Sulfur-mediated intramolecular double Michael-type reaction: synthesis of trans-hydroindanes and mechanism
A sulfur-mediated intramolecular double Michael-type reaction stereoselectively producing trans-hydroindanes was developed. Îµ-Caprolactone 3 was transformed into four [(E,E), (E,Z), (Z,E) and (Z,Z)] isomers of methyl 7-methyl-8-oxo-10-(phenylthio)deca-2,9-dienoate (14, 16, 18 and 19). Treatment of these four isomers, 14, 16, 18 and 19, respectively, with tert-butyldimethylsilyl trifluoromethanesulfonate in the presence of triethylamine gave the trans-hydroindane 20 as the main product together, with a small amount of the cis-isomer 22. The annulation was accelerated by the presence of an electron-donating group on the phenylthio group. Substrates 8, 34, 36 and 37, having hydrogen, isopropyl, trimethylsilyl and phenylsulfinyl instead of the sulfenyl group at the 10-position, provided no cyclised product under the same reaction conditions.