3-Substituted 4-bromo-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1- yloxyl radicals as versatile synthons for synthesis of new paramagnetic heterocycles

Article abstract of DOI:10.1055/s-1998-2178

β-Bromo-α, β-unsaturated electron withdrawing group containing dihydropyrrole nitroxides 2, 3, 4b, and 5b were used for synthesis of dihydropyrrol-1-yloxyl radicals annulated to thiazepine 7, 13 and thiophene 14a-g rings. The 4-bromo-3-formylsubstituted 2,5-dihydropyrrole 5b was also a synthon for the synthesis of 3,4-disubstituted nitroxides 17a-d, 18a-d, 19a- c, 20a-c.

Full text of DOI:10.1055/s-1998-2178

SYNTHESIS
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3-Substituted4-Bromo-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxylRadicalsas
Versatile Synthons for Synthesis of New Paramagnetic Heterocycles
Tamás Kálai,a Mária Balog,a József Jekö,b Kálmán Hideg*a
a Institute of Organic and Medicinal Chemistry, University of Pécs, H-7643 Pécs, P.O. Box 99, Hungary
E-mail: KHIDEG@main.pote.hu
b ICN Alkaloida Co. Ltd., H-4440 Tiszavasvári, P.O. Box 1, Hungary
Received 18 December 1997; revised 13 March 1998
pyrrol-1-yloxyl radical (1),15,16 4-bromo-3-cyano-2,2,5,5-
Abstract: β-Bromo-α,β-unsaturated electron withdrawing group
containing dihydropyrrole nitroxides 2, 3, 4b, and 5b were used
for synthesis of dihydropyrrol-1-yloxyl radicals annulated to thi-
and 4-bromo-3-formyl-2,2,5,5-tetramethyl-2,5-dihydro-
azepine 7, 13 and thiophene 14a–g rings. The 4-bromo-3-formyl-
substituted 2,5-dihydropyrrole 5b was also a synthon for the syn-
tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxyl radical (4b)17
1H-pyrrol-1-yloxyl radical (5b)15, 16 proved useful syn-
thons which conveniently gave bifunctionalized spin la-
bels when they were treated with aliphatic or aromatic thi-
ols. However, when the amino group or an activated me-
thylene group containing thiols was used, dihydro-
thiazepine or thiophene fused spin labels were obtained,
respectively. The treatment of activated ester 2 with aque-
ous ammonia solution gave amide 4a which was treated
with p-toluenesulfonyl chloride in pyridine and yielded
nitrile 4b. Reduction of compound 2 with NaBH4 led to an
allylic alcohol 5a which was oxidized with activated
MnO2 to give aldehyde 5b.18, 19 The allylic bromide 5c
could be obtained from the allylic alcohol 5a via the me-
sylate and the methylthiosulfonate 5d was obtained by
treatment of bromide 5c with NaSSO2CH3.12 An irrevers-
ible thiol specific allylic mercury iodide label 5e was
achieved when compound 5c was treated with NaI fol-
lowed by mercury in THF.20 It is worth noting that com-
pounds 2 and 3 should behave as amino group specific
labels,17, 21 however in a biochemical labelling protocol
they gave ambiguous data, while compound 5d proved to
be a clearly thiol specific label (Scheme 1).
thesis of 3,4-disubstituted nitroxides 17a–d, 18a–d, 19a–c, 20a–c.
Key words: spin labels, thiazepines, thiophenes, sulfides, organo-
mercury compounds
The synthesis of nitroxide stable free radicals has attracted
attention in various fields of chemistry and biological sci-
ences during the past decades. Nitroxides are widely used
as spin labels,1, 2 as co-oxidants,3 as MRI reagents,4 as free
radical scavangers,5 and several spin labelled antitumor
agents have more significant activity with decreased tox-
icity compared to the parent compound.6, 7 Application of
spin label technique very recently had an important devel-
opment with the introduction of site directed spin label-
ling.8 This technique requires the design of lipophilic or
hydrophilic spin labels for site and function directed label-
ling of biomolecules. Introduction of a second arm into
the 2,5-dihydropyrrole ring was achieved by Michael
reaction9 or Claisen rearrangement10 or Grignard reac-
tion.11 The spin labels which proved to be the most useful
protein reagents were those in which reactivity of the thiol
specific arm was increased by an allylic double bond.12, 13
However, the synthesis of bifunctionalized reagents hav-
ing a second substituent at the γ-carbon of the allylic
group proved rather laborious. In this paper we report that
the mixed anhydride ester 2 and the imidazolide 314 of
4-bromo-3-carboxy-2,2,5,5-tetramethyl-2,5-dihydro-1H-
To investigate the thiol versus amino group reactivity we
studied the reaction of compounds 3, 4b, and 5b in the
presence of the amino and thiol group. Reaction of com-
pound 3 with glycine methyl ester in aqueous dioxane af-
forded the corresponding amino acid labelled at the amino
(a) ClCO2Et, Et3N, Et2O, 0°C to r.t., 3 h (80%); (b) 1,1'-carbonyldiimidazole, THF, 65°C, 30 min (72%); (c) aq NH3, r.t., 40 min (45%);
(d) TsCl, pyridine, 0°C to 80°C, 2 h then KOH, r.t., 48 h (60%); (e) NaBH4, EtOH, 0°C, 4 h (62%); (f) MnO2, CHCl3, 61°C, 1 h (44%); (g)
MsCl, CH2Cl2, Et3N, 0°C to r.t., 1 h then LiBr, acetone, 56°C, 45 min (67%); (h) NaSSO2CH3, EtOH, H2O, 78°C, 30 min (44%); (i) NaI, THF,
65°C, 1 h then Hg, r.t., 3 h (38%)
Scheme 1

SYNTHESIS
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1477
group 6 fulfilling our expectation; however, when com- teamine the formation of a 3,4-disubstituted product com-
pound 3 was treated with cysteamine, reaction was ob- pound 11 and 6,6,8,8-tetramethyl-2,3,6,8-tetrahydro-7H-
served not only with the amino group, but also with the pyrrolo[3,4-f]-1,4-thiazepin-7-yloxyl radical 13, respec-
sulfhydryl group shown by formation of 6,6,8,8-tetrame- tively, were observed (Scheme 2).
