Unprecedented nucleophile-promoted 1,7-S or Se shift reactions under Pummerer reaction conditions of 4-alkenyl-3-sulfinylmethylpyrroles

Article abstract of DOI:10.3762/bjoc.14.250

A unique 1,7-S- and Se-shift reaction under Pummerer reaction conditions of 4-alkenyl-3-sulfinyl- and seleninylpyrroles was described. The usual Pummerer reaction of 4-(alkenylaminomethyl)-3-phenylsulfinylpyrroles and a successive reaction with tetrabutyl

Full text of DOI:10.3762/bjoc.14.250

Unprecedented nucleophile-promoted 1,7-S or Se shift
reactions under Pummerer reaction conditions of
4-alkenyl-3-sulfinylmethylpyrroles
Takashi Go1, Akane Morimatsu1, Hiroaki Wasada2, Genzoh Tanabe3, Osamu Muraoka3,
Yoshiharu Sawada4 and Mitsuhiro Yoshimatsu*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2722–2729.
1Department of Chemistry, Faculty of Education, Gifu University,
doi:10.3762/bjoc.14.250
Yanagido 1-1, Gifu 501-1193, Japan, 2Department of Chemistry,
Faculty of Regional Study, Gifu University, Yanagido 1-1, Gifu
Received: 30 June 2018
Accepted: 03 October 2018
Published: 29 October 2018
501-1193, Japan, 3School of Pharmacy, Kindai University, 3-4-1
Kowakae, Higashi-osaka, Osaka 577-8502, Japan and 4Life Science
Research Center, Gifu University, Yanagido 1-1, Gifu 501-1193,
Japan
Associate Editor: B. Stoltz
Email:
© 2018 Go et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Mitsuhiro Yoshimatsu* - yoshimae@gifu-u.ac.jp
*
Corresponding author
Keywords:
hydroamination; Pummerer reaction; pyrrole; pyrroloazepine;
1,7-sulfur shift
Abstract
A unique 1,7-S- and Se-shift reaction under Pummerer reaction conditions of 4-alkenyl-3-sulfinyl- and seleninylpyrroles was de-
scribed. The usual Pummerer reaction of 4-(alkenylaminomethyl)-3-phenylsulfinylpyrroles and a successive reaction with tetra-
butylammonium hydroxide (TBAH) yielded either pyrrolo[3,2-c]azepines or N-pyrrol-3-ylmethyl-N-(4-hydroxy-3-sulfanylpropyl)-
p-toluenesulfonamides (diols). Seleno-Pummerer reactions of 3-selanylmethylpyrroles also proceeded via in situ generation of
selenoxides, followed by a treatment with TBAH.
Introduction
Pyrrolo- and indoloazepine skeletons are chemical frameworks pounds operate by inhibiting proteins or enzymes that regulate
present in marine natural products [1,2] such as hymeniald- cell cycles, including cyclin-dependent kinases [9], tyrosine
isines [3,4], latonduines [5], paullones, kenpaullones and alster- kinase [10], glycogen-synthase kinase and mitochondrial malate
paullones [6,7]. These compounds can be used for various phar- dehydrogenase [11]. These interesting biological activities led
maceutical applications [8] such as for the treatment of us to develop one-pot sequential or cascade protocols for the
neurodegenerative and proliferative disorders. These com- synthesis of [2,3-b]-, [2,3-c]- and [3,4-b]pyrrolo- and
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indoloazepines [12-20]. However, the yields of the pyrrolo- and foxides to obtain their corresponding pyrroloazepines, we ob-
indolo[3,2-c]azepines were relatively low. One of the most effi- served unprecedented events, i.e., 1,7-sulfanyl group (1,7-S)
cient procedures for preparing pyrrolo[3,2-c]azepin-4-ones shift reactions, on the cation intermediates. Herein, we report
involves the Beckmann rearrangement of 6,7-dihydroindol- the unique 1,7-S and 1,7-Se shift reactions through Pummerer
4
(5H)-ones. However, after implementing this approach, an reactions of alkenyl pyrrolomethyl sulfoxides and the corre-
undesirable isomer was obtained [21]. Although a number of sponding successive cyclisation reactions.
