The facile synthesis of 1-benzoazepine derivatives via gold-catalyzed regioselective cycloisomerization reactions of N-(o-alkynylaryl)-N-vinyl sulfonamides

Article abstract of DOI:10.1039/c6cc01061j

Gold-catalyzed, regioselective cycloisomerization of N-(o-alkynylaryl)-N-vinyl sulfonamides afforded high yields of 2-sulfonylmethyl-1-benzoazepine derivatives. This 7-endo-dig selective cyclization proceeds via the incorporation of an exocyclic double bond by a labile 1-benzoazepine intermediate. The cyclization substrates were assembled in two steps from readily available materials using Sonogashira coupling and a Cs₂CO₃-mediated formal vinylic substitution.

Full text of DOI:10.1039/c6cc01061j

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Facile synthesis of 1-benzoazepine derivatives via gold-catalyzed
regioselective cycloisomerization reactions of N-(o-alkynylaryl)-N-
vinyl sulfonamides
Received 00th January 20xx,
Accepted 00th January 20xx
Sridhar Undeela,a Gurram Ravikumar,a Jagadeesh Babu Nanubolu,b Kiran Kumar Singarapuc and
Rajeev S. Menon*d
DOI: 10.1039/x0xx00000x
www.rsc.org/
Gold-catalyzed, regioselective cycloisomerization of N-(o-alkynylaryl)-N- intramolecular addition of nitrogen nucleophiles to gold-activated
vinyl sulfonamides afforded high yields of
2-sulfonylmethyl-1- π-systems (carbon-nitrogen bond formation) or cyclization of
benzoazepine derivatives. This 7-endo-dig selective cyclization proceeds nitrogen tethered enynes (carbon-carbon bond formation).11
via a labile 1-benzoazepine intermediate incorporating an exocyclic double Formation of seven-membered nitrogen heterocycles in gold-
bond. The cyclization substrates were assembled in two steps from readily catalyzed cyclization of nitrogen bearing π-systems, however, are
available materials using Sonogashira coupling and a Cs2CO3-mediated relatively rare.12 In general, nitrogen tethered 1,6- and 1,7-enynes
formal vinylic substitution.
are known to cycloisomerize in the presence of gold catalysts to
afford five or six-membered nitrogen heterocycles.13
Benzo-fused, seven membered nitrogen heterocycles, commonly
known as benzoazepines, are found in a variety of biologically active
synthetic compounds.1 Benzoazepines are further classified based
on the extent of unsaturation as well as the position of the nitrogen
atom. Among them, the 1-benzoazepine moiety forms the core
units of antagonists of N-methyl-D-aspartate (NMDA),2 vasopressin
V2 receptor,3 CC chemokine receptor-54 as well as anti-parasitic
agents5 potentially useful for the treatment of leishmaniasis.
Despite their importance, the 1-benzoazepine class of compounds
has received scant synthetic attention when compared to 2- and 3-
benzoazepines. Only a few methods based on approaches such as
intramolecular Heck coupling,6 iridium-catalyzed asymmetric allylic
amination,7 cyclization reactions of suitably substituted Morita-
Baylis-Hillman adducts8 and insertion of diazocarbonyl compounds
to quinolinium salts (Scheme 1a)9 have been described for the
construction of 1-benzoazepines. Evidently, there is a demand for
general and convenient protocols to access substituted 1-
benzoazepine derivatives from readily available precursors.