thyl-5-oxo-2,3,4,5,6,8-hexahydro-7H-pyrrolo[3,4-f]-1,4-
thiazepin-7-yloxyl radical (7). When imidazolide 3 was Consequently, compounds 3 and 5b cannot be regarded
treated with an equivalent amount of benzyl mercaptan for a selective spin label reagent, because they react with
(phenylmethanethiol) in acetonitrile in the presence of both thiol and amino groups. This dual behavior of these
DBU two benzyl thiol esters 8 and 9 were obtained accom- compounds offers an excellent possibility for the synthe-
panied by the starting material 3. The appearance of com- sis of heterocyclic rings fused to the dihydropyrrole ni-
pound 9 showed that not only transesterification, but also troxide ring, such as the Fiesselmann thiophene synthe-
an addition–elimination process, has taken place. Com- sis.23–25 Compound 5b was reacted with thioglycolic acid
pound 9 could be obtained directly from compound 8 with methyl ester in acetonitrile in the presence of equimolar
benzyl mercaptan under the same reaction conditions de- DBU to give 2-(methoxycarbonyl)-4,4,6,6-tetramethyl-
scribed above. These observations mean that imidazolide 3 4,6-dihydro-5H-thieno[2,3-c]pyrrol-5-yloxyl radical (14a)
can be regarded as an unselective short distance cross-link- the dihydrothiophene analog of which was synthesized
ing spin label reagent. The conjugated addition of the sulf- earlier,26 however its aromatization failed in our hands.
hydryl group of N-acetyl-L-cysteine methyl ester22 to com- Compound 14a was hydrolyzed to carboxylic acid 14b
pound 4b and 5b followed by elimination yielded the spin which could be decarboxylated when heated in quinoline
labelled cysteines 10 and 12 in the presence of DBU in ac- with Cu powder.27 From compound 14b the hydroxy-
etonitrile. However, when 4b and 5b were treated with cys- methyl compound 14d was achieved via the mixed anhy-
•
•
(a) H2NCH2CO2CH3 HCl, KHCO3, dioxane, H2O, r.t., 12 h (30%); (b) H2NCH2CH2SH HCl, DBU, MeCN, r.t., 1 h (42–52%); (c) PhCH2SH,
DBU, MeCN, r.t., 1 h (15–54%); (d) N-acetyl-L-cysteine methyl ester, DBU, MeCN, r.t., 1 h (27–31%)
Scheme 2

SYNTHESIS
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1478
dride ester. The oxidation of alcohol 14d with activated 15 and 16. This method allowed us to introduce a great
MnO2 gave spin labelled thiophene aldehyde 14e. The hy- variety of aliphatic or aromatic moieties, the preparation
droxymethyl compound 14d was converted to the bro- of which would otherwise be rather laborious. For spin
momethyl compound 14f when the mesylate was treated label applications, the steric and electronic influence of
with LiBr, 14f was converted with NaSSO2CH3 to give the 4-substituent on immobilization of labelled biomole-
reversible thiol specific methylsulfonylthio label 14g cules will help in obtaining more information about the
(Scheme 3).
vicinity of the coupling site.
Aldehydes 15 and 16 were reduced with NaBH4 to give
alcohols 17a and 18a which could be converted to the cor-
responding allylic bromides 17b and 18b via mesylate.
Then bromides 17b and 18b could be substituted with the
methylthiosulfonyl group to give 3,4-disubstituted thiol-
specific cleavable reagents 17c and 18c or could be con-
verted to thiol specific mercury iodides 17d and 18d. 4-
Ethylthio-substituted alcohol 17a and 4-phenylthio alco-
hol 18a could be oxidized selectively with H2O2 in the
presence of a catalytic amount of Na2WO4 to the corre-
sponding sulfones 19a and 20a, respectively.28 The hy-
droxy group of 19a and 20a could be replaced with LiBr
via the mesylate to give bromides 19b and 20b from
which methylthiosulfonates 19c and 20c were obtained
(Scheme 4).
In conclusion, β-bromo-α,β-unsaturated electron with-
drawing group containing dihydropyrrole nitroxides were
found to react with both thio and amino groups. There-
fore, these compounds cannot be used as selective spin
label reagents, however they are versatile synthons for the
synthesis of 3,4-disubstituted spin labels and paramagnet-
ic annulated heterocycles, dihydrothiazepines or dihydro-
thienopyrroles.
