A Short Synthesis of the 2-Bromo-N,9-dimethyl-6,7,8,9-tetrahydro-5 H-pyrido[2,3-b]indol-6-amine Building Block

Article abstract of DOI:10.1021/acs.oprd.9b00222

A concise synthesis of pharmaceutically useful (R)-tert-butyl N-(2-bromo-9-methyl-6,7,8,9-tetrahydro-5H-pyrido[2,3-b]indol-6-yl)-N-methylcarbamate building block 11 is described. The racemic intermediate 17 was prepared in a single step from 2-bromo-6-(1-methylhydrazinyl)pyridine sulfate salt (14) and N,3,3-trimethyl-1,5-dioxaspiro[5.5]undecan-9-amine hydrochloride salt (16). Chiral separation of racemic intermediate 17 by diasteromeric salt recrystallization afforded the diasteromeric salt 18 in 37% yield, which was Boc-protected to afford building block 11. Thus, the process for the synthesis and chiral separation by diasteromeric salt crystallization allowed the synthesis of chiral building block 11 in kilogram quantities in 18% overall yield.

Full text of DOI:10.1021/acs.oprd.9b00222

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Communication
A short synthesis of the 2-bromo-N,9-dimethyl-6,7,8,9-
tetrahydro-5H-pyrido[2,3-b]indol-6-amine building block
Nampally Sreenivasachary, Heiko Kroth, Pascal Benderitter,
Wolfgang Barth, Andrea Pfeifer, and Andreas Muhs
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.9b00222 • Publication Date (Web): 20 Sep 2019
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Organic Process Research & Development
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A short synthesis of the 2-bromo-N,9-dimethyl-6,7,8,9-tetrahydro-
5H-pyrido[2,3-b]indol-6-amine building block
Nampally Sreenivasacharya, Heiko Krotha*, Pascal Benderittera,b, Wolfgang Bartha, Andrea Pfeifera,
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Andreas Muhsa†
aAC Immune SA, EPFL Innovation Park, Building B, 1015 Lausanne, Switzerland.
bPresent address: Oncodesign, 20, rue Jean Mazen, 21076 Dijon, France; †Deceased on December 6, 2018.
Original synthetic route: The initial synthesis of racemic 10
involved nine linear steps (Scheme 1), and full experimental
details are provided in the Supporting Information. In brief, the
first step was the reduction of commercially available 1,4-
cyclohexadione monoethylene acetal with sodium borohydride in
methanol as described2 to afford alcohol 2 in 94 % yield. The
synthesis of compound 3 was already reported as a two-step
process, involving the activation of the alcohol by tosylation
followed by nucleophilic displacement with phthalimide.3
However we prepared compound 3 from 2 in one step using
Mitsunobu reaction4 condition with phthalimide to afford 3 after
chromatography on silica gel in 50 % yield.
ABSTRACT: A concise synthesis of pharmaceutically useful (R)-
tert-butyl(2-bromo-9-methyl-6,7,8,9-tetrahydro-5H-pyrido[2,3-
b]indol-6-yl)(methyl)carbamate building block 11 is described.
The racemic intermediate 17 was prepared in a single step from 2-
bromo-6-(1-methylhydrazinyl)pyridine sulfate salt 14 and N,3,3-
trimethyl-1,5-dioxaspiro[5.5]undecan-9-amine hydrochloride salt
16. Chiral separation of racemic intermediate 17 by diasteromeric
salt recrystallization afforded the diasteromeric salt 18 in 37 %
yield, which was Boc-protected to afford building block 11. Thus,
the process for the synthesis and chiral separation by
diasteromeric salt crystallization allowed the synthesis of chiral
building block 11 in kilogram quantities with 18 % overall yield.
KEYWORDS: neurodegenerative diseases, Fischer indole
synthesis, enantiomer separation, diasteromeric salts
Scheme 1: Initial synthesis of racemic building block 10a
OH
O
O
O
c
b
a
O
O
O
N
N
N
O
O
2
O
O
O
O
1
3
4
1. INTRODUCTION
NH2
d
A drug discovery programs at AC Immune required the multi-kilo
gram synthesis of enantiopure heterocyclic building block 11 to
prepare an advanced lead compound for toxicology and in vivo
proof-of-concept studies. The initial synthesis1 shown in Scheme
1, gave racemic 10 in ∼4.6 % overall yield after nine linear steps
starting from compound 1. One lead compound1 containing
racemic 10 utilized a chiral column for enantiomer separation,
rendering this process not suitable for the preparation of the lead
compound1 on a multi-kilogram scale as: (1) several steps
required chromatographic purification; (2) the non-acid-catalyzed
thermal Fischer-indole cyclization step required high temperature
microwave conditions; (3) the deprotection of the phthalimide
moiety with hydrazine hydrate is hazardous, and not an
environmentally friendly step; (4) N-bis-methylation required the
use of sodium hydride and methyl iodide, both hazardous
reagents, and not always easy to handle on large scale; 5)
enantiomerically pure lead compound1 required separation by
chiral HPLC.
