Synthesis of the β2-Agonist Tulobuterol and Its Metabolite 4-Hydroxytulobuterol

Article abstract of DOI:10.1134/S1070428020030045

Abstract: Alternative methods have been developed for the synthesis of theβ₂-agonist tulobuterol and its metabolite4-hydroxytulobuterol with a similar activity. The proposed procedures utilizeavailable reagents, and the key stage in the synthesis is the formation ofintermediate oxirane according to the Corey–Chaykovsky reaction, followed byopening of the oxirane ring by the action of excess tert-butylamine. In the synthesis of 4-hydroxytulobuterol, thehydroxy group was protected by benzylation, and the protecting group was removedin the final stage by hydrogenation over carbon-supported palladium.

Full text of DOI:10.1134/S1070428020030045

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 3, pp. 390–394. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Organicheskoi Khimii, 2020, Vol. 56, No. 3, pp. 379–383.
Synthesis of the β2-Agonist Tulobuterol
and Its Metabolite 4-Hydroxytulobuterol
M. A. Glushkovaa,*, S. V. Popkova, and M. L. Burdeinyia
a Mendeleev University of Chemical Technology of Russia, Moscow, 125047 Russia
*e-mail: maria-n1002@mail.ru
Received September 13, 2019; revised January 18, 2020; accepted January 20, 2020
Abstract—Alternative methods have been developed for the synthesis of the β2-agonist tulobuterol and its
metabolite 4-hydroxytulobuterol with a similar activity. The proposed procedures utilize available reagents, and
the key stage in the synthesis is the formation of intermediate oxirane according to the Corey–Chaykovsky
reaction, followed by opening of the oxirane ring by the action of excess tert-butylamine. In the synthesis of
4-hydroxytulobuterol, the hydroxy group was protected by benzylation, and the protecting group was removed
in the final stage by hydrogenation over carbon-supported palladium.
Keywords: β2-agonists, tulobuterol, metabolite, Corey–Chaykovsky reaction, benzyl protection
DOI: 10.1134/S1070428020030045
Tulobuterol (1) is a long-acting selective β2-adren-
ergic receptor agonist used in the treatment of chronic
obstructive pulmonary disease in humans and animals
[1–3]. β2-Agonists also contribute to acceleration of
growth and reduction of depot fat and are therefore
used as food additives for cattle, pigs, sheep, and fowl
to increase lean meat production [4]. However, residual
amounts of β2-agonists present in agricultural products
pose serious risks to consumer health. The use of
β2-agonists as growth stimulants is prohibited in the
Russian Federation. In this connection, there is a need
of developing analytical procedures for the determina-
tion of residual amounts of such compounds in im-
ported meat products. Study of biotransformations of
β2-agonists is also important from the viewpoint of
revealing the instances of their prohibited use and
estimating their effect on human and animal health,
since their metabolites often exhibit similar biological
activity and can be identified in biological media over
a longer time than the parent compounds.
relatively recently introduced into the pharmaceutical
market under the trade name meluadrine tartrate
(β2-agonist) [8].
Reference samples are required for the determina-
tion of β2-agonists and their metabolites in biological
media. The goal of the present work was to develop
alternative procedures for the synthesis of compounds
1 and 2 using available reagents.
Two methods of synthesis of tulobuterol (1) have
been reported [9]. According to the first of these,
2-chloroacetophenone is oxidized with selenium oxide
to the corresponding aldehyde which then reacts with
tert-butylamine, and the resulting Schiff base is
reduced with sodium tetrahydridoborate to obtain the
target product. The second method involves bromina-
tion of 2-chloroacetophenone to ω-bromo-2-chloro-
acetophenone which is reduced with sodium tetrahy-
dridoborate to bromohydrin, and the latter is converted
to 2-(2-chlorophenyl)oxirane by treatment with alkali.
The final stage is opening of the oxirane ring with
tert-butylamine.
4-Hydroxytulobuterol (2) is one of the main metab-
olites of tulobuterol 1 [5, 6]. Study of the mechanism
of action of 1 showed [2] that 4-hydroxytulobuterol
was more potent in relaxing guinea pig tracheae than
tulobuterol and was more active. After oral administra-
tion of 1 at a dose of 1 mg, compounds 1 and 2 can
be identified in human urine over a period of longer
than 32 h [7]. The (R)-enantiomer of 2 has been
We focused on the second method, but intermediate
oxirane 4 was synthesized by the Corey–Chaykovsky
reaction of 2-chlorobenzaldehyde (3) with dimethyl-
sulfonium methylide generated in situ from trimethyl-
sulfonium iodide by the action of potassium tert-but-
oxide by analogy with [10, 11] (Scheme 1). The yield
of 4 isolated by vacuum distillation was 26%. Com-
390

