for the synthesis of midazolam
and desmethyl midazolam has been reported. Benzodi-
azepine nucleus (3a) was used as the starting material
for this synthetic route. The imidazole ring closure on
benzodiazepine is carried out similarly to above modified
one-pot reaction. The carbanion of ethyl isocyanoacetate
condensed with less stable iminophosphates intermediate
to give the target imidazobenzodiazepine framework (5e).
After workup, the desired imidazo product was precipi-
tated from diethyl ether. The ethyl ester moiety on the
imidazole ring hydrolyzes to carboxylic acid via basic
condition. Lastly, the process for decarboxylation of the
imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid was
carried out in high boiling solvent such as N,N-dimethy-
lacetamide, mineral oil [63]. In another research, decar-
boxylation of this ligand performed in n-butanol at high
pressure and temperature [64]. However, decarboxylation
reaction of this compound led to formation of the isomer
impurity that affected the yield of the product [65]. In
order to prevent the formation of impurity, this problem is
overcome by the conversion of intermediate (6e) to its salt
[66]. Acid hydrolysis of the derivative (6e) can be per-
formed by dissolving in an inorganic acid in the presence
of alcohol at ambient temperature. The thermal decar-
boxylation of the salt (7e) in NMP to avoid the forma-
tion of the isomer impurity and significant percentage of
desmethyl midazolam (8e) prepared. Finally, methylation
at position 2 of the imidazole ring can be proceed with
butyllithium at − 78 °C, followed by addition of methyl
iodide in excellent yield.
2‑(2‑Bromoacetamido)‑5‑chloro‑2′‑fluorobenzophe
none (2a)
To a solution of 2-amino-5-chloro-2′-fluorobenzophenone
1a (0.15 g, 0.6 mmol) in dry CH2Cl2 (10 mL) at 0 °C was
added dropwise bromoacetyl bromide (0.12 mL, 2.3 mmol).
After stirring the mixture at 0 °C for 4 h, the reaction solu-
tion was quenched with ammonia solution 5% (30 mL) and
extracted with CH2Cl2 (2×25 mL). The combined organic
layers were dried over MgSO4, concentrated in vacuum, and
the white product was washed with diethyl ether and cold
methanol for further purification. Yield: 0.2 g (0.54 mmol,
90%); pale white solid. Mp 131–133 °C. IR (νmax, KBr):
1648, 1690 cm−1 2(C=O), 3014 cm−1 (–CH2), 3387 cm−1
1
(N–H). HNMR (250 MHz, CDCl3, δppm): 4.24 (s, 2H),
7.17–7.35 (m, 2H), 7.49–7.64 (m, 4H), 8.72 (d, 1H, J=7.5),
11.96 (s, 1H). 13CNMR (62.5 MHz, CDCl3, δppm): 43.2,
116.5, 116.8, 122.5, 124.6, 124.7, 124.9, 126.5, 126.7,
128.6, 130.5, 130.6, 133.1, 133.1, 133.9, 134.1, 135.0,
138.2, 157.6, 161.6, 165.7, 195.5.
2‑(2‑Bromoacetamido)‑5‑chlorobenzophenone (2b)
This product was prepared from 1b according to the gen-
eral procedure described above. Yield: 0.324 g (0.92 mmol,
92%); pale yellow solid. Mp 100–102 °C. IR: (νmax, KBr):
1629, 1680 cm−1 2(C=O), 3018 cm−1 (–CH2), 3221 cm−1
1
(N-H). HNMR (250 MHz, CDCl3, δppm): 4.03 (s, 2H),
7.50–7.56 (m, 4H), 7.63–7.69 (m, 1H), 7.73–7.76 (m,
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Journal of the Iranian Chemical Society
2H) 8.56–8.60 (d, 1H, J = 10), 11.33 (s, 1H). 13CNMR
(62.5 MHz, CDCl3, δppm): 29.5, 123.1, 125.5, 128.4, 128.7,
130.1, 132.7, 133.2, 133.9, 137.6, 137.9, 165.1, 197.9.
70%); white solid. Mp 204–206 °C. IR (νmax, KBr): 1614 cm−1
(C=N), 1688 cm−1 (C=O), 2967 cm−1 (–CH2), 3184 cm−1
1
(N–H). HNMR (250 MHz, CDCl3, δ ppm): 4.40 (s, 2H),
7.06–7.30 (m, 4H), 7.44–750 (m, 2H), 7.57–7.63 (m, 1H), 9.68
(s, 1H). 13CNMR (62.5 MHz, CDCl3, δ ppm): 56.7, 116.2,
116.5, 122.9, 124.4, 124.5, 127.1, 127.3, 129.2, 129.3, 129.4,
131.5, 131.5, 132.1, 132.3, 132.4, 136.5, 158.4, 162.5, 166.8,
171.6. Anal. Calcd for C15H10ClFN2O: C, 62.40; H, 3.49; N,
9.70. Found: C, 61.52; H, 3.28; N, 9.90.
2‑(2‑Chloroacetamido)‑5‑nitro‑2′‑fluorobenzophen
one (2c)
To a solution of 2-amino-5-nitro-2′-fluorobenzophenone
1c (0.52 g, 2 mmol) in dry CH2Cl2 (20 mL) at 0 °C was
added dropwise chloroacetyl chloride (0.63 mL, 8 mmol).
After stirring the mixture at 0 °C for 2 h, the reaction mix-
ture was warmed to room temperature and stirred for 10 h.
After this time, the reaction solution was quenched with
ammonia solution 5% (30 mL) and extracted with CH2Cl2
(2×25 mL). The combined organic layers were dried over
MgSO4, concentrated in vacuum, and the white product was
washed with cold methanol for further purification. Yield:
0.639 g (1.9 mmol, 95%); white solid. Mp 160–162 °C. IR
(νmax, KBr): 768 cm− 1 (C-Cl) 1345, 1510 cm− 1 (–NO2),
7‑Chloro‑5‑phenyl‑1,3‑dihydro‑2H‑benzo[e][1,4]
diazepin‑2‑one (3b)
This product was prepared from 2b according to the gen-
eral procedure described above to afford after washing cool
toluene for further purification. Yield: 0.332 g (1.23 mmol,
82%); pale yellow solid. Mp 195–197 °C. IR (νmax, KBr):
702 cm− 1 (C-Cl), 1680 cm− 1 (C=O), 2958 cm− 1 (CH2),
1
3105 cm− 1 (N-H). HNMR (250 MHz, CDCl3, δ ppm):
1
1642, 1692 cm− 1 2(C=O), 3215 cm− 1 (N-H). HNMR
4.25 (s, 2H), 7.14–7.22 (m, 3H), 7.33–7.47 (m, 5H), 10.14
(s, 1H). 13CNMR (62.5 MHz, CDCl3, δ ppm): 56.6, 122.9,
128.5, 128.8, 129.7, 130.7, 130.8, 131.9, 137.5, 138.7,
170.1, 172.1.
(250 MHz, CDCl3, δppm): 4.08 (s, 2H), 7.19–7.38 (m, 2H),
7.55–7.66 (m, 2H), 8.43–8.46 (m, 2H), 8.93–8.97 (d, 1H),
12.16 (s, 1H). 13CNMR (62.5 MHz, CDCl3, δppm): 43.2,
116.6, 116.9, 121.2, 123.1, 125.0, 125.1, 125.8, 126.0,
127.5, 129.0, 129.1, 129.8, 130.6, 130.7, 134.2, 134.6,
134.8, 142.4, 144.6, 157.6, 161.6, 166.2, 195.2.
7‑Nitro‑5‑(2‑fluorophenyl)‑1,3‑dihydro‑2H‑benzo[e]
[1,4]diazepin‑2‑one (3c)
Amidobenzodiazepine: general procedure
A mixture of hexamine (0.631 g, 4.5 mmol) and ammonium
chloride (0.201 g, 3.75 mmol) in ethanol (50 ml) was heated
at 70 °C for 1 h. After this time, the 2-(2-chloroacetamido)-
5-nitro-2′-fluorobenzophenone 2c (0.505 g, 1.5 mmol) was
added slowly and the reaction mixture was refluxed for
4 h. The resultant yellow solution was evaporated under
vacuum. The solid residue was dissolved in water (40 ml)
and extracted with CH2Cl2 (3 × 20 mL). The combined
organic phases were dried over Na2SO4 and concentrated
to dryness under vacuum. The desired yellow product 3c
obtained in good yield. Yield: 0.391 g (1.3 mmol, 87%);
Mp: 200–203 °C. IR (νmax, KBr): 1342, 1534 cm−1 (NO2),
1698 cm− 1 (C=O), 2925, 2964 cm− 1 (-CH2-), 3096 cm− 1
A solution of α-(haloacetamido)-benzophenone (1.0 mmol)
in methanol (20 ml) was cooled to 0 °C with stirring. A
moderate current of in situ-formed ammonia gas is bub-
bled through the solution (500–600 ml. per minute), for 2 h
longer (over a 2-h period). Then, the reaction mixture was
heated under reflux overnight and cooled to room tempera-
ture. The solvent was removed under vacuum to yield com-
bined organic layers as a yellow-white solid. This combined
organic layer was washed with cool toluene for further puri-
fication (yields 70–85%).
1
7‑Chloro‑5‑(2‑fluorophenyl)‑1,3‑dihy‑
dro‑2H‑benzo[e][1,4]diazepin‑2‑one (3a)
(N-H). HNMR (250 MHz, CDCl3, δ ppm): 4.39 (s, 2H),
6.97–7.04 (t, 1H), 7.20–7.30 (m, 2H), 7.43–7.46 (q, 1H),
7.57–7.63 (t, 1H), 8.07 (s, 1H) 8.24–8.27 (d, 1H), 10.33
(s, 1H). 13CNMR (62.5 MHz, CDCl3, δ ppm): 56.7, 116.3,
116.6, 122.2, 124.8, 124.9, 126.1, 126.4, 126.6, 128.2,
131.5, 133.0, 133.1, 142.5, 143.2, 158.4, 162.4, 166.6,
171.4.
A solution of 2-(2-bromoacetamido)-5-chloro-2′-
fluorobenzophenone 2a (0.370 g, 1 mmol) in methanol
(20 ml) was cooled to 0 °C with stirring. A moderate current
of in situ-formed ammonia gas is bubbled through the solution
(500–600 ml per minute), for 2 h longer (over a 2 h period),
then the reaction mixture was heated under reflux overnight
and cooled to room temperature. The solvent was evaporated
under vacuum to yield combined organic layers as a yellow-
white solid. This combined organic layer was washed with
cool toluene for further purification. Yield: 0.202 g (0.7 mmol,
Tosylimidazo[1,5‑a][1,4]benzodiazepines (5): gen‑
eral procedure
To a stirred solution of amidobenzodiazepine (1 mmol) in dry
THF (20 mL) at 0 °C under argon, t-BuOK (1.1 mmol) was
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Journal of the Iranian Chemical Society
added portionwise. After a further 20 min at 0 °C, the reaction
mixture was cooled to −20 °C with stirring. Diethyl chloro-
phosphate (1.4 mmol) was added dropwise over 5 min. After
stirring this mixture at 0 °C for 30 min, the reaction flask was
cooled to −20 °C and Tos-MIC (1.1 mmol) was added por-
tionwise, followed by the addition of t-BuOK (1.21 mmol).
The resulting solution was stirred at room temperature for 4 h.
Then, the reaction mixture was quenched with sat.aq NaHCO3
(30 mL) and extracted with CH2Cl2 (3×50 mL). The com-
bined organic phases were dried over Na2SO4 and concen-
trated in vacuum and the resulting solid residue was added
with Et2O (50 mL). The suspension was stirred at 25 °C for
10 min and the resultant precipitate was filtered, washed with
Et2O (50 mL). The desired product 5 obtained in good yields.
(Overall yield 70–85%).
with diethyl ether (50% hexanes–EtOAc). Yield: 0.632 g
(1.41 mmol, 80%); yellow-orange solid. Mp: 157–159 °C.
IR (νmax, KBr): 609 cm−1 (C-Cl), 1145, 1324 cm−1 (S=O),
1489, 1611 cm−1 (C=C arom). 1H NMR (250 MHz, CDCl3,
δ ppm): 2.31 (s, 3 H), 3.99 (d, J= 12.9 Hz, 1 H), 6.07 (d,
J=12.9 Hz, 1 H), 7.22 (d, J=7.5 Hz, 2 H), 7.29–7.45 (m,
7 H), 7.55–7.59 (m, 1 H), 7.83 (s, 1 H), 7.93 (d, J=8 Hz,
2 H). 13C NMR (62.5 MHz, CDCl3, δ ppm): 21.6, 44.3,
124.4, 128.0, 128.4, 129.4, 129.5, 129.8, 130.8, 131.9,
132.4, 133.6, 135.1, 136.7, 138.0, 138.9, 144.3, 168.2.
8‑Nitro‑6‑(2‑fluorophenyl)‑3‑(4‑toluenesulfonyl)‑4H
‑imidazo[1,5‑a][1,4]benzodiazepine (5c)
This product was prepared from 3c according to the
general procedure described above to afford after chro-
matography separation (50% hexanes–EtOAc). Yield:
0.345 g (0.72 mmol, 62%); pale yellow-orange solid. Mp:
225–228 °C. IR (νmax, KBr): 1154, 1321 cm−1 (S=O), 1354,
1531 cm−1 (NO2). 1H NMR (250 MHz, CDCl3, δ ppm): 2.39
(s, 3 H), 4.13 (br s, 1 H), 6.24 (br s, 1 H), 6.98–7.05 (t, 1 H),
7.23–7.34 (m, 3 H), 7.50–7.55 (dd, 1 H), 7.74–7.78 (d, 2 H),
8‑Chloro‑6‑(2‑fluorophenyl)‑3‑(4‑toluenesulfonyl)‑4
H‑imidazo[1,5‑a][1,4]benzodiazepine (5a)
t-BuOK (0.86 g, 7.7 mmol) was added portionwise to a solu-
tion of amidobenzodiazepine 3a (2.02 g, 7 mmol) in dry THF
(120 mL) at 0 °C under argon. After 20 min at 0 °C, the mix-
ture was cooled to −20 °C with stirring, then diethyl chlo-
rophosphate (1.41 ml, 9.8 mmol) was added dropwise over
5 min. After stirring this mixture at 0 °C for 30 min, the result-
ing yellow solution was cooled to −20 °C and Tos-MIC (1.5 g,
7.7 mmol) was added portionwise, followed by the addition
of t-BuOK (0.951 g, 8.47 mmol). The resulting red-brown
solution was stirred at room temperature for 4 h. Then, the
resultant light yellow reaction solution was quenched with sat.
aq NaHCO3 (150 mL) and extracted with CH2Cl2 (3×40 mL).
The combined organic phases were dried over Na2SO4 and
concentrated in vacuum and the resulting solid residue was
treated with Et2O (30 mL). The suspension was stirred at 25 °C
for 5 min and the resultant precipitate was filtered, washed
with Et2O (50 mL). The desired light yellow-white product 5a
obtained with an overall yield of 60%. Mp: 250–253 °C. IR
(νmax, KBr): 612 cm−1 (C–Cl), 1148, 1305 cm−1 (SO2), 1487,
1614 cm−1 (C=C arom) 1633 cm−1 (C=N), 2924 cm−1 (-CH2-
). 1H NMR (250 MHz, CDCl3, δ ppm): 2.37 (s, 3 H), 4.12 (br
s, 1 H), 6.13 (br s, 1 H), 6.98–7.05 (m, 1 H), 7.24–7.31 (m,
4 H), 7.48–7.52 (m, 2 H), 7.60–7.70 (m, 2 H), 7.93–8.02 (m,
3 H). 13C NMR (62.5 MHz, CDCl3): 21.6, 44.3, 116.0, 116.4,
124.5, 124.6, 127.2, 127.4, 127.9, 129.8, 130.2, 130.4, 131.4,
132.4, 132.6, 133.9, 135.3, 136.2, 137.2, 138.1, 144.3, 158.1,
162.1, 165.2. Anal. Calcd for C24H17ClFN3O2S: C, 61.87; H,
3.68; N, 9.02. Found: C, 61.67; H, 3.67; N, 9.20.