Br
N
N
H
O
NH2
N
O
O
f
e
N
N
N
N
H
Br
Br
N
N
Br
N
6
O
H
7
5
g
NBoc
NHBoc
NH
x 2 HCl
j
h, i
N
N
H
N
Br
N
Br
N
8
Br
N
9
10
NBoc
N
Br
N
11
(required chiral building block)
Thus, it was necessary to develop a new scalable and efficient
synthesis of building block 11 (Scheme 1), because the respective
(R)-enantiomer of the lead compound1 showed the higher
physiological activity. Herein, we describe a concise synthesis of
racemic 17 as free amine that proceeds in one step with 69 %
yield from commercially available building block 16 with
compound 14, enantiomer separation of racemic 17 via
diastereomeric salts, followed by Boc-protection to yield 18 in 18
% overall yield and >99 % ee chiral purity.
aReagents and conditions: (a) Sodium borohydride, methanol,
room temperature, 2 h, 94 % yield; (b) Diethyl azodicarboxylate,
triphenylphosphine, tetrahydrofurane, 0-5 °C to room
temperature, 16 h, 50 % yield; (c) Tetrahydrofurane, water, p-
toluene sulfonic acid, 115 °C, 2 h, quantitative yield; (d) Ethanol,
room temperature, 1 h, quantitative yield; (e) Diethylene glycol,
microwave, 250 °C, 35 minutes, 49 % yield; (f) 50-60 %
Hydrazine hydrate, ethanol, room temperature, 16 h, 70 % yield;
(g) Di-tert-butyldicarbonate, triethylamine, tetrahydrofurane,
room temperature, 16 h, 33 % yield; (h) Sodium hydride, N, N’-
dimethylacetamide, 0-5 °C, 2 h; (i) Methyl iodide, 0-5 °C to room
temperature, 16 h, 85 % yield; (j) 2 M Hydrochloric acid/diethyl
2. RESULTS AND DISCUSSION
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ether, dichloromethane, room temperature, 16 h, quantitative
yield.
Information for details), the overall process would not be
improved as a methylation reaction would still be required after
cleavage of the phthalimide moiety.
1
2
3
Cleavage of the acetal moiety of compound 3 was achieved
according to
a
literature procedure5 by heating
3
in
a
Scheme 3: Synthesis of building block 15a
4
5
6
7
8
9
tetrahydrofurane/water mixture in the presence of a catalytic
amount of p-toluene sulfonic acid, to afford the keto-compound 4
in quantitative yield. The synthesis of keto-compound 4 from the
corresponding alcohol by oxidation was already described.6 The
hydrazone derivative 5 was prepared according to a literature
procedure7 via condensation reaction of keto-compound 4 and 2-
bromo-6-hydrazineylpyridine, prepared as described8, in ethanol
and isolated in quantitative yield. The resulting hydrazone
derivative 5 was cyclized under non-acid-catalyzed thermal
Fischer-indole synthesis according to a literature procedure7 in
diethylene glycol to afford tricycle 6 in 49 % yield, after treatment
of the crude reaction product with methanol. The cleavage of the
phthalimide protecting group in 6 according to a literature
procedure9 with hydrazine-hydrate in ethanol afforded amine 7 in
70 % yield. Protection of the aliphatic amine group in 7 according
to a literature procedure9 with di-tert-butyl dicarbonate afforded
Boc-protected 8 in 33 % yield, after treatment of the crude
reaction mixture with diethyl ether and collection of the
precipitate. Bis-methylation of compound 8 according to a
literature procedure10 using sodium hydride and methyl iodide in
N, N’-dimethylacetamide afforded racemic compound 9 in 85 %
yield after chromatography on silica gel. Cleavage of the Boc-
O
N
O
O
a, b
+
O
N
Br
N
N
NH2
N
Br
N
O
13
4
15
10
11
12
13
14
15
16
17
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aReagents and conditions: (a) Ethanol, room temperature, 1 h,
quantitative yield; (b) Diethylene glycol, microwave, 250 °C. 45
minutes, 42 % yield.
