1,3,4-Thiadiazolium-3-methanide 1,3-dipole 6 and the 2,5-diphenyl and 2,5-dimethyl derivatives, 4 and 5, were generated at -60degreesC in dichloromethane. Cycloaddition reactions with substituted alkenes gave many new derivatives of the pyrrolo[2,1-b][1,3,4]thiadiazole ring system. The first examples of a bowl-shaped tricyclic nitrogen-sulfur analogue of the tripentagon bowl, a 3,4,10-triaza-6-thiatricyclo[6,3,0,0 3,7]undecane ring system were obtained from N-substituted maleimide dipolarophiles. The reactions displayed predominantly endo-stereochemistry but with decreasing size of the substituent at the incipient 7a-fusion bridgehead in the cycloaddition transition state, the extent of exo-cycloaddition increased. The cycloadduct endo : exo ratio was reduced from exclusively endo to ca. 2 : 1 on changing the 1,3-dipole from the 2,5-diphenyl derivative 4 to the unsubstituted case 6. X-Ray crystal structures are reported for 2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][1,3,4]thiadiazole-endo-6,7-N-methyldicarboxyimide 7a,2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][ 1,3,4]thiadiazole-endo-6,7-N-phenyldicarboxyimide 9e and 2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][ 1,3,4]thiadiazole-7-endo-carbonitrile 13.