(a) HSCH2CO2CH3, DBU, MeCN, r.t., 1 h (65%); (b) MeOH, 10%
NaOH, r.t., 2 h then 2 M HCl (74%); (c) Cu, quinoline, 237°C, 3 min,
(20%); (d) ClCO2Et, Et3N, Et2O, 0°C to r.t., 2 h then NaBH4, EtOH,
0°C, 2 h (48%); (e) MnO2, CHCl3, 61°C, 1 h (40%); (f) MsCl, Et3N,
CH2Cl2, 0°C then LiBr, acetone, 56°C, 30 min (53%); (g)
NaSSO2CH3, EtOH, H2O, 78°C, 30 min (25%)
Scheme 3
Mps were determined on a Boetius micro melting point apparatus and
are uncorrected. Elemental analyses (C, H, N, S) were performed on
a Fisons EA 1110 CHNS elemental analyzer. The IR (Specord 75)
spectra were in each case consistent with the assigned structure. MS
were recorded on a VG TRIO-2 instrument in the EI mode (70 eV,
direct inlet) or with thermospray technique. Samples were analyzed in
In the absence of an activated methylene group, such as
the ethanethiol or thiophenol group, the ring closure does
not take place, however conjugated addition of thiol fol-
lowed by HBr elimination resulted in formation of 3-
formyl-4-ethylthio- or 4-phenylthio-2,5-dihydropyrroles
(a) EtSH or PhSH, MeCN, DBU, 0°C to r.t., 1 h (75–82%); (b) NaBH4, EtOH, 0°C to r.t., 30 min (65–75%); (c) MsCl, Et3N, CH2Cl2, 0°C to
r.t., 1 h then LiBr, acetone, 56°C, 30 min (45–60%); (d) NaSSO2CH3, EtOH, H2O, 78°C (26–45%); (e) NaI, THF, 65°C, 1 h then HgI, 50°C,
•
3 h (32–38%); (f) H2O2, Na2WO4 2 H2O, MeOH, H2O, r.t., 8 h (50–55%)
Scheme 4

SYNTHESIS
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1479
Table. Compounds 3, 5–20 Prepareda
Product
Yield (%)
(Method)
mp
(°C)
IR (Nujol)
MS
m/z (%)
ν (cm–1)
3
72
44
38
30
42
40
190–192
95–97
1705 (C=O), 1620 (C=C)
1640 (C=C)
312 (M+, 1), 107 (7), 91 (7), 68 (100)
5d
5e
6
342 (M+, 29), 328 (14), 218 (16), 154 (100)
561 (M+H+, 100) TSP
145–147
92–94
1610 (C=C)
3410 (NH), 1740, 1650 (C=O), 1605 (C=C)
3400 (NH), 1630 (C=O), 1605 (C=C)
1630 (C=O), 1580 (C=C)
1630 (C=O), 1575 (C=C)
333 (M+, 5), 303 (19), 135 (98), 107 (100)
241 (M+, 7), 226 (19), 211 (32), 68 (100)
368 (M+, 7), 338 (10), 215 (20), 91(100)
412 (M+, 5), 398 (6), 382 (12), 91 (100)
7
190–192
147–149
73–75
8
9
15 (A)
54 (B)
10
27
52
31
48
65
74
20
48
40
53
25
75
82
65
55
35
32
75
60
40
38
55
45
30
50
53
26
oil
3450 (NH), 2200 (C≡N), 1740, 1655 (C=O)
3365, 3300 (NH2), 2200 (C≡N), 1610 (C=C)
3440 (NH), 1730, 1650 (C=O)
1610 (C=C), 1570 (C=N)
1710 (C=O), 1525 (C=C)
3500 (OH), 1660 (C=O), 1530 (C=C)
1615 (C=C)
340 (M+, 1), 310 (2), 177 (15), 43 (100)
240 (M+, 19), 210 (14), 181 (60), 44 (100)
343 (M+, 3), 329 (4), 313 (3), 43 (100)
225 (M+, 95), 211 (39), 195 (100), 180 (49)
254 (M+, 9), 240 (10), 224 (100), 209 (97)
240 (M+, 9), 226 (34), 210 (100), 195 (87)
196 (M+, 10), 166 (78), 151 (100), 135 (10)
226 (M+, 6), 212 (15), 196 (100), 181 (90)
224 (M+, 8), 210 (12), 194 (100), 179 (91)
288 (M+, 1) 258 (6), 210 (14), 179 (100)
320 (M+, 3), 290 (38), 210 (61), 179 (100)
228 (M+, 38), 198 (83), 169 (69), 108 (100)
276 (M+, 30), 262 (22), 246 (66), 109 (100)
230 (M+, 85), 215 (47), 200 (59), 41 (100)
292 (M+, 9), 248 (55), 218 (66), 138 (100)
324 (M+, 44), 294 (15), 215 (52), 122 (100)
542 (M+H+, 100) TSP
11
47–48
12
193–195
102–104
116–118
160
13
14a
14b
14c
14d
14e
14f
14g
15
110–112
117
3390 (OH), 1655 (C=C)
1670 (C=O)
159
89–91
1660 (C=C)
102–104
83–84
1630 (C=C)
1665 (C=O), 1560 (C=C)
1660 (C=O), 1560 (C=C)
3370 (OH), 1620 (C=C)
16
96–98
17a
17b
17c
17d
18a
18b
18c
18d
19a
19b
19c
20a
20b
20c
75–77
34–36
83–84
96–98
109–111
73–75
3330 (OH), 1565 (C=C)
1565 (C=C)
278 (M+, 48), 264 (35), 248 (38), 43 (100)
340 (M+, 18), 326 (15), 296 (13), 231 (100)
372 (M+, 42), 358 (23), 342 (32), 122 (100)
590 (M+H+, 100) TSP
114–116
146–148
106–108
108–110
118–120
146–148
115–117
58–60
1565 (C=C)
1565 (C=C)
3380 (OH), 1610 (C=C)
1610 (C=C)
262 (M+, 44), 248 (21), 121 (62), 41 (100)
324 (M+, 7), 280 (28), 157 (48), 121 (100)
356 (M+, 5), 342 (5), 153 (65), 65 (100)
310 (M+, 19), 296 (13), 280 (12), 43 (100)
372 (M+, 1), 358 (1), 196 (4), 43 (100)
404 (M+, 2), 294 (13), 151 (89), 65 (100)
1600 (C=C)
3340 (OH), 1620, 1570 (C=C)
1620, 1570 (C=C)
1600, 1575 (C=C)
a Satisfactory elemental analyses obtained.

SYNTHESIS
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the by-pass mode. 10 µL of the sample solution in MeOH were intro-
duced via the thermospray interface. The mobile phase was MeOH/
H2O 1:1 solution containing 0.1 M NH4Ac. The capillary tip tempera-
ture was 230 °C, the electrode voltage was 180 V and the source tem-
perature was 210 °C. The ESR spectra were obtained from a 10–5 molar
solution (CHCl3), using a BRUKER 300-E spectrometer. All monorad-
icals exhibit three equidistant lines with aN = 15.1–15.5 G. Flash col-
umn chromatography was performed on Merck Kieselgel 60 (0.040–
0.063 mm). Quantitative TLC was carried out on commercially pre-
rified by flash column chromatography (hexane/EtOAc) to give the
title compound as a yellow crystalline solid.
4-Bromo-3-(iodomercuriomethyl)-2,2,5,5-tetramethyl-2,5-dihydro-
1H-pyrrol-1-yloxyl Radical (5e): yield: 1.06 g (38%); Rf 0.46 (hex-
ane/EtOAc 2:1); mp 145–147°C.
4-Ethylthio-3-(iodomercuriomethyl)-2,2,5,5-tetramethyl-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radical (17d): yield: 864 mg (32%); Rf 0.55
(hexane/EtOAc 2:1); mp 96–98°C.
pared plates (20 × 20 × 0.2 cm) coated with Merck Kieselgel GF254
.
Compounds 1,15 2,16 4a,17 4b,17 5a,16 5b,16 and 5c16 were prepared ac-
cording to published procedures as indicated in Scheme 1. The physical
and spectral data of all new compounds are listed in the Table.
3-(Iodomercuriomethyl)-2,2,5,5-tetramethyl-4-phenylthio-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radical (18d): yield: 1.11 g (38%); Rf 0.60
(hexane/EtOAc 2:1); mp 146–148°C.
Methyl 2-(4-Bromo-2,2,5,5-tetramethyl-1-oxyl-2,5-dihydro-1H-
pyrrol-3-ylcarbonylamino)acetate Radical (6):
4-Bromo-3-(imidazol-1-ylcarbonyl)-2,2,5,5-tetramethyl-2,5-di-
hydro-1H-pyrrol-1-yloxyl Radical (3):
A mixture of 3 (313 mg, 1.0 mmol) and glycine methyl ester hydro-
chloride (250 mg, 2.0 mmol) and KHCO3 (200 mg, 2.0 mmol) in wa-
ter (5 mL) and dioxane (10 mL) was allowed to stand at r.t. for 12 h.
The solvents were evaporated off in vacuo, the residue was dissolved
in CHCl3 (2 × 10 mL), the organic phase was washed with brine
(10 mL), dried (MgSO4), filtered and purified by flash column chro-
matography (CHCl3/Et2O) to give 6 as a yellow solid; yield: 100 mg
(30%); Rf 0.42 (CHCl3/Et2O 2:1); mp 92–94°C.
To a solution of carboxylic acid 1 (2.63 g, 10.0 mmol) in anhyd THF
(30 mL) was added 1,1'-carbonyldiimidazole (1.62 g, 10 mmol) and
the mixture refluxed for 30 min. After cooling to r.t., Et2O (30 mL)
was added, the organic phase was washed with sat. NaHCO3, cooled
to 0°C, the organic phase was immediately separated, dried (MgSO4),
filtered and evaporated to give yellow crystalline solid; yield: 2.25 g
(72%); Rf 0.35 (CHCl3/Et2O 2:1); mp 190–192°C.
Methylsulfonylthiomethyl Radicals 5d, 14g, 17c, 18c, 19c, and
20c; General Procedure:
Reaction of 3, 4b, and 5b with Cysteamine Hydrochloride (7, 11,
13); General Procedure:
To a solution of bromide 5c or 14f or 17b or 18b or 19b or 20b
(3.0 mmol) in EtOH (15 mL) was added NaSSO2CH3 (804 mg,
6.0 mmol) in water (5 mL) and the mixture was refluxed for 30 min.
The EtOH was evaporated, the residue was taken up in CHCl3
(20 mL), washed with brine, dried (MgSO4), filtered, evaporated and
purified by flash column chromatography (hexane/EtOAc, CHCl3/
Et2O) to give the methylsulfonylthiomethyl radicals.
To mixture of 3 or 4b or 5b (1.0 mmol) and cysteamine hydrochloride
(227 mg, 1.0 mmol) in MeCN (10 mL) was added DBU (500 mg,
3.28 mmol) and the mixture stirred at r.t. for 1 h. Then MeCN was
evaporated off, the residue was taken up in CHCl3 (20 mL), washed
with brine (10 mL), the organic phase was separated, dried (MgSO4),
filtered, evaporated and the residue was purified by flash chromatog-
raphy (CHCl3/MeOH).
6,6,8,8-Tetramethyl-5-oxo-2,3,4,5,6,8-hexahydro-7H-pyrrolo[3,4-
f]-1,4-thiazepin-7-yloxyl Radical (7): yield: 101 mg (42%); Rf 0.34
(CHCl3/MeOH 9:1); mp 190–192°C.
4-Bromo-2,2,5,5-tetramethyl-3-(methylsulfonylthiomethyl)-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radical (5d): yield: 453 mg (44%); Rf 0.50
(CHCl3/Et2O 2:1); mp 95–97 °C.
4-(2-Aminoethylthio)-3-cyano-2,2,5,5-tetramethyl-2,5-dihydro-1H-
pyrrol-1-yloxyl Radical (11): yield; 125 mg (52%); Rf 0.29 (CHCl3/
MeOH 9:1); mp 47–48°C.
4,4,6,6-Tetramethyl-2-(methylsulfonylthiomethyl)-4,6-dihydro-5H-
thieno[2,3-c]pyrrol-5-yloxyl Radical (14g): yield: 240 mg (25%); Rf
0.64 (CHCl3/Et2O 2:1); mp 102–104 °C.
6,6,8,8-Tetramethyl-2,3,6,8-tetrahydro-7H-pyrrolo[3,4-f]-1,4-thiaz-
epin-7-yloxyl Radical (13): yield: 108 mg (48%); Rf 0.26 (CHCl3/
MeOH 20:1); mp 102–104°C.
4-Ethylthio-2,2,5,5-tetramethyl-3-(methylsulfonylthiomethyl)-2,5-di-
hydro-1H-pyrrol-1-yloxyl Radical (17c): yield: 340 mg (35%); Rf
0.29 (hexane/EtOAc 2:1); mp 83–84 °C.
3-(Benzylthiocarbonyl)-4-bromo-2,2,5,5-tetramethyl-1-oxyl-2,5-
dihydro-1H-pyrrole Radical (8) and 4-Benzylthio-3-(benzylthio-
carbonyl)-2,2,5,5-tetramethyl-1-oxyl-2,5-dihydro-1H-pyrrole
Radical (9):
2,2,5,5-Tetramethyl-3-(methylsulfonylthiomethyl)-4-phenylthio-2,5-
dihydro-1H-pyrrol-1-yloxyl Radical (18c): yield: 447 mg (40%); Rf
0.33 (hexane/EtOAc 2:1); mp 114–116 °C.
Method A (from compound 3):
4-Ethylsulfonyl-2,2,5,5-tetramethyl-3-(methylsulfonylthiomethyl)-
2,5-dihydro-1H-pyrrol-1-yloxyl Radical (19c): yield: 320 mg (30%);
Rf 0.28 (CHCl3/Et2O 2:1); mp 118–120 °C.
To a solution of imidazolide 3 (626 mg, 2.0 mmol) and benzyl mer-
captan (248 mg, 2.0 mmol) in MeCN (10 mL) was added DBU
(334 mg, 2.2 mmol) and the mixture was allowed to stand at r.t. for
30 min. Then MeCN was evaporated off, the residue was taken up in
EtOAc (20 mL), washed with 5% aq H2SO4, dried (MgSO4), filtered,
evaporated and the residue was purified by flash column chromatog-
raphy (hexane/Et2O) to give compound 8; yield: 295 mg (40%); Rf
0.60 (hexane/Et2O 2:1); mp 147–149°C; and compound 9; yield:
123 mg (15%); Rf 0.47 (hexane/Et2O 2:1); mp 73–75°C.
2,2,5,5-Tetramethyl-3-(methylsulfonylthiomethyl)-4-phenysulfonyl-
2,5-dihydro-1H-pyrrol-1-yloxyl Radical (20c): yield: 312 mg (26%);
Rf 0.62 (CHCl3/Et2O 2:1); mp 58–60 °C.
Iodomercuriomethyl Radicals 5e, 17d, and 18d; General Proce-
dure:
4-Benzylthio-3-(benzylthiocarbonyl)-2,2,5,5-tetramethyl-1-oxyl-
2,5-dihydro-1H-pyrrole Radical (9):
Method B (from compound 8):
To a solution of thioester 8 (73.8 mg, 0.2 mmol) and benzyl mercap-
tan (25 mg, 0.2 mmol) in MeCN (5 mL) was added DBU (40 mg,
0.26 mmol) and the mixture was allowed to stand at r.t. for 30 min.
To a solution of bromide 5c or 17b or 18b (5.0 mmol) in anhyd THF
(40 mL) was added NaI (1.50 g, 10.0 mmol) and the mixture was
stirred and refluxed for 1 h, then Hg (2.0 g, 10.0 mmol) was added and
stirring was continued for 3 h at r.t. The mixture was decanted from
the remaining Hg, the organic phase was washed with water (2 ×
10 mL), dried (MgSO4), filtered, evaporated and the residue was pu-

SYNTHESIS
October 1998
1481
Then MeCN was evaporated off, the residue was taken up in EtOAc
(10 mL) washed with 5% aq H2SO4, dried (MgSO4), filtered, evapo-
rated and the residue was crystallized from hexane to give 9; yield: 45
mg (54%); Rf 0.47 (hexane/Et2O 2:1); mp 73–75°C.
organic phase was dried (MgSO4), filtered and evaporated. After
chromatography (CHCl3/Et2O) the product was obtained as a crystal-
line solid; yield: 1.08g (48%); Rf 0.37 (CHCl3/Et2O 2:1); mp 117°C.
2-Formyl-4,4,6,6-tetramethyl-4,6-dihydro-5H-thieno[2,3-c]pyr-
rol-5-yloxyl Radical (14e):
Reaction of Compounds 4b and 5b with N-Acetyl-L-cysteine
Methyl Ester; General Procedure:
To a solution of alcohol 14d (452 mg, 2.0 mmol) in CHCl3 (30 mL)
was added activated MnO2 (2.50 g, 28.7 mmol) and the mixture
stirred and refluxed for 1 h. Then MnO2 was filtered off through a
Celite pad, the CHCl3 was evaporated off, the residue was purified by
flash chromatography to give aldehyde 14e; yield: 180 mg (40%); Rf
0.40 (hexane/EtOAc 2:1); mp 159°C.
To a stirred mixture of 4b or 5b (1.0 mmol) and N-acetyl-L-cysteine me-
thyl ester (177 mg, 1.0 mmol) was added DBU (167mg, 1.1 mmol) and
the mixture was allowed to stand at r.t. for 1 h. Then MeCN was evapo-
rated off, the residue was taken up in CHCl3 (20 mL), washed with brine
(10 mL), the organic phase was separated, dried (MgSO4), filtered, and
purified by flash column chromatography (CHCl3/MeOH).
Synthesis of Allylic Bromides 14f, 17b, 18b, 19b, and 20b; Gener-
al Procedure:
Methyl 2-Acetamido-3-(4-cyano-2,2,5,5-tetramethyl-1-oxyl-2,5-di-
hydro-1H-pyrrol-3-ylthio)propanoate Radical (10): yield: 92 mg
(27%); Rf 0.48 (CHCl3/MeOH 9:1); oil.
To a mixture of alcohol 14d or 17a or 18a or 19a or 20a (5.0 mmol)
and Et3N (555 mg, 5.5 mmol) in CH2Cl2 (20 mL) was added methane-
sulfonyl chloride (630 mg, 5.5 mmol) dropwise at 0°C and the mix-
ture stirred at r.t. for 1 h. The organic phase was washed with brine,
dried (MgSO4), filtered and evaporated. To the solution of crude me-
sylate in acetone (30 mL) was added LiBr (868 mg, 10.0 mmol) and
the mixture refluxed for 30 min. The acetone was evaporated off, the
residue was dissolved in Et2O (30 mL), the organic phase was washed
with brine (10 mL), dried (MgSO4), filtered and evaporated. Chroma-
tography (hexane/Et2O, hexane/EtOAc) gave the bromides.
Methyl 2-Acetamido-3-(4-formyl-2,2,5,5-tetramethyl-1-oxyl-2,5-di-
hydro-1H-pyrrol-3-ylthio)propanoate Radical (12): yield: 106 mg
(31%); Rf 0.65 (CHCl3/MeOH 2:1); mp 193–195°C.
4,4,6,6-Tetramethyl-2-methoxycarbonyl-4,6-dihydro-5H-
thieno[2,3-c]pyrrol-5-yloxyl Radical (14a):
To stirred mixture of aldehyde 5b (2.47 g, 10.0 mmol) and methyl
thioglycolate (1.06 g, 10.0 mmol) in anhyd MeCN (30 mL) was added
DBU (1.67 g, 11.0 mmol) dropwise at 0°C and the mixture was al-
lowed to stand at r.t. for 1 h. Then MeCN was evaporated off in vacuo,
the residue was dissolved in EtOAc (30 mL) and washed with 5% aq
H2SO4. The organic phase was separated, dried (MgSO4), filtered,
evaporated and purified by flash column chromatography (hexane/
Et2O) to give compound 14a as a yellow crystalline solid; yield:
1.65 g (65%); Rf 0.34 (hexane/Et2O 2:1); mp 116–118°C.
2-(Bromomethyl)-4,4,6,6-tetramethyl-4,6-dihydro-5H-thieno[2,3-
c]pyrrol-5-yloxyl Radical (14f): yield: 766 mg (53%); Rf 0.65 (hex-
ane/EtOAc 2:1); mp 89–91°C.
3-(Bromomethyl)-4-ethylthio-2,2,5,5-tetramethyl-2,5-dihydro-1H-
pyrrol-1-yloxyl Radical (17b): yield: 806 mg (55%); Rf 0.33 (hexane/
Et2O 2:1); mp 34–36°C.
2-Carboxy-4,4,6,6-tetramethyl-4,6-dihydro-5H-thieno[2,3-c]pyr-
rol-5-yloxyl Radical (14b):
3-(Bromomethyl)-2,2,5,5-tetramethyl-4-phenylthio-2,5-dihydro-1H-
pyrrol-1-yloxyl Radical (18b): yield: 1.16 g (60%); Rf 0.26 (hexane/
Et2O 2:1); mp 73–75°C.
