Intramolecular sulfoxide electrophilic sulfenylation in 1- and 3-acylindoles

Heating 1-(2-ethylsulfinyl)benzoylindole (1) in refluxing p-xylene [thermal Sulfoxide Electrophilic Sulfenylation (SES)] produces indolo[2,1-b][1,3]benzothiazin-12-one (2) in 66% yield. Similar treatment of 1-methyl-3-(2-ethylsulfinyl)benzoylindole (8) provides three products: sulfide 7 and cyclized products 9 and 10 in 10, 19 and 15% yield, respectively. Conversion of 10 to 9 under the reaction conditions is demonstrated and a spirocyclic intermediate 12 , which may form from both 9 and 10 but undergoes only preferential S-migration, is postulated to account for the rearrangement.