Recently,
rearrangement of N-propargyl-N-vinyl sulfonamides leading to the
divergent synthesis of 2-sulfonylmethylpyrroles and
we
reported
gold-catalyzed
propargyl-Claisen
dihydropyridines.14 The aza-enyne precursors were, in turn,
conveniently prepared from 2-bromoallyl sulfones and
sulfonamides via a base-mediated formal vinylic displacement
reaction that was developed in our group.15 This metal-free union
of a vinyl bromide with sulfonamides affords N-vinyl sulfonamides
that incorporate an allyl sulfone moiety. The conter-intuitive higher
thermodynamic stability of allyl sulfones vis-à-vis vinyl sulfones
presumably prevents the isomerization of this double bond to form
the more substituted and conjugated vinyl sulfone.16 We surmised
that the nucleophilicity of these N-vinyl sulfonamides could be
exploited in gold-catalyzed synthesis of higher nitrogen
heterocycles. Our efforts along this direction culminated in the
development of a gold-catalyzed, regioselective cycloisomerization
of N-(o-alkynylaryl)-N-vinyl sulfonamides
1
to afford 1-
benzoazepine derivatives 2 (Scheme 1b). Intriguingly, the more
commonly observed and entropy-favored 6-exo-dig mode of
cyclization did not occur. The preliminary results of this study
constitute the subject matter of this communication.
Homogeneous gold catalysis constitutes a versatile method for the
synthesis of carbocycles and heterocycles of varying ring sizes and
oxidation levels.10 Gold-catalyzed construction of nitrogen
heterocycles follows one of the following two routes;
a.Medicinal Chemistry and Pharmacology Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad, 500 007, India.
b.Centre for X-ray Crystallography, CSIR-Indian Institute of Chemical Technology,
Hyderabad, 500 007, India.
c. Centre for Nuclear Magnetic Resonance, CSIR-Indian Institute of Chemical
Technology, Hyderabad, 500 007, India
d.Department of Chemistry, Central University of Haryana, Mahendergarh, Haryana
123 029, India. Email: rajeevmenon@cuh.ac.in
Electronic Supplementary Information (ESI) available: [General experimental
procedures, NMR data, and single crystal X-ray data for CCDC 1446305 and CCDC
1446306] For ESI and crystallographic data, See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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complete isomerization of the initially formed intermediate to
afford a single, stable product (entry 10). The latter was assigned
benzoazepine structure 2a based on standard spectroscopic
analysis. We observed that both acids and bases promoted the
isomerization to various extents. Triethylamine was chosen for this
purpose because it promoted an instantaneous and clean
isomerization (˂ 1 min). Addiꢀonally, its use allowed a simple work-
up procedure which involved the addition of silica gel to the
reaction mixture, evaporation of solvent and subsequent
chromatography.
Table 1. Optimization of reaction conditions for cycloisomerization of N-vinyl
sulfonamide 1a.a
Scheme 1: (a) Yadav’s method for the synthesis of fully unsaturated 1-benzoazepine
derivatives. (b) The present work
The aza-enynes 1 employed in the present study were conveniently
prepared in two steps from readily available precursors. The union
of bromoallyl sulfones 3a-b and o-iodoaniline derivatives 4a-b was
achieved by the cesium carbonate-mediated formal vinylic
displacement reaction that was recently developed in our group.
Bromoallyl sulfones 3a-b function as stable synthetic equivalents of
allenyl sulfones in this reaction.17 The N-vinyl sulfonamides 5a-d
were then subjected to Sonogashira coupling with various terminal
alkynes to install the alkynyl functionality (Scheme 2). It is
noteworthy that the potentially base-sensitive allylic sulfone unit of
5 was unaffected during the Sonogashira reaction.16
Entry
1c
Catalyst (loading, mol%)
AuCl3 (2)
Time (h)
12
Product
(Yield)b
1a
2c
PPh3AuCl (2)
12
1a
3c
12
1a
AgSbF6 (5)
4c
12
1a
AgOTf (5)
5c
12
1a
AuCl3 (2) & AgSbF6 (5)
PPh3AuCl (2) & AgSbF6 (5)
PPh3AuCl (2)
6c
12
1a
7d
2
6 (44%)
6 (53%)
-
8d
2
AgOTf (5)
9e
12
(JohnPhos)Au(CH3CN)SbF6 (2 )
(JohnPhos)Au(CH3CN)SbF6 (2 ),
then 2 equiv Et3N
10
12
2a (89%)
Scheme 2: Synthesis of cyclization substrates
aReaction conditions: 1a, catalyst, CH2Cl2, 25 °C; bisolated yields; c1a was
isolated unchanged; dreaction at 40 °C; e1a was consumed and the product
isomerized during chromatography to afford a mixture of products.
With an assortment of 3-aza-1,6-enynes 1a-s at hand, the π-
activation of their alkyne subunits by gold and silver catalysts were
then explored. Results of the optimization study carried out on the
representative substrate 1a are summarized in Table 1. At ambient
temperature, 1a was unchanged on treatment with AuCl3,
PPh3AuCl, AgSbF6 and AgOTf even after 12h (entries 1-4). The
combination of AgSbF6 and two gold catalysts (AuCl3 and PPh3AuCl)
was also ineffective in promoting any reaction (entries 5-6). On
heating, PPh3AuCl and AgSbF6 catalyzed the undesired hydrolytic
cleavage of 1a affording low yields of 2-(phenylsufonyl)acetone 6
The results depicted in Table 1 clearly demonstrate the superior
catalytic ability of Echavarren’s catalyst [JohnPhosAu(CH3CN)]SbF6
when compared to AuCl3 and PPh3AuCl.18 Interestingly, the 7-endo
mode of cylization is preferred here over the kinetically favored 6-
exo pathway leading to quinoline-class of products. The efficiency
and exclusive regioselectivity of the pilot reaction prompted us to
explore the method’s scope and generality for the construction of
benzoazepine system. Thus, all the available 3-aza-1,6-enynes 1a-s
were subjected to the optimized conditions for cycloisomerization.
Pleasingly, these substrates emulated their congener 1a in gold-
catalysed cycloisomerization reactions to afford excellent yields of
corresponding benzoazepines 2a-r (Table 2). Single crystal X-ray
analysis of the representative benzoazepine 2r confirmed the
assigned structure.19 Disappointingly, the trimethylsilyl-substituted
(entries
7-8).
Echavarren’s
gold(I)
catalyst
[JohnPhosAu(CH3CN)]SbF6, on the other hand, promoted the
conversion of 1a into a labile product that appeared to isomerize
partially during chromatography on silica (entry 9). Further
experiments revealed that the addition of
2 equivalents of
triethylamine to the reaction mixture after 12h caused rapid and
2 | J. Name., 2012, 00, 1-3
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alkyne 1s afforded an intractable mixture of products on exposure in solution (see ESI for details). Treatment of 7 with triethylamine
to the gold catalyst. Attempts to generate the corresponding caused rapid isomerization to afford quantitative yield of the
terminal alkyne by desilylation of 1s also furnished a mixture of benzoazepine 2d. By analogy, it may be deduced that the gold-
products presumably formed via the base-mediated isomerization catalyzed cycloisomerizations of aza-enynes 1a-s generate vinylic
of the allyl sulfone moiety.
sulfone products (such as 7) that are then transformed into the
more stable allylic sulfone 2a-s via base (or acid) mediated
isomerization.16
Table 2. Scope of gold-catalyzed cycloisomerization of 3-aza-1,6-enynes 1a-s.a,b
Scheme 3
.
Isolation of intermediate 7 and its Et3N-mediated isomerization
The
above-described
regioselective
gold-catalysed
cycloisomerization of benzenoid 3-aza-1,6-enynes 1a-s inspired us
to examine the reaction of an aliphatic analogue under similar
conditions.
A representative aliphatic 3-aza-1,6-enyne 8 was
prepared via the cesium carbonate-mediated vinylic displacement
reaction of bromoallyl sulfone 3b with yne-sulfonamide 9 (Scheme
4). On subjecting
8 to the conditions of gold-catalyzed
cycloisomerization, the 4-methylene tetrahydropyridine derivative
10 was obtained in 75% yield. Evidently, 10 is formed via a 6-exo-
dig mode of cyclization. Presumably, this is the result of bonding of
the gold catalyst to the less hindered terminal carbon of the alkyne
unit. The flexible bis-methylene tether in 8 could also impose a
higher entropic demand on the 7-endo mode of cyclization.
2 mol% [Au]
CH2Cl2, 30 min.
Ts
Ts
Cs2CO3
Br
NHTs
Ts
CH3CN
25 °C, 2h
then Et3N
NTs
N
1 min, 25 °C
Ts
8
10
75%
3b
9
[Au] = [JohnPhosAu(CH3CN)]SbF6
Scheme 4. Gold-catalyzed cycloisomerization of aliphatic 3-aza-1,6-enyne
afford a 4-methylene tetrahydropyrdine derivative 10
8 to
.
aReaction conditions: 1a-s (0.20 mmol), Au-catalyst (3 mg, 2 mol %), CH2Cl2 (2
mL), 25 °C, 12h, then Et3N (0.06 mL, 0.40 mmol), 1 min. bYields of products
isolated after column chromatography.
It was of interest to identify the unstable intermediate that
presumably isomerized on exposure to triethylamine to afford the
benzoazepines 2a-r. Our efforts along this direction met with
fortuitous success in the case of the p-methoxyphenyl group-
bearing aza-enyne 1d. Flash chromatography of the reaction
mixture after gold-catalyzed reaction (without triethylamine
treatment) of the latter afforded a single compound. The latter was
stable enough to permit complete spectroscopic analysis and it was
assigned the benzoazepine structure 7 in which the double bond
occupied an exocyclic position (Scheme 3). Unambiguous evidence
for the assigned structure and geometry of the exocyclic double
bond was obtained by single crystal X-ray analysis.19 It was
observed that 7 was stable in crystalline state but isomerized slowly
Scheme 5. A mechanistic hypothesis for the Au-catalyzed cycloisomerization
A
simplified mechanistic rationalization for the gold-catalyzed
cycloisomerization can be advanced as depicted in Scheme 5. The
alkyne unit activated by co-ordination to the Au(I) catalyst suffers a
nucleophilic attack via the electron rich end of the enamine-like
olefin of 1a. Loss of the acidic α-sulfonyl proton from 11 generates
the exocyclic olefin 12. Exposure to triethylamine promotes the
isomerization of the double bond into the endocyclic position to
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8
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afford the final product 2a. The 7-endo-dig ring closure is
uncommon in gold-catalyzed cycloisomerizations of enynes,
however, a preference for this pathway has been reported in
isolated cases of formation of 7-membered lactams from aryl ring-
bearing alkynes.20
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In conclusion, an exclusively 7-endo-dig-selective, gold-catalyzed
cycloisomerization of N-(o-alkynylaryl)-N-vinyl sulfonamides to
afford 2-sulfonylmethyl-4-arylbenzoazepines in excellent yields is
developed. This unique class of nitrogen heterocycles can be
conveniently synthesized from readily available precursors that are,
in turn, assembled via a base-mediated vinylic substitution reaction
of 2-bromoallyl sulfones and aniline-sulfonamides. The present
protocol allows the generation of a library of hitherto inaccessible
benzoazepines for biological evaluation. The well-known
pharmacological importance of closely related benzodiazepines
makes such an endeavor worthwhile and efforts along this direction
are currently underway.
11 Selected reviews: (a) S. Nayak, B. Prabagar and A. K. Sahoo,
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The Department of Science and Technology (DST), India is
acknowledged for the award of Ramanujan fellowships to both RSM
(SR/S2/RJN-05/2011) and KKS. DST is thanked for the award of a
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2996; (b) A. Kumar, Z. Li, S. K. Sharma, V. S. Parmar and E. V.
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Start-up Research Grant (CS-141/2011) for RSM.
Research
Fellowships for SU and GR from the Council of Scientific and
Industrial Research (CSIR), India are also acknowledged.
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