Efficient and practical synthesis of enantioenriched 2,3-dihydropyrroles through gold-catalyzed anti-Markovnikov hydroamination of chiral homopropargyl sulfonamides

Article abstract of DOI:10.1039/c4cc09245g

A direct gold-catalyzed 5-endo-dig cycloisomerization of chiral homopropargyl sulfonamides has been developed. A range of enantioenriched 2,3-dihydropyrroles are readily accessed by utilizing this approach. Importantly, this gold-catalyzed cycloisomerization reaction proceeds through an anti-Markovnikov addition by using a catalytic base as the additive, which completely suppresses the undesired dimerization. This journal is

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Efficient and practical synthesis of enantioenriched
2,3-dihydropyrroles through gold-catalyzed
anti-Markovnikov hydroamination of chiral
homopropargyl sulfonamides†
Cite this:DOI: 10.1039/c4cc09245g
Received 19th November 2014,
Accepted 13th December 2014
Yong-Fei Yu,a Chao Shu,a Bo Zhou,a Jian-Qiao Li,a Jin-Mei Zhoua and
DOI: 10.1039/c4cc09245g
Long-Wu Ye*ab
www.rsc.org/chemcomm
A direct gold-catalyzed 5-endo-dig cycloisomerization of chiral
homopropargyl sulfonamides has been developed. range
A
of enantioenriched 2,3-dihydropyrroles are readily accessed by
utilizing this approach. Importantly, this gold-catalyzed cyclo-
isomerization reaction proceeds through an anti-Markovnikov
addition by using a catalytic base as the additive, which completely
suppresses the undesired dimerization.
Scheme 1 Gold-catalyzed nucleophilic addition to terminal alkynes.
During the past decade, gold-catalyzed addition of a heteroatom
nucleophile to a C–C multiple bond, in most cases an alkyne, has
proven to be an extremely powerful approach in organic synthesis,1
and an incredible variety of efficient synthetic methods have been
developed for the construction of intricate scaffolds based on this
study.2 It is surprising, however, that only a few examples have
been reported about gold-catalyzed 5-endo-dig cyclization of terminal
alkynes except for indole formation.3 In particular, to the best
of our knowledge, the gold-catalyzed 5-endo-dig cyclization of
chiral homopropargyl alcohols or amides towards the synthesis
of 2,3-dihydrofurans or 2,3-dihydropyrroles has not been reported
(Scheme 1). This could be explained by the point that the gold-
catalyzed cycloisomerization reaction4 involves an anti-Markovnikov
addition, while a Markovnikov regioselectivity was normally
observed for the gold-catalyzed nucleophilic addition to a terminal
alkyne (Scheme 1).
2,3-Dihydropyrroles constitute an important category of hetero-
cyclic ring systems which exist in a large number of bioactive natural
and synthetic molecules.5 In addition, they are also widely employed
as valuable building blocks for the construction of complex
molecules due to their latent reactivity and the large panel of h
ighly selective transformations they can undergo.6 For example,
2,3-dihydropyrrole 2aa is the key intermediate for the synthesis of
the antifungal pyrrolidinol alkaloid (+)-preussin (Scheme 2).7 How-
ever, despite the fact that numerous preparative methods have been
developed in the past decade,8 there are only limited examples of
enantioselective synthesis of 2,3-dihydropyrroles,9 including those
based on other metal-catalyzed cycloisomerization towards chiral
2,3-dihydropyrroles.9c,g,h In particular, these chiral compounds are
generally prepared through multistep routes. For example, a typical
route starts from reduction of chiral g-lactams with superhydride to
form the lactamols, which then undergo the subsequent protection
and elimination to deliver the final 2,3-dihydropyrrole compounds
(Scheme 2).7,10 Therefore, the development of novel methods for the
synthesis of chiral 2,3-dihydropyrroles is highly desirable, especially
those with high enantioselectivity, flexibility and good modu-
larity. As part of our continuous efforts to study gold-catalyzed
cycloisomerization reactions,11 we reported gold-catalyzed
tandem cycloisomerization–oxidation11d (Scheme 2) and tandem
a State Key Laboratory for Physical Chemistry of Solid Surfaces & The Key
Laboratory for Chemical Biology of Fujian Province, Department of Chemistry,
Xiamen University, Xiamen, 361005, Fujian, P. R. China.
