A synthetic approach to methyl thieno[2,3-d][1,2,3]thiadiazole carboxylates via diazotation

Article abstract of DOI:10.1007/PL00010236

A route to methyl thieno[2,3-d][1,2,3]thiadiazole carboxylates 3 as highly potent inducers of systemic acquired resistance (SAR) is reported. The construction of the appropriately substituted thiophene precursors is elaborated. Cyclization towards the thi

Full text of DOI:10.1007/PL00010236

Monatshefte fuÈr Chemie 130, 573±580 (1999)
A Synthetic Approach to Methyl Thieno[2,3-
d][1,2,3]thiadiazole Carboxylates via Diazotation
Ã
Peter Stanetty and Marko D. Mihovilovic
Institute of Organic Chemistry, Vienna University of Technology, A-1060 Vienna, Austria
Summary. A route to methyl thieno[2,3-d][1,2,3]thiadiazole carboxylates 3 as highly potent
inducers of systemic acquired resistance (SAR) is reported. The construction of the appropriately
substituted thiophene precursors is elaborated. Cyclization towards the thiadiazole ring system was
carried out using diazotation techniques.
Keywords. Plant activators; Systemic acquired resistance; Diazotation.
È
È
Ein Syntheseweg zu Thieno[2,3-d][1,2,3]thiadiazolcarbonsauremethylestern uber
Diazotierungsreaktionen
È
Zusammenfassung: Eine Herstellungsmethode von Thieno[2,3-d][1,2,3]thiadiazolcarbonsaure-
methylestern 3 als hochwirksame Induktoren fuÈr chemisch induzierte Resistenz (SAR) wird be-
schrieben. Der Aufbau von entsprechend substituierten Thiophenvorstufen wird beschrieben. Der
Ringschluû zum Thiadiazolringsystem erfolgte uÈber Diazotierungstechniken.
Introduction
In many cases the local infection of a plant leads to the development of resistance
to subsequent challenges by a variety of pathogens. The phenomenon of systemic
acquired resistance (SAR) can also be induced by heterocyclic compounds called
plant activators like 2,6-dichloroisonicotinic acid 1 or various 1,2,3-benzothiadia-
zole-7-carboxylic acid derivatives such as 2 (Scheme 1). Both approaches lead to
the same biochemical reactions within the plant strongly correlating with the
expressions of PR-proteins. As a result, the whole plant develops a general
resistance against a large variety of different pathogens [1]. This strategy was
successfully introduced as a new concept in plant protection chemistry recently
by Novartis with compound 2 representing the ®rst commercial product called
Bion1 [2].
Based on the bioisosteric effect [3] of thiophene and benzene rings, we became
interested in the development of synthetic routes to thieno [2,3,-d][1,2,3]thiadia-
zole-6-carboxylic esters 3 as potential activators for SAR [4].
Ã
Corresponding author

574
P. Stanetty and M. D. Mihovilovic
Scheme 1
Scheme 2
Diazotation of amine precursors and trapping of the ionic intermediate by an
adjacent sulfur functionality represents a general route to 1,2,3-thiadiazole ring
systems [5]. The retrosynthetic analysis suggested two straightforward approaches
to the construction of the key species A: introduction of the nitrogen functionality
could be achieved either after (B) or before (C) implementation of the protected
sulfur moiety into the required substitution pattern (Scheme 2).
Results and Discussion
Following the ®rst route towards the intermediate of type B, the enol form of the
easily accessible compound 4 [6] was trapped with tosyl chloride to give 5
(Scheme 3). Introduction of the protected sulfur substituent was carried out by an
addition/elimination process with benzylthiol. The obtained dihydro compound 6a
could be aromatized according to a general procedure by treatment with SO2Cl2
[7]. Electrophilic chlorination followed by elimination led to 7a.
For completion of the construction of the substitution pattern, we wanted to
utilize the electronic effects present in product 7a. However, the attempted
nitration reaction with HNO3/H2SO4 did not lead to the expected compound 8a but
gave the sulfoxide 9a instead. Several different nitration procedures failed to give
substitution in position 5, but all led to oxidation of the sulfur group. In order to
prove the nature of 9a the compound was deliberately synthesized from 7a by
treatment with m-chloroperbenzoic acid.
Changing our approach towards the heterocyclic system 3 we aromatized
compound 5 to obtain 10 (Scheme 4). On this route, the following nitration gave
the expected product 11 with the tosyloxy moiety as highly activated leaving group
for a nucleophilic substitution reaction with benzylthiol. Reduction of the NO2
group in 8 was performed with iron/AcOH.