acid- [22,23] and metal-catalysed cascade processes [24,25]
have been developed (for example, Pictet–Spengler [26] and Results and Discussion
Ugi-type reactions [27], the acyl radical cyclisations [28] and We prepared sulfoxides as substrates for the Pummerer reac-
the 1,7-electrocyclisation of azomethine ylides or ring expan- tions according to the aforementioned method [32,33]; subse-
sion sequences [29]), most of these approaches are ineffective quently, oxidation with mCPBA was performed (Scheme 2).
for synthesising pyrrolo[3,2-c]azepines. Recently, Echavarren Both the allylic sulfides 4a–f and their sulfoxides 5a–f were ob-
and Beller reported a novel Au- or Pt-catalysed cycloisomerisa- tained in good to high yields. Corresponding pyrroloselenides
tion reaction of pyrrole-2-carboxamides that produces 6a–f were prepared by applying a similar synthetic sequence;
pyrrolo[2,3-c]- and [2,3-d]azepinones [30,31]. However, this however, the selenoxides could not be obtained by imple-
reaction is not suitable for the synthesis of pyrrolo[3,2- menting the usual reaction conditions. The structure of the prod-
c]azepines in viewpoint of the difficulties in preparing the ucts derived from the amination–cyclisation reaction of sulfanyl
starting materials.
1,6-diyne 1a was determined by analysing the single-crystal
X-ray diffraction spectrum of the corresponding sulfone of 4a;
We previously reported that the hydroamination/cyclisation the structure was a pyrrole bearing both 3-sulfanylmethyl and
reaction of sulfanyl-1,6-diynes with secondary amines resulted 4-allylaminomethyl groups (see Supporting Information File 1).
in the formation of 3-sulfanylmethyl-4-aminomethylpyrroles
(
Scheme 1) [32,33]. The sulfanyl group of pyrroles is easily To develop a useful Pummerer reaction procedure, we selected
oxidised to produce their corresponding sulfoxides. When we allylamine derivative 5a as representative substrate and per-
performed the Pummerer reaction and cyclization of pyrrolesul- formed a screening to find suitable conditions for its reaction
Scheme 1: Our synthetic plan for pyrrolo[3,2-c]azepines.
Scheme 2: Preparation of precursors for the Pummerer reactions.
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Table 1: Screening the reaction conditions for the Pummerer reactions.
entry
anhydride [equiv]
solvent
T [°C]
time [h]
5a
yields [%] 7a
8a
9a
1
2
3
4
5
6
7
8
9
Ac2O (5)
AcOH
DCM
MeCN
DCM
DCM
DCE
THF
120
0
6
86
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
TFAA (1.5)
2
–
–
Tf2O (1.5)
0
1
–
TFAA (8), Et3N (3)
TFAA (5)
0
1.3
0.5
0.3
0.5
2
36
–
–
−20
−40
−20
−20
−20
−20
−20
0
–a
TFAA (5)
–
–a
–
TFAA (5)
86
–
TFAA (5), Bu4NOH aq
TFAA (5), K2CO3 aq
TFAA (5), Bu4NOH aq
TMSOTf (3)
DCM
DCM
DCM
DCE
DCE
67
46
93
–
1.5
0.5
0.7
0.3
–
10
11
12
–
–
(COCl)2 (5), Et3N (5)
74
18
a9a was obtained contaminated with traces of other unknown compounds.
with anhydrides (Table 1). Since the reaction of 5a with acetic (refer the NMR studies of 9a in Supporting Information File 1).
anhydride/acetic acid completely stagnated at room tempera- To clarify the details of the S-shift reactions, we performed the
ture, we performed the same reaction at 120 °C. However, the usual Pummerer reaction, followed by treatment with bases at
reaction conditions did not yield the desired cyclic products and −20 °C (Table 1, entries 8–10). We determined that the S-shift
ester 7a was instead produced in good yield (entry 1 in reaction was strongly affected by the base. Tetrabutylammoni-
Table 1). We next examined the reaction of 5a with 1.5 equiv of um hydroxide was found to be the best base for the formation of
trifluoroacetic anhydride (TFAA) in dichloromethane at 0 °C; 9a (Table 1, entry 10). Neither trimethylsilyl trifluoromethane-
however, the reaction yielded a complex mixture (Table 1, entry sulfonate nor oxallyl chloride/triethylamine proved effective for
2
). Since the reaction with trifluoromethanesulfonic anhydride the formation of 9a (Table 1, entries 11 and 12).
resulted in the recovery of 5a (Table 1, entry 3), we reinvesti-
gated the reaction of 5a with TFAA in detail. The use of an Although the synthesis of the desired pyrrolo[3,2-c]azepines
excess of TFAA, followed by the treatment with triethylamine was unsuccessful, we discovered the unique 1,7-S shift reac-
yielded aldehyde 8a instead of alcohol 7a (Table 1, entry 4). tions. We next examined the scope of 1,7-S- and 1,7-Se-shift
These results show that the reaction conducted in the presence reactions under Pummerer reaction conditions of sulfoxides and
of an excess of TFAA was indeed successful and the Pummerer the results are shown in Scheme 3. We first examined the
reaction products such as α-trifluoroacetoxysulfide underwent following one-pot sequential reactions of the selenium analog:
hydrolysis to yield aldehyde 8a. In entries 5 and 6, we present the mCPBA oxidation of N-allylpyrrolomethylselenide 6a, the
the results from performing the reactions at lower temperatures. following Pummerer reaction, and final treatment with TBAH.
Fortunately, we found the formation of unprecedented product The stepwise procedure succeeded to give 7-phenylselenodiol
9
a, which was contaminated with unknown products at both 10a in 91% yield. Next, we performed the Pummerer reaction
20 °C and −40 °C. The Pummerer rearrangement and succes- of N-β-methallyl sulfoxide 5b in order to clarify the substituent
−
sive cyclisation with either the intramolecular C–C double bond effects on the N-alkenyl groups. Surprisingly, the reaction of 5b
or the pyrrole were unsuccessful; however, N-(3-hydroxy-2- afforded the intramolecular cyclised pyrroloazepine 11b. The
(
phenylthio)propyl)-N-((4-(hydroxymethyl)-1-tosyl-1H-pyrrol- N-methallylselenopyrrole 6b also gave the pyrroloazepine 10b,
-yl)methyl)-4-methylbenzenesulfonamide (9a) was apparently which was formed via the 1,7-Se shift reaction. While N-3-
obtained via the sulfanyl group migration (S-shift reaction) methylbut-2-enyl derivatives 4c and 6c yielded 1,7-S- and
3
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Scheme 3: Substrate scope of 1,7-S and 1,7-Se shift reactions.
Se-shifted diols 9c and 10c, respectively. Interestingly, the reac- through detrifluoroacetoxylation of 14 (path b: normal
tion of N-cinnamylpyrroles 4d and 6d exclusively afforded Pummerer reactions) and undergoes intramolecular cyclisation
trans-azepines 11d and 12d via intramolecular cyclization. The on the pyrrole ring of 19. Aromatisation of dihydropyrrole inter-
same procedure applied to the bulky N-(3-methylbut-2-enyl) de- mediate 20 leads to the formation of pyrroloazepinic cation 21,
rivatives 4e and 6e yielded the corresponding diols 9e and 10e. which is stabilised by the phenylsulfanyl group at its 3-position.
Whether the Pummerer reactions yielded cyclised products or The S-shift reaction involves the transannular sulfonium inter-
not was strongly affected by the substituent patterns. If the mediate 22, which is attacked by a hydroxide anion from the
azepinium cation intermediates were stabilised by either anti-direction. Both products, diols and azepinopyrroles, are ob-
α-methyl or β-phenyl substituents, the intramolecular cyclisa- tained in a stereoselective manner. In order to confirm the reac-
tion with the pyrrole ring proceeded to afford azepinoindoles. If tion mechanism, we performed crossover experiments with
both substituents stabilised the intermediates, N-allyl (n = 1; arylthiol (Scheme 5). First, we examined the Pummerer reac-
R1 = R2 = H), N-2-butenyl (n = 1; R1 = H, R2 = Me) and N-3- tions in the presence of p-methoxybenzenethiol or p-chloroben-
methyl-2-butenyl derivatives (n = 1; R1 = R2 = Me) exclusively zenethiol; however, the reactions yielded the reductive sulfide
gave the diols 9a, 9c, 9e, 10a, 10c and 10e. In particular, all 4a. We next examined the stepwise process: i) the Pummerer
products were obtained through 1,7-S- or 1,7-Se shift reactions. reaction of 5a with TFAA, followed by ii) the usual work-up
The yields of the Pummerer reactions of both N-3-butenyl de- and successive treatment with p-methoxybenzenethiol/TBAH,
rivatives 9f and 10f were very low. These products were con- which exclusively produced 9a (the diol 24a was not detected
firmed by X-ray spectra and/or single-crystal X-ray diffraction by ESI mass spectroscopy). A similar result was obtained from
patterns.
the reaction of 5a with p-chlorobenzenethiol/TBAH. These
results support the hypothesis that the reaction proceeds via the
The unique S-shift reactions in the formation of either initial formation of the key intermediate 15, of which the
pyrrolo[3,2-c]azepines or diols proceeded according to the sulfanyl group would intramolecularly migrate to produce 16
mechanism we proposed, which is depicted in Scheme 4. Sulf- stereoselectively (path c, not d). The hydrolysis of 16 using
oxide 13 is activated by TFAA to form sulfonium salt 14, which TBAH yields diol 17. We further confirmed the mechanism of
is characterised by an intramolecular double bond [34,35]. A the 1,7-S shift reaction by performing density functional theory
less substituted double bond easily approaches the sulfonium (DFT)-based calculations (Table 2) [36]. Whether the reactions
ion and undergoes an addition reaction to produce intermediate lead to the formation of pyrroloazepines or diols clearly
1
5. The trifluoroacetate anion attacks the posterior side of the depends on the substituents in the alkenyl group. We per-
α-carbon of the sulfur atom. Bis(trifluoro)acetate 16 resulted formed DFT calculations for the reaction pathways starting
from the opening of the ring is hydrolysed by TBAH to yield from the key intermediate 14 bearing methyl and phenyl groups.
diol 17. Conversely, sterically hindered double bonds of 14 are The DFT-calculated ΔG values of 15a is by 24 kcal/mol lower
not easily accessible to the sulfonium ion; therefore, the than that of 21a (R1 = R2 = H), which is in accordance with the
Pummerer intermediate α-thio carbenium ion 18 is generated experimental result that the reaction of 4a leads to the forma-
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Beilstein J. Org. Chem. 2018, 14, 2722–2729.
Scheme 4: Proposed mechanism.
Scheme 5: Crossover experiment.
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Beilstein J. Org. Chem. 2018, 14, 2722–2729.
sequential process of sulfanyl-1,6-diynes to the oxazine deriva-
tives (Scheme 7). Hydroamination–cyclisation of diyne 1 with
N-allyl-p-bromobenzenesulfonamide 3g and the successive oxi-
dation proceeded to give the corresponding sulfoxide 5g in high
yield. The sequential process, Pummerer reaction of 5g with
TFAA, followed by treatment with TBAH to afford the 1,7-
sulfur-shifted product 9g as crystals; however, the yield was rel-
atively low. The final ytterbium-catalysed intramolecular cycli-
sation afforded 25g with a 56% yield. We obtained the
Table 2: DFT Calculation of the two pathways: from 14 to 15x or from
4 to 21x.
1
4
H-pyrrolo[3,4-g]oxazine derivative as crystals. The structure
R1
R2
15x (ΔG kcal/mol)
21x (ΔG)
of these products was determined through NMR spectroscopy
experiments (COSY and HMBC data are reported and dis-
cussed in Supporting Information File 1).
H
H
15a (−27.16)
15b (−28.25)
15c (−12.82)
15d (−18.59)
21a (−3.17)
21b (−16.55)
21c (−26.02)
21d (−24.51)
Me
H
H
Me
H
Ph
tion of diol 9a. The calculated results related to 15b,c were also
found to match the experimental data. The S-shift process of
pyrrolo[3,2-c]azepinium cation 21x enables the reaction to
proceed via an intramolecular 1,7-S shift, followed by a hydrox-
ide attack on 22 (relevant details are reported in Supporting
Information File 1).
Scheme 6: Lewis acid-catalysed cyclization of diols.
We further investigated the cyclisation of diols to synthesise
pyrrolo[3,2-c]azepines (Scheme 6). However, the products ob-
tained were not the desired pyrrolo[3,2-c]azepines but were Conclusion
2
,6,7,8,9,10-hexahydro-4H-pyrrolo[3,4-g][1,5]oxazonines We found unique 1,7-S and 1,7-Se shift reactions by investigat-
2
5a–e, which were formed via a reaction route that involved the ing the synthesis of pyrroloazepines through amination–cyclisa-
intramolecular dehydration of diols. To confirm the structure of tion of 1,6-diynes, followed by the Pummerer reaction and
H-pyrrolo[3,4-g]oxazines, we prepared an N-bromobenzene- TBAH hydrolysis. We are now investigating the syntheses of
4
sulfonyl derivative, which was expected to act as a good sub- pyrroloazepines using 4-heteroarylmethyl-3-sulfanylpyrroles,
strate for the isolation of crystals, and performed the same which could be easily obtained by hydroamination–cyclization
Scheme 7: Sequential process of sulfanyl-1,6-diyne 1 to 4H-pyrrolo[3,4-g]oxazine 25g.
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1
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1
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2000, 7, 51–63. doi:10.1016/s1074-5521(00)00063-6
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Supporting Information File 1
Schemes S1 and S2 on the syntheses of compounds 1, 2,
and 3a–g; the NMR study for the structure determinations,
further DFT calculations, the ORTEP drawing of both
sulfone of 5a, 11d and the 1H and 13C NMR charts.
doi:10.1021/acs.jmedchem.5b01882
1
3.Kumar, A.; Vachhani, D. D.; Modha, S. G.; Sharma, S. K.;
Parmar, V. S.; Van der Eycken, E. V. Eur. J. Org. Chem. 2013,
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supplementary/1860-5397-14-250-S1.pdf]
2
288–2292. doi:10.1002/ejoc.201300132
4.Haibach, M. C.; Deb, I.; De, C. K.; Seidel, D. J. Am. Chem. Soc. 2011,
33, 2100–2103. doi:10.1021/ja110713k
1
Supporting Information File 2
1
Crystallographic information of compound 11d.
15.Modha, S. G.; Kumar, A.; Vachhani, D. D.; Sharma, S. K.;
[https://www.beilstein-journals.org/bjoc/content/
Parmar, V. S.; Van der Eycken, E. V. Chem. Commun. 2012, 48,
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0916–10918. doi:10.1039/c2cc35900f
6.Yokosaka, T.; Nemoto, T.; Hamada, Y. Chem. Commun. 2012, 48,
431–5433. doi:10.1039/c2cc31699d
supplementary/1860-5397-14-250-S2.txt]
1
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Supporting Information File 3
17.Zhu, W.; Zhao, L.; Wang, M.-X. J. Org. Chem. 2015, 80, 12047–12057.
Crystallographic information of the sulfone of compound
doi:10.1021/acs.joc.5b02021
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