Reactions of nitroxides. Part X: Antifungal activity of selected sulfur and selenium derivatives of 2,2,6,6-tetramethylpiperidine

Article abstract of DOI:10.1016/j.bmcl.2010.10.092

The antifungal activity of nitroxyl radicals - derivatives of 2,2,6,6-tetramethylpiperidine-1-oxyl with reactive substituents 4-isothiocyanato-, 4-isocyano-, and 4-isoselenocyanato- and of N-formyl-, N-thioformyl-, N-selenoformyl-derivatives of 2,2,6,6-tetramethylpiperidine was investigated. Those of the above compounds, which contain a sulfur or selenium atom are the most active against four fungus plant patogens: Botrytis cinerea, Fusarium culmorum, Phytophthora cactorum, Rhizoctonia solani. 4-Isoselenocyanato-2,2,6,6-tetramethylpiperidine-1-oxyl proved to be the most active compound.

Full text of DOI:10.1016/j.bmcl.2010.10.092

Bioorganic & Medicinal Chemistry Letters 21 (2011) 514–516
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Reactions of nitroxides. Part X: Antifungal activity of selected sulfur
and selenium derivatives of 2,2,6,6-tetramethylpiperidine q
⇑
Jerzy Zakrzewski , Maria Krawczyk
Institute of Industrial Organic Chemistry, Annopol 6, 03-236 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The antifungal activity of nitroxyl radicals—derivatives of 2,2,6,6-tetramethylpiperidine-1-oxyl with
reactive substituents 4-isothiocyanato-, 4-isocyano-, and 4-isoselenocyanato- and of N-formyl-,
N-thioformyl-, N-selenoformyl-derivatives of 2,2,6,6-tetramethylpiperidine was investigated. Those of
the above compounds, which contain a sulfur or selenium atom are the most active against four fungus
plant patogens: Botrytis cinerea, Fusarium culmorum, Phytophthora cactorum, Rhizoctonia solani. 4-Isose-
lenocyanato-2,2,6,6-tetramethylpiperidine-1-oxyl proved to be the most active compound.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 12 July 2010
Revised 18 October 2010
Accepted 19 October 2010
Available online 25 October 2010
Keywords:
Nitroxyl radicals
Isothiocyanates
Isoselenocyanates
Isocyanides
Organothio and organoselenium compounds exhibit many inter-
esting biological properties. Aliphatic isothiocyanates are present as
glucosinolates in food in cruciferous vegetables (horseradish, mus-
tard, cabbage, broccoli, Brussels sprouts). The breakdown of gluco-
sinolates liberates dietary isothiocyanates: sulforaphane (CH3
S(@O)(CH2)4NCS), allyl isothiocyanate (mustard oil), benzyl and
phenethyl isothiocyanates. The chemistry and mechanisms of anti-
carcinogenic activities of dietary isothiocyanates are reviewed.2 In
particular, the activities of sulforaphane (present in broccoli)3 and
aralkyl isothiocyanates4,5 were reported. Lower alkyl isothiocya-
nates, allyl isothiocyanate, benzyl and phenethyl isothiocyanates
were tested as protective compounds against gastric lesions induced
by ethanol or acetylosalicylic acid. Allyl isothiocyanate exhibits
strong protective activity against such lesion factors.6 Sulforaphane
and other isothiocyanates exhibit direct antibacterial activity
against Helicobacter pylori and other bacterial and fungal patho-
gens,7 and may prevent gastric cancer.8 The effect of 2-(3,4-dihy-
droxyphenyl)ethyl isothiocyanate on a bacterial virus K was
described.9
isothiocyanates were investigated as potential anticancer drugs
due to their capability to create covalent bonds with biore-
ceptors.12
Selenium plays an important role as a trace element in living
organisms (reviews13). Traces of selenium are important in oxygen
metabolism. Organoselenium compounds are considered to be
inhibitors of free radicals, as well as anti-inflammatory and antiox-
idant agents. Recently, selenoureas have been found to be effective
superoxide radical scavengers.14 Heterocyclic compounds contain-
ing the selenium atom may restrain cancer cell growth.15 More
generally, selenium and its relationship to cancer was reviewed.16
The capability of organoselenium compounds to undergo biotrans-
formations was also described.17
In general, isoselenocyanates are more active than their dietary
thio analogs described above. The comparison of the same series of
aralkyl isoselenocyanates with analogous aralkyl isothiocyanates
shows that the antitumor activity of selenium compounds is higher
than the activity of thio analogs.18 Aralkyl isoselenocyanates
(C6H5(CH2)nNCSe n = 1, 2, 4, 6) are therapeutically effective for
inhibiting melanoma tumor development by inducing the apopto-
sis in melanoma cells.19
The selenium analog of sulforaphane [CH3S(@O)(CH2)4NCSe,
‘selenoraphane’] seems to be more toxic to cancer cells than
sulforaphane itself, but less toxic to normal cells.20
In our earlier papers, we described the synthesis and some
pesticidal properties of thio- and seleno-analogs of some com-
monly known N,N-dimethyl-N0-aryl urea herbicides,21 as well as
thio and selenoureas bearing 2,2,6,6-tetramethylpiperidine-1-oxyl
(nitroxyl) moiety.22,23 In this Letter, we present the antifungal activ-
ity of reactive nitroxyl radicals with isothiocyanato, isocyano, and
Aromatic isothiocyanates were tested in a wide spectrum of
biological activities. Anthelmintic properties of a series of isothioc-
yanatophenyl derivatives of 1,2,4-oxadiazoles were reported.10
A
symmetric aryl diisothiocyanate (CH2)n(NHC(@S)NHC6H4NCS)2)
and diisothiocyanatostilbenedisulfonic acid derivatives (DIDS and
H2DIDS) were examined as inhibitors of phospholipase C.11 Aryl
q
See Ref. 1 for Part IX of this series.
⇑
Corresponding author.
E-mail addresses: zakrzewski@ipo.waw.pl, jerzy_z@wp.pl (J. Zakrzewski).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.10.092

