Synthesis of Camalexin

- In this paper we describe a new method for the synthesis of camalexin (1) based on the reaction of 1-(tert-butoxycarbonyl)indole-3-carboxaldehyde with methyl L-cysteinate hydrochloride, followed by oxidation and decarboxylation. Compounds 1, and intermediates 5-7 were identified by elemental analysis, 1H NMR, 13C NMR and mass spectroscopy.


Introduction
Camalexin [3-(2 ' -thiazolyl)indole] (1) is a natural phytoalexin, isolated for the first time from the leaves of Camelina sativa and elicited by the fungus Alternaria brassicae [1]. Camalexin is also the principal phytoalexin found in Arabidopsis thaliana [2]. It exhibits antifungal activity similar to the systemic fungicide thiabendazole (2) [1,3] and also has antitumor activity [4]. In the literature there are described four methods for synthesis of camalexin, based on the reaction of indolylmagnesium iodide with 2-bromothiazole [3], heating of indole-3-carboxamide with P 2 S 5 and chloroacetaldehyde diethyl acetal in ethanol [5], reductive cyclization of 2-formamidophenyl-2´-thiazolylketone upon heating with TiCl 3 and zinc dust [6] and reaction of 1-sulfonyl-3-iodoindole with active zinc and following Pd catalyzed arylation with 2-iodothiazole [7]. Recently, it has been suggested that the biosynthesis of camalexin involves the condensation of indole-3-carboxaldehyde with cysteine followed by a two-step oxidation and decarboxylation [8,9]. In the presence work we have studied the synthesis of camalexin according to this biosynthetic scheme.

Conclusions
In this contribution we report a biomimethic synthesis of camalexin (1) according to the proposed biosynthetic scheme. The formation of the thiazole ring involves only one oxidation step followed by decarboxylation to camalexin.

Acknowledgments
We thank the Grant Agency for Science of the Slovak Republic (grant No. 1/6080/99) for financial support of this work.

General
Melting points were measured on a Koffler hot stage apparatus and are uncorrected. Purity of compounds was confirmed by elemental analysis on a Perkin-Elmer, model 2400 analyzer. The reaction course was monitored by TLC on Silufol (Kavalier) and Alumina 60 F 254 neutral (Merck) TLC plates. Preparative column chromatography was performed on Kavalier 40/100 µm silica gel and Merck Kieselgel 60 F25. The infrared absorption spectra of compounds 1, and 5-7 were measured in CHCl 3 on an IR75 (Zeiss Jena) spectrometer in the region 400-4000 cm -1 . The 1 H-NMR spectrum of 5 was measured on a TESLA BS 487 A (80 MHz), 1 H-and 13 C-NMR spectra of compounds 1, 6, 7 on a Varian Gemini 2000 (300 MHz) in deuterochloroform, using tetramethylsilane as an internal standard. The electron impact mass spectra of 7 were recorded on a Finnigan SSQ 700 spectrometer at an ionization energy of 70 eV. Methyl L-cysteinate hydrochloride, pyridinium chlorochromate (PCC), pchloranil and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) from Fluka, Merck and Avocado were used as obtained without further purification. 1-(tert-Butoxycarbonyl)indole-3-carboxaldehyde (3) was prepared according to the described procedure [11].
To a solution of NaOH (22.0 mg, 5.6 mmol) in water (2 mL) was added a solution of thiazole 7 (120 mg, 0.46 mmol) in methanol (2 mL) and the reaction mixture was refluxed for 1 hour. After cooling and evaporation of the methanol, NaHCO 3 (660 mg, 7.86 mmol) was added and the reaction mixture was refluxed for 1 hour. The product separated after cooling and was collected on filter paper and dried. Crystallization from a mixture of diethylether/hexane yielded 10 mg (12%); M.p. 140-141 o