Catalyst-controlled divergence in cycloisomerisation reactions of N-propargyl-N-vinyl sulfonamides: Gold-catalysed synthesis of 2-sulfonylmethyl pyrroles and dihydropyridines

Article abstract of DOI:10.1039/c5cc04871k

Gold-catalysed, divergent synthesis of 2-sulfonylmethyl pyrroles and dihydropyridines from N-propargyl-N-vinyl sulfonamides has been achieved. Echavarren's gold(i) catalyst promoted the formation of pyrrole derivatives whereas the combination of PPh₃AuCl and AgSbF₆ afforded dihydropyridines. The aza-enyne precursors for the cycloisomerisation reaction were prepared by a base-mediated formal vinylic substitution reaction of 2-bromoallyl sulfones.

Full text of DOI:10.1039/c5cc04871k

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Catalyst-controlled divergence in cycloisomerisation reactions of
N-propargyl-N-vinyl sulfonamides: Gold-catalysed synthesis of 2-
sulfonylmethyl pyrroles and dihydropyridines
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
c
DOI: 10.1039/x0xx00000x
Sridhar Undeela, Srinivas Thadkapally, Jagadeesh Babu Nanubolu, Kiran Kumar Singarapu and
a
Rajeev S. Menon*
www.rsc.org/
Gold-catalysed, divergent synthesis of 2-sulfonylmethyl pyrroles propargyl-N-vinyl sulfonamides
1 to afford 2-sulfonylmethyl
and dihydropyridines from N-propargyl-N-vinyl sulfonamides has pyrroles 2 or 2-sulfonylmethyl-1,2-dihydropyridines 3 (Scheme 1b).
been achieved. Echavarren’s gold(I) catalyst promoted the Remarkably, complete selectivity can be achieved by the choice of
formation of pyrrole derivatives whereas the combination of catalysts under otherwise identical conditions. Additionally, the
PPh
3
AuCl/AgSbF
6
afforded dihydropyridines. The aza-enyne cyclisation precursors were assembled via a novel, base-mediated
precursors for the cycloisomerisation reaction were prepared by a formal vinylic substitution reaction of propargyl sulfonamides 4 and
base-mediated formal vinylic substitution reaction of 2-bromoallyl 2-bromoallylsulfones 5 (Scheme 1b).
sulfones.
a) Divergent catalysis-concept
The development of suitable catalysts and conditions for promoting
cat.
cat.
a desired reaction pathway of multifunctional substrates is one of
path a
path b
1
the fundamental challenges of catalysis. An additional level of
product a
b) This work
R1
a
product b
b
selectivity may be contemplated wherein a common substrate can
be transformed into more than one products by careful choice of
catalysts/conditions. Such endeavors, commonly known as
divergent catalysis, are highly attractive for the discovery of drugs
R1
[Au]
4
SO R
2
R1
N
2
CH Cl
2
R2
R2
SO
2
R3
2
NH
SO
and functional materials (Scheme 1a). Although the benefits of
R3
Cs
2
CO
3
2 (87-99%)
2
4
CH
2
3
CN
SO R
4
R
1
divergent catalytic processes are obvious, their development is
5 °C R2
2
N
3
R4
Br
SO
2
3
[Au-Ag]
CH Cl
extremely challenging.
SO R
2
R4
SO
2
Catalytic cylcoisomerisation of enynes constitute
a
powerful,
1
2
2
N
5
5-94%
SO
3
R3
modern method for the construction of a variety of carbocycles and
2
5
(78-98%)
Scheme 1. a) Conceptual framework of divergent catalysis. b) Synthesis of aza-enynes
and their divergent cycloisomerisation. [Au] = [JohnPhosAu(CH CN)]SbF6. [Au-Ag] =
Ph PAuCl/ AgSbF
4
heterocycles. Among the various catalysts that are employed for
enyne cycloisomerisation, gold complexes stand out owing to their
3
5
ability to selectively activate alkynes in complex molecular settings.
