Stereoselective synthesis of 1-methoxyspiroindoline phytoalexins and their amino analogues Aneta Salayová a , Zuzana Kudlic ˇková a , Matej Baláz ˇ a , Mariana Budovská a,⇑ , Martina Pilátová b , Ján Mojz ˇiš b , Kenji Monde c , Ján Petrovaj d , Tat ˇ ána Gondová d a Department of Organic Chemistry, 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 c Division of Biological Sciences, Graduate School of Science, Frontier Research Center for Post-Genomic Science and Technology, Hokkaido University, Kita-ku, Sapporo 001-0021, Japan d Department of Analytical Chemistry, Institute of Chemical Sciences, Faculty of Science, P.J. Šafárik University, Moyzesova 11, 040 01 Košice, Slovak Republic article info Article history: Received 22 June 2014 Accepted 15 July 2014 Available online xxxx abstract The stereoselective synthesis of the spiroindoline phytoalexin (R)-(+)-1-methoxyspirobrassinin and its unnatural (S)-(À)-enantiomer were achieved by the bromine-induced spirocyclization of 1-methoxy- brassinin using chiral auxiliaries (+)- and (À)-menthol, followed by oxidation of the obtained menthyl ethers. The TFA-catalyzed methanolysis of chiral 1-methoxyspirobrassinol menthyl ethers provided (2R,3R)-(À)-1-methoxyspirobrassinol methyl ether and its other three unnatural stereoisomers. The enantiomers of the 2-amino analogues of the indole phytoalexin (2R,3R)-(À)-1-methoxyspirobrassinol methyl ether were prepared by the TFA-catalyzed replacement of the chiral alkoxy group with an amine. The synthesized compounds were tested in vitro on cancer cell lines and examined with enantiopure 2- amino analogues of the indole phytoalexin (2R,3R)-(À)-1-methoxyspirobrassinol methyl ether, which showed in most cases, lower activity than the corresponding racemates. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Indole phytoalexins are low molecular weight secondary metabolites produced by plants of the family Cruciferae, after their exposure to physical, biological, or chemical stress. 1 Among them several spiroindoline[3,5]thiazolidine-type phytoalexins, such as (S)-(À)-spirobrassinin 1, from Japanese radish (Raphanus sati- vus), 2,3 (R)-(+)-1-methoxyspirobrassinin 2, from kohlrabi (Brassica oleracea var. gongylodes) 4,5 , or (2R,3R)-(À)-1-methoxyspirobrassi- nol methyl ether 3, from Japanese radish (Raphanus sativus) 5,6 have been isolated (Fig. 1), their absolute configuration determined. The optically inactive spiroindoline phytoalexin 1-methoxyspirobrass- inol 4 was isolated from Japanese radish and exists in solution as a mixture of diastereoisomers 4a and 4b in a 1:4 ratio due to its unstable hemiaminal structure. 6 (+)-Erucalexin 5, with an as yet unknown absolute stereochemistry, have been isolated as a phyto- alexin of dog mustard. 7 In 2002, a spirocyclization strategy toward spiroindoline phyto- alexins was developed based on the treatment of 1-methoxybrass- inin 6 with dioxane dibromide in the presence of 5% of a nucleophile. 8 A mixture of racemic trans- and unnatural cis-diaste- reoisomer of 1-methoxyspirobrassinol methyl ether 3a and 3b in a 1:2 ratio was prepared using methanol as the nucleophile. Spiro- cyclization in the presence of water afforded a mixture of 4a and 4b in a 2:1 ratio. The trans-diastereoisomer is regarded as the one with the sulfur of the thiazoline ring and a group at the 2-posi- tion being located on the opposite sides of the indoline ring, http://dx.doi.org/10.1016/j.tetasy.2014.07.006 0957-4166/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +421 552341196. E-mail address: mariana.budovska@upjs.sk (M. Budovská). N H S N O SCH 3 (S)-(-)-1 N S N O SCH 3 (R)-(+)-2 N S N SCH 3 (2R, 3R)-(-)-3 OCH 3 OCH 3 OCH 3 N S N OH SCH 3 (±)-4a OCH 3 N S N OH SCH 3 (±)-4b OCH 3 N (+)-5 OCH 3 O N S SCH 3 Figure 1. Selected indole phytoalexins. Tetrahedron: Asymmetry xxx (2014) xxx–xxx Contents lists available at ScienceDirect Tetrahedron: Asymmetry journal homepage: www.elsevier.com/locate/tetasy Please cite this article in press as: Salayová, A.; et al. Tetrahedron: Asymmetry (2014), http://dx.doi.org/10.1016/j.tetasy.2014.07.006