J. Zakrzewski, M. Krawczyk / Bioorg. Med. Chem. Lett. 21 (2011) 514–516
515
Table 1
isoselenocyanato substituents, as well as of N-formyl, N-thioformyl,
Evaluation of antifungal activity of 1–6
and N-selenoformyl derivatives of 2,2,6,6-tetramethylpiperidine.
4-Isothiocyanato-, 4-isocyano-, 4-isoselenocyanato-2,2,6,6-tet-
ramethylpiperidine-1-oxyl (TEMPO-NCS (1), TEMPO-NC (2), TEM-
PO-NCSe (3), respectively), and N-formylo-, N-thioformylo-,
N-selenoformylo-2,2,6,6-tetramethylpiperidine (TMP-CHO (4),
TMP-CHS (5), TMP-CHSe (6), respectively) were synthesized
according to the methods presented in Scheme 1.
2,2,6,6-Tetramethyl-4-piperidinone (triacetonamine, 7) was ob-
tained from acetone, concentrated aqueous ammonia solution and
ammonium chloride.24–26 The Wolf–Kishner reduction of 7 with
hydrazine in the presence of potassium hydroxide in diethylene
glycol affords 2,2,6,6-tetramethylpiperidine (8).25–27 Formylation
of 8 with chloroform using PTC conditions affords TMP-CHO
(4).22 4 was either thionized with Lawesson reagent to TMP-CHS
(5),22 or selenized with either P/Se or Woollins reagent to TMP-
CHSe (6).23 7 was converted in a multistep synthesis to 4-amino-
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-NH2, 13).1,29–34
Reaction of 13 with thiophosgene affords TEMPO-NCS (1).34
Formylation of 13 with ethyl formate35 followed by dehydration
of 14 with phosgene or diphosgene results in TEMPO-NC (2).35 Sel-
enization of 2 with metallic selenium affords TEMPO-NCSe (3).23,36
The details of the synthetic protocols are presented in the above-
mentioned references.
Compd B. cinerea, F. culmorum, P. cactorum, R. solani,
B. graminis,
200 mg/L 1000 mg/L
200 mg/L
in vitro
200 mg/L
in vitro
200 mg/L
in vitro
in vitro
in vivo
1
2
3
4
5
6
60
5
94
0
77
80
67
0
100
0
44
62
97
35
97
10
100
100
100
60
100
13
100
86
35
37
40
12
17
43
Reduction of colonies growth (%).
Table 2
IC50 values in mg/L of sulfur and selenium derivatives 1, 3, 5, 6
Compd
B. cinerea
F. culmorum
P. cactorum
R. solani
1
3
5
6
102.2
19.3
110.1
121.5
95.3
17.5
221.3
132.7
41.7
6.3
48.4
59.4
61.8
5.1
65.1
68.7
The pesticidal activity of TEMPO-NCS (1), TEMPO-NC (2), TEM-
PO-NCSe (3), TMP-CHO (4), TMP-CHS (5), TMP-CHSe (6) was inves-
tigated. The results of the antifungal activity against Botrytis
cinerea, Fusarium culmorum, Phytophthora cactorum, Rhizoctonia
solani, Blumeria graminis are presented in Table 1. The detailed pro-
cedure was presented in the previous paper.22 The values of the
inhibition index IC50 for the most active compounds 1, 3, 5, 6 are
presented in Table 2. The inhibition index IC50 (the concentration
that inhibits 50% of mycelial growth) was calculated by the probit
analysis.
All synthesized compounds (1–6) were inactive as herbicides
and insecticides, but some of them showed antifungal activity.
