The enthalpies of formation AH, and the ring strain energies E, were calculated for the nine isomeric dithiacyclooctynes 1-9 by applying the MNDO method. Reaction Scheme 2 shows the synthesis of 1,4-dithia-2-cyclooctyne (l), which was the highest E, value in this series. It dimerizes to 13 in the absence of reaction partners. Scheme 4 summarizes the Chern.

Abstract1‐Thiacyclooct‐4‐yne (=5,6‐didehydro‐3,4,7,8‐tetrahydro‐2H‐thiocin; 9) can be prepared from thiocan‐5‐one (6) in three steps by applying the so‐called selenadiazole method. The heterocyclic alkyne can be oxidized to the corresponding sulfoxide 16 and sulfone 17. Due to their geometrical strain, all three cyclic alkynes show high reactivities in DielsAlder and 1,3‐dipolar cycloadditions. Moreover, tetrathiafulvalenes can be prepared from 9 and 16 by the reaction with CS2.

The isomeric thiacyclooctanones 1b-d are transformed via the semicarbazones Z-2b,c, E-2b,c and 2d to the bicyclic 1,2,3-selenadiazoles 3b,c, 3b´,c´ and 3d. The ring closure reaction with SeO2 is highly regioselective and doesn’t depend on the Z /E -ratio of the semicarbazones. A detailed 1H and 13C NMR study was made for 1, 2 and 3.

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