Synthesis of Thietanes from Saturated Three‐membered Heterocycles

π-Stacking-Controlled Dearomatic Sulfur-Shifted Ene Reaction of Ketenes and Polycyclic Arylthiiranes: Access to Areno[d]-ε-thiolactones

Electrophilic ring-expansion of polycyclic arylthiiranes and ketenes generated from alkoxy/aryloxyacetyl chlorides in the presence of triethylamine (TEA) is developed and provides a new strategy for the synthesis of areno[d]-ε-thiolactones, areno[d]thiepinones, directly without catalysts or additives. This strategy features atom- and step-economic one-pot characteristic via a tandem sequence of in situ ketene generation, π-stacking-controlled dearomatic sulfur-shifted ene, and aromatization. The current reaction is a novel strategy of electrophilic ring expansions of three-membered saturated heterocycles.

Thietanes and derivatives thereof in medicinal chemistry

Unlike the oxetane ring, which, as evidenced by numerous studies, is known to play an increasingly important role in medicinal chemistry, the thietane ring has thus far received comparatively limited attention. Nonetheless, a growing number of reports now indicate that this 4-membered ring heterocycle may provide opportunities in analog design. In the present review article, we discuss the possible use and utility of the thietane fragment in medicinal chemistry and provide an overview of its properties and recent applications with a focus on isosteric replacements.

Regio- and stereoselective synthesis of thiazoline derivatives via the thioketene-induced ring expansion of aziridines

Metal-free thioketene-induced ring expansion of aziridines gave 4-alkylthiazolines stereospecifically from 2-alkylaziridines through an intramolecular substitution at the less substituted ring carbon and 5-arylthiazolines stereoselectively from 2-arylaziridines via tandