Radical-mediated dehydrative preparation of cyclic imides using (NH4)2S2O8-DMSO: Application to the synthesis of vernakalant

Article abstract of DOI:10.3762/bjoc.11.113

Ammonium persulfate-dimethyl sulfoxide (APS-DMSO) has been developed as an efficient and new dehydrating reagent for a convenient one-pot process for the synthesis of miscellaneous cyclic imides in high yields starting from readily available primary amines and cyclic anhydrides. A plausible radical mechanism involving DMSO has been proposed. The application of this facile one-pot imide forming process has been demonstrated for a practical synthesis of vernakalant.

Full text of DOI:10.3762/bjoc.11.113

Radical-mediated dehydrative preparation of cyclic imides
using (NH4)2S2O8–DMSO: application to the synthesis
of vernakalant
Dnyaneshwar N. Garad, Subhash D. Tanpure and Santosh B. Mhaske*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1008–1016.
CSIR-National Chemical Laboratory, Division of Organic Chemistry,
Pune 411 008, India
doi:10.3762/bjoc.11.113
Received: 09 April 2015
Accepted: 23 May 2015
Published: 12 June 2015
Email:
Santosh B. Mhaske* - sb.mhaske@ncl.res.in
* Corresponding author
Associate Editor: B. Stoltz
Keywords:
© 2015 Garad et al; licensee Beilstein-Institut.
License and terms: see end of document.
APS-DMSO; imides; practical synthesis; radical-mediated;
vernakalant
Abstract
Ammonium persulfate–dimethyl sulfoxide (APS–DMSO) has been developed as an efficient and new dehydrating reagent for a
convenient one-pot process for the synthesis of miscellaneous cyclic imides in high yields starting from readily available primary
amines and cyclic anhydrides. A plausible radical mechanism involving DMSO has been proposed. The application of this facile
one-pot imide forming process has been demonstrated for a practical synthesis of vernakalant.
Introduction
Cyclic imides are privileged pharmacophores and important cide), procymidone (pesticide) and cinidon-ethyl (herbicide)
building blocks for the synthesis of natural products, drugs, [18,19]. Imides are the backbones of several commercially
agrochemicals, advanced materials and polymers. Migrastatin, available high-performance polymers [20,21] and many other
lamprolobine, julocrotine, cladoniamide A, palasimide and advanced materials [22-25].
salfredin C-1 are few of the important natural products having
the imide motif [1-4]. Imides have been extensively used in the The most commonly used method to construct the imide func-
synthesis of several bioactive natural products [5-12]. Various tionality is to react the corresponding primary amines with
drugs such as lurasidone, phensuximide, buspirone, (R/S)- cyclic anhydrides to form amic acids, which are then cyclized
thalidomide, lenalidomide and apremilast contain cyclic imide using acetic anhydride and sodium acetate in a separate step
moieties and possess a wide range of biological properties [26]. Hexamethyldisilazane–zinc chloride has also become one
(Figure 1) [13-17]. Cyclic imides have found immense applica- of the most widely used dehydrating reagent for imide syn-
tions in agrochemicals such as chlorophthalim (herbicide), thesis [27]. Several dehydrating conditions such as heating in
captan (fungicide), flumipropyn (herbicide), flumioxazin (herbi- ionic liquid at 140 °C, heating at 150–180 °C under microwave
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
Figure 1: Natural products and drugs featuring imide core.
in various solvents, reaction with N,N'-disuccinimidyl oxalate processes to such an important building block is an area of
followed by heating in trichloroethylene with 4-(dimethyl- persistent interest [28-54].
amino)pyridine, Nb2O5, hydrothermal cyclization, H2SO4 in
acetic acid, PEG-600, silica supported TaCl5, esterification Results and Discussion
using inorganic base and alkylating agent followed by heating While working on the development of a palladium-catalyzed
in the presence of tetrabutylammonium bromide, diphenyl decarboxylative C–H activation methodology to access the
2-oxo-3-oxazolinylphosphonate and Et3N as well as several important core structure dihydroquinolone 1 from succinanilic
other conditions have been reported in the literature [28-45]. acid (2), we observed dehydrative cyclization to furnish
Thermal cyclization processes are still the preferred way for the N-phenylsuccinimide (3) instead of the expected product 1
synthesis of polyimides from the intermediate polyamic acid, (Scheme 1).
which is heated in high boiling solvents (>200 °C) with contin-
uous removal of water by distillation [46]. Imide synthesis We were curious to find the actual reagents responsible for this
methodologies starting from different types of starting ma- serendipitous facile transformation. We found that the cycliza-
terials also have been reported [47-54]. However, low yields, tion reaction works in the absence of a palladium catalyst and
use of acidic conditions, lack of generality, use of expensive without the need of inert atmosphere or Schlenk tube, however,
and toxic metal catalysts or reagents, formation of hazardous the presence of both APS and DMSO was necessary. Optimiz-
byproducts, difficult to access starting materials, azeotropoic ation of the protocol using various permutations and combina-
removal of water, column purification, harsh reaction condi- tions provided the ideal reaction conditions for imide synthesis
tions, longer reaction time and lack of scalability are some of (Table 1, entry 9). To the best of our knowledge, although, APS
the drawbacks of prior methods. The development of improved is being used in several commercial applications [55] and in
Scheme 1: Attempted methodology and its outcome (reaction conditions: (a) Pd(OAc)2 (10 mol %), ammonium persulfate (APS) (2 equiv), 1,4-
dioxane (0.1 M), DMSO (5% v/v), 100 °C, 3 h in a Schlenk tube).
