Reaction of ArSCl (Ar = phenyl or ortho-nitrophenyl) withendo-tricyclo[6.2.1.02,7]- undeca-4,9-diene-3,6-dione (1) results in anti addition across the norbornene carbon-carbon double bond with concomitant aromatization of the cyclohexenedione ring, thereby affording 4 (87%) and 5 (63%), respectively. The corresponding reaction of PHSCl with 1 in the presence of added Ag(I) proceeds via polar addition across the norbornene carbon-carbon double bond with concomitant intramolecular nucleophilic trapping of an intermediate episulfonium ion by a distant CO group, thereby affording 6 (57% yield). The reaction of PhICl2 with 1, when carried out in the presence of SbCl5 or AgBF4, proceeds with concomitant Wagner-Meerwein rearrangement and aromatization to afford adducts 7a and 8, respectively. © 1994.