7.99–8.03 (d, 3 H), 8.22 (s, 1 H), 8.47–8.51 (d, 1 H), 13
C
NMR (62.5 MHz, CDCl3, δ ppm): 21.7, 44.3, 116.2, 116.5,
124.2, 124.9, 126.1, 126.5, 126.7, 126.9, 128.1, 129.9,
130.1, 131.5, 133.1, 133.2, 135.3, 136.1, 137.6, 138.2,
144.6, 146.3, 158.1, 162.1, 164.7.
8‑ Nitro‑6‑(2‑chlorophenyl)‑3‑(4‑toluenesulfonyl)‑4
H‑imidazo[1,5‑a][1,4]benzodiazepine (5d)
t-BuOK (0.246 g, 2.2 mmol) was added portionwise to
a stirred solution of clonazepam 3d (0.631 g, 2 mmol)
in dry THF (40 mL) at 0 °C under argon. After 20 min
at 0 °C, the mixture was cooled to − 20 °C with stirring,
then diethyl chlorophosphate (0.404 ml, 2.8 mmol) was
added dropwise over 5 min. After stirring this mixture at
0 °C for 30 min, the resulting yellow solution was cooled
to − 20 °C and Tos-MIC (0.429 g, 2.2 mmol) was added
portionwise, followed by the addition of t-BuOK (0.271 g,
2.42 mmol). The resulting red-brown solution was stirred
at room temperature for 4 h. Then, the resultant light yel-
low reaction solution was quenched with sat.aq NaHCO3
(150 mL) and extracted with CH2Cl2 (3 × 40 mL). The
combined organic phases concentrated in vacuum and the
resulting dark brown solid residue was dissolved in Et2O
(30 mL) and the resultant solution was filtered from vis-
cose residue. The organic phases were dried over Na2SO4
and concentrated in vacuum and the resulting solid residue
was chromatographed to afford the desired orange prod-
uct 5d with an overall yield of 85%. Mp: 145–147 °C. IR
(νmax, KBr): 609 cm− 1 (C–Cl), 1145, 1330 cm− 1 (SO2),
8‑Chloro‑6‑phenyl‑3‑(4‑toluenesulfonyl)‑4H‑imidaz
o[1,5‑a][1,4]benzodiazepine (5b)
This product was prepared from 3b according to the gen-
1
eral procedure described above to afford after washing
1348, 1486 cm− 1 (NO2), 2922 cm− 1 (CH3). H NMR
1 3

Journal of the Iranian Chemical Society
(250 MHz, CDCl3, δ ppm): 2.41 (s, 3 H), 4.17 (br s, 1 H),
6.24 (br s, 1 H), 7.27–7.35 (m, 3 H), 7.45–7.49 (m, 2 H),
7.67–7.70 (m, 1 H), 7.73–7.77 (d, 1 H), 7.99–8.07 (m,
4 H), 8.46–8.51 (dd, 1 H), 13C NMR (62.5 MHz, CDCl3, δ
ppm): 21.7, 44.3, 124.0, 125.8, 126.9, 127.6, 128.1, 129.9,
130.0, 130.3, 131.3, 131.8, 132.3, 135.2, 136.1, 137.7,
138.9, 144.6, 146.3, 167.4.