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
Table 1: Optimization studiesa.
Entry
Oxidant/DMSO
(equiv)
Solvent
(3 mL)
Temp/Timeb
(°C/h)
Yield
(%)c
1
2
APS(2)/ 5% v/v
APS (2)/ -
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
–
100/3
100/3
100/4
100/6
100/6
100/6
100/8
100/8
100/6
100/6
25/4
72
10
–
3
– /5% v/v
4
APS (1.2)/ 4
APS (1.5)/ 4
APS (2)/ 4
40
75
94
72
78
93
15
–
5
6
7
APS (2)/ 1
8
APS (2)/ 1.5
APS (2)/2
9
10
11
12
13
14
K2S2O8 (1.2)/ 2
APS (2)/ excess
APS (2)/ 2
toluene
water
111/7
100/6
100/6
67
–
APS (2)/ -
APS (2)/2
water
–
aAll reactions were performed on 60 mg scale of amine 4 under air atmosphere in a round bottom flask equipped with a water condenser; btime
required for the radical cyclization step; cisolated yields.
organic [56-60] as well as polymer [55] chemistry as an ortho-position due to a probable formation of a radical;
oxidizing agent, it has been never reported to work as a dehy- however, we did not observe such effect (Table 2, entry 9). The
drating reagent via a radical pathway.
polyaromatic amine 2-aminoanthracene also reacted smoothly
under the optimized conditions (Table 2, entry 10). We studied
In this context, reported herein is a convenient one-pot process the effect of substituted succinic anhydrides and observed that
for the preparation of structurally diverse cyclic imides starting mono- and di-substituted succinic anhydride provides the
from readily available primary amines and cyclic anhydrides corresponding succinimides in excellent yield (Table 2, entries
using APS–DMSO as an efficient and novel dehydrating 11 and 12). Interestingly, the N-phenyl analogue of Captan, a
reagent and its application to a drug synthesis.
commercially used fungicide could be synthesized in excellent
yield (Table 2, entry 12) [18,19].
The scope of the developed protocol was studied on varyingly
substituted aliphatic/aromatic primary amines and saturated/ The reaction of aromatic amines with unsaturated anhydrides to
unsaturated cyclic anhydrides. The generalization of the form maleimides was investigated (Table 3). Aniline, p-tolui-
protocol was first studied on succinic anhydrides and various dine and p-cyanoaniline furnished the corresponding male-
aromatic amines (Table 2, entries 1–10). The reaction works imides from maleic anhydride (Table 3, entries 1–3). 4,4’-
well with aniline, alkyl-substituted aniline and aniline with elec- Oxydianiline and maleic anhydride also reacted well to provide
tron donating or withdrawing substituents at various positions 4,4’-bis(maleimidodiphenyl) ether in excellent yield (Table 3,
of the aromatic ring (Table 2, entries 1–5). The reaction entry 4). This bismaleimide and its analogues are important
performed equally well on 1 g scale (Table 2, entry 3). The monomers for the synthesis of polymers used in high tempera-
steric hindrance or electronic factors did not show much effect ture applications [61-63]. Methyl maleic anhydride and
on the yield of the reaction. Anilines having halogen p-bromoaniline smoothly provided the corresponding male-
substituents at various positions of the aromatic ring furnished imide (Table 3, entry 5). The reaction of p-toluidine and
the corresponding succinimides in high yields (Table 2, entries phthalic anhydride was very facile and the corresponding
6–9). We were expecting some interference by the iodine in the phthalimide was obtained in very good yield (Table 3, entry 6).
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
Table 2: Imides from substituted/unsubstituted aromatic amines and succinic anhydridesa.
Entry
1
Product
Timeb
6 h
Yieldc
93%
2
3
3 h
5 h
8 h
9 h
6 h
7 h
6 h
7 h
7 h
90%
90%
92%d
4
81%
93%
92%
95%
91%
89%
76%
5
6
7
8
9
10
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
Table 2: Imides from substituted/unsubstituted aromatic amines and succinic anhydridesa. (continued)
11
9 h
4 h
94%
97%
12
aReaction conditions: (i) amine (1 equiv, 60 mg scale), anhydride (1.1 equiv), 1,4-dioxane (0.1 M), 25/100 °C, 10 min–12 h; (ii) APS (2 equiv), DMSO
(2 equiv), 100 °C; btime required for the radical cyclization step; cisolated yields; dyield for the reaction using 1 g of amine.
