At -20°C, SO 2 undergoes homocheletropic addition with 7,7-dimethyl[2.2.1]hericene (18) to give (1R,5S)-6,10-dimethylidene-8-isopropylidene-3-thiatetracyclo[5.2.1.0 1 , 5 .0 5 , 9 ]decane 3,3-dioxide (21). This sulfolane is isomerized into (1R,7S)-8,9-dimethylidene-10-isopropylidene-4-thiatricyclo[5.2.1.0 2 , 6 ]dec -2(6)-ene 4,4-dioxide (22). At 20°C, the equilibrium ratio [21]/[22] = 4.1, the sulfolane being slightly more stable than the isomeric sulfolene, in contrast with the reactions of SO 2 with 2,3,5,6-tetramethylidenebicyclo[2.2.1]heptane and 5,6,7,8-tetramethylidenebicyclo[2.2.2]oct-2-ene. While 2,3,5,6,7-pentamethylidenebicyclo[2.2.2]octane (19) and [2.2.2]hericene (20) react with SO 2 to give first the corresponding monosulfolenes (1SR,7RS)-8,9,10-trimethylidene-4-thiatricyclo[5.2.2.0 2 , 6 ]undec-2(6)-ene 4,4-dioxide (24) and 8,9,10,11-tetramethylidene-4-thiatricyclo[5.2.2.0 2 , 6 ]undec-2(6)-ene 4,4-dioxide (27) respectively, these adducts undergo homocheletropic additions concurrently with the cheletropic additions of SO 2 under conditions of kinetic control, giving (1SR,2SR,8RS,10SR)-9-methylidene-5,12-dithiapentacyclo[6.5.1.0 1 , 1 0 .0 2 , 1 0 ,0 3 , 7 ]tetradec-3(7)-ene 5,5,12,12-tetraoxide (25) and (1R,10S)-9,14-dimethylidene-5,12-dithiapentacyclo[6.5.1.0 1 , 1 0 .0 2 , 1 0 . 0 3 , 7 ]tetradec-3(7)-ene 5,5,12,12-tetraoxide (28), respectively. At 20°C 25 was partly isomerized into the bissulfolene 13-methylidene-4,10-dithiatetracyclo[5.5.2.0 2 , 6 .0 8 , 1 2 ]tetradeca-2(6), 8(12)-diene 4,4,10,10-tetraoxide (26) ([25]/[26] = 2:1), whereas 28 was completely converted to the more stable bissulfolene 13,14-dimethylidene-4,10-dithiatetracyclo-[5.5.2.0 2 , 6 .0 8 , 1 2 ]tetradeca -2(6),8(12)-diene 4,4,10,10-tetraoxide (29). Single crystal X-ray diffraction studies on 6,6-dimethyl-3-thiabicyclo[3.1.0]hexane 3,3-dioxide (4) and 6,11-dimethylidene-3-thiatetracyclo-[5.3.1.0 1 , 5 .0 5 , 1 0 ]undec-8-ene 3,3-dioxide (12) revealed unusual bond elongations for the σ bond connecting C(1) and C(5) in 3-thiabicyclo[3.1.0]hexane 3,3-dioxide systems.