<p>Heterocyclization reactions of unsubstituted 2-alkenylsulfanyl derivatives of 1,3,4-thiadiazole with bromine and iodine were studied for the first time in inert solvents (acetonitrile and dichloromethane) under mild conditions. The annulation pathways of the thiazole and thiazine rings were analyzed. The structures of all synthesized compounds were determined by ¹H and ¹³C NMR spectroscopy and GC-MS. The structures of 6-iodo-5-phenyl-6,7-dihydro-5<i>H</i>-[1,3,4]thiadiazolo[2,3-<i>b</i>][1,3]thiazinium triiodide and 5-bromomethyl-5-methyl-5,6-dihydrothiazolo[2,3-<i>b</i>][1,3,4]thiadiazolium tribromide were studied by X-ray diffraction. For the tribromide, the polarized vibrational spectra and the tensor of elastic moduli were calculated to evaluate the response of the system to external mechanical stimuli and investigate the directionality of the main structural moieties in the crystal. 6-Iodo-5-phenyl-6,7-dihydro-5<i>H</i>-[1,3,4]thiadiazolo[2,3-<i>b</i>][1,3]thiazinium triiodide was shown to exhibit moderate fungicidal activity against <i>Botrytis cinerea</i>, the causative agent of gray mold, and the phytopathogen <i>Sclerotinia sclerotiorum</i>.</p>