Thus, we employed commercially available ketal derivative 16 as
reaction partner for the Fischer-indole synthesis as it already
contained the N-methyl moiety. As derivative 16 also contained a
ketal-protecting group, we employed acidic reaction conditions to
allow in situ cleavage of the ketal-protecting group during the
acid-catalyzed Fischer-indole synthesis.12 When a mixture of 13
and 16 in 1,4-dioxane in the presence of concentrated sulfuric acid
was heated under reflux, the desired racemic product 17 was
isolated with 75 % yield (Scheme 4, see Supporting Information
for details).
protection group in
9 with acid afforded racemic 10 as
Scheme 4: Improved synthesis of racemic building blocks 9 and
hydrochloric acid salt in quantitative yield.
17a
In order to reduce the number of steps, improve the synthetic
yields, and to avoid involvement of hazardous material such as
methyl iodide and hydrazine hydrate (Scheme 1), we first
envisioned that the bis-methylation of building block 8 could be
avoided by using 2-bromo-6-(1-methylhydrazinyl)pyridine 13 or
its corresponding sulfate salt 14 (Scheme 2) as one of the starting
materials for the Fischer-indole synthesis together with an
appropriate keto-derivative. To this end building block 13 was
prepared by nucleophilic substitution reaction (Scheme 2) of 2, 6-
dibromopyridine 12 with N-methyl hydrazine as described11,
except ethanol was used as co-solvent, to afford the required
NH
O
a, b
HN
+
O
Br
N
N
NH2
x.HCl
N
Br
N
13
16
17 (free amine)
c
Boc
N
N
Br
N
9
building block 13 as free amine in 89
% yield after
chromatography on silica gel (Supporting Information).
aReagents and conditions: (a) Sulfuric acid, 1,4-dioxane, 140 °C,
4 h; (b) Sodium hydroxide, dichloromethane, room temperature,
Scheme 2: Synthesis of 2-bromo-6-(1-methylhydrazinyl)pyridine
building blocks 13 and 14a
75
%
yield; (c) Di-tert-butyldicarbonate, triethylamine,
tetrahydrofurane, room temperature, 16 h, 47 % yield.
Compound 17 was then Boc-protected9 to yield
9 after
chromatography on silica gel, which matched compound 9
obtained via the initial synthetic route. Thus, the successful
preparation of 9 in a two steps from 13 and 16 not only improved
the yield of the Fischer-indole synthesis, but also eliminated seven
synthetic steps from the initial synthesis, including the use of
microwave conditions, and usage of hazardous reagents, like
sodium borohydride, sodium hydride, methyl iodide, hydrazine
hydrate, and several purification steps for each intermediate.
Though the initial trial reactions of the Fischer-indole synthesis on
small scale gave the required product 17, the reaction mixture was
not homogeneous, which proved difficult for the workup of the
reaction mixture on larger scale.
a
b, c
NH2
NH2
Br
N
Br
Br
N
N
Br
N
N
12
13
14
x H2SO4
aReagents and conditions: (a) 85 % N-methyl hydrazine, ethanol,
reflux, 48 h, 89 % yield; (b) 85 % N-methyl hydrazine, 1,4-
dioxane, 85-90 °C, 8 h; (c) Dichloromethane, sulfuric acid, 0-5 °C
to room temperature, 2-3 h, 63-68 % yield.
The 2-bromo-6-hydrazineylpyridine free amine building block 13
was obtained as reddish, light sensitive oil. The quality of 13 had
a significant influence on the purity of the subsequent Fischer-
indole products. Thus, compound 13 was isolated as a sulfate salt
14 to improve its stability. The synthesis of sulfate salt 14 was
accomplished up to 8 kg scale in a single batch by treating 2,6-
dibromo pyridine 12 with N-methyl hydrazine in 1,4-dioxane
under reflux conditions, followed by treatment with sulfuric acid
to obtain the sulfate salt 14 in 63-68 % yield as off-white solid
(Scheme 2). Though the non-acid-catalyzed thermal Fischer-
indole synthesis of 13 with keto intermediate 4 produced
compound 15 in 42 % yield (Scheme 3, see Supporting
Therefore, the process was further optimized in a step wise
manner. Thus, several solvents as N, N’-dimethylacetamide, N-
methyl-pyrrolidone and sulfolane were screened to identify a
more suitable solvent with respect to reaction time and workup
procedure. N, N’-dimethylacetamide and N-methyl-pyrrolidone
were identified to be the best choices regarding conversion time,
yield and costs. While using N-methyl-pyrrolidone as solvent, the
product recovery during the workup was less attractive compared
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Organic Process Research & Development
to N, N’-dimethylacetamide as solvent. To avoid the strong
room temperature; (e) Di-tert-butyldicarbonate, triethylamine,
dichloromethane, room temperature, 2 h, 93 % yield.
