Synthesis of Pyrrolo[2,3-d][1,2,3]thiadiazole-6-carboxylates via the Hurd-Mori Reaction. Investigating the Effect of the N‑Protecting Group on the Cyclization.

A route to methyl pyrrolo[2,3-d][1,2,3]thiadiazole-6-carboxylates as potential plant activators and inducers of systemic acquired resistance (SAR) is reported. A synthetic strategy based on cyclization of the thiadiazole ring system utilizing thionyl chloride via the Hurd-Mori protocol as key step was developed. Success of the ring closure reaction turned out to be highly dependent on the nature of the N-protecting group of the pyrrolidine precursor. While electron donors such as alkyl gave only poor conversion to the required 1,2,3-thiadiazoles, an electron withdrawing substituent such as methyl carbamate gave superior yields.


Introduction
Systemic acquired resistance (SAR) represents a novel concept in plant protection chemistry. This approach takes advantage of the plant's own defense mechanisms. In previous studies it was demonstrated that the local infection of a plant may result in developing a resistance to subsequent challenges by a variety of pathogens throughout the whole organism. While on the molecular level this response is linked to the expression of PR-proteins, the complete mechanism is not fully understood, so far, and intensive scientific research is currently under way to elucidate this intricate effect [1].
In addition to the "traditional" mode of infection with a pathogen, the stimulation of this effect can also be induced by several non-natural compounds. Such agents became known as plant activators, and heterocyclic molecules like 2,6-dichloroisonicotinic acid 1, various 1,2,3-benzothiadiazole-7carboxylic acid derivatives such as 2, [2] or thiophene fused thiadiazoles 3 [3,4] are capable to trigger the same biochemical cascade subsequently resulting in an immunization of the plant (Scheme 1). Previously, Bion ® 2 became the first commercialized product with this novel mode of action. Recent efforts to find new heterocyclic systems based on the lead structure 2 [5,6] suggested the 1,2,3-thiadiazole system [7] as an integral motif for biological activity. Consequently, we became interested in modifying the benzo-part of Bion ® 2, a strategy successfully applied in the development of biologically active thieno compounds 3. In the present paper we present a synthetic route to pyrrolofused analogs 4. Based on structure activity relationships the position of the ester group adjacent to the annelation site is an important structural feature for the activity as an inducer of systemic acquired resistance, narrowing the number of first choice candidates in the pyrrolo series.
In our quest to develop an optimized approach to the corresponding thieno targets 3 [3] we became interested in the Hurd-Mori cyclization [8] as a most simple and versatile protocol for the generation of 1,2,3-thiadiazoles. [9] In the particular case of 3, the reaction with thionyl chloride gave a fully aromatized product, which was an especially intriguing aspect to us. We have suggested a mechanistic rational for our observations in the thieno series, previously. [3,10] For the construction of the analogous pyrrolo products we envisioned a similar approach.

Results and Discussion
Synthesis of alkyl substituted pyrrolo-thiadiazoles 4a (R = Bn) and 4b (R = Me) started from itaconic acid or its dimethylester (Scheme 2). Cyclization towards the pyrrolidine system 5a/b was optimized based on a protocol by Feldkamp and coworker [11].
From our previous experiences in the thieno-series we had already expected that efficient conversion to the hydrazone precursor for the Hurd-Mori reaction required formation of a thiocarbonyl species. Consequently, lactams 5a/b were transformed into the corresponding thiolactams 6a/b using Lawesson's reagent. [12] As work-up conditions for reactions involving this reagent are troublesome, sometimes, we found Kugelrohr distillation as a most convenient form to purify the products.
Thiolactams 6a/b gave clean conversion to the corresponding hydrazono cyclization precursors 7a/b by treatment with ethyl carbazate upon refluxing in THF in the presence of Hg(OAc) 2 . In the absence of this reagent, the reaction progress was significantly decreased. In addition, equilibrium concentrations are shifted by precipitation of HgS.
The actual Hurd-Mori cyclization towards the alkyl-thiadiazoles gave fully aromatized products 4a/b with both precursors. However, it turned out to be disappointing with yields of 25% (4a) and 15% (4b), respectively, even under optimized reaction conditions. The pyrrolidino-precursors required significantly harsher conditions (chloroform at reflux) to display reasonable conversion rates. However, at such temperatures the stability of the starting material and/or of intermediate products becomes a limiting factor. In all optimization experiments significant decomposition was observed by darkening of the reaction mixtures and/or precipitation of insoluble material.
Although we could demonstrate access to N-alkyl-pyrrolo-thiadiazole-carboxylates 4a and 4b in principal, the chosen strategy did not seem suitable for scale-up in order to provide larger quantities for derivatization studies. We attribute the limited stability to the presence of a relatively electron rich nitrogen species in all cyclization precursors and intermediates. Consequently, another option to overcome the poor cyclization yields could be by changing the protecting group at the pyrrolidine system to an electron withdrawing substituent.
In contrast to the alkyl precursors, Hurd-Mori reaction with 7c required cooling and progressed smoothly to fully aromatized thiadiazole 4c. No significant side products were observed and compound 4c was isolated after simple recrystallization in 94% yield. Final deprotection of the pyrrolo nitrogen was accomplished by treatment of a methanolic solution of 4c with silica gel to afford 4d quantitatively.

Conclusions
Basicity of the ring nitrogen atom in precursors for the Hurd-Mori reaction has a major effect on the success of the transformation. Presence of an electron withdrawing protecting group at the nitrogen plays a significant role for a successful cyclization. We have successfully established a synthetic route to pyrrolo [2,3-d] [1,2,3]thiadiazole-6-carboxylates via this versatile cyclization protocol. Best results were observed for electron withdrawing protecting groups at the pyrrolo nitrogen. In all cases, fully aromatized target compounds were obtained. Further optimization efforts for the synthetic steps towards the cyclization precursors are currently underway in our laboratory.

General
Unless otherwise noted, chemicals were purchased from commercial suppliers and used without further purification. All solvents were distilled prior to use. Flash column chromatography was performed on silica gel 60 from Merck (40-63µm). Melting points were determined using a Koflertype Leica Galen III micro hot stage microscope and are uncorrected. NMR-spectra were recorded from CDCl 3 or DMSO-d 6 solutions on a Bruker AC 200 (200 MHz) spectrometer and chemical shifts are reported in ppm using TMS as internal standard. Combustion analysis was carried out in the Microanalytic Laboratory, University of Vienna.

1-Methyl
Thiolactam 6b (11.0 g, 63 mmol) and ethyl carbazate (6.6 g, 63 mmol) were dissolved in dry THF (100 mL) and treated with Hg(OAc) 2 (20.24 g, 63 mmol). The mixture was stirred at room temperature for 16 hours, then charcoal was added and the suspension was filtered through a pad of Celite ® . The resulting solution was concentrated to give 12.35 g (81%) of 7b as orange, highly viscous oil, which is sufficiently pure for the subsequent cyclization. Additional chromatographic purification (silica gel, LP/EtOAc