To solution of ester 14a (2.54 g, 10.0 mmol) in MeOH (30 mL) was
added 10% aq NaOH (10 mL) and the mixture was allowed to stand
at r.t. for 2 h. Then MeOH was evaporated off and the mixture was
acidified with 2 M HCl, extracted with CHCl3 (2 × 20 mL), the organ-
ic phase was separated, dried (MgSO4), filtered and evaporated to
give a pale yellow solid; yield: 1.77 g (74%); Rf 0.22 (CHCl3/MeOH
9:1); mp 160°C (dec).
3-(Bromomethyl)-4-ethylsulfonyl-2,2,5,5-tetramethyl-2,5-dihydro-
1H-pyrrol-1-yloxyl Radical (19b): yield: 730 mg (45%); Rf 0.50 (hex-
ane/EtOAc 2:1); mp 108–110°C.
3-(Bromomethyl)-2,2,5,5-tetramethyl-4-phenylsulfonyl-2,5-dihydro-
1H-pyrrol-1-yloxyl Radical (20b): yield1.0 g (53%); Rf 0.58 (hexane/
Et2O 2:1); mp 115–117°C.
4,4,6,6-Tetramethyl-4,6-dihydro-5H-thieno[2,3-c]pyrrol-5-ylox-
yl Radical (14c):
A mixture of acid 14b (480 mg, 2.0 mmol) and Cu powder (20 mg,
0.3 mmol) in quinoline (5 mL) was heated to reflux and kept at this
temperature for 3 min, then the mixture was immediately cooled to r.t.
The mixture was poured onto ice water (50 mL), the aqueous phase
was extracted with Et2O (3 × 20 mL), the organic phase was washed
with 2 M HCl (3 × 20 mL), the organic phase was separated, dried
(MgSO4), filtered, evaporated, and the residue was chromatographed
(hexane/Et2O) to give the title compound; yield: 79 mg (20%); 0.53
(hexane/Et2O 2:1); mp 110–112°C.
Synthesis of 4-Alkyl(Aryl)thio-Substituted 3-Formyl-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radicals 15, 16; General Procedure:
To a solution of aldehyde 5b (2.47 g, 10.0 mmol) and ethanethiol or
thiophenol (10.0 mmol) in MeCN (40 mL) was added DBU (1.67 g,
11.0 mmol) at 0°C. The mixture was allowed to stand at r.t. for 1 h,
then MeCN was evaporated off, the residue was taken up in EtOAc
(40 mL), washed with 5% H2SO4 (20 mL). The organic phase was
separated, dried (MgSO4), filtered and evaporated. The residue was
chromatographed (hexane/Et2O) to give the aldehydes.
2-(Hydroxymethyl)-4,4,6,6-tetramethyl-4,6-dihydro-5H-
4-Ethylthio-3-formyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-
yloxyl Radical (15): yield: 1.71 g (75%); Rf 0.37 (hexane/Et2O 2:1);
mp 83–84°C.
thieno[2,3-c]pyrrol-5-yloxyl Radical (14d): Typical Procedure:
To a solution of carboxylic acid 14b (2.40 g, 10.0 mmol) and Et3N
(1.11 g, 11.0 mmol) in anhyd Et2O (40 mL) was added ethyl chloro-
formate (1.20 g, 11.0 mmol) at 0 °C. The mixture was stirred at r.t. for
2 h. The triethylamine hydrochloride was filtered off, the Et2O was
evaporated in vacuo. The residue, the crude mixed anhydride ester
was immediately dissolved in EtOH (30 mL) and treated with NaBH4
(600 mg, 15.8 mmol) at 0°C. The solution was kept at this tempera-
ture for 3 h, then EtOH was evaporated off. To the residue was added
brine (15 mL) and it was then extracted with CHCl3 (3 × 10 mL). The
3-Formyl-2,2,5,5-tetramethyl-4-phenylthio-2,5-dihydro-1H-pyrrol-
1-yloxyl Radical (16): yield: 2.26 g (82%); Rf 0.29 (hexane/Et2O 2:1);
mp 96–98°C.
Synthesis of Alcohols 17a and 18a; General Procedure:
A solution of aldehyde 15 or 16 (7.0 mmol) in EtOH (30 mL) was
treated with NaBH4 (378 mg, 10.0 mmol) at 0°C. Then the mixture

SYNTHESIS
Papers
1482
was allowed to stand at r.t. for 30 min, the EtOH was evaporated off,
the residue was taken up in CHCl3 (30 mL), washed with brine, the
organic phase was separated, dried (MgSO4), filtered and evaporated.
The residue was crystallized from hexane to give the alcohols.
(4) Couet, W. R.; Eriksson, U. G.; Brasch, R. G.; Tozer, T. N. Phar-
maceutical Res. 1985, 69.
(5) Connolly, T. J.; Scaiano, J. C. Tetrahedron Lett. 1997, 38, 1133.
(6) Sosnovsky, G.; Maheswara Rao, N. O.; Li, S. W. J. Med. Chem.
1986, 29, 2225.
4-Ethylthio-3-(hydroxymethyl)-2,2,5,5-tetramethyl-2,5-dihydro-1H-
pyrrol-1-yloxyl Radical (17a): yield: 1.04 g (65%); Rf 0.37 (hexane/
Et2O 2:1); mp 75–77°C.
(7) Tian, X.; Wang, Y.; Yang, M.; Chen, Y. Life Sci. 1997, 60, 511.
(8) Steinhoff, H.; Mollaaghababa, R.; Altenbach, C.; Khorana, H.