1
2
3
4
5
6
7
8
oxidizing sulfuric acid, several alternatives were also screened
such as acetic acid, methane sulfonic acid, and p-toluene sulfonic
acid. Methane sulfonic acid gave 80 % conversion, but a lot of
unspecific impurities were generated. It was also tried to reduce
the excess of sulfuric acid, i.e. 3 eq., 1.5 eq., 0.3 and 0.05 eq. were
screened. However, 3 eq. of sulfuric acid were required for
complete and fast conversion to the indole derivative. Thus, we
have identified N, N’-dimethylacetamide as the most suitable
solvent, and sulfuric acid as the most suitable acid, to obtain a
homogenous reaction mixture, leading to a simplified workup
procedure and resulting in consistent yields of the Fisher-indole
synthesis product 17.
Further optimization of the process by using N, N’-
dimethylformamide as solvent allowed the isolation of 18 in
kilogram quantities with a yield of 37 % and an enantiomeric
excess of >99 % ee. Compound was 18 then Boc-protected9 to
afford chiral building block 11 in >93 % yield (Scheme 5).
3. CONCLUSIONS
We optimized the conditions with the preparation of building
block 14 as sulfate salt, then use of commercially available 16,
and N, N’-dimethylacetamide as the solvent proved to be the best
options for the Fischer-Indole synthesis of racemic 17.
Furthermore, we identified (R)-mandelic acid as chiral acid which
worked best in the presence of N, N’-dimethylformamide as
solvent for diastereomeric salt separation, leading to an optimized
process suitable for the synthesis of chiral building block 11 in
kilogram quantities in 18 % overall yield (Scheme 5).
9
10
11
12
13
14
15
16
17
18
19
20
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The synthesis of racemic building block 17 was then carried out
by Fischer-indole synthesis12 on multi-kilograms scale (Scheme 5)
with compounds 14 and 16, using N, N’-dimethylacetamide as
solvent in the presence of sulfuric acid at 100-110 °C. Though
building block 16 is commercially available, it can also be
prepared in one step from commercially available 1,4-
cyclohexanedione mono-2-(2',2'-dimethyltrimethylene) ketal as
described in the literature.13 In order to separate racemic 17,
diastereomeric salt screening was employed for the separation of
the enantiomers, see Supporting Information for details (Table
S1). In brief, several chiral acids ((S)-(+)-mandelic acid, (+)-
camphoric acid, (+)-glutamic acid, (1S)-(+)-10-camphorsulfonic
acid, D-(−)-quinic acid, (−)-O,O′-di-p-toluoyl-L-tartaric acid, L-
(+)-lactic acid, D-(+)-malic acid, L-(+)-tartaric acid, (+)-O,O′-
dibenzoyl-D-tartaric acid) and solvents (methanol, ethyl acetate,
acetone, tetrahydrofurane, dichloromethane, toluene, ethanol, 2-
propanol) were screened and the best chiral resolutions were
achieved with (S)-(+)-mandelic acid in acetone, tetrahydrofurane,
dichloromethane, ethanol, and 2-propanol (Table S1). Ethyl
acetate and toluene proved to be poor solvents leading to no
enantiomeric enrichment for any of the chiral acids (Table S1).
Though methanol gave no precipitate in the initial screening, it
was possible from racemic 17 to isolate the late eluting
enantiomer of 19 with >97 % ee using (S)-(+)-mandelic acid, and
the early eluting enantiomer of 18 with >97 % ee using (R)-(-)-
mandelic acid, see Supporting Information for details. However,
the initial enantiomeric purity of >97 % ee could not be
reproduced on larger scale as the desired diastereomeric salt 18
was obtained with a yield of 12 % and an enantiomeric excess of
93 % ee by using (R)-(-)-mandelic acid in methanol. Employing
40 g of racemic 17 and changing the solvent to ethanol for the
crystallization increased the yield to 23 % and the enantiomeric
excess to 96 % ee, see Supporting Information for details.
4. SUMMARY
We have developed a short and efficient synthesis of chiral
building block 11, which was prepared in 4 steps and 18 % overall
yield from 14 and 16. The significant improvements over the
initial synthesis allowed us to manufacture up to 7.1 kilograms of
the chiral building block 11. Building block 11 has been utilized
for multiple discovery programs at AC Immune targeting protein
aggregates linked to neurodegenerative diseases.
EXPERIMENTAL SECTION
All reagents and solvents were obtained from commercial sources
and used without further purification. Proton (1H) NMR and
carbon (13C) NMR spectra were recorded on a Bruker DRX 400
MHz NMR spectrometer in deuterated solvents. Typical spectral
parameters: spectral width 10 ppm; Description of signals: s =
singlet; brs = broad singlet; d = doublet; t = triplet; q = quartet, m
= multiplet; dd =doublet of doublet: dt = doublet of triplet; tt =
triplet of triplets; ddd = doublet of double doublet. Mass spectra
(MS) were recorded on an Advion CMS spectrometer.