TEMPO-NCSe (3) proved to be the most active compound: it
showed the maximum inhibitory effect on the mycelium growth
for B. cinerea, F. culmorum, P. cactorum, R. solani. In particular, the
strongest activity was found against two fungi P. cactorum and R.
solani with IC50 values of 6.3 and 5.1, respectively.
Z
4,TMP-CHO
Z
Z
Ar
P
P Ar
Z
N
iv
Lawesson reagent:
Ar=4-CH3OC6H4, Z=S
Woollins reagent:
Ar=C6H5, Z=Se
iii
CHO
8
5 X=S TMP-CHS
6 X=Se TMP-CHSe
N
NH
O
ii
CHX
7
NH
3,TEMPO-NCSe
HO
v
i
N
NH2
NCSe
O
vi
9
10
NH
NH
N
TEMPO-NCS (1), TMP-CHS (5) and TMP-CHSe (6) exhibited sim-
ilar activities: they induced a good inhibitory effect againstP. cacto-
rum and R. solani, whereas the growth of B. cinerea and F. culmorum
was inhibited by approximately 44–80%. Generally, all compounds
tested showed weak to moderate activity against powdery mildew
on wheat (B. graminis) at concentration of 1000 mg/L.
•
xii
vii
NHCOCH3
NC
O
11
NH
N
•
NH2
2,TEMPO-NC
viii
O
In conclusion, 2,2,6,6-tetramethylpiperidine derivatives con-
taining sulfur and selenium—nitroxyl radicals: 4-isothiocyanato-2,
2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-NCS, 1), 4-isoseleno-
cyanato-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-NCSe, 3),
and N-substituted compounds: 1-thioformyl-2,2,6,6-tetramethyl-
piperidine (TMP-CHS, 5), and 1-selenoformyl-2,2,6,6-tetramethyl-
piperidine (TMP-CHSe, 6) were found to exhibit the antifungal
activity in vitro against four fungus species. In particular, TEM-
PO-NCSe (3) proved to be the most active among all compounds
tested. The comparison of the antifungal activity of 4–6 indicates
that incorporating a selenium or sulfur atom instead of the oxygen
one may have a positive influence on the enhanced antifungal
activity of the tested molecules.
xi
NHCOCH3
NHCHO
N
x
•
ix
O
13
N
O
N
xiii
•
•
NCS
12
O
14
N
•
1,TEMPO-NCS
O
Scheme 1. Synthesis of compounds 1–14. Reagents and conditions: (i) NH3 (aq),
NH4Cl, three days, 26–33%;24–26 (ii) 50% aqueous NH2NH2, potassium hydroxide,
diethylene glycol, 127–200 °C, 75–82%;25–27 (iii) CHCl3, 50% NaOH aq, TEBACl, 50 °C,
5.5 h, 45%;22 (iv) X = S: Lawesson reagent, toluene, reflux, 3.5 h, rt overnight, 28%;22
X = Se: P/Se, xylene, reflux, 18% or Woollins reagent, toluene, rt, 18 h, 83%;23 (v)
NH2OH ꢀ HCl, CH3COONa, H2O, 18%;28–30 (vi) Na, n-C3H7OH, reflux, 1 h, 80%;30–33
(vii) (CH3CO)2O, diethyl ether, 5–10 °C, 3.5 h, 100%;1,34 (viii) H2O2, WO42ꢁ, rt, six
days, 41%;1,34 (ix) H2O, KOH, reflux, 30 h, 93%;1,34 (x) HCOOC2H5 (excess), reflux, 5 h,
96%;35 (xi) COCl2, N(C2H5)3, CH2Cl2, 5–8 °C, 84%35 or ClCOOCCl3 (diphosgene),
N(C2H5)3, CH2Cl2, <10 °C, 78%;23 (xii) Se, CHCl3 reflux, 70 h, 38%;23,36 (xiii) CSCl2,
N(C2H5)3, benzene, 10–17 °C, 67%.34
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