Methyl Thieno[2,3-d][1,2,3]thiadiazole Carboxylates
575
Scheme 3
Cyclization to 3 under acidic diazotation conditions gave only a very poor yield
(16%) in the case of amine 12a. Further optimization efforts changing several
reaction parameters including the nitrosation source failed. However, when using
the fully substituted precursor 12b, the yield of cyclized product 3b was improved
substantially. We attribute this different behavior to the fact that the intermediate
ionic species I is also activated at position 2 as shown in the mesomeric structure II
(Scheme 5). As a consequence, side reactions such as nitrosations, azo-coupling
reactions, and polymerizations can occur in the case of an unsubstituted position 2
in precursor 12a resulting in a low yield of 3a. A similar effect was observed in the
synthesis of thieno[2,3-d]triazinones via diazotation techniques [8]. Blocking
position 2 by introducing a methyl group as in 12b prevents the side reactions, and
the required ring system 3b is accessible in good yields.
Scheme 4

576
P. Stanetty and M. D. Mihovilovic
Scheme 5
Experimental
General
All solvents were distilled prior to use. Dry CH2Cl2 was prepared by distillation from P2O5 and dry
MeOH by distillation from Mg. Commercially available dry DMF was treated with molecular sieves
Ê
(4 A). TLC was performed on Merck precoated silica gel plates (5554), and ¯ash column
chromatography on silica gel 60 from E. Merck (40±63 mm, 9385). Melting points were determined
using a Reichert micro hot stage apparatus and are uncorrected. Elemental analyses were carried out
at the Microanalytical Laboratory, University of Vienna. The results are in good agreement with the
calculated values. The NMR spectra were recorded in CDCl3 solution with a Bruker AC 200
(200 MHz) spectrometer; chemical shifts are reported in ppm relative to TMS as internal standard.
2,5-Dihydro-2-methyl-4-((4-methylphenyl)sulfonyl)oxy-3-thiophenecarboxylic acid methyl ester
(5b; C14H16O5S2)
To a suspension of NaH (0.17 g, 7.00 mmol) in 10 ml of dry DMF, compound 4b (1.11 g, 6.37 mmol)
in 10 ml of dry DMF was added dropwise. After addition stirring was continued for 20 min at room
temperature then solid p-toluenesulfonyl chloride (1 eq) was added slowly. When TLC indicated
complete conversion, the reaction mixture was hydrolyzed with water and extracted with diethyl
ether. The combined organic layers were repeatedly washed with water to remove DMF, dried over
Na2SO4, ®ltered, and evaporated to dryness to give 1.53 g (73%) of crude 5b as a brown oil.
According to TLC and NMR, the isolated material was suf®ciently pure and was therefore directly
used in the next step due to the instability of the compound.
1H NMR (200 MHz, ꢀ, CDCl3): 1.45 (d, J ˆ 7 Hz, 3H), 2.47 (s, 3H), 3.60 (s, 3H), 3.85
(dd, J1 ˆ 3 Hz, J2 ˆ 16 Hz, 1H), 4.02 (dd, J1 ˆ 5 Hz, J2 ˆ 16 Hz, 1H), 4.25±4.42 (m, 1H), 7.39
(d, J ˆ 7 Hz, 2H), 7.87 (d, J ˆ 7 Hz, 2H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 21.6 (q), 23.8 (q), 35.1
(t), 43.2 (d), 51.6 (q), 125.5 (s), 128.1 (d), 129.7 (d), 132.4 (s), 145.8 (s), 151.3 (s), 162.1 (s) ppm.
2,5-Dihydro-4-(phenylmethyl)thio-3-thiophenecarboxylic acid methyl ester (6a; C13H14O2S2)
Benzylthiol (5.92 g, 47.7 mmol) was added to a solution of NaOH (1 eq) in a mixture of MeOH/
water ˆ 8/1 and stirred until the emulsion became homogeneous. Compound 5a (15.00 g, 47.7 mmol)
was dissolved in 100 ml of acetone and treated with the prepared solution of thiolate at 0ꢀC. After the

Methyl Thieno[2,3-d][1,2,3]thiadiazole Carboxylates
577
addition was complete, the reaction mixture was warmed to room temperature, and stirring was
continued for 20 h. The solution was concentrated, hydrolyzed with water, and extracted with diethyl
ether. The combined organic layers were washed with 2 N NaOH and water, dried over Na2SO4,
®ltered, and evaporated. Recrystallization of the crude product gave 11.43 g (90%) of pure 6a as
colorless crystals.
1
M.p.: 71±74ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 3.40 (s, 3H), 3.92±4.02 (m, 2H), 4.06±4.21
(m, 4H), 7.20±7.43 (m, 5H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 36.2 (t), 37.7 (t), 41.2 (t), 51.5 (q),
121.3 (s), 127.5 (d), 128.6 (d), 128.7 (d), 136.2 (s), 153.5 (s), 164.3 (s) ppm.
Aromatization of compounds 5 and 6 using SO2Cl2
A 10% solution of dihydro compound 5 or 7 (1 eq) in dry CH2Cl2 was treated with a 10% solution of
SO2Cl2 (1 eq) in dry CH2Cl2 below 25ꢀC. The mixture was stirred at room temperature until TLC
indicated complete conversion. After hydrolysis with ice/water the organic layer was washed with
water and satd. NaHCO3 solution, dried over Na2SO4, ®ltered, and concentrated to give the crude
aromatized product.
4-(Phenylmethyl)thio-3-thiophenecarboxylic acid methyl ester (7a; C13H12O2S2)
Compound 6a (4.50 g, 16.89 mmol) was treated with SO2Cl2 to give 3.47 g (78%) of 7a as beige
crystals after recrystallization from diisopropyl ether.
1
M.p.: 100±102ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 3.87 (s, 3H), 4.15 (s, 2H), 6.81 (d, J ˆ 3 Hz,
1H), 7.20±7.46 (m, 5H), 8.17 (d, J ˆ 3Hz, 1H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 38.1 (t), 51.7
(q), 118.5 (d), 127.2 (d), 128.5 (d), 128.8 (d), 130.7 (s), 134.4 (d), 135.4 (s), 136.3 (s), 162.4 (s) ppm.
4-((4-Methylphenyl)sulfonyl)oxy-3-thiophenecarboxylic acid methyl ester (10a; C13H12O5S2)
Treatment of 5a (5.00 g, 15.90 mmol) according to the above procedure for aromatization afforded
4.64 g (94%) of 10a as colorless crystals after recrystallization from diisopropyl ether.
M.p.: 88±89ꢀC; 1H NMR (200 MHz, ꢀ, CDCl3 ): 2.44 (s, 3H), 3.74 (s, 3H), 7.06 (d, J ˆ 4 Hz, 1H),
7.33 (d, J ˆ 8 Hz, 2H), 7.78 (d, J ˆ 8 Hz, 2H), 8.00 (d, J ˆ 4 Hz, 1H) ppm; 13C NMR (50 MHz, ꢀ,
CDCl3): 21.5 (q), 51.6 (q), 116.0 (d), 125.8 (s), 128.5 (d), 129.5 (d), 132.0 (s), 132.8 (d), 143.2 (s),
145.5 (s), 160.8 (s) ppm.
2-Methyl-4-((4-methylphenyl)sulfonyl)oxy-3-thiophenecarboxylic acid methyl ester
(10b; C14H14O5S2)
Compound 5b (2.94 g, 8.95 mmol) was treated with SO2Cl2 to give 2.07 g (71%) of 10b as yellow oil
after puri®cation by ¯ash column chromatography (silica gel: substance ˆ 15:1, petroleum ether/
ethyl acetate ˆ 10/1) which slowly crystallized.
1
M.p.: 89±92ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 2.45 (s, 3H), 2.62 (s, 3H), 3.76 (s, 3H), 6.67
(s, 1H), 7.32 (d, J ˆ 7 Hz, 2H), 7.78 (d, J ˆ 7 Hz, 2H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 16.6 (q),
21.6 (q), 51.3 (q), 111.3 (d), 121.8 (s), 128.4 (d), 129.5 (d), 132.2 (s), 142.7 (s), 145.3 (s), 148.1 (s),
162.1 (s) ppm.
4-(Phenylmethyl)sul®nyl-3-thiophenecarboxylic acid methyl ester (9a; C13H12O3S2)
Compound 7a (0.50 g, 1.89 mmol) was dissolved in 10 ml of dry CH2Cl2, cooled to ꢁ60ꢀC, and
treated with a solution of MCPBA (1 eq) in 10 ml dry CH2Cl2. After the addition was completed, the
reaction mixture was warmed to room temperature and stirred for 3 h. The solution was washed with

578
P. Stanetty and M. D. Mihovilovic
satd. NaHCO3 solution, dried over Na2SO4, ®ltered, and evaporated to give 0.32 g (60%) of pure
9a as colorless crystals after ¯ash column chromatography (silica gel: substance ˆ 40:1, petroleum
ether/ethyl acetate ˆ 1/1).
1
M.p.: 85±88ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 3.92 (s, 3H), 4.01 (d, J ˆ 13 Hz, 1H), 4.38 (d,
J ˆ 13 Hz, 1H), 7.01±7.14 (m, 2H), 7.19±7.31 (m, 3H), 7.50 (d, J ˆ 4 Hz, 1H), 8.22 (d, J ˆ 4 Hz, 1H)
ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 52.4 (q), 61.1 (t), 128.0 (d), 128.1 (d), 128.3 (d), 128.9 (s),
130.0 (s), 130.4 (d), 136.0 (d), 144.2 (s), 162.2 (s) ppm.
Nitration of compounds 10a,b
A 10% solution of 10 (1 eq) in Ac2O was cautiously treated with a mixture of fuming HNO3 (3 eq)
and conc. H2SO4 (3 eq) keeping the temperature below ‡ 40ꢀC. After the highly exothermic reaction
ceased the solution was stirred at room temperature until completion (TLC control), hydrolyzed with
ice/water, and alkalized with solid NaHCO3. After extraction with diethyl ether the combined
organic layers were washed with satd. NaHCO3 solution and water, dried over Na2SO4, ®ltered, and
evaporated to dryness.
4-((4-Methylphenyl)sulfonyl)oxy-5-nitro-3-thiophenecarboxylic acid methyl ester (11a; C13H11NO7S2)
Compound 10a (1.99 g, 6.37 mmol) gave 1.37 g (60%) of pure 11a as faint yellow crystals according
to the above procedure after recrystallization from diisopropyl ether.
1
M.p.: 122±124ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 2.48 (s, 3H), 3.80 (s, 3H), 7.36 (d, J ˆ 8 Hz,
2H), 7.80 (d, J ˆ 8 Hz, 2H), 8.17 (s, 1H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 21.9 (q), 52.5 (q),
128.1 (s), 128.8 (d), 130.0 (d), 132.5 (s), 133.8 (d), 140.0 (s), 141.3 (s), 146.6 (s), 160.0 (s) ppm.
2-Methyl-4-((4-methylphenyl)sulfonyl)oxy-5-nitro-3-thiophenecarboxylic acid methyl ester
(11b; C14H13NO7S2)
Compound 10b (2.00 g, 6.13 mmol) gave 1.50 g (66%) of 11b as faint yellow crystals according to
the above procedure after recrystallization from diisopropyl ether. In order to get an analytically pure
sample, a small amount was puri®ed by ¯ash column chromatography (silica gel: substance ˆ 50:1,
petroleum ether/ethyl acetate ˆ 5/1).
1
M.p.: 82±85ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 2.45 (s, 3H), 2.72 (s, 3H) 3.81 (s, 3H), 7.35
(d, J ˆ 7 Hz, 2H), 7.78 (d, J ˆ 7 Hz, 2H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 16.6 (q), 21.6 (q), 52.0
(q), 124.6 (s), 128.3 (d), 129.9 (d), 132.2 (s), 136.7 (s), 139.7 (s), 146.3 (s), 150.0 (s), 161.2 (s) ppm.
Nucleophilic substitution of 11 with benzylthiol
A 10% solution of benzylthiol (1 eq) in dry DMF was treated with K2CO3 (1 eq) at 0ꢀC. To this
mixture, a 10% solution of 11 (1 eq) in dry DMF was added maintaining the temperature below 0ꢀC.
After TLC indicated complete conversion, hydrolysis was carried out with ice/water and the mixture
was extracted with diethyl ether. The combined organic layers were washed with 2 N HCl, satd.
NaHCO3, and water, dried over Na2SO4, ®ltered, and concentrated in vacuo.
5-Nitro-4-(phenylmethyl)thio-3-thiophenecarboxylic acid methyl ester (8a; C13H11NO4S2)
Precursor 11a (0.89 g, 2.49 mmol) gave 0.46 g (60%) of 8a after puri®cation by ¯ash column
chromatography (silica gel: substance ˆ 30:1, petroleum ether/ethyl acetate ˆ 5/1).
1
M.p.: 75±77ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 3.92 (s, 3H), 4.26 (s, 2H), 7.13±7.30 (m, 5H),
8.09 (s, 1H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 40.1 (t), 52.4 (q), 127.5 (d), 128.5 (d), 128.9 (d),
134.4 (s), 135.6 (d), 135.8 (s), 136.4 (s), 150.0 (s), 161.2 (s) ppm.

Methyl Thieno[2,3-d][1,2,3]thiadiazole Carboxylates
579
2-Methyl-5-nitro-4-(phenylmethyl)thio-3-thiophenecarboxylic acid methyl ester (8b; C14H13NO4S2)
Compound 11b (0.54 g, 1.45 mmol) gave 0.40 g (85%) of 8b as yellow crystals after puri®cation by
¯ash column chromatography (silica gel: substance ˆ 40:1, petroleum ether/ethyl acetate ˆ 10/1).
1
M.p.: 97±100ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 2.56 (s, 3H), 3.90 (s, 3H), 4.16 (s, 3H), 7.15±
7.32 (m, 5H) ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 15.7 (q), 39.9 (t), 52.3 (q), 127.6 (d), 128.5 (d),
129.0 (d), 133.1 (s), 135.7 (s), 136.4 (s), 145.8 (s), 148.6 (s), 163.0 (s) ppm.
Reduction to amines 12
Iron powder (4 eq) was added slowly to a 10% solution of compound 8 (1 eq) in acetic acid, keeping
the reaction temperature below 30ꢀC. After TLC indicated complete conversion the mixture was
hydrolyzed with ice/water and extracted with diethyl ether. The combined organic layers were
washed with satd. NaHCO3 solution and water, dried over Na2SO4, ®ltered, and concentrated.
5-Amino-4-(phenylmethyl)thio-3-thiophenecarboxylic methyl ester (12a; C13H13NO2S2)
8a (6.00 g, 19.39 mmol) gave 4.45 g (82%) of 12a as red oil following the above protocol. Crude 16a
was suf®ciently pure for the following conversion according to TLC and NMR.
B.p.: 198±203ꢀC (0.05 mbar, Kugelrohr distillation with partial decomp.); 1H NMR (200 MHz, ꢀ,
CDCl3): 3.86 (s, 3H), 3.90 (s, 2H), 4.10 (bs, 2H), 7.04±7.13 (m, 2H), 7.16±7.25 (m, 3H), 7.31 (s, 1H)
ppm; 13C NMR (50 MHz, ꢀ, CDCl3): 40.4 (t), 51.5 (q), 105.3 (s), 118.7 (d), 126.7 (d), 128.1 (d),
128.7 (d), 132.7 (s), 138.6 (s), 156.5 (s), 162.8 (s) ppm.
5-Amino-2-methyl-4-(phenylmethyl)thio-3-thiophenecarboxylic acid methyl ester
(12b; C14H15NO2S2)
Compound 8b (400 mg, 1.24 mmol) gave 150 mg (42%) of 12b as orange oil according to the general
reduction procedure after puri®cation by ¯ash column chromatography (silica gel: substance ˆ 30:1,
petroleum ether/ethyl acetate ˆ 10/1).
1H NMR (200 MHz, ꢀ, CDCl3): 2.45 (s, 3H), 3.77±3.95 (2s and bs, 7H), 7.03±7.30 (m, 5H) ppm;
13C NMR (50 MHz, ꢀ, CDCl3): 15.2 (q), 40.7 (t), 51.3 (q), 105.2 (s), 126.7 (d), 128.2 (d), 128.8 (d),
129.3 (s), 131.7 (s), 138.8 (s), 152.8 (s), 164.4 (s) ppm.
Cyclization to 3
An approx. 3% solution of the precursor 12a,b (1 eq) in a 1:1 mixture of acetic acid and conc. HCl
was cooled below 10ꢀC and slowly treated with an approx. 10% solution of NaNO2 (1.1 eq) in water.
The progress of the reaction was monitored by TLC. After hydrolysis with ice/water the mixture was
extracted either with diethyl ether or CH2Cl2. The combined organic layers were washed with satd.
NaHCO3 solution and water, treated with charcoal, dried over Na2SO4, ®ltered, and evaporated
in vacuo.
Thieno[2,3-d][1,2,3]thiadiazole-6-carboxylic acid methyl ester (3a; C6H4N2O2S2)
Amino compound 12a (3.14 g, 11.24 mmol) gave 0.35 g (16%) 3a as colorless crystals after ¯ash
column chromatography (silica gel: substance ˆ 30:1, petroleum ether/ethyl acetate ˆ 5/1) and
recrystallization from diisopropyl ether.
1
M.p.: 140±142ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 3.98 (s, 3H), 8.57 (s, 1H) ppm; 13C NMR
(50 MHz, ꢀ, CDCl3): 52.7 (q), 122.3 (s), 141.3 (d), 145.8 (s), 161.0 (s), 163.1 (s) ppm.

580
P. Stanetty and M. D. Mihovilovic: Methyl Thieno[2,3-d][1,2,3]thiadiazole Carboxylates
5-Methyl-thieno[2,3-d][1,2,3]thiadiazole-6-carboxylic acid methyl ester (3b; C7H6N2O2S2)
Precursor 12b (150 mg, 0.51 mmol) gave 60 mg (55%) of 3b as colorless crystals after puri®cation by
¯ash column chromatography (silica gel: substance ˆ 60:1, petroleum ether/ethyl acetate ˆ 10/1)
according to the general cyclization procedure.
1
M.p.: 89±91ꢀC; H NMR (200 MHz, ꢀ, CDCl3): 2.95 (s, 3H), 3.98 (s, 3H) ppm; 13C NMR
(50 MHz, ꢀ, CDCl3): 17.4 (q), 52.4 (q), 118.1 (s), 147.5 (s), 158.0 (s), 160.3 (s), 161.8 (s) ppm.
Acknowledgements
Financial support of this project by Novartis Crop Protection AG, Basel, Switzerland, is highly
appreciated.
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Received October 16, 1998. Accepted October 26, 1998