3
6
.
As a result, tremendous progress has been made in the recent years
4
,5a-b,6
Discovery of the above-described vinylic substitution has its origins
in our recent investigations on the cyclocondensation reactions of
in this area.
Often, two or more cyclisation pathways are
avilable for substituted enynes and a complex interplay of inherent
substrate bias and catalysts dictate the outcome of the
9
unsaturated sulfones. We were interested in allenyl sulfones as
7
potential building blocks for sulfone-containing cyclic systems. In
cycloisomerisation. While it is highly desirable, control over the
1
0
8
view of the rich and versatile chemistry of allenoates, allenyl
sulfones appear to hold significant but largely untapped synthetic
potential. The relative scarcity of studies involving allenyl sulfones
may be attributed to their base-sensitivity and propensity to
available divergent pathways is realised rather rarely. Here we
report gold-catalysed divergent cycloisomerisation reactions of N-
1
1
a.
undergo nucleophile-triggered oligomerisation reactions. Our
efforts to circumvent this problem culminated in the development
of a base-mediated formal vinylic substitution reaction of the 2-
Medicinal Chemistry and Pharmacology Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad, 500 007, India.
Centre for X-ray Crystallography, CSIR-Indian Institute of Chemical Technology,
Hyderabad, 500 007, India.
Centre for Nuclear Magnetic Resonance, CSIR-Indian Institute of Chemical
Technology, Hyderabad, 500 007, India.
b.
12
c.
bromoallyl sulfones 5a-b and various sulfonamide nucleophiles
4a-h to afford the N-propargyl-N-vinyl sulfonamide 1aa-hb (Table
Electronic Supplementary Information (ESI) available: [General experimental
procedures, NMR data, and single crystal X-ray data for CCDC 1063242.] For ESI
and crystallographic data, See DOI: 10.1039/x0xx00000x
1
). The intermediacy of 1-(phenylsulfonyl)allene was confirmed by
further experiments (See supporting information for details).
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The N-propargyl-N-vinyl sulfonamides 1aa-hb constitute an
assortment of 3-aza-1,5-enynes with structural features that make
them attractive substrates for catalytic processes for the synthesis
Table 2. Optimisation of reaction conditions for cycloisomerisation of N-propargyl-
a
N-vinyl sulfonamide 1aa.
1
3
of nitrogen heterocycles. The activation of the alkyne subunit by
π-acids such as gold and silver catalysts was investigated with a
view to promote cyclisation reactions. Screening of a handful of
gold and silver catalysts revealed that two cyclisation pathways are
operative leading to the formation of the pyrrole derivative 2aa and
the dihydropyridine product 3aa (Table 2).
Entry
Catalysts (mol%)
AuCl (5)
(JohnPhos)Au(CH CN)SbF
Ph PAuCl (5)
AuCl (5) & AgSbF
Time Yield of
Yield of
Table 1. CS
2 3
CO -mediated formal vinylic substitution of 2-bromoallyl sulfones 5a-b for
b
b
a
(h)
3
2aa
3aa
the synthesis of N-propargyl-N-vinyl sulfonamides 1aa-hb.
1
2
3
4
5
3
78%
-
3
6
(2)
1
2
2
1
89%
-
3
-
Traces
-
58%
46%
48%
3
6
(5)
a
(JohnPhos)Au(CH
AgSbF
3
CN)SbF
(5)
6
(2) &
Entry
Sulfonamides
Bromoallyl
sulfones
Product
Yield
6
4
a-h
5
a-b
6
7
Ph
3
PAuCl (2) & AgSbF
6
(5)
1
4
-
-
-
92%
1
2
3
4
1
2
3
4
5
6
7
8
9
4a, R = R = H, R
5a, R = Ph
1aa
1ab
1ba
1bb
1ca
1cb
1da
1db
1ea
94%
89%
96%
92%
93%
90%
95%
90%
82%
AgSbF (10)
6
66%
=
Me
c
8
AgOTf (10)
No catalyst
2
-
-
4a
5b,
4
R = p-tolyl
d
1
2
3
9
24
-
4b, R = R = H, R
5a
5b
5a
5b
5a
5b
5a
a
b
=
Ph
Reaction conditions: 1aa (0.5 mmol), catalysts, CH Cl (2 mL). Isolated yield
2
2
c
d
4b
after chromatography. Hydrolysis of 1aa was observed. 1aa was unchanged
Ms = methanesulfonyl, JohnPhos = (2-biphenyl)di-tert-butylphosphine].
[
1
2
4c, R = R = H,
3
R = p-tolyl
Gold(III) chloride (entry 1) and Echavarren’s gold(I) catalyst
[JohnPhosAu(CH CN)]SbF (entry 2) promoted the formation of the
pyrrole derivative 2aa in 78% and 89% yields respectively, whereas
Ph PAuCl catalysed the formation of the dihydropyridine 3aa in
8% yield (entry 3). Interestingly, dihydropyridine 3aa was
selectively formed when a silver co-catalyst AgSbF was used with
4c
3
6
1
2
4d, R = Ph, R =
3
3
H, R = Me
5
4d
6
any of these gold catalysts (entries 4-6). Selective formation of the
1
4e, R =
dihydropyridine 3aa in a reasonably good yield (66%) was also
thiophene-2-yl,
2
3
observed when AgSbF
Silver triflate, however, failed to promote the cyclisation reaction
altogether (entry 8). The combination of 2 mol% Ph PAuCl and 5
mol% AgSbF afforded the best yield of dihydropyridine 3aa, while
6
alone was employed as a catalyst (entry 7).
R = H, R = Me
1
1
0
1
4e
5b
5a
1eb
1fa
94%
93%
3
1
4f, R = 2-
6
bromophenyl,
Echavarren’s catalyst (2 mol%) furnished the highest yield of the
pyrrole 2aa. It may be noted that, in almost all cases, the cyclisation
afforded either the dihydropyridine or the pyrrole exclusively.
The position of substituents on the pyrrole ring of 2aa indicated
2
3
R = H, R = Me
1
1
2
3
4f
5b
5a
1fb
1ga
89%
88%
1
4g, R = 4-
that 1aa has undergone
a gold-catalysed propargyl-Claisen
methoxyphenyl,
14
rearrangement prior to the final cyclisation, whereas the skeletal
reorganisation was not apparent in the structure of dihydropyridine
2
3
R = H, R = Me
1
1
4
5
4g
5b
5b
1gb
1hb
84%
55%
3
aa. The intriguing catalyst-controlled divergence in the cyclisation
1
2
pathways called for further investigations. Thus, the N-propargyl-N-
vinyl sulfonamides 1aa-bh (table 1), prepared via the formal vinylic
substitution method, were separately subjected to the optimised
conditions for both the cycloisomerisation reactions. A variety of
pyrrole derivatives 2aa-hb and dihydropyridines 3aa-hb were
4h, R = H, R =
3
Ph, R = Me
a
Reaction conditions: 4a-g (1mmol), 5a-b (1.2 mmol), Cs
2
CO
3
(2.2 mmol),
b
acetonitrile (5mL), 25 °C, 4 h. Yields of products isolated after column
c
chromatography. Reaction conditions 4h (1mmol), 5b (1.2 mmol), NaH (2.2
mmol), THF (5mL), 0 °C, 0.5 h and 1hb obtained as a mixture of isomers (see formed in these reactions and they are listed in Table 3. It was
supporting information for details).
evident that the cycloisomerisation reactions and the catalyst-
2
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a
controlled divergence were general. The sulfonyl groups on the Table 3. Gold and silver-catalysed divergent cycloisomerisation of aza-enynes 1aa-hb.
nitrogen (from alkyne component) as well as carbon (allene
component) can be varied. Cycloisomerisations of aza-enynes with
1
arenes or a heteroarene (2-thienyl) as the alkyne substituent (R )
afforded
hetero)aryldihydropyridines (3da-fb) in excellent yields. Single
crystal X-ray analysis of the representative pyrrole 2da confirmed
3-(hetero)arylpyrroles
(2da-fb)
and
4-
(
15
the assigned structure.
Interestingly, dihydropyridine formation was not observed when
2
the aza-enyne 1hb carrying a propargylic phenyl substituent (R =
SO2R4
S
Me
Ph) was subjected to either of the conditions for cyclisation. In both
instances, 2-benzyl-5-sulfonylmethyl pyrrole 2hb was formed as the
sole product. Presumably, presence of the propargylic phenyl
substituent imparts a substrate bias for pyrrole formation that
overrides the catalyst-control of cycloisomerisation pathways.
It is important to note that 3-arylpyrroles are much-preferred
intermediates for the synthesis of a number of natural products
N
4
SO2R4
2
SO R
SO2R3
Me
Me
N
N
4
2
aa, R3 = Me, R = Ph, 89%
SO2Me
SO2Me
ab, R = Me, R4 = p-tolyl, 96%
3
2
2ea, R4 = Ph, 95%
2eb, R = p-tolyl, 97%
2da, R4 = Ph, 96%
4
2ba, R3 = Ph, R4 = Ph, 98%
2bb, R3 = Ph, R4 = p-tolyl, 97%
ca, R3 = p-tolyl, R4 = Ph, 97%
4
2
db, R = p-tolyl, 92%
2
2
cb, p-tolyl, R4 = p-tolyl, 99% MeO
1
6
Br
such as rhazinilam, lamellarins and pyrrolnnitrin. Similarly, 1,2-
dihydropyridines are useful precursors for the preparation of
polysubstituted 6-membered nitrogen heterocycles and
2
SO Ph
SO2R4
SO2R4
N
Me
N
Me
Ph
Ms
N
SO2Me
2ga, R4 = Ph, 91%
2gb, R = p-tolyl, 98%
2hb, R4 = p-tolyl, 87%
1
7
SO2Me
isoquinuclidines. Therefore, the quick and efficient assembly
reported here, of pyrroles and 1,2-dihydropyridines armed with
readily malleable substituents at 2- and 4-positions, has the
potential to find applications in targeted synthesis.
4
2
fa, R = Ph, 87%
2fb, R4 = p-tolyl, 89%
4
4
S
SO R
2
N
A tentative mechanistic rationalisation for the formation and
cycloisomerisation of the representative aza-enyne 1da is depicted
in Scheme 2. The control experiments indicated that the allenyl
sulfone 6 is a plausible intermediate in the formation of the aza-
enyne 1. The latter undergoes a metal-mediated propargyl-Claisen
SO R3
2
3
4
4
3aa, R = Me, R = Ph, 92%
SO2R4
2
SO R
3
4
N
N
3
ab,R = Me,R = p-tolyl, 95%
SO2Me
SO Me
3
ba, R3 = Ph, R4 = Ph, 97%
3bb, R = Ph, R = p-tolyl, 96%
ca, R3 = p-tolyl, R4 = Ph, 98%
cb, R = p-tolyl, R = p-tolyl, 96%
2
3
4
3da, R4 = Ph, 93%
3db, R = p-tolyl, 89%
3ea, R4 = Ph, 78%
3eb, R = p-tolyl, 96%
4
3
3
4
3
4
1
4
rearrangement to generate the β-allenyl imine 7A. It may be
noted that the aza-enyne 1aa did not undergo any rearrangement
in the absence of catalysts (table 2, entry 9). Gold-mediated 5-exo-
dig cyclisation of 7A and subsequent aromatisation via hydrogen
shift affords the pyrrole derivative 2da. The dihydropyridine 3da is
OMe
SO2R4
Br
Ph
N
2
SO2R4
N
SO Me
SO2R4
N
2hb. R4 = p-tolyl, 47%
1
8
SO2Me
fa, R4 = Ph, 98%
fb, R = p-tolyl, 97%
presumably formed via the tautomerisation of 7A to the aza-
SO2Me
3ga, R4 = Ph, 98%
3gb, R = p-tolyl, 97%
3
3
triene 7B and subsequent 6π-aza-electrocyclisation.
