A facile method for the synthesis of indole phytoalexin rutalexin Mariana Budovská a,⇑ , Zuzana Kudlic ˇková a , Peter Kutschy a , Martina Pilátová b , Ján Mojz ˇiš b a Institute of Chemical Sciences, Faculty of Science, P. J. Šafárik University, Moyzesova 11, 040 01 Košice, Slovak Republic b Department of Pharmacology, Faculty of Medicine, P. J. Šafárik University, SNP 1, 040 66 Košice, Slovak Republic article info Article history: Received 12 January 2015 Revised 11 April 2015 Accepted 2 May 2015 Available online 8 May 2015 Keywords: Indoles Phytoalexins Rutalexin Natural products abstract Novel methods for the preparation of the indole phytoalexin rutalexin (8) in high yields are presented. The synthesis of rutalexin (8) was achieved from previously synthesized 9-tert-butoxycarbonyl-2-meth- oxy-4-oxo-[1,3]thiazino[6,5-b]indole (5) by its hydrolysis, methylation, and deprotection. A second method starting from 9-Boc-cyclobrassinin (9) involved oxidation, methylation, and deprotection. Ó 2015 Elsevier Ltd. All rights reserved. Phytoalexins are defined as antimicrobial low molecular weight secondary metabolites, that are produced by plants upon exposure to physical, biological, or chemical stress. 1 About 40 indole phytoalexins have been isolated from economically and dietary important plants of the family Cruciferae, which are cultivated worldwide. 2 Among them, several 1,3-thiazino[6,5-b]indole derivatives such as cyclobrassinin (3a) from the Chinese cabbage (Brassica campestris L. ssp. pekinensis), 3a and sinalbin B (3b) from the white mustard (Sinapis alba) 3b have been described. Indole phytoalexins demonstrate in vitro cytotoxic/cytostatic activities against human solid tumor and leukemia cell lines. 2c,e Indole phytoalexins have also been shown to exhibit cancer chemopreventive activity in 7,12-dimethylbenz[a]anthracene (DMBA)-induced precancerous lesions in mouse mammary gland organ cultures. 2f Compounds 3a and 3b have been prepared by oxidative bromocyclization of the corresponding indole phytoalexins brassinin (1a) or 1-methoxybrassinin (1b) using pyridinium tribromide (3a, 34%), 3a NBS (3a, 35%; 3c 3b, 41% 3b ), dioxane dibromide (3a, 45%) 3d or phenyltrimethylammonium tribromide (3a, 59%, Scheme 1). 3e In 1994 Gross et al. described the isolation and characterization of a new phytoalexin from kohlrabi (Brassica oleracea var. gongy- lodes) which was assigned as structure 4 and named cyclobrassi- non. 4a However only the 1 H NMR (CDCl 3 ) and mass spectrum were published and a melting point was not given. 4a We have previously studied the synthesis of thiazino[6,5-b]in- dole derivative 4 and its analogs (Scheme 1). 4b–d Although our synthetic product 4 showed an identical mass spectrum to the natural product, it was poorly soluble in CDCl 3 and its 1 H NMR spectrum in DMSO-d 6 exhibited differences in the chemical shifts for OMe (d 4.18 4b vs d 3.55 4a in CDCl 3 ) and NH (d 12.69 4b vs d 8.56 4a in CDCl 3 ) which we attributed to the different solvent used. Although an original sample of the natural product isolated from kohlrabi was not available for comparison, we correctly assigned the structure of synthetic product 4 by 1 H NMR, 13 C NMR, and IR spectra. This assignment was also based on the experience we had gained from previous syntheses of analogous 2-substituted-4H-benzo[4,5]thieno[2,3-e]-1,3-thiazin-4-ones. 4e,f It was later shown that the structure of the natural product cyclobrassinon had originally been misassigned, thus accounting http://dx.doi.org/10.1016/j.tetlet.2015.05.001 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +421 552341196. E-mail address: mariana.budovska@upjs.sk (M. Budovská). N 3 S N SCH 3 N H S N O OCH 3 4 N 1 N H N H H O 2 SCH 3 S R R Cl Cyclization 1a, 3a: R = H; 1b, 3b: R = OCH 3 Scheme 1. Tetrahedron Letters 56 (2015) 3945–3947 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet