3,5-Dihydro-2H-thieno[2,3-c]pyrrole 1,1-dioxide - A new simple pyrrole unit. Preliminary communication

Article abstract of DOI:10.1002/hlca.200900362

2,3-Dihydrothiophene 1,1-dioxide ('2-sulfolene') reacted with tosylmethyl isocyanide (TsMIC) in the presence of a base to give the hitherto unknown 3,5-dihydro-2H-thieno[2,3-c]pyrrole 1,1-dioxide ('β′- sulfolenopyrrole') from the expected cyclocondensation. A serendipitous formation of this β′-sulfolenopyrrole was found earlier, when we investigated synthetic routes to a 3,5-dihydro-1H-thieno[ 3,4-c]pyrrole 2,2-dioxide (a 'β″-sulfolenopyrrole') from TsMIC and 2,5-dihydrothiophene 1,1-dioxide ('3-sulfolene'). Here, we present the synthesis and characterization of β′-sulfolenopyrrole. The X-ray crystal-structure analyses of β′-sulfolenopyrrole and the isomeric β″-sulfolenopyrrole are also reported here. This β′- sulfolenopyrrole is a new type of a functionalized pyrrole, which is likely to be of interest for pharmaceutical purposes.

Full text of DOI:10.1002/hlca.200900362

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3,5-Dihydro-2H-thieno[2,3-c]pyrrole 1,1-Dioxide – A New Simple Pyrrole
Unit
Preliminary Communication
by Srinivas Banala*a)1), Klaus Wurstb), and Bernhard Krꢀutlera)
a) Institute of Organic Chemistry and Centre of Molecular Biosciences, University of Innsbruck,
A-6020 Innsbruck (e-mail: banala1@gmail.com)
b) Institute of General, Inorganic, and Theoretical Chemistry, University of Innsbruck, A-6020 Innsbruck
Dedicated to Professor Klaus Mꢀller, Basel
2,3-Dihydrothiophene 1,1-dioxide (ꢀ2-sulfoleneꢁ) reacted with tosylmethyl isocyanide (TsMIC) in
the presence of a base to give the hitherto unknown 3,5-dihydro-2H-thieno[2,3-c]pyrrole 1,1-dioxide (ꢀb’-
sulfolenopyrroleꢁ) from the expected cyclocondensation. A serendipitous formation of this b’-
sulfolenopyrrole was found earlier, when we investigated synthetic routes to a 3,5-dihydro-1H-
thieno[3,4-c]pyrrole 2,2-dioxide (a ꢀb’’-sulfolenopyrroleꢁ) from TsMIC and 2,5-dihydrothiophene 1,1-
dioxide (ꢀ3-sulfoleneꢁ). Here, we present the synthesis and characterization of b’-sulfolenopyrrole. The
X-ray crystal-structure analyses of b’-sulfolenopyrrole and the isomeric b’’-sulfolenopyrrole are also
reported here. This b’-sulfolenopyrrole is a new type of a functionalized pyrrole, which is likely to be of
interest for pharmaceutical purposes.
Introduction. – Pyrroles are the building blocks of porphyrins, chlorins, and other
tetrapyrrolic compounds, which are the core moieties of the physiologically important
ꢀpigments of lifeꢁ [1 – 4]. The synthesis of pyrroles, their chemistry, and their further use
in the production of various materials have received considerable interest [5 – 8]. Apart
from classical pyrrole syntheses, such as the Paal – Knorr synthesis [9][10], a variety of
other important methods have been developed [11 – 14]. We were interested in
employing cyclocondensation reactions, in particular by the so-called ꢀ[3 þ 2]-
approachꢁ [15] which provides access to various 2,3,4-substituted pyrroles [8][15 –
17]. For example, van Leusen and co-workers [18 – 20] reported the reaction of
tosylmethyl isocyanide (TsMIC) and a,b-unsaturated ketones/esters (Michael accept-
ors; Scheme 1,a) for the preparation of 3,4-disubstituted pyrroles [21 – 24]. Similarly,
alkyl a-isocyano esters and nitro-olefins can be reacted using the robust Barton – Zard
protocol, to produce various substituted pyrroles (Scheme 1,b) [25] subsequently used
in the assembly of porphyrinoid compounds [26 – 28].
A rational synthesis of functionalized porphyrinoid compounds via the reactive
tetra-b’’-sulfolenoporphyrin 1 [29 – 31] required the synthesis of the b’’-sulfolenopyrrole
2 [32 – 34] as a building block (Scheme 2). Several multi-step sequences for the
1
)
Current address: Technical University of Berlin, Institut fꢃr Chemie, FG Organische Chemie,
Strasse des 17. Juni 124/TC 2, D-10623 Berlin.
ꢂ 2010 Verlag Helvetica Chimica Acta AG, Zꢃrich