4
4
The transformations listed in Scheme 3 provide a glimpse of the
synthetic potential embedded in the heterocycles generated in this
study. A regioselective bromination of the pyrrole 2aa to afford 3-
a
Reaction conditions: 1aa-hb (0.5mmol), catalysts, CH
Isolated yields. [Au] = 2 mol% (JohnPhos)Au(CH CN)SbF
Ph PAuCl & 5 mol% AgSbF
2
Cl
2
(2 mL), 25 °C, 1 h.
3
6
; [Au-Ag] = 2 mol%
3
6
.
bromopyrrole
8
was achieved by treatment with N-
bromosuccinimide in THF. Similarly, site-selective catalytic
hydrogenation of the dihydropyridine 3ab furnished the
tetrahydropyridine 9. The dihydropyridine 3ab was also converted
1
9
smoothly into the corresponding pyridine derivative 10 via base-
promoted elimination of methanesulfinate group. The
phenylsulfonyl group has been termed ‘arguably the most versatile
2
0
functional group’ by Fuchs and it is conceivable that its potential
may be exploited in further synthetic elaboration at the
methylenesulfonyl unit of the products.
Scheme 2. Mechanistic rationalisation for the formation and cycloisomerisation
of aza-enyne 1da.
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Chem. Soc. Rev., 2012, 41, 2448; (d) H. C. Shen, Tetrahedron,
2
1
008, 64, 3885; (e) Z. Li, C. Brouwer, C. He, Chem. Rev. 2008
08, 3239; (f) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180.
,
6
7
For selected reviews, see: (a) L. Fensterbank, M. Malacria,
Acc. Chem. Res., 2014, 47, 953; (b) C. Obradors, A. M.
Echavarren, Acc. Chem. Res., 2014, 47, 902; (c) A. Fürstner,
Acc. Chem. Res., 2014, 47, 925.
(a) E. Jiménez-Núñez, A. M. Echavarren, Chem. Rev., 2008,
1
(
08, 3326; (b) D. J. Gorin, F. D. Toste, Nature, 2007, 446, 395;
c) A. Gimeno, A. B. Cuenca, S. Suárez-Pantiga, C. R. de
Arellano, M. Medio-Simón, G. Asenio, Chem. –Eur. J., 2014,
, 683.
Scheme 3. Synthetic applications of the pyrrole 2aa and dihydropyridine 3ab.
2
0
In summary, a catalyst-controlled divergent cycloisomerisation
reaction of N-propargyl-N-vinyl sulfonamides to selectively afford
either 2-sulfonylmethyl pyrroles or dihydropyridines was
developed. The N-propargyl-N-vinyl sulfonamides were in turn
prepared via a novel, base-mediated formal vinylic substitution
reaction of 2-bromoallyl sulfones. A variety of 2,3,4-substituted
pyrroles and 2,4-substituted dihydropyridines were assembled in
two steps from propargyl sulfonamides and 2-bromoallyl sulfones.
It is presumable that the rich chemistry of the methylenesulfonyl
moiety may be exploited for further elaboration of the heterocycles
into important natural products and analogues. Efforts along this
direction and a detailed investigation of the cesium carbonate
mediated vinylic substitution reaction are currently underway.
8
(a) W. Rao, W. Susanti, B. J. Ayers, P. W. H. Chan, J. Am.
Chem. Soc., 2015, 137, 6350; (b) C. Wang, X. Xie, J. Liu, Y. Liu,
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Li, J. Sun, Angew. Chem. Int. Ed., 2015, 54, 883; (d) R. K.
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Zhang, Chem. Commun., 2014, 50, 4203; (f) W. Rao, M. J.
Koh, P. W. H. Chan, J. Org. Chem., 2013, 78, 3183; (g) P.
Mauleón, R. M. Zeldin, A. Z. González, F. D. Toste, J. Am.
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(a) P. R. Joshi, S. Undeela, D. D. Reddy, K. K. Singarapu, R. S.
Menon, Org. Lett., 2015, 17, 1449; (b) S. Reddy, S.
Thadkapally, M. Mamidyala, J. B. Nanubolu, R. S. Menon,
RSC Adv., 2015, 5, 8199.
0 (a) A. Jose, A. J. Jayakrishnan, B. Vedhanarayana, R. S.
9
1
Menon, S. Varughese, E. Suresh, V. Nair, Chem. Commun.,
2
014, 50, 4616; (b) E. Li and Y. Huang, Chem. Commun.,
The Department of Science and Technology (DST), India is
acknowledged for the award of Ramanujan fellowships to both RSM
and KKS. DST is thanked for the award of a Start-up Research Grant
2014, 50, 948; (c) S. Yu and S. Ma, Angew. Chem. Int. Ed.,
2012, 51, 3074; (d) A. Jose, K. C. S. Lakshmi, E. Suresh, V.
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1
1 (a) A. Núñez Jr., M. R. Martín, A. Fraile, J. L. G. Ruano, Chem.
–
(young scientists) for RSM. Financial support in part from the XIIth
Eur. J., 2010, 16, 5443; (b) C. Lu, X. Lu, Tetrahedron, 2004,
0, 6575.
five year plan project “Affordable Cancer Therapeutics (ACT)” and
6
Research Fellowship for SU from the Council of Scientific and 12 S. Ma, Q. Wei, J. Org. Chem., 1999, 64, 1026.
Industrial Research (CSIR), India is also acknowledged.
13 For relevant examples, see: (a) A. Saito, T. Konishi, Y.
Hanzawa, Org. Lett., 2010, 12, 372; (b) T. Harschneck, S. F.
Kirsch, J. Org. Chem., 2011, 76, 2145.
1
1
4 B. D. Sherry, F. D. Toste, J. Am. Chem. Soc., 2004, 126, 15978.
5 CCDC 1063242 (2da) contains the supplementary
Notes and references
1
(a) Sharpless’s deliberation, in his Nobel-prize lecture, on the
regio- and stereoselective oxidation of the olefinic units of
geraniol may be recalled here: K. B. Sharpless, Angew.Chem.
Int. Ed., 2002, 41, 2024; Also see; (b) C. A. Lewis, S. J. Miller,
Angew.Chem. Int. Ed., 2006, 45, 5616; (c) C. A. Lewis, K. E.
crystallographic data for this compound. This data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
16 (a) For a synthesis of rhazinilam family of alkaloids, see: L.
McMurray, E. M. Beck, M. J. Gaunt, Angew. Chem. Int. Ed.,
2012, 51, 9288; (b) For the synthesis of a lamellarin alkaloid,
see: K. Hasse, A. C. Willis, M. G. Banwell, Eur. J. Org. Chem.,
2011, 88; (c) For pyrrolnitrin synthesis, see: M. D. Morrison,
J. J. Janthorn, D. A. Pratt, Org. Lett., 2009, 11, 1051.
Longcore, S. J. Miller, P. A. Wender, J. Nat. Prod., 2009, 72
864.
,
1
2
3
J. Mahatthananchai, A. M. Dumas, J. W. Bode, Angew. Chem.
Int. Ed. 2012, 51, 10954 and references cited therein.
For selected recent examples of catalyst-controlled divergent 17 (a) E. M. P. Silva, P. A. M. M. Varandas, A. M. S. Silva,
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