SYNTHESIS OF THE β2-AGONIST TULOBUTEROL
391
Scheme 1.
OH
H
N
CHO
Cl
O
Me3SI, t-BuOK, DMSO
t-BuNH2, H2O, Δ
Bu-t
Cl
Cl
3
4
1
OH
H2
N
HCl, i-PrOH
Bu-t
Cl–
Cl
5
pound 1 was obtained in 59% yield by ring-opening
reaction of oxirane 4 with excess tert-butylamine and
was converted to hydrochloride 5 (yield 91%).
the yield of benzyloxy derivative 7 was 53%. The next
stage was Corey–Chaykovsky reaction of aldehyde 7
with dimethylsulfonium methylide generated in situ
from trimethylsulfonium iodide and potassium tert-
butoxide [10, 11] to afford 93% of 2-(4-benzyloxy-2-
chlorophenyl)oxirane (8). Opening of the oxirane ring
in 8 with excess tert-butylamine in water gave
1-(4-benzyloxy-2-chlorophenyl)-2-(tert-butylamino)-
ethanol (9). The benzyl protecting group was removed
from 9 under milder conditions than those described in
[9], by hydrogenation over Pd/C. The yield of 2 was
35%. When the hydrogenolysis of 9 was carried out at
a hydrogen pressure of 5.1 atm, the reaction was
accompanied by hydrodechlorination with the forma-
tion of about 30% of 2-tert-butylamino-1-(4-hydroxy-
phenyl)ethanol in addition to the target product. Initial
2-chloro-4-hydroxybenzaldehyde (6) was prepared by
the Reimer–Tiemann condensation of 3-chlorophenol
with chloroform according to [13].
Three alternative methods for the synthesis of
4-hydroxytulobuterol (2) are known [9]. The first
procedure is similar to the first method of synthesis of
compound 1, but it utilizes 4-hydroxy-2-chloroaceto-
phenone as starting material. The second method
involves the same transformations starting from 4-ben-
zyloxy-2-chloroacetophenone, and the benzyl protec-
tion is removed in the final stage by treatment with
aqueous HBr. The starting compound in the third
method is 2-chloro-4-nitroacetophenone which is
subjected to the same transformations as in the first and
second methods to obtain 4-nitro analog of 2. Its
reduction with hydrogen generated in situ by reaction
of zinc with aqueous HCl gives 4-amino derivative
which is converted to the target compound 2 via
diazotization and decomposition of the diazonium salt.
The structure of the isolated compounds was con-
We have developed an alternative four-step syn-
thesis of metabolite 2 (Scheme 2). In the first stage, the
hydroxy group in 2-chloro-4-hydroxybenzaldehyde (6)
was protected by benzylation with benzyl chloride in
the presence of potassium carbonate according to [12];
1
firmed by H NMR spectra. Compounds 5 and 2 were
additionally characterized by 13C NMR spectra, low-
and high-resolution mass spectra (HPLC/MS), and
GC/MS data.
Scheme 2.
PhCH2Cl, K2CO3
BTEAC, MeCN
CHO
Cl
CHO
O
Me3SI, t-BuOK, DMSO
HO
BzlO
OH
Cl
BzlO
Cl
6
7
8
OH
H
N
H
N
t-BuNH2, H2O, Δ
H2, Pd/C, EtOH
Bu-t
Bu-t
BzlO
Cl
HO
Cl
9
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 3 2020

GLUSHKOVA et al.
392
EXPERIMENTAL
3.20 d.d (1H, CH2, J = 5.5, 4.0 Hz), 4.22 m (1H, CH),
7.18–7.28 m (3H, Harom), 7.36 d.d. (1H, Harom, J =
5.1, 2.9 Hz).
Commercially available reagents were used without
further purification. The progress of reactions was
monitored by TLC. The 1H and 13C NMR spectra were
recorded on a Bruker AM-300 spectrometer
(300.13 MHz) from solutions in DMSO-d6 or CDCl3
using tetramethylsilane as internal standard.
2-(tert-Butylamino)-1-(2-chlorophenyl)ethanol
(1). tert-Butylamine, 1.3 g (17.5 mmol, 1.9 mL), was
added to a mixture of 2.1 g (13.5 mmol) of compound
4 and 5.5 mL of water, and the mixture was stirred at
room temperature for 48 h. The mixture was diluted
with 5.5 mL of water and extracted with diethyl ether
(2×20 mL), the combined extracts were dried over
magnesium sulfate and evaporated under reduced
pressure (water-jet pump), and the residue was purified
by column chromatography using chloroform as eluent.
Yield 1.8 g (59%), white crystals, mp 89–91°C (89–
The low-resolution mass spectra (atmospheric
pressure chemical ionization, positive and negative
ion detection) were measured on a Surveyor MSQ
HPLC/MS instrument (C18 column, 250×4.6 mm,
particle size 5 μm; eluents 0.1% formic acid in water
and 0.1% formic acid in acetonitrile, gradient elution,
flow rate 1.5 mL/min; column temperature 25°C). The
high-resolution mass spectra (atmospheric pressure
electrospray ionization, positive and negative ion
detection) were obtained using an Ultimate 3000RS
chromatograph coupled with a QExactive mass spec-
trometric detector (C18 column, 150× 2.1 mm, particle
size 3 μm; gradient elution with 0.1% formic acid
in acetonitrile–water (95:5 to 5:95), flow rate 0.5 mL×
min–1; column temperature 35°C). The GC/MS data
were obtained with an Agilent 7890A chromatograph
coupled with an Agilent 5975C mass-selective detector
(electron impact ionization; HP-5ms quartz capillary
column, 30 m×0.25 mm, film thickness 0.25 μm;
carrier gas helium, flow rate 1 mL/min; oven tem-
perature programming from 40 to 270°C at a rate of
10 deg/min). Compound 2 was preliminarily converted
into bis-O-trimethylsilyl derivative by treatment with
N-methyl-N-(trimethylsilyl)trifluoroacetamide
(MSTFA) as derivatizing agent.
1
90°C [9]), Rf 0.10 (CHCl3). H NMR spectrum
(CDCl3), δ, ppm: 1.10 s (9H, t-Bu), 2.47 d.d (1H, CH2,
J = 12.5, 8.8 Hz), 3.13 br.s (2H, NH, OH), 3.05 d.d
(1H, CH2, J = 12.1, 3.3 Hz), 5.01 d.d (1H, CH, J = 8.4,
3.3 Hz), 7.25 d.d (1H, CHarom, J = 7.3, 1.5 Hz), 7.28–
7.40 m (2H, Harom), 7.64 d (1H, Harom, J = 8.8 Hz).
2-(tert-Butylamino)-1-(2-chlorophenyl)ethanol
hydrochloride (5). Compound 1, 1.8 g (7.9 mmol),
was dissolved in 30 mL of diethyl ether, 0.9 mL
(7.9 mmol) of a 8.5 M solution of HCl in propan-2-ol
was added, and the precipitate was filtered off, washed
with diethyl ether, and dried over phosphoric anhy-
dride. Yield 1.9 g (91%), white crystals, mp 165–
1
167°C. H NMR spectrum (DMSO-d6), δ, ppm: 1.31 s
(9H, t-Bu), 2.70 d (1H, CH2, J = 12.3 Hz), 3.07 d (1H,
CH2, J = 12.4 Hz), 5.31 d (1H, CH, J = 10.1 Hz), 6.37 s
(1H, OH), 7.37 d.d (1H, Harom, J = 7.3, 1.5 Hz), 7.41–
7.49 m (2H, Harom), 7.69 d (1H, Harom, J = 8.8 Hz),
8.64 br.s (1H, NH), 9.20 br.s (1H, NH). 13C NMR spec-
trum (DMSO-d6), δC, ppm: 25.4 [(CH3)3C], 47.1
[C(CH3)3], 57.0 (CH2), 66.3 (CHOH), 128.0 (C4′, C5′),
128.5 (C3′), 129.7 (C6′), 131.4 (C2′), 139.5 (C1′). Low-
resolution mass spectrum: m/z: 228.23 [M + H]+. High-
resolution mass spectrum: m/z 228.11490 [M + H]+.
Mass spectrum (EI), m/z: 141 [M – CH2=NHC(CH3)3]+,
86 [CH2=NHC(CH3)3]+.
2-(2-Chlorophenyl)oxirane (4). A solution of 7.0 g
(62.2 mmol) of potassium tert-butoxide in 30 mL of
DMSO was added dropwise to a cold mixture of 7.0 g
(49.8 mmol, 5.6 mL) of 2-chlorobenzaldehyde (3) and
14.2 g (69.7 mmol) of trimethylsulfonium iodide in
30 mL of DMSO so that the temperature did not exceed
5°C. The mixture was stirred at room temperature for
30 min and cooled, and 150 mL of water was added
dropwise, maintaining the temperature below 5°C. The
mixture was extracted with diethyl ether (3×30 mL),
the combined extracts were washed with water and
dried over magnesium sulfate, the solvent was evap-
orated under reduced pressure (water-jet pump), and
the residue was distilled under reduced pressure to
collect a fraction boiling at 98–102°C (20 mm Hg).
4-Benzyloxy-2-chlorobenzaldehyde (7). Benzyl
chloride, 1.2 g (18.8 mmol, 1.1 mL), was added drop-
wise with stirring to a mixture of 2.0 g (12.8 mmol)
of 2-chloro-4-hydroxybenzaldehyde (6), 3.4 g
(24.6 mmol) of potassium carbonate, and 2.0 g
(5.4 mmol) of benzyl(triethyl)ammonium chloride in
80 mL of anhydrous acetonitrile. The mixture was left
overnight and filtered, the filtrate was evaporated under
reduced pressure (water-jet pump), and the residue was
extracted with ethyl acetate to isolate 1.8 g of crude
1
Yield 2.0 g (26%), yellowish oil. H NMR spectrum
(CDCl3), δ, ppm: 2.67 d.d (1H, CH2, J = 5.5, 2.6 Hz),
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 3 2020

