Higher-order [8+2]-cycloadditions of tropothione with levoglucosenone (LGO) and structurally similar exo-cyclic enones derived from cyrene.

Levoglucosenone (LGO) and structurally similar exo-cyclic enones derived from cyrene (dihydrolevoglucosenone) react with tropothione following the higher-order [8 + 2]-cycloaddition pathway. Reactions were performed at room temperature in CHCl solutions in absence of any activating reagent. Whereas reaction of tropothione with LGO occurred with complete stereoselectivity, leading to a single, sterically favored exo cycloadduct, identified as polycylic thiophene derivative, reactions performed with exo-cyclic enones yielded in some instances mixtures of two isomeric exo and endo cycloadducts, derived from spiro-tetrahydrothiophene as major and minor components, respectively, of the studied reaction mixtures. Exo and endo [8 + 2] cycloadducts differ in absolute configuration at the newly created chiral centers. Structures of exo and endo cycloadducts were confirmed by single crystal X-ray diffraction analysis.