2-Bromo-6-(1-methylhydrazinyl) pyridine sulfate (14). 2, 6–
Di-bromo pyridine 12 (50 g; 0.2109 mol; 1eq.) and 1, 4-dioxane
(500 mL) were charged into a 2 L round bottom flask equipped
with mechanical stirrer, thermo packet and condenser at 25-35 °C.
Then 85 % N-methyl hydrazine (22.74 g; 0.4935 mol; 2.34 eq.)
was added to the reaction mixture at room temperature (~25 °C),
and heated under stirring at 85-90 °C for 8 h. Then the reaction
mass was cooled to 25 °C, distilled water (500 mL), and
dichloromethane (500 mL) were added, and the reaction mixture
was stirred for 15 minutes. The organic and aqueous layers were
separated; the procedure was repeated for 3 times, and the
combined organic layer was washed with distilled water (3 x 500
mL), followed by 10 % brine solution (500 mL). The organic
layer was separated, dried with sodium sulfate, and the mixture
was filtered. The organic layer was concentrated under reduced
pressure at 40-45 °C to afford a thick liquid (note: 1, 4-dioxane
should be completely distilled off). The residue was re-dissolved
in dicholoromethane (500 mL) and cooled to 0-5 °C. Then
sulfuric acid (5 mL, 0.1 w/v) was added to the reaction mass via
an addition funnel at 0-5 °C over a period of 10-15 minutes. The
mixture was allowed to warm to room temperature and stirred or
2-3 h. The solid material was filtered off, washed with
dichloromethane (50 mL), and dried for 1 h. The solid was
suspended in acetonitrile (350 mL) and stirred for 1 h at 25 °C.
The solid was filtered, washed with acetonitrile (50 mL), and suck
dried for 1 h to yield 14 as white to yellow solid (42 g, 68 %). As
per the above process, compound 14 was produced on plant scale
Scheme 5: Optimized synthesis of chiral building block 11a
NH
O
a, b
HN
+
Br
N
N
O
N
NH2
Br
c
N
xHCl
xH2SO4
14
17 (free amine)
OH
16
COOH
O
O
N
N
NH
HO COOH
x
d, e
N
Br
N
Br
N
11
18
aReagents and conditions: (a) Sulfuric acid, N, N’-
dimethylacetamide, 100-110 °C, 4 h; (b), Sodium hydroxide,
dichloromethane, room temperature, 63 % yield; (c) (R)-2-
hydroxy-2-phenylacetic acid, N, N’-dimethylformamide, 80-85
°C, 6 h, 37 % yield; (d) Sodium hydroxide, dichloromethane,
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using compound 12 as starting material, to afford 16.7 kg (63 %)
of compound 14 with 99.66 % HPLC purity.
5.24 kg of a brown/dark brown colored solid (63 % yield; 97.8 %;
HPLC purity). According to this procedure, racemic 17 was
produced as free amine in three batches to obtain 34 kg with >97
1
2
3
4
5
6
7
8
9
2-Bromo-N,9-dimethyl-6,7,8,9-tetrahydro-5H-pyrido[2,3-
b]indol-6-amine (17). 8.5 kg of 14 was suspended in 45 L of N,
N’-dimethylacetamide at 25-35 °C in a 250 L Glass Lined Reactor
under nitrogen atmosphere and 7.06 kg of 16 was added to obtain
a heterogeneous mass. The mixture was cooled to 0-5 °C and 9.6
kg of concentrated sulfuric acid was slowly added to yield thick
syrup. The temperature during the concentrated sulfuric acid
addition should be kept below 10 °C The reaction mixture was
heated to 75 °C to become a clear solution. Then the temperature
was raised to 105-111 °C and the reaction mixture stirred for 4 h
(conversion was checked by IPC by HPLC until not more than 1
% of 14 was present). The resulting brown suspension was cooled
to 0-10 °C, and 47 L water and 34 L dichloromethane were added,
and the reaction mass was stirred at 0-10 °C for 15 minutes. The
reaction mass was transferred to a 250 L Stainless Steel Reactor
and the pH of the suspension was adjusted to pH 11-13 by adding
a 30 % sodium hydroxide solution, prepared by adding 10.2 kg of
sodium hydroxide to 24 L of water, while the temperature of the
suspension was kept below 10 °C. The reaction mass temperature
was allowed to warm to 25-35 °C and stirred for 30 minutes. The
reaction mass was filtered through a Nutsche filter (500 mm
diameter), and the solid washed with 21 L of dichloromethane.
