A minor byproduct in the reaction of (S)-prolinol with thiophosgene in the presence of triethylamine is identified as a novel tricyclic dipyrrolidino-1,3,6-oxadiazocane-2-thione, the first example of such a ring system, and a representative of the uncommon, but useful 1,3,6-oxadiazocanes. A mechanism is proposed for its formation.