SYNTHESIS OF THE β2-AGONIST TULOBUTEROL
393
product 7 which was purified by column chromatog-
raphy using chloroform–methanol (10:1) as eluent.
Yield 1.7 g (53%), white crystals, mp 71–72°C (68–
69°C [14]), Rf 0.70 (CHCl3–MeOH, 10:1). H NMR
spectrum (DMSO-d6), δ, ppm: 5.28 s (2H, CH2O),
a hydrogen pressure of 5 atm. The mixture was filtered
through a layer of celite, the filtrate was evaporated
under reduced pressure (water-jet pump), and the
residue (0.4 g) was purified by column chromatography
using chloroform–methanol–25% aqueous ammonia
(70:10:1) as eluent. Yield 0.15 g (35%), light yellow
crystals, mp 79–80°C, Rf 0.33 (CHCl3–MeOH–25%
1
7.18 d.d (1H, Harom, J = 8.8, 2.2 Hz), 7.30 d (1H, Harom
,
J = 2.2 Hz), 7.33–7.57 m (5H, Harom), 7.86 d (1H,
Harom, J = 8.8 Hz), 10.21 s (1H, CHO).
1
aq. NH3, 70:10:1). H NMR spectrum (DMSO-d6), δ,
ppm: 1.03 s (9H, t-Bu), 2.44 d.d (1H, CH2N, J = 11.7,
8.8 Hz), 2.61 d.d (1H, CH2N, J = 11.7, 2.9 Hz),
3.35 br.s (2H, NH, OH), 4.78 d.d (1H, CHO, J = 8.8,
2-[4-(Benzyloxy)-2-chlorophenyl]oxirane (8).
A solution of 1.0 g (8.4 mmol) of potassium tert-
butoxide in 5 mL of DMSO was added to a mixture of
1.7 g (6.7 mmol) of compound 7 and 1.9 g (9.4 mmol)
of trimethylsulfonium iodide in 5 mL of DMSO so that
the temperature did not exceed 5°C. The mixture was
stirred at room temperature for 30 min, and 20 mL of
water was added dropwise, maintaining the temperature
not higher than 5°C. The mixture was extracted with
diethyl ether (3×5 mL), the combined extracts were
washed with water and brine, dried over magnesium
sulfate, and evaporated under reduced pressure (water-
2.9 Hz), 6.70–6.80 m (2H, Harom), 7.37 d (1H, Harom
,
J = 9.5 Hz). 13C NMR spectrum (DMSO-d6), δC, ppm:
29.4 [(CH3)3C], 49.7 [C(CH3)3], 50.1 (CH2), 69.6
(CHOH), 114.9 (C5′), 115.7 (C6′), 128.9 (C3′), 131.4
(C2′), 132.4 (C1′), 157.4 (C4′). Low-resolution mass
spectrum (APCI): m/z 243.85 [M + H]+. High-
resolution mass spectrum (ESI), m/z: 244.10941
[M + H]+, 242.09538 [M – H]+. Mass spectrum (EI)
of bis-O-trimethylsilyl derivative, m/z: 301 [M –
CH2=NHC(CH3)3]+, 86 [CH2=NHC(CH3)3]+.
1
jet pump). Yield 1.5 g (93%), mp 39–40°C. H NMR
spectrum (DMSO-d6), δ, ppm: 2.74 d.d (1H, CH2O,
J = 5.1, 2.9 Hz), 3.11 t (1H, CH2O, J = 5.1 Hz), 4.05 t
(1H, CHO, J = 2.9 Hz), 5.11 s (2H, CH2O), 6.97 d.d
(1H, Harom, J = 8.8, 2.2 Hz), 7.10 d (1H, Harom, J =
8.8 Hz), 7.13 d (1H, Harom, J = 2.2 Hz), 7.27–7.45 m
(5H, Harom).
CONFLICT OF INTEREST
The authors declare the absence of conflict of interest.
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