Synthesis of 3,7-disubstituted 10-methylphenothiazines

Article abstract of DOI:10.1055/s-1998-2121

Lithiation at the C-3 and C-7 positions of N-methylphenothiazine (1) can be achieved by halogen metal exchange reaction starting from 3,7-dibromo-10-methylphenothiazine (2). Reaction of the lithiated species with carbon, sulfur and silicon electrophiles results in new 3,7-disubstituted 10-methylphenothiazines 4a-g. The described procedure gives high yield and an easy excess to important enzyme mediators.

Full text of DOI:10.1055/s-1998-2121

SYNTHESIS
August 1998
1107
Synthesis of 3,7-Disubstituted 10-Methylphenothiazines
Søren Ebdrup
Novo Nordisk A/S, Novo Nordisk Park, D9, DK-2760 Måløv, Denmark
Fax +45(44)434742; E-mail: sebd@novo.dk.
Received 24 November 1997; revised 30 January 1998
Abstract: Lithiation at the C-3 and C-7 positions of N-meth-
mation of byproducts was observed. GC/MS of a repre-
sentative crude reaction mixture indicated that the main
by-products were 3,10-dimethylphenothiazine (5) and 10-
methyl-phenothiazine (1). No byproducts formed as a re-
sult of a rearrangement of the C-3 lithium anion to the
more stable C-4 anion8,10,11 were detected.
ylphenothiazine (1) can be achieved by halogen metal exchange
reaction starting from 3,7-dibromo-10-methylphenothiazine (2).
Reaction of the lithiated species with carbon, sulfur and silicon
electrophiles results in new 3,7-disubstituted 10-methylpheno-
thiazines 4a–g. The described procedure gives high yield and an
easy excess to important enzyme mediators.
Key words: 10-methylphenothiazine, dilithiation, 3,7-disubsti-
tuted 10-methylphenothiazines
10-Substituted phenothiazine derivatives can be used to
reversibly mediate the reaction of different oxidative en-
zymes like peroxidases1,2 and laccases3 by radical forma-
tion. The concept is well suited for important industrial
applications such as, e.g. dye transfer inhibition during
wash, denim bleaching, waste water purification and as
glucose sensors.1–4 The radicals are capable of forming
oligomers (waste reaction) by carbon–carbon bond for-
mation (ortho and para positions are reactive).5
Scheme
N-H and N-alkylphenothiazines can be metalated at dif-
ferent positions. A C-1 anion of phenothiazine can be gen-
erated by in situ protection of the N-10 position followed
by deprotonation with t-BuLi.6 Reaction of phenothiazine
with 2 equivalents of BuLi gives access to a C-1 and N-10
doubly lithiated species.7–9 10-Alkylphenothiazines can
be lithiated at C-4 with BuLi8,10 or preferably with s-BuLi/
TMEDA.11 Metalation at C-3 can be accomplished under
Grignard conditions from 3-iodo-10-ethylphenothiazine10
or from 3-bromo-10-ethylphenothiazine12 both in moder-
ate yield. Different substituted phenothiazine derivatives
can also be formed from substituted diphenylamines by
cyclization with sulfur,13,14 or from phenothiazine by
electrophilic aromatic substitution.14,15
Table 1. Optimization of the Conditions for the BuLi/Br Exchange
Reaction of 2 and Subsequent Reaction with Methyl Iodide
Entry
Base
Temp
(°C)
Products (%)a
4b
1
5
1
2
3
4
5
BuLi
20
20
20
–78
–78
89
92
92
97
98
11
5
3
2
2
1
4
5
0.2
0.3
s-BuLi
t-BuLi
BuLi
s-BuLi
a Product distribution based on GC data.
The aim of this work was to investigate the possibilities of Based on the results in Table 1 it can be concluded that the
introducing different substituents into both the C-3 and optimal reaction condition is to apply 4 equivalents of
C-7 position of 10-alkylphenothiazine in one step (3,7-di- s-BuLi in THF at –78°C followed by addition of an elec-
bromo-10-methylphenothiazine 2 used as an example) by trophile after 0.5 hour. The resulting different 3,7-disub-
a double bromine-lithium exchange reaction.
stituted 10-methylphenothiazines can be prepared in high
yields (78–90%) by this procedure (Scheme and Table 2).
