New Synthesis and Spectroscopic Studies of Thialene (Cyclopentathiapyran)

Article abstract of DOI:10.1021/jo00374a027

A new five-step synthesis of thialene, cyclopentathiapyran (2c), an intensely blue-violet pseudoazulene, is described.The critical final step in the reaction sequence gave 35percent yield of the title compound via rapid sequential thermolysis and dehydrogenation of cyclopentano-2,4-dihydro-3-(N,N-dimethyl(thiocarbamoyl)thiapyran (8), an intermediate which allowed transient formation of the highly unstable tetrahydro derivative(s).Theoretical and spectroscopic studies of 2c indicate extensive ?-delocalization.The 600.6-MHz 1H NMRspectrum displays peaks from 6.78 to 7.84 ppm, confirming a sustained ring current and establishing the molecule as aromatic based on a diatropic definition.The 1H-decoupled 75.47-MHz 13C NMR spectrum displays peaks from 110.01 to 132.09 ppm.The 70-eV EI-MS is startlingly similar to benzothiophene, indicating probable rearrangement to the benzenoid heterocycle prior to fragmentation.The molecule displays weak emission from S₂; λ_max = 338 nm.The IR, (+) and (-) chemical ionization MS, and solvatochromic shifts are reported.The conflict between aromaticity and chemical reactivity is discussed.