Helvetica Chimica Acta – Vol. 93 (2010)
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Scheme 1. Two Examples of the ꢁ(3 þ 2)-Approachꢂ to Pyrroles: a) TsMIC-Based Approach [18], b) a-
Isocyano Ester-Based Strategy [25]
synthesis of 2 have been developed [30][35][36]. However, an interest to improve the
overall yield2) prompted us to look for more efficient alternative methods to prepare
the pyrrole 2.
Scheme 2. Structures of Tetra-b’’-sulfolenoporphyrin 1, and of Its Precursors b’’-Sulfolenopyrrole 2, and
of b’-Sulfolenopyrrole 3
Results and Discussion. In the course of a search for alternative methods for 2,
combining van Leusen method with the Barton – Zard approach, cyclocondensation
2
)
Benzyl b’’-sulfolenopyrrole-2-carboxylate was prepared in up to 60% yield by treating benzyl
isocyanoacetate with a,b-unsaturated sulfones in the presence of a base [35]. However, the acid-
catalyzed decarboxylation of the b’’-sulfolenopyrrole-2-carboxylic acid to give 2 proved to be
problematic. The conditions used earlier for decarboxylation (hot CF3COOH) gave 2 in up to 56%
yield [30].

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reaction of TsMIC with the a,b-unsaturated ꢀ2-sulfoleneꢁ (¼2,3-dihydrothiophene 1,1-
dioxide; as electron deficient Michael acceptor) was to be explored. The effectiveness
of the reaction was tested with the readily accessible 2-sulfolene 4 (Scheme 3), which
was prepared via base-catalyzed isomerisation of ꢀ 3-sulfoleneꢁ (¼2,5-dihydrothio-
phene 1,1-dioxide; 5) [37].
Scheme 3. Cyclocondensation of TsMIC with 2-Sulfolene gives b’-Sulfolenopyrrole 3.
The mixture of 4 and TsMIC in THF was added to a suspension of tBuOK in THFat
ꢀ 208, warmed up to room temperature, and stirred for additional 30 min. The reaction
progress was monitored by TLC; the formation of 3 was identified by the characteristic
blue stain with Ehrlichꢁs reagent. Aqueous workup and chromatographic purification
gave up to 63% of 3,5-dihydro-2H-thieno[2,3-c]pyrrole 1,1-dioxide (3; the conven-
tional non-IUPAC numbering is shown in Fig. 1). Change of the addition sequence,
such as the addition of TsMIC (in THF) to the mixture of 4 and tBuOK (in THF) at ꢀ
208, or the addition of 4 to the mixture of TsMIC and tBuOK (in THF) at ꢀ 208 gave
lower yields (ca. 40%) of 3.
Fig. 1. X-Ray crystal structure of b’-sulfolenopyrrole 3.
Mass-spectrometric analysis of the pyrrole 3 showed the molecular ion ([M þ H]þ
as base peak) at m/z 158.0 Da, corresponding to the molecular formula C6H8NO2Sþ,
and little fragmentation. A 300-MHz 1H-NMR spectrum of 3 (in (D6)acetone) showed
two signals of pyrrole H-atoms, at 6.69 (HꢀC(5)) and 7.15 ppm (HꢀC(2)), consistent
with an unsymmetrical substitution pattern (see Exper. Part). The constitution of 3 was
1
established by heteronuclear H,13C coupling. The b’’-sulfolenopyrrole 2 showed the
spectrum of a symmetrical pyrrole: in CD3OD the two pyrrolic H-atoms gave a signal at
6.75 ppm [32].