The combined filtrate was washed with 21 L of water and the
organic layer separated. The aqueous layer was extracted with 17
L of dichloromethane and the organic layer separated. The
combined organic layers were transferred to a 250 L Stainless
Steel Reactor and washed with 47 L of water and the organic
layer separated. The organic layer was transferred to a 250 L
Stainless Steel Reactor and the solvents were removed by
evaporation under vacuum, initially below 45 °C to remove
dichloromethane, then the reaction mass was dried at 70 °C in
high vacuum to remove traces of N, N’-dimethylacetamide. The
reaction mass was re-suspended in 42.5 L of water, transferred to
a 250 L Glass Lined Reactor and cooled to 0-10 °C. Then 8.5 L of
concentrated hydrochloric acid was added while the temperature
was kept below 10 °C until pH<2. The reaction mass was allowed
to warm to 25-35 °C and stirred for 1 h. The precipitated solid
was isolated by centrifugation (18’’) and washed with 8.5 L of
water and 8.5 L of n-hexane. After drying the solid by
centrifugation for 1-2 h, the solid was transferred to 250 L
Stainless Steel Reactor and re-suspended in (42.5 L) water and
(42.5 L) dichloromethane. The reaction mass was cooled to 0-5
°C and the pH was adjusted to pH 12-13 by adding a 30 % sodium
hydroxide solution, prepared by adding 4.25 kg of sodium
hydroxide to 9.35 L of water while the temperature was kept
below 10 °C. After the pH was adjusted, the temperature of the
reaction mass was allowed to warm to 25-35 °C. The organic
layer was separated and the aqueous layer extracted with 42.5 L
of dichloromethane. The combined organic layers were
transferred to a 250 L Stainless Steel Reactor and washed with
brine, prepared by adding 2.12 kg of sodium chloride to 21.25 L
of water. The organic layer separated, transferred to a 250 L
Stainless Steel Reactor, and dried over 2.21 kg of sodium sulfate.
The reaction mass was filtered through a Nutsche filter (500 mm
diameter), the filter washed with 8.5 L of dichloromethane, the
combined filtrates transferred to a 250 L Stainless Steel Reactor
and concentrated under vacuum below 45 °C. To the residue was
then added 4.25 L of n-hexane and the reaction mass was
concentrated under vacuum below 45 °C. The obtained solid was
re-suspended in 4.25 L of n-hexane and the reaction mass was
concentrated under vacuum below 45 °C. The obtained solid was
re-suspended in 8.5 L of n-hexane, stirred for 1 h, filtered through
a Nutsche filter (500 mm diameter), washed with 4.25 L of n-
hexane and then suck-dried for 1 h. The wet product was dried at
50-55 °C for 6 h using a vacuum tray dryer (12’’ trays) to afford
1
% HPLC purity. H-NMR (400 MHz, DMSO-d6) δH = 7.74 (d, J
= 8.0 Hz, 1H), 7.16 (d, J = 8.0 Hz, 1H), 3.62 (s, 3H), 3.04–2.60
(m, 5H), 2.36 (s, 4H), 2.08 (dt, J = 11.6, 5.6 Hz, 1H), 1.64 (m,
1H). 13C-NMR (101 MHz, DMSO-d6): δ = 147.62, 137.28,
132.45, 128.33, 118.74, 118.09, 106.19, 55.38, 34.17, 28.49,
28.04, 27.40, 20.32. ESI-MS m/z: 294.3/296.3 (M+H)+.
2-Bromo-N,
9-dimethyl-6,7,8,9-tetrahydro-5H-pyrido[2,3-
b]indol-6-amine (R)-2-hydroxy-2-phenylacetate (18). 7.5 kg of
17 was suspended in 22.5 L of N, N’-dimethylformamide at 25-35
°C in a 100 L Stainless Steel Reactor. The reaction mixture was
then heated to 80-85 °C and stirred for 10-15 minutes. At this
temperature a solution of 3.88 kg (R)-(-)-mandelic acid in 15 L of
N, N’-dimethylformamide was slowly added to the reaction mass.
