Olefins mesoionic ring systems

An attractive approach toward the preparation of polycyclic systems containing a thiophene ring involves the intramolecular [3 - - 2] cycloaddition of thiocarbonyl ylides. A number of representative examples were reported using mesoionic compounds. Gotthardt et al. (151) used l,3-dithiolium-4-olates such as 89 bearing an olefinic side chain. Upon heating to 120 °C in xylene, the polycyclic tetrahy-drothiophene 90 was formed (Scheme 5.33). 

Ring-fused mesoionic anhydro triazolium hydroxide systems 131 (84M11) that incorporate the thiocarbonyl ylide dipole undergo cycloaddition with both olefinic and acetylenic dipolarophiles the unstable 1 1 cycloadducts 132 and 133 yield ring-fused a-pyridinones 134 by elimination of hydrogen sulfide or sulfur, respectively (Scheme 42) (79JOC3803). 

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