E-mail: longwuye@xmu.edu.cn; Fax: +86-592-218-5833
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc09245g
Scheme 2 Synthesis design for the formation of 2,3-dihydropyrroles through
gold-catalyzed cycloisomerization of homopropargyl sulfonamides.
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cycloisomerization–dimerization11c from readily available chiral increased to 65% (entry 9). In addition, various typical gold catalysts
homopropargyl sulfonamides, leading to the efficient formation of with a range of electronic and steric characteristics were screened
enantioenriched g-lactams and pyrrolidines, respectively. Inspired by (entries 10–14) and the desired product 2a was formed in quantita-
these results, we envisioned that by fine tuning the basicity of the tive yield by using BrettPhosAuNTf2 as the catalyst (entry 14). Finally,
reaction, the preparation of 2,3-dihydropyrroles 2 might be achieved it should be mentioned that the reaction failed to give even a trace of
directly through the gold-catalyzed cycloisomerization of chiral 2a by employing AgNTf2 as the catalyst (entry 15) and AgOAc was
homopropargyl sulfonamides 1 (Scheme 2). Herein, we report also not effective in promoting this reaction even at 40 1C for 10 h
the first gold-catalyzed synthesis of 2,3-dihydropyrroles directly with or without base as the additive (entries 16 and 17).12
from homopropargyl sulfonamides by combining Ellman’s tert-
The chiral homopropargyl sulfonamide substrates were readily
butylsulfinimine chemistry and gold catalysis. Importantly, prepared with excellent enantiomeric excesses by using Ellman’s tert-
this gold-catalyzed hydroamination reaction proceeds through an butylsulfinimine chemistry.13 With these substrates in hand, we then
anti-Markovnikov addition by using a catalytic base as the additive, turned our attention to survey the generality of the current reaction
which completely suppresses the undesired dimerization.
under the optimized reaction conditions. As summarized in Table 2,
At the outset, homopropargyl sulfonamide 1a was chosen as a all of the homopropargyl sulfonamides 1 underwent smooth cyclo-
model substrate and a series of experiments were performed in isomerization to produce the corresponding 2,3-dihydropyrroles 2 in
order to validate our approach (Table 1). As expected, only dimer 2ab excellent yields (91–99%). Moreover, excellent enantioselectivities
was obtained by employing 5 mol% IPrAuNTf2 as the catalyst could be achieved in all cases and essentially no epimerization was
(entry 1). We then sought to use additives to suppress the unwanted detected, therefore constituting a good combination of chiral tert-
dimer byproduct (entries 2–8). As seen from Table 1, the screening butylsulfinimine chemistry with gold catalysis. In addition, the use of
of different inorganic or organic bases revealed that the use of (S)-(+)-tert-butylsulfinamide-derived homopropargyl sulfonamide 1a0
0.5 equiv. of 2,6-dibromopyridine or 2 mol% Et3N could give the also delivered the anticipated 2,3-dihydropyrrole 2a0 with the opposite
desired 2,3-dihydropyrrole 2a in 40% and 41% yields, respectively enantioselectivity (entry 16). Thus, this approach provides a highly
(entries 5 and 7). Notably, the use of 0.5 equiv. of 2.6-lutidine or efficient and practical route for the preparation of both enantiomers
5 mol% Et3N failed to give any product (entries 4 and 8). To our of 2,3-dihydropyrrole 2 just by a simple choice of the starting chiral
delight, by combining the Et3N (2 mol%) and 2,6-dibromopyridine source. The product configuration was assumed based on the reaction
(0.5 equiv.) as the additives, the yield of product 2a could be mechanism involving the gold-catalyzed cycloisomerization reaction
Table 1 Optimization of reaction conditionsa
Yieldb (%)
Entry
Metal catalyst
Additive
2a
2b
65
15
13
o1
o1
28
1a
1
2
3
4
5
6
7
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
PPh3AuNTf2
Et3PAuNTf2
(4-CF3C6H4)3PAuNTf2
XPhosAuNTf2
BrettPhosAuNTf2
AgNTf2 (5 mol%)
AgOAc (20 mol%)
AgOAc (20 mol%)
—
o1
14
27
o1
NaOAc (0.5 equiv.)