The same procedure can also be applied for the syntheses
of other C-3 and -7 disubstituted 10-alkylphenothiazines.5
Phenothiazine was alkylated to give 10-methylphenothi-
azine (1)10,16 which was brominated with bromine to give
3,7-dibromo-10-methylphenothiazine (2).17 Halogen-
metal exchange in 2 and subsequent reaction with methyl The site of substitution was confirmed to be C-3 and C-7
iodide was investigated under different conditions based on recorded spectroscopic data, literature data11,18
(Scheme, Table 1). As can be seen from Table 1 the yield and comparisons with the assignment of 4g. Unambi-
of 3,7,10-trimethylphenothiazine (4b) is in the same range guous signals were assigned via a 2D heterocorrelated
when BuLi (89%), s-BuLi (92%) or t-BuLi (92%) was HSQC, a long-range HMBC (optimized for 7 Hz) and a
used for experiments performed at room temperature in NOE spectra (details in experimental part). The coupling
1
diethyl ether (Entries 1–3). By comparing Entries 4 and 5 constants measured for the H NMR spectra (Table 3)
it can be concluded that changing the solvent to THF and demonstrate a symmetric meta or para substitution pat-
reducing the temperature to –78°C increases the yield of tern. The 13C NMR data (Table 4) show the absence of
4b to 97–98% (BuLi and s-BuLi). In all experiments, for- protonated carbon atoms at about δ =122 and the appear-

SYNTHESIS
Short Papers
Table 2. Preparation of 3,7-Disubstituted Phenothiazines 4a–g
1108
–78°C followed by warming to r.t. and stirring for 4 h. After evapo-
ration of the solvent, CH2Cl2 (30 mL) and H2O (25 mL) were added,
the phases were separated and the aqueous phase was extracted with
CH2Cl2 (2 × 30 mL). The combined organic phases were dried and
evaporated to dryness to give the crude product. The composition of
the crude mixtures were analyzed by GC (Table 1).
Prod-
ucta
Electro-
phile
El
Yield
(%)
mp (˚C)b
4ac
4b
4c
4d
4e
4f
D2O
MeI
Me2S2
DMF
CO2
D
Me
SMe
CHO
CO2H
CH(OH)But
SiMe3
88
89
85
88
90
85
78
94–95
77–77.5d
125–125.5
196.5–197e
310 (dec)
153.5–154
128.5–129
3,7-Disubstituted 10-Methylphenothiazines 4a–g; General Proce-
dure:
Compound 2 (1.0 g, 2.7 mmol) was dissolved in anhyd THF (20 mL)
under an atmosphere of N2. s-BuLi (7.7 mL of a 1.4 M solution in
hexane, 11 mmol) was added slowly at –78°C and the mixture (sus-
pension) stirred for 30 min, followed by addition of an electrophile
(12 mmol) and stirring for 1 h. The mixture was allowed to warm to
r.t. followed by stirring for an additional 4 h. The mixture was poured
into satd NH4Cl solution (30 mL), the phases were separated and the
aqueous phase extracted with Et2O (3 × 25 mL). The combined or-
ganic phases were dried and evaporated to give the crude product. Pu-
rification was performed by flash chromatography for 4a, 4f and 4g
using the following eluents: EtOAc/heptane (1:8) (4a); EtOAc/hep-
tane (1:6 → 1:4) (4f); heptane → EtOAc/heptane (1:4) (4g). Com-
pounds 4b and 4c were recrystallized from EtOH and 4d and 4e were
suspended in EtOH and filtered. Spectroscopic data for 4g: Long
range HMBC, optimized for 6 Hz: C-10a with NCH3, H-2 and H-4;
t-BuCHO
Me3SiCl
4g
a
Satisfactory microanalyses obtained: C ± 0.30, H ± 0.20, N ± 0.30,
S ± 0.20. Compounds 4e and 4f contained 0.25 and 0.5 equiv of oc-
cluded H2O.
b
Uncorrected; recrystallized from EtOH. Compounds 4a–c, f, g are
colorless to light yellow crystals, 4d, e are yellow crystals.
