Stereospecific transannular cyclization of (Z)- and (E)-thiacyclooct-4-enes: synthesis and carbon-13 nuclear magnetic resonance spectra of substituted cis-1-thioniabicyclo[3.3.0]octane salts. Conformational analysis (force field) of (E)-thiacyclooct-4-ene and its 2- and 3-methyl derivatives