After the addition was completed, the reaction mass was heated at
80-85 °C for 6 h. Then the reaction mass was cooled to 25-35 °C
and stirred at this temperature for 1 h. The precipitate was isolated
by centrifugation (18’’), the solid washed with N, N’-
dimethylformamide and suck dried. Then the wet product was
dried at 50-55 °C for 4 h under vacuum to afford the crystalline
mandelic acid salt 18. The mandelic acid salt 18 was re-suspended
in 37.5 L of N, N’-dimethylformamide in a 100 L Stainless Steel
Reactor, and then heated at 105-115 °C for 1 h to become a clear
solution. The reaction mass was allowed to cool to 85-100 °C and
7.5 g mandelic acid salt 18 crystal seeds were added. Then the
reaction mass was allowed to cool to 25-35 °C and was stirred for
an additional 1 h at this temperature. The precipitate was isolated
by centrifugation (18’’), the solid washed with N, N’-
dimethylformamide and spin-dried. Then the wet product was
dried at 50-55 °C for 4 h using a vacuum tray dryer (12’’ trays) to
afford 4.26 kg of the re-crystallized mandelic acid salt 18 (37.4 %
yield; 98.22 % HPLC purity; 99.68 % chiral purity). According to
this procedure, (R)-(-)-mandelic acid salt 18 was produced in two
batches to obtain 8.68 kg with >97 % HPLC purity and >99 % ee
chiral purity.
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(R)-Tert-butyl(2-bromo-9-methyl-6,7,8,9-tetrahydro-5H-
pyrido[2,3-b]indol-6-yl)(methyl)carbamate (11). To a solution
of 19 L of dichloromethane in a 100 L Stainless Steel Reactor was
added 4.246 kg of 18 at 25-35 °C. The reaction mass was cooled
to 20-25 °C and a 2 M sodium hydroxide solution, prepared from
2.72 kg of sodium hydroxide and 34 L of water, was slowly
added. After the addition was completed, the reaction mass was
stirred at this temperature for 30 minutes. The organic phase was
separated and the aqueous layer extracted with 12.7 L of
dichloromethane. The combined organic layers were transferred
to a 100 L Stainless Steel Reactor, dried over 0.42 kg of sodium
sulfate, filtered through a Nutsche filter (500 mm diameter), and
the filter washed with 4.2 L of dichloromethane. The combined
organic filtrates were transferred to a 100 L Stainless Steel
Reactor and 2.166 kg of triethylamine added at 20-25 °C. Then
6.02 kg of di-tert-butyldicarbonate was slowly added at 20-25 °C
and the reaction mass was stirred at the same temperature for 1 h
(conversion was checked by IPC by HPLC until not more than 1
% of 18 was present). Then 42.5 L water was added to the
reaction mass at 20-25 °C and the organic layer was separated.
The organic layer was transferred to a 100 L Stainless Steel
Reactor, washed with 42.5 L of water, the organic layer separated
and transferred to a 100 L Stainless Steel Reactor, and dried over
dried over 0.42 kg of sodium sulfate. The sodium sulfate was
removed by filtration through a Nutsche filter (500 mm diameter),
and the filter washed 4.2 L of dichloromethane. The combined
organic filtrate was transferred to a 100 L Stainless Steel Reactor
and the solvent distilled under vacuum below 45 °C. The residue
was treated with 2.12 L of n-hexane and the solvents distilled
under vacuum below 45 °C. The reaction mass was then treated
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Organic Process Research & Development
(4) Mitsunobu, O. The use of diethyl azodicarboxylate and
triphenylphosphine in synthesis and transformation of natural products.
Synthesis 1981, 1-28.
with 6.5 L of n-hexane and the mixture stirred at 25-35 °C for 15-
30 minutes. The precipitate was collected by filtration through a
Nutsche filter (500 mm diameter), the solid washed with 2.12 L of
n-hexane and suck-dried. Then the wet product was dried at 50-55
°C for 6 h using a vacuum tray dryer (12’’ trays) to afford 3.504
kg of compound 11 (93.4 % yield; 99.14 % HPLC purity).
According to this procedure, title compound 11 was produced in
1
2
3
4
5
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7
8
(5) Majewski, M.; MacKinnon, J. Enantioselective deprotonation of
protected 4-hydroxycyclohexanones. Can. J. Chem. 1994, 72, 1699-1704.
(6) Glennon, R. A.; Hong, S. S.; Bondarev, M.; Law, H.; Dukat, M.;
Rakhit, S.; Power, P.; Fan, E.; Kinneau, D.; Kamboj, R.; Teitler, M.;
Herrick-Davis, K.; Smith, C. Binding of O-alkyl derivatives of serotonin
at human 5-HT1Dβ receptors. J. Med. Chem.1996, 39, 314-322.
(7) Crooks, P. A.; Robinson, B. Azaindoles. Part I. The syntheses of 5-
aza- and 5,7-diazaindoles by the non-catalytic thermal indolization of 4-
pyridyl- and 4-pyrimidylhydrazones respectively Can. J. Chem. 1969, 47,
2061-2067.