Helvetica Chimica Acta – Vol. 93 (2010)
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The structure of 3 was established by its single-crystal X-ray structure (Fig. 1)3).
The N(1)ꢀC(2) bond (1.354(3) ꢄ) is slightly shorter than the N(1)ꢀC(5) bond
(1.365(3) ꢄ), whereas the C(2)ꢀC(3) bond (1.376(3) ꢄ) is a little longer than
C(4)ꢀC(5) bond (1.366(3) ꢄ), as rationalized by the electron-withdrawing effect of
the SO2 group. The C(3)ꢀC(4) bond is 1.411(3) ꢄ long. The bond lengths in 2 are
1.362(2) (N(1)ꢀC(2)), 1.369(2) (C(2)ꢀC(3)), and 1.421(2) ꢄ (C(3)ꢀC(4)), respec-
tively (Fig. 2). The pyrrol 2 [30] was crystallized by slow evaporation of its MeOH
solution.
Fig. 2. X-Ray crystal structure of b’’-sulfolenopyrrole 2.
As the anion derived from TsMIC is very reactive in cyclocondensation reaction
with electron-deficient alkenes [20], 3-sulfolene (5) was also tested for its tendency to
react with TsMIC. To the suspension of NaH in Et2O at 08, a mixture of TsMIC and 5 in
DMSO/Et2O was added. Aqueous workup and chromatographic purification yielded
the b’-sulfolenopyrrole 3 (instead of 2) in low (up to 32%) yield (Scheme 4), and no
trace of the isomeric 2 was found. Change of the addition of the reagents, i.e., first
deprotonating TsMIC with NaH and then adding 5, did not yield any 2 but only 3.
Scheme 4. Cyclocondensation of TsMIC with 3-Sulfolene.
The formation of 3 (under these conditions) can be explained as follows: the 3-
sulfolene 5 is a poor reactant in the cyclocondensation reaction with the anion of
TsMIC. Deprotonation of 5 at the CH2(2) group (adjacent to the sulfone function) and
in-situ reprotonation at the 4-position leads to the 2-sulfolene 4 faster than nucleophilic
attack of the anion of TsMIC on 5 could take place (Scheme 4). Indeed, alkylation of
the anion derived from deprotonation of the 3-sulfolene 5 with NaH/DMF, was
observed to lead to the formation of a mixture of the 2-alkyl-2-sulfolene and 2-alkyl-3-
sulfolene, in a 1:3 to 1:1 ratio, depending on the alkylating agent used [38].
3
)
CCDC-748499 and CCDC-748498 contain the supplementary crystallographic data for 3 and 2,
respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/
cif.

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Conclusions. – We reported herein a cyclocondensation reaction between the 2-
sulfolene 4 and the anion of TsMIC, which provided efficient access to the b’-
sulfolenopyrrole 3. To the best of our knowledge 3 is a new compound. Modifications
introduced in the assembly, such as a-substituted TsMIC [39], 4- or 5-substituted 2-
sulfolenes [38], would generate substituted b’-sulfolenopyrroles under similar reaction
conditions.
The conjugated sulfolene functionality is expected to modulate the reactivity of the
pyrrole moiety in a predictable fashion. The b’-sulfolenopyrrole 3 thus has fine-tuned
electronic properties and an interesting reactivity for further exploitation, which could
assist in selective functionalization at C(2) and C(5). It may thus be of interest for
further synthetic transformations, e.g., for further assembly (to porphyrinoid com-
pounds), as well as a basic unit, to be explored for pharmaceutical purposes.
Experimental Part
General. Tosylmethyl isocyanide (TsMIC), 3-sulfolene, NaH, tBuOK, abs. DMSO were from Fluka,
abs. THF from Acros, and they were used as received. CH2Cl2, AcOEt, petroleum ether (PE 40 – 60), and
Et2O were from Acros and were distilled before use. Glassware for all reactions was oven-dried at 1108
and cooled under N2 flow prior to use. Column chromatography (CC): Fluka silica gel 60 (230 – 400
mesh). High vacuum: ca. 0.05 mbar. 1H- and 13C-NMR spectra: Bruker (1H: 300 MHz, 13C: 75 MHz) at
300 K; chemical shifts d in ppm, J in Hz, with d(CHD2COCD3) 2.05 ppm, d(CD3COCD3) 29.9 and
206.7 ppm. FAB-MS: Finnigan MAT-95, positive-ion mode, glycerine matrix; m/z (rel. intensity %).
3,5-Dihydro-2H-thieno[2,3-c]pyrrole 1,1-Dioxide (3). To the suspension of 230 mg of tBuOK
(2 mmol, 2 equiv.) in 7 ml of dry THFat ꢀ 208, a soln. of 195 mg of TsMIC (1 mmol, 1 equiv.) and 135 mg
of 2,3-dihydrothiophene 1,1-dioxide (4; 1.15 mmol, 1.1 equiv.) in 10 ml of dry THF was added dropwise
over 15 min with rigorous stirring. The resulting pale yellow suspension was stirred for 20 min at ꢀ 208
and then left to warm up to r.t., while stirring for another 30 min. The reaction progress was monitored by
TLC (CH2Cl2/AcOEt 9 :1) for the consumption of TsMIC and product formation with Ehrlichꢁs reagent
(0.1% 4-(dimethylamino)benzaldehyde in conc. HCl).
To the mixture, 5% aq. Na2CO3 (15 ml) was added, and the mixture was extracted with CH2Cl2 (3 ꢁ
20 ml). The org. extracts were washed with H2O (20 ml), dried (Na2SO4), filtered, and the solvents were
evaporated in vacuo. The mixture was purified by CC (15 ꢁ 2 cm) with AcOEt/CH2Cl2 gradient. The
product 3 was dried in high vacuum at 408 overnight to obtain 98.6 mg of crude 3 (0.628 mmol, 63%). The
crude 3 was suspended in acetone (1 ml) and precipitated by addition of Et2O (6 ml). The precipitated
product was filtered, dried overnight in high vacuum at 408 to obtain 94.8 mg (0.604 mmol, 60.4%) of 3.
TLC (CH2Cl2/AcOEt 9 :1): Rf 0.25. M.p. 208 – 2098. 1H-NMR ((D6)acetone): 3.14 (t, J ¼ 6.8, CH2(41)),
1
3.61 (t, J ¼ 6.8, CH2(42)), 6.69 (br. s, HꢀC(5)), 7.15 (br. s, HꢀC(2)), 10.80 (br. s, NH(1)). H-NMR
(300 MHz, CD3OD): 3.17 (t, J ¼ 6.5, CH2(41)), 3.69 (t, J ¼ 6.8, CH2(42)), 6.62 (br. s, HꢀC(5)), 7.09 (br. s,
HꢀC(2)). 13C-NMR: (75 MHz): 20.2 (C(41); 59.7 (C(42); 112.2 (C(2)); 112.9 (C(5)); 126.0 (C(4)); 127.4
(C(3)). FAB-MS: 159.0 (11), 158.0 (100, [M þ 1]þ, C6H8NSO2þ ; calc. 158.02).
Suitable crystals of 3 for X-ray diffraction were obtained from (D6)acetone by slow evaporation of
the solvent at r.t.
Crystal Data and Details of Structure Refinement for 3. Crystals were grown from C2D6O. Crystals
data at 233(2) K for C6H7NO2S (Mr 157.19). Crystal system, monoclinic; space group, P21/n (no. 14); a ¼
6.8399(5), b ¼ 10.5559(8), c ¼ 9.0749(4) ꢄ, a ¼ 908, b ¼ 99.479(4)8, g ¼ 908; V¼ 646.27(7) ꢄ3. q Range
for data collection, 2.98 – 24.998; wavelength 0.71073 ꢄ. Z, 4; density (calc.), 1.616 g/cm3; absorption
coefficient, 0.427 mmꢀ1; F(000), 328; crystal size 0.28 ꢁ 0.1 ꢁ 0.06 mm3; index ranges, ꢀ 8 ꢂ h ꢂ 7, ꢀ 12 ꢂ
k ꢂ 11, ꢀ 10 ꢂ l ꢂ 10; reflections collected, 3215; independent reflections, 1131 [Rint ¼ 0.0229]; reflections
[I > 2s(I)], 1007; completeness to q ¼ 24.998 99.4%; absorption correction: none. Refinement method:
full-matrix least-squares on F2, data/restraints/parameters 1131/0/96; goodness-of-fit on F2 1.089; final R