G.; Hubbell, W. L. Biophys. Chem. 1995, 56, 89.
(9) Hideg, K.; Hankovszky, H. O.; Halász, H. A.; Sohár, P.
J. Chem. Soc., Perkin 1. 1988, 2905.
3-(Hydroxymethyl)-2,2,5,5-tetramethyl-4-phenylthio-2,5-dihydro-
1H-pyrrol-1-yloxyl Radical (18a): yield: 1.46 g (75%); Rf 0.40 (hex-
ane/EtOAc 2:1); mp 109–111°C.
(10) Hideg, K.; Sár, P. C.; Hankovszky, H. O.; Tamás, T.; Jerkovich,
Gy. Synthesis 1993, 390.
(11) Kálai, T.; Bárácz, M. N.; Jerkovich, Gy.; Hankovszky, H. O.;
Hideg, K. Synthesis 1995, 1278.
(12) Berliner, L. J.; Grünwald, J.; Hankovszky, H. O.; Hideg, K.
Anal. Biochem. 1982, 119, 450.
Oxidation of Sulfides 17a and 18a to Sulfones 19a and 20a; Gen-
eral Procedure:
To a solution of alcohol 17a or 18a (10.0 mmol) in MeOH (30 mL)
•
were added 30% H2O2 (3 mL) and Na2WO4 2 H2O (165 mg,
(13) Oh, J. K.; Zhan, H.; Cui, C.; Hideg, K.; Collier, R. J.; Hubbell,
W. L. Science 1996, 273, 810.
(14) Barratt, M. D.; Dodd, G. H.; Chapman, D. Biochem. Biophys.
Acta 1969, 194, 600.
(15) Zhdanov, R. I. In Bioactive Spin Labels; Zhdanov, R. I. Ed.;
Springer: Berlin, 1992; p 23.
0.5 mmol) and the mixture was allowed to stand overnight at r.t. The
solution was extracted with CHCl3 (3 × 20 mL), the organic phase
was dried (MgSO4), filtered and evaporated. The residue was purified
by flash chromatography (CHCl3/Et2O).
4-Ethylsulfonyl-3-(hydroxymethyl)-2,2,5,5-tetramethyl-2,5-dihydro-
1H-pyrrol-1-yloxyl Radical (19a): yield: 1.44 g (55%); Rf 0.25
(CHCl3/Et2O 2:1); mp 106–108°C.
(16) Chudinov, A. V.; Rozantsev, E. G.; Rosinov, B. V. Izv. Akad.
Nauk. Ser.Khim. 1983, 409; Chem. Abstr. 1983, 98, 197927.
(17) Chudinov, A. V.; Rozantsev, E. G. Izv. Akad. Nauk. Ser.Khim.
1983, 400; Chem. Abstr. 1983, 98, 179180.
3-(Hydroxymethyl)-2,2,5,5-tetramethyl-4-phenylsulfonyl-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radical (20a): yield: 1.55 g (50%); Rf 0.36
(CHCl3/Et2O 2:1); mp 146–148°C.
(18) Hideg, K.; Hankovszky, H. O.; Lex, L.; Kulcsár, Gy. Synthesis
1980, 911.
(19) Hankovszky, H. O.; Hideg, K.; Lex, L. Synthesis 1980, 914.
(20) Larock, R. C.; Lu, Y. J. Org. Chem. 1993, 58, 2846.
(21) Griffith, O. H.; Keana, J. F. W.; Noall, D. L.; Ivey, J. L. Bio-
chem. Biophys. Acta 1967, 148, 683.
(22) Bruice, T. W.; Kenyon, G. L. J. Protein Chem. 1982, 1, 47.
(23) Fiesselmann, H.; Schipprak, P. Chem. Ber. 1954, 87, 835.
(24) Gronowitz, S. In The Chemistry of Heterocyclic Compounds:
Thiophene and its Derivatives; Gronowitz, S. Ed.; Wiley: New
York, 1984; Vol. 1; p 88.
This work was supported by grants from the Hungarian National
Research Foundation (T021277). The authors wish to express their
thanks to V. Csokona for technical assistance, M. Szabó (ICN Alka-
loda Co. Ltd.) for mass spectral measurements and to B. Rozsnyai
for microanalyses.
(25) Cagniant, P.; Kirsch, G. C. R. Hebd. Seances Acad. Sci., Ser. C
1975, 281, 35; Chem. Abstr. 1975, 83, 178681.
(1) Hideg, K.; Hankovszky, H. O. In Spin-Labeling Theory and Ap-
plications; Berliner, L. J.; Reuben, J. Eds.; Plenum Press: New
York, 1989; p 427.
(2) Rozantsev, E. G. Free Nitroxide Radicals; Plenum Press: New
York, 1970.
(26) Keana, J. F. W.; Hideg, K.; Birrell, G. B.; Hankovszky, H. O.;
Ferguson, G.; Parvez, M. Can. J. Chem. 1982, 60, 1439.
(27) Barker, J. M.; Huddleston, P. R. In The Chemistry of Hetero-
cyclic Compounds: Thiophene and its Derivatives; Gronowitz,
S. Ed.; Wiley: New York, 1986; Vol. 3; p 565.
(3) Nooy, A. E. J.; Besemer, A. C.; Bekkum, H. Synthesis 1996,
1153.
(28) Zhdanov, R. I.; Kapitanova, N. G.; Rozantsev, E. G. Synthesis
1977, 312.