1
two batches to obtain 7.098 kg with >98 % HPLC purity. H-
NMR (400 MHz, DMSO-d6) δH = 7.79 (d, J = 8.1 Hz, 1H), 7.18
(d, J = 8.1 Hz, 1H), 4.24 (s, 1H), 3.63 (s, 3H), 3.08–2.66 (m, 7H),
2.15–1.82 (m, 2H), 1.42 (s, 9H). 13C-NMR (101 MHz, CDCl3): δ
= 155.65, 147.98, 135.32, 133.29, 127.48, 118.40, 118.31, 106.40,
79.62, 28.70, 28.54, 27.98, 27.02, 23.93, 22.00. ESI-MS m/z:
394.4/396.4 (M+H)+.
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(8) Boyer, T.; Dodic, N.; Evans, B.; Kirk, B. E. 2, 6-disubstituted
pyridines and 2,4-disubstituted pyrimidines as soluble guanylate cyclase
activators. Patent WO2009068652A1, 2009.
(9) King, F. D.; Gaster, L. M.; Kaumann, A. J.; Young, R. C. Use of
tetrahydrocarbazone derivatives as 5HT1 receptor agonists. Patent
WO199300086A1, 1993.
(10) Marugán, J. J.; Manthey, C.; Anaclerio, B.; Lafrance, L.; Lu, T.;
Markotan, T.; Leonard, K. A.; Crysler, C.; Eisennagel, S.; Dasgupta, M.;
Tomczuk, B. Design, synthesis, and biological evaluation of novel potent
and selective αvβ3/αvβ5 integrin dual inhibitors with improved
bioavailability. Selection of the molecular core. J. Med. Chem. 2005, 48,
926-934.
(11) Guzzo, P, R.; Surman, M. D.; Zhu, L. Azinone-substituted
azabicycloalkane-indole and azabicycloalkane-pyrrolo-pyridine MCH-1
antagonists, methods of making, and use thereof. Patent
WO2011003021A1, 2011.
(12) (a) Robinson, B. The Fischer indole synthesis Chem. Rev. 1963, 63,
373-401. (b) Robinson, B. Studies on the Fischer indole synthesis Chem.
Rev. 1969, 69, 227-250.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and analytical data of compounds 2-10
(Scheme S1). Experimental procedures and analytical data of
compounds 13 and 14 (early synthesis) (Scheme S2).
Experimental procedure and analytical data of compound 15
(Scheme S3). Experimental procedures and analytical data of
compounds 9 and 17 prepared with the improved synthesis
(Scheme S4). Experimental procedures and test runs to identify
suitable chiral acids and solvents for the diastereomeric salt
separation of racemic 2-bromo-N,9-dimethyl-6,7,8,9-tetrahydro-
5H-pyrido[2,3-b]indol-6-amine 17.
(13) Brackenridge, I.; McGee, C.; McIntyre, S.; Knight, J.; Hartley, D.
Process for the production of R-(+)-6-carboxamido-3-N-methylamino-
1,2,3,4-tetrahydrocarbazole. Patent WO199954302A1, 1999.
AUTHOR INFORMATION
Corresponding Author
*E-mail: heiko.kroth@acimmune.com
Present Addresses
bOncodesign, 20, rue Jean Mazen, 21076 Dijon, France.
Conflict of Interest Disclosure
Nampally Sreenivasachary is an employee of AC Immune and
holds shares of AC Immune.
Heiko Kroth is an employee of AC Immune and holds shares of
AC Immune.
Pascal Benderitter holds shares of AC Immune.
Wolfgang Barth has no conflict of interest.
Andrea Pfeifer is an employee of AC Immune and holds shares of
AC Immune.
Andreas Muhs was an employee of AC Immune and holds shares
of AC Immune.
ACKNOWLEDGMENT
We dedicate this publication to Dr. Andreas Muhs, who provided
insight and expertise that greatly assisted the research at AC
Immune .The authors also wish to thank Laurus Labs team DVL
Rao, Naveen Kumar Reddy, Suresh Kumar Oladri, Manibushan
Kotala for their contributions.
REFERENCES
(1) Kroth, H.; Hamel, A.; Benderitter, P.; Froestl, W.; Sreenivasachary,
N.; Muhs, A. Novel compounds for the treatment of diseases associated
with amyloid or amyloid-like proteins. Patent WO2011128455A1, 2011.
(2) Uyanik, M.; Akakura, M.; Ishihara, K. 2-Iodoxybenzenesulfonic acid
as an extremely active catalyst for the selective oxidation of alcohols to
aldehydes, ketones, carboxylic acids, and enones with oxone. J, Am.
Chem. Soc. 2009, 131, 251-262.
(3) Zhang, X.; Hufnagel, H. R.; Sui, Z. 4-azetidinyl-1-heteroatom linked-
cyclohexane antagonists of CCR2. Patent WO2010121036A1, 2010.
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