Helvetica Chimica Acta – Vol. 93 (2010)
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indices [I > 2s(I)], R1 ¼ 0.0315, wR2 ¼ 0.0750; R indices (all data), R1 ¼ 0.0364, wR2 ¼ 0.0774; extinction
coefficient, 0.008(4); largest diff. peak and hole, 0.251 and ꢀ 0.369 e ꢄꢀ3. CCDC-748499.
Compound 3 from the Reaction of 3-Sulfolene (5) with TsMIC. To the suspension of 132 mg of NaH
(ca. 55% in mineral oil, 2.75 mmol, 2.4 equiv.) in 5 ml of dry Et2O at 08, a soln. of 227 mg of TsMIC
(1.16 mmol, 1 equiv.) and 275 mg of 5 (2.32 mmol, 2 equiv.) in 15 ml of dry Et2O/abs. DMSO (2 :1) was
added. After 30 min, consumption of TsMIC and pyrrolic product (Rf 0.25) formation were observed by
TLC (CH2Cl2/AcOEt 9 :1). The mixture was worked up similarly, and CC gave 58.3 mg (0.37 mmol,
32%) of 3 after drying in high vacuum at 408. The anal. data were identical to those given above for 3.
X-Ray Structure Analysis of 2. The b’’-sulfolenopyrrole 2 was synthesized according to the reported
procedure [30]. Suitable crystals of 2 for X-ray diffraction were obtained from methanol by slow
evaporation of the solvent at r.t.
Crystal Data and Details of Structure Refinement for 2. Crystals were grown from MeOH. Crystals
data at 233(2) K for C6H7NO2S, (Mr 157.19). Crystal system, orthorhombic; space group, Pbca (no. 61);
unit cell dimensions, a ¼ 9.6000(2), b ¼ 9.4868(2), c ¼ 14.6556(4) ꢄ, a ¼ 908, b ¼ 908, g ¼ 908; V¼
1334.73(5) ꢄ3; wavelength 0.71073 ꢄ Z, 8; density (calc.), 1.564 g/cm3 ; absorption coefficient,
0.414 mmꢀ1; F(000), 656; crystal size, 0.35 ꢁ 0.15 ꢁ 0.15 mm3; q range for data collection, 2.78 – 27.008;
index ranges, ꢀ 12 ꢂ h ꢂ 0, ꢀ 12 ꢂ k ꢂ 12, ꢀ 18 ꢂ l ꢂ 18; reflections collected, 7852; independent
reflections, 1449 [Rint ¼ 0.0222]; reflections [I > 2s(I)], 1345; completeness to q ¼ 27.008 99.5%;
absorption correction, none. Refinement method full-matrix least-squares on F2; data/restraints/
parameters, 1449/0/96; goodness-of-fit on F2, 1.069; final R indices [I > 2s(I)], R1 ¼ 0.0295, wR2 ¼ 0.0850;
R indices (all data), R1 ¼ 0.0318, wR2 ¼ 0.0865; extinction coefficient, 0.005(3); largest diff. peak and
hole, 0.301 and ꢀ 0.299 e ꢄꢀ3. CCDC-748498.
We would like to thank Prof. Karl-Hans Ongania for mass-spectrometric analysis, the Austrian
Science Foundation (FWF, project No. P17437 to B. K.) for financial support. Dr. Luke Green (F.
Hoffmann La Roche, Basel) is acknowledged for proof-reading this manuscript.
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Received October 13, 2009