Material 97% deuterated as shown by 1H NMR analysis.
Lit.19 mp 81°C.
c
d
e
Lit.20 mp 202–203°C, Lit.21 mp 204°C.
ance of nonprotonated carbons at δ = 122–136 (C-3 and
C-7). The melting point of 3,7,10-trimethylphenothiazine C-2 with H-4; C-3 with Si(CH3)3, and H-2; C-4 with H-2 and C-4a
(4b) corresponded to that reported19 for 4b synthesized by
with H-1. NOE: NCH3 with H-1 and H-1 with H-2.
Friedel–Crafts formylation of the not readily available 5
followed by Wolf–Kishner reduction. The melting point
of 10-methylphenothiazine-3,7-dicarboxaldehyde (4d)
was in agreement with that reported20 for 4d synthesized
in 3.5% yield by Vilsmeier formylation of 1 and in a 86%
yield by a two step procedure21 via a bisimidazoline deriv-
ative from 1. Compounds 4b and 4d have not yet been
characterized in the literature.
The above general procedure was modified for three of the com-
pounds. For 4d the reaction mixture was warmed to 0°C, poured into
HCl (4.5% w/v; 170 mL) and stirred for an additional 30 min (suspen-
sion of yellow crystals). The Et2O phase was separated and the aque-
ous phase extracted with CH2Cl2 (5 x 75 mL). The organic phase was
dried and evaporated to give the crude product. For 4f the reaction
mixture was allowed to warm to r.t. and poured into HCl (5% w/v;
50 mL) instead of satd NH4Cl solution. For 4e the lithiated interme-
diate 3 was poured into solid CO2 (40 g) in Et2O (40 mL). When the
CO2 had evaporated, aq NaOH (8% w/v; 100 mL) was added and the
two phases separated. The aqueous phase was washed with CH2Cl2
(3 × 50 mL), acidified with HCl (37% w/v) and the precipitated crude
product was filtered. The aqueous phase was extracted with CH2Cl2
All reactions involving air-sensitive reagents were performed under
N2 or Ar using syringe-septum cap techniques. The glassware was
flame dried prior to use. MgSO4 was used for drying the organic sol-
vents and the solvents were evaporated under reduced pressure. Melt-
ing points are uncorrected. The 1H NMR spectra were recorded on a
400 MHz instrument using TMS as internal standard. The 13C NMR
spectra were recorded at 100 MHz using CDCl3 (76.90 ppm) or
DMSO-d6 (39.60 ppm) as internal standards. Coupling constants (J)
are given in Hz. Flash chromatography22 was performed using silica
gel Merck 60 size 40–63 µm. THF was dried by distillation from so-
dium/benzophenone ketyl. Phenothiazine was obtained from
Hoechst. Trimethylacetaldehyde was distilled prior to use.
Compound 1 was prepared with a slight modification of the published
procedure:10,16 A mixture of phenothiazine (18 g, 90 mmol), MeOH
(8 mL) and MeI (7 mL, 110 mmol) was heated in a glass screw cap
vessel23 for 18 h at 105°C. Standard workup gave 1 (32.6 g, 79%), mp
99–100°C (EtOAc/EtOH); (Lit.16 mp 102–104°C). 3,7-Dibromo-10-
methylphenothiazine (2) was prepared with slight modification of the
published method17 by changing the solvent sytem to AcOH/AcONa;
mp 146–147°C (EtOAc/EtOH) (Lit.24 mp 151–152°C). BuLi, s-BuLi
and t-BuLi were titrated25 prior to use.
Table 3. 1H NMR Data of Compounds 4a–ga,b
Prod- δ, J (Hz)
uct
1-H, 9-H
2-H, 8-H
4-H, 6-H
NCH3
4a
4b
4c
4d
4e
4f
6.80 (d, 2H, 7.15 (m, 2H,
J = 8.0) J = 8.0)
6.67 (d, 2H, 6.94 (dd, 2H,
J = 8.5) J = 2.0, 8.5)
6.70 (d, 2H, 7.10 (dd, 2H,
J = 8.5) J = 8.0, 2.0)
6.92 (d, 2H, 7.68 (dd, 2H,
J = 8.5) J = 8.5, 2.0)
7.06 (d, 2H, 7.79 (d, 2H,
J = 8.5) J = 8.5)
7.14 (m, 2H) 3.34 (s, 3H)
6.95 (m, 2H) 3.31 (s, 3H)
7.08 (d, 2H, 3.31 (s, 3H)
J = 2.5)
7.60 (d, 2H, 3.49 (s, 3H)
J = 2.0)
7.63 (s, 2H) 3.41 (s, 3H)
6.74 (d, 2H, 7.07–7.10 (m, 4H)
J = 8.5)
3.35 (s, 3H)
4g
6.79 (d, 2H, 7.28 (dd, 2H,
7.25 (d, 2H, 3.36 (s, 3H)
J = 8.0)
J = 8.0, 0.5)
J = 1.0)
Bromine-Lithium Exchange Reaction of 3,7-Dibromo-10-meth-
ylphenothiazine (2):
a
Recorded in CDCl3 except for 4e which was recorded in DMSO-d6.
Assignment based on HSQC, HMBC and NOE spectra recorded for
4f,11,18 and the observed coupling constants.
Additional peaks not listed in Table 3 are as follows. 4b: 2.23 (s,
6H, ArCH3); 4c: 2.42 (s, 6H, SCH3); 4d: 9.83 (s, 2H, CHO); 4e:
12.81 (br s, 1H, CO2H); 4f: 0.90 [s, 18H, C(CH3)3], 1.82 (br s, 2H,
CHOH), 4.29 (s, 2H, ArCH); 4g: 0.22 [s, 18H, Si(CH3)3].
Compound 2 (1.0 g, 2.7 mmol) was dissolved in Et2O ( 100 mL) or
THF (20 mL) under an atmosphere of N2. The base BuLi, s-BuLi or
t-BuLi (11 mmol, 1.4 M solution in hexane, 1.3 M solution in cyclo-
hexane and 1.5 M solution in pentane, respectively) was added slowly
at r.t. or at –78°C and the mixture was stirred for 30 min. The elec-
trophile was added slowly and the mixture stirred for 4 h or for 1 h at
b

SYNTHESIS
C-9a, C-10a
August 1998
1109
Table 4. 13C NMR Data of Compounds 1, 2, and 4a–ga,b
Product
δ
C-1, C-9
C-2, C-8
C-3, C-7
C-4, C-6
C-4a, C-5a
NCH3
1
2
113.9
115.2
113.9
113.5
114.2
114.5
114.9
112.9
113.5
127.0
129.3
126.9*
127.5*
126.7
128.0
127.7
126.1*
132.5
122.3
124.7
122.0c
131.6
131.2
132.0
121.5
136.4
133.6
127.3
130.2
127.2*
127.7*
127.5
130.4
129.8
126.6*
131.9
123.2
114.8
123.2
123.1
123.7
123.4
125.5
122.4
122.7
145.7
144.4
145.7
143.5
143.7
149.3
148.1
144.7
146.1
35.1
35.3
35.1
35.0
35.2
36.2
35.9
35.6
35.0
4a
4b
4c
4d
4e
4f
4g
a
Recorded in CDCl3 except for 4e which was recorded in DMSO-d6. Assignment based on HSQC, HMBC and NOE spectra recorded for
4f.11,18 Chemical shift values marked * may be interchangeable.
Additional peaks not listed in Table 4 are as follows. 4b: 20.2 (ArCH3); 4c: 17.4 (SCH3); 4d: 189.8 (CHO); 4e: 166.5 (CO2H); 4f: 25.8
[C(CH3)3], 35.2 [C(CH3)3], 81.6 (CHOH); 4g: –1.2 [Si(CH3)3].
b
c
(t, J = 98.5 Hz)
(3 × 50 mL), the crystals and the organic phases were combined and
evaporated. The main characteristics of the synthesized compounds
are presented in Tables 2–4.
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