Na2CO3 (0.5 equiv.)
2,6-Lutidine (0.5 equiv.)
2,6-Dibromopyridine (0.5 equiv.)
1 mol% Et3N
2 mol% Et3N
5 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
2 mol% Et3N
—
43
35
o1
42
42
20
40
495
o1
45
50
48
40
20
41
6
8
o1
o1
o1
36
27
32
9c
65
10c
11c
12c
13c
14c,d
15c
16c,e
17e
o1
o1
o1
o1
99
40
43
o1
o1
o1
o1
o1
495
65
o1
23
20
66
a
b
Reaction conditions: [1a] = 0.05 M; DCE: 1, 2-dichloroethane. Estimated by 1H NMR using diethyl phthalate as the internal reference.
c
d
e
0.5 equiv. of 2,6-dibromopyridine was added. 1 h. 40 1C, 10 h.
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Table
2
Reaction scope for the formation of enantioenriched 2,3-
Table 2 (continued)
dihydropyrrolesa
ee
Yield ee
(%) (%)
ee
Yield ee
(%) (%)
Entry Substrate
15
1
(%) Product
2
Entry Substrate
1
1
(%) Product
2
1o 98
1a0 99
2o 95
2a0 99
98
99
1a 99
1b 99
2a 99
2b 92
99
99
16b
2
a
Reactions run in vials; [1] = 0.05 M; isolated yields are reported; ees are
b
determined using HPLC on a chiral stationary phase. Using (S)-(+)-tert-
butylsulfinamide-derived homopropargyl amide 1a0 as the substrate.
3
4
5
6
7
1c 99
1d 98
1e 97
1f 97
1g 99
2c 99
2d 99
2e 99
2f 94
2g 91
98
98
98
98
99
and further confirmed by comparing the specific rotation and the
HPLC profile of compound 2h with those of the reported compound
in the literature.12a
Besides the tosyl group, it was found that the reaction could
work well for Bs ( p-bromobenzenesulfonyl) and Ns (o-nitrobenzene-
sulfonyl) protected substrates 1p–1q, leading to the efficient
formation of the corresponding 2p and 2q in excellent yields
and excellent ees (eqn (1)), thus providing an easier way for its
later removal. In addition, this chemistry can also be extended to
the preparation of 2,2-disubstituted 2,3-dihydropyrrole 2r in 91%
yield with well-maintained enantioselectivity (eqn (2)).
8
1h 99
1i 99
1j 98
2h 99
2i 95
2j 99
99
99
97
(1)
9
(2)
10
However, attempts to expand this chemistry to homopropargyl
alcohols were not successful. As shown in eqn (3), the reaction
only gave a complicated mixture of products and no desired 4a
was obtained under the relevant reaction conditions.14
11
12
13
14
1k 99
1l 97
1m 99
1n 99
2k 99
2l 98
2m 99
2n 98
98
96
97
98
(3)
Finally, we performed deuterium labeling studies. It was found
that when substrate 1a0 (88% D) was treated under the optimal
reaction conditions, no deuterium loss was detected (eqn (4)),
indicating that the reaction presumably proceeds through a gold-
catalyzed direct 5-endo-dig cyclization of homopropargyl amides
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In summary, we have developed a flexible and general solution
for the enantioselective synthesis of various 2,3-dihydropyrroles via a
gold-catalyzed cycloisomerization of chiral homopropargyl sulfon-
amides. Most importantly, this gold-catalyzed hydroamination reac-
tion proceeds through an anti-Markovnikov addition by using a
catalytic base as the additive, which completely inhibits the for-
mation of unwanted dimers. The use of readily available substrates,
a simple procedure, and mild reaction conditions and, in particular,
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potentially useful in organic synthesis.
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We are grateful for the financial support from the National Natural
Science Foundation of China (No. 21102119 and 21272191), the
Program for Changjiang Scholars and Innovative Research
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