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 CH₂Cl₂ 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.

Asymmetric Catalytic (2+1) Cycloaddition of Thioketones to Synthesize Tetrasubstituted Thiiranes

Herein, we report the first example of enantioselective (2+1) cycloaddition of thioketones with α-diazo pyrazoleamides for the direct synthesis of tetrasubstituted thiiranes. In the presence of chiral N,N'-dioxide/cobalt(ΙΙ) complexes (2-5 mol%), excellent efficiency (up to 99 % yield within 15 mins) and high stereoselectivity (up to >19 : 1 dr and 97 % ee) are available. Elaborations of thiiranes via desulfuration have also been conducted to deliver tetrasubstituted olefins. Density functional theory calculations reveal that the reaction initiates from a doublet state cobalt(ΙΙ) carbenoid, which is followed by a quartet cobalt(ΙΙ)-bound thiocarbonyl ylide pathway. This work provides a route for the selective construction of tetrasubstituted thiiranes and olefins that are otherwise difficult to access.

Unveiling the molecular mechanisms of the cycloaddition reactions of aryl hetaryl thioketones and C,N-disubstituted nitrilimines

Many synthetic routes to constructing biologically active heterocyclic compounds are made feasible through the (3 + 2) cycloaddition (32CA) reactions. Due to a large number of possible combinations of several heteroatoms from either the three-atom components (TACs) or the ethylene derivatives, the potential of the 32CA reactions in heterocyclic syntheses is versatile. Herein, the cycloaddition reaction of thiophene-2-carbothialdehyde derivatives and C,N-disubstituted nitrilimines have been studied through density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. In the present study, a one-step 32CA and two-step (4 + 3) cycloaddition (43CA) reaction mechanisms involved in TACs reactions and ethylene derivative have been investigated. In all reactions considered, the one-step 32CA cycloaddition is preferred over the two-step 43CA. The TAC chemoselectively adds across the thiocarbonyl group present in the ethylene derivative in a 32CA fashion to form the corresponding cycloadduct. Analysis of the electrophilic [Formula: see text] and nucleophilic [Formula: see text] Parr functions at the various reaction centers in the ethylene derivative show that the TAC adds across the atomic centers with the largest Parr functions, which is in total agreement with the experimental observation. The selectivities observed in the titled reactions are kinetically controlled.