Ethyl 8‑Chloro‑6‑(2′‑fluorophenyl)‑4H‑imidazo[1,
5‑a][1,4]benzodiazepine‑3‑carboxylate (5e)
t-BuOK (0.427 g, 3.81 mmol) was added portionwise to a
stirred solution of amidobenzodiazepine 3a (1 g, 3.46 mmol)
in dry THF (80 mL) at 0 °C under argon. After 30 min at
0 °C, the mixture was cooled to −20 °C with stirring, then
diethyl chlorophosphate (0.7 ml, 4.85 mmol) was added
dropwise over 5 min. After stirring this mixture at 0 °C for
30 min, the resulting yellow solution was cooled to −20 °C
and ethyl isocyanoacetate (0.42 ml, 3.81 mmol) was added
dropwise, followed by the addition of t-BuOK (0.466 g,
4.16 mmol). The reaction mixture was stirred at −20 °C
for 1 h. Then, the solution was stirred at room temperature
for 4 h. The resultant light yellow reaction solution was
quenched with sat.aq NaHCO3 (150 mL) and extracted with
EtOAc (3 × 30 mL). The combined organic phases were
dried over Na2SO4 and concentrated in vacuum and the
resulting solid residue was treated with Et2O (30 mL). The
suspension was stirred at 25 °C for 5 min and the result-
ant precipitate was filtered, washed with Et2O (50 mL) to
give most of the desired tricyclic systems 3e. The mother
liquor was further purified by chromatography on silica gel
(40:60 EtOAc:petroleum ether) to afford additional prod-
uct. The desired light yellow-white (pale yellow) product
8‑Chloro‑6‑(2‑fluorophenyl)‑1‑methyl‑3‑(4‑toluenes
ulfonyl)‑4H‑imidazo[1,5‑a][1,4]benzodiazepine (6a)
To a solution of 5a (0.341 g, 0.729 mmol) in dry THF
(20 ml) at −78 °C, n-Butyllithium (0.45 mL, solution 1.7 M
in hexanes, 0.765 mmol) was added dropwise under argon
gas. After stirring the mixture at −78 °C for 15 min, the
methyl iodide (0.2 ml, 2.92 mmol) was added dropwise. The
resulting black solution was stirred vigorously for 4 h at
− 78 °C. After this time, the resultant green mixture was
then allowed to warm to room temperature. The solution was
then stirred at 25 °C overnight. After this time, the result-
ant light brown reaction solution was quenched with sat. aq
NaHCO3 (50 mL) and extracted with CH2Cl2 (3×50 mL).
The combined organic phases were dried over Na2SO4 and
concentrated in vacuum. The resulting solid residue was
purified by chromatography on silica gel (20:80 EtOAc:
Petroleum ether). The desired white product 6a obtained
with an overall yield of 90% (0.314 g). Mp: 269–271 °C. IR
(νmax, KBr): 1610 cm−1 (S=O). 1HNMR (250 MHz, CDCl3,
δ ppm): 2.37 (s, 3H), 2.50 (s, 3H), 3.97 (d, 1H, J=13), 6.10
(d, 1H, J = 13), 7.01 (m, 1H), 7.27 (m, 4H), 7.37 (d, 1H,
J=7.5), 7.46 (m, 1H), 7.58 (d, 1H. J=7.5), 7.72 (m, 1H),
8.00 (d, 2H, J=7.5).13CNMR (62.5 MHz, CDCl3, δ ppm):
14.1, 21.6, 44.7, 116.0, 116.4, 124.6, 126.0, 128.0, 129.5,
129.7, 131.3, 131.3, 131.3, 132.2, 132.3, 132.5, 132.7,
134.0, 135.2, 137.4, 138.4, 144.0, 145.1, 158.3, 162.3,
165.0.
5e obtained with an overall yield of 60% (0.79 g). IR (νmax
,
KBr): 2928, 2960 cm−1 (-C2H5) 1563, 1613 cm−1 2(C=N),
1722 cm−1 (C=O).1HNMR (250 MHz, CDCl3, δ ppm): 1.34
(t, 3H, J = 7.5), 4.03 (br s, 1H), 4.33 (q, 2H, J = 7.5), 6.05
(br s,1H), 6.94 (m, 1H), 7.14–7.25 (m, 2H), 7.34–7.47 (m,
1H), 7.50–7.59 (m, 3H), 7.89 (s, 1H). 13CNMR (62.5 MHz,
CDCl3, δ ppm): 14.5, 45.0, 60.8, 116.1, 116.4, 124.0, 124.6,
124.6, 128.8, 129.5, 130.3, 131.3, 132.2, 132.4, 132.5, 132.9,
133.6, 134.4, 138.3, 158.2, 162.2, 162.9, 165.1. Anal. Calcd
for C20H15ClFN3O2: C, 62.59; H, 3.94; N, 10.95. Found: C,
61.40; H, 3.70; N, 10.99.
8‑Chloro − 6‑(2′‑fluorophenyl)‑4H‑imidazo[1,5‑a]
[1,4]benzodiazepine‑3‑carboxylic acid (6e)
Midazolam (7a): 8‑chloro‑6‑(2‑fluorophenyl)‑1‑me‑
thyl‑4H‑imidazo[1,5‑a][1,4]benzodiazepine
The ester 5e (1.0 g, 2.6 mmol) from the previous step was
dissolved in EtOH (80 mL) and 2 N aq NaOH (8 mL) was
added dropwise to the solution. The reaction mixture was
stirred at room temperature for 4 h. The solvent evaporated
and dried under reduced pressure. This solid residue was
cooled to 0 °C. The mixture was adjusted at the same tem-
perature to pH 4 by the dropwise addition of a solution of
1 N HCl. The resultant precipitate was filtered under vacuum
and dried at room temperature. The precipitate was crushed
and then suspended in diethyl ether (100 ml). After stirring
at 0 °C for 1 h, the mixture was filtered under vacuum. The
solid was dried to afford 6e as a yellow solid with an overall
yield of 98% (0.906 g). IR (νmax, KBr): 1611 cm−1 2(C=O),
This product was prepared from 6a according to the pro-
cedure described in the literature [9]. Yield: 0.117 g
(0.36 mmol, 72%); yellow solid. Mp 158–160 °C. 1HNMR
(250 MHz, DMSO, δ ppm): 2.85 (s, 3H), 4.22 (d, 1H, J=13),
5.27 (d, 1H, J=13), 7.29–7.41 (m, 3H), 7.58–7.76 (m, 3H),
7.94 (m, 1H), 8.10 (m, 1H) .13CNMR (62.5 MHz,CDCl3, δ
ppm): 13.1, 44.8, 116.1, 116.5, 116.8, 125.0, 127.1, 127.2,
127.8, 129.8, 131.4, 131.9, 132.2, 132.6, 133.1, 133.3,
134.0, 135.4, 145.3, 158.2, 162.2, 164.0. Anal. Calcd for
C18H13ClFN3: C, 66.36; H, 4.02; N, 12.90. Found: C, 66.32;
H, 4.04; N, 13.01.
1 3

Journal of the Iranian Chemical Society
1
2400–3600 cm− 1 (O-H). HNMR (250 MHz, DMSO, δ
the resultant light brown reaction solution was quenched
with sat. aq NaHCO3 (50 mL) and extracted with CH2Cl2
(3×50 mL). The combined organic phases were dried over
Na2SO4 and concentrated in vacuo. The resulting solid resi-
due was purified by chromatography on silica gel (20:80
EtOAc:petroleum ether). The desired white product 7a
ppm): 4.12 (br s, 1H), 6.09 (br s, 1H), 7.05 (m, 1H), 7.30 (m,
2H), 7.50 (m, 1H), 7.66 (m, 2H), 8.08 (m, 1H), 8.98 (m, 1H).
13CNMR (62.5 MHz, CDCl3, δ ppm): 45.0, 116.4, 125.2,
125.9, 127.9, 129.3, 129.8, 130.2, 131.9, 132.3, 132.6,
133.0, 133.5, 136.1, 138.2, 158.0, 160.1, 162.2, 164.5.
1
obtained with an overall yield of 90% (0.938 g). HNMR
5‑(Aminomethyl)‑1‑{4‑chloro‑2‑[(2‑fluorophenyl)
carbonyl]phenyl}‑1H‑imidazole‑4‑carboxylic acid
dihydrochloride (7e)
(250 MHz, DMSO, δ ppm): 2.85 (s, 3H), 4.22 (d, 1H,
J=13), 5.27 (d, 1H, J=13), 7.29–7.41 (m, 3H), 7.58–7.76
(m, 3H), 7.94 (m, 1H), 8.10 (m, 1H) .13CNMR (62.5 MHz,
CDCl3, δ ppm): 13.1, 44.8, 116.1, 116.5, 116.8, 125.0,
127.1, 127.2, 127.8, 129.8, 131.4, 131.9, 132.2, 132.6,
133.1, 133.3, 134.0, 135.4, 145.3, 158.2, 162.2, 164.0.
The tricyclic acid 6e (1 g, 2.81 mmol) from the previous step
was dissolved in EtOH (80 mL) at room temperature and
aqueous solution of 2 M HCl (8 mL) was added dropwise to
the solution. The ring closure dihydrochloride intermediate
precipitated from the reaction mixture after addition of the
acid solution. The reaction mixture was stirred for 4 h. The
resultant salt was filtered under vacuum, washed with cold
ethanol (3 ×20 mL). The solid was dried to afford 7e as a
light yellow product with an overall yield of 83% (1.04 g).
Conclusion
We have presented two different and improved multistep
methods for the synthesis of midazolam and its analogous.
These synthetic methods included the formation of an imi-
dazole ring fused in the 1,2-position benzodiazepines which
had been developed from a one-pot annulation process. This
procedure permitted the carbon–carbon bond forming reac-
tion and condensation of the less stable iminophosphates
with mono-anion of tosylmethyl isocyanide or ethyl isocy-
anoacetate under basic (moderate) conditions. In the first
method (route I), tosylmethyl isocyanide (Tos-MIC) was
used and the number of synthetic steps are decreased to 5
step in comparison to previous report and the overall yield
of this method was 29%. In the second method (route II),
the synthesis of midazolam with ethyl isocyanoacetate was
accomplished in 6 step with 82% yield.
8‑Chloro‑6‑(2‑fluorophenyl)‑4H‑imidazo[1,5‑a][1,4]
benzodiazepine (8e)
The dihydrochloride salt 7e (1.0 g, 2.23 mmol) from the
previous was dissolved in NMP (40 mL) at room temper-
ature. The reaction mixture was reflux at 200 °C for 2 h.
Then, the reaction mixture was cooled to room tempera-
ture with stirring. The resulting solution was adjusted to
PH=11 by the dropwise addition of 10% Na2CO3. The solu-
tion was extracted with EtOAc (3 × 25 mL), washed with
water (2×40 ml). The combined organic phases were dried
over Na2SO4, evaporated in vacuum and the resulting crude
mixture was chromatographed to afford 8e as a yellow prod-
uct with an overall yield of 70% (0.488 g). IR (νmax, KBr):
758 cm−1 (C-Cl), 3080 cm−1 (CH arom), 1487, 1606 cm−1
(C=C arom), 1HNMR (250 MHz, CDCl3, δ ppm): 4.21 (br s,
1H), 5.22 (br s,1H), 6.98–7.07 (m, 2H), 7.23–7.34 (m, 2H),
7.43 (m, 1H), 7.55–7.62 (m, 3H), 7.97 (s, 1H) .13CNMR
(62.5 MHz, CDCl3, δ ppm): 45.5, 116.1, 116.4, 123.8, 124.5,
126.2, 127.8, 127.9, 129.9, 130.3, 131.1, 132.0, 132.1,
132.3, 132.5, 133.2, 133.7, 134.8, 158.2, 162.2, 164.4.
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Affiliations
Mohammad Javad Taghizadeh1 · Gholam reza malakpouri1 · Abdollah Javidan1,2
2
* Mohammad Javad Taghizadeh
Eyvanekey University, Eyvanekey, Islamic Republic of Iran
mohammadjavadtaghizadeh31@yahoo.com
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Department of Chemistry, Faculty of Science, University
of Imam Hossein, Tehran, Islamic Republic of Iran
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