Table 3: Imides from substituted/unsubstituted aromatic amines and unsaturated anhydridesa.
Entry
1
Product
Timeb
7 h
Yieldc
84%
2
3
4
5
6
10 h
7 h
7 h
6 h
3 h
87%
94%
96%
80%
90%
aReaction conditions: (i) amine (1 equiv, 60 mg scale), anhydride (1.1 equiv), 1,4-dioxane (0.1 M), 25/100 °C, 15 min–24 h; (ii) APS (2 equiv), DMSO
(2 equiv), 100 °C; btime required for the radical cyclization step; cisolated yields.
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
Furthermore, the scope of the protocol to obtain imides from entries 1–3). Interestingly, the use of secondary and tertiary ali-
aliphatic amines and saturated anhydride was explored phatic amines also worked equally well (Table 4, entries 4 and
(Table 4). Primary aliphatic amines with short and long alkyl 5). Benzylamine was reacted with succinic anhydride and
chains were treated with succinic anhydride and they were diacetoxysuccinic anhydride to obtain the respective succin-
found to give succinimides in good to excellent yields (Table 4, imides in high yields (Table 4, entries 6 and 7). The formation
Table 4: Imides from aliphatic amines and saturated anhydridesa.
Entry
1
Product
Timeb
7 h
Yieldc
85%
2
3
4
6 h
7 h
7 h
99%
99%
75%
5
6
7
8
7 h
7 h
8 h
4 h
65%
95%
85%
98%
aReaction conditions: (i) amine (1 equiv, 60 mg scale), anhydride (1.1 equiv), 1,4-dioxane (0.1 M), 25/100 °C, 5 min–1 h; (ii) APS (2 equiv), DMSO
(2 equiv), 100 °C; btime required for the radical cyclization step; cisolated yields.
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
of imide from benzylamine and glutaric anhydride worked t-BuOK in THF. Nucleophilic ring opening of the aziridine 7 by
smoothly (Table 4, entry 8).
homoveratryl alcohol in the presence of a catalytic amount of
the Lewis acid BF3·OEt2 furnished compound 8. The -NHBoc
Our protocol worked efficiently with all types of amines and was deprotected using 4 M HCl in 1,4-dioxane. The free amine
anhydrides (Tables 2–4), but unfortunately it could not be 9 obtained after neutralization was directly subjected to the
applied successfully to the synthesis of imides from the combi- developed protocol, wherein the solution of (R)-acetoxy
nation of aliphatic amines and unsaturated anhydrides. Plau- succinic anhydride and the amine 9 was stirred at room
sibly, the intermediate amic acid in these cases may be prone to temperature for 30 min followed by the addition of
decarboxylation [57] and radical polymerization similar to the APS–DMSO and further heating the reaction mixture at 100 °C
acrylamide polymerization using APS as a radical initiator [55]. for 8 h to give succinimide 10 without any racemization at the
C-3 centre. Finally, reduction of the imide and deprotection of
Encouraged by this elegant transformation (Tables 2–4), we the acetyl moiety was done in a single step by LiAlH4 to obtain
planned to explore our imide forming protocol to the synthesis vernakalant (11) as the free base. The synthesis was completed
of the drug vernakalant (11). It was discovered by Cardiome/ in 5 steps with 51% overall yield.
Astellas Pharma Inc. and later developed as a novel antiar-
rhythmic agent for the treatment of atrial fibrillation (cardiac A preliminary mechanistic study revealed that the transforma-
arrhythmia leading to strokes) in collaboration with Merck & tion developed herein may be proceeding via a radical pathway.
Co., Inc. Its intravenous formulation has been approved as a The reaction was conducted with the substrate 2 using our stan-
drug by the European agency (EMEA) under the trade name dard protocol in the presence of TEMPO (Figure 2), wherein
Brinavess (Cardiome/Merck) [64]. Few synthetic routes to complete inhibition of the reaction was observed, thus indi-
vernakalant (11) and its intermediates have been reported in the cating involvement of radical intermediates [57]. Similarly,
literature [64]. Our plan was to devise a concise and practical
synthetic route. The planned synthetic strategy is illustrated in
Scheme 2.
The synthesis began from Boc-protected trans-amino alcohol 6
[65]. The two-step sequence – tosylation of the secondary
alcohol 6 followed by elimination to form the Boc- protected
Figure 2: Radical trapping experiment.
aziridine 7 – was optimized in a one-pot procedure using
Scheme 2: A practical synthesis of vernakalant (11).
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Beilstein J. Org. Chem. 2015, 11, 1008–1016.
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In conclusion, we have developed a novel and efficient protocol
“APS–DMSO” for the synthesis of cyclic imides. The scope of
the developed protocol is wide and pure products could be
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D.N.G. thanks UGC-New Delhi for the research fellowship.
S.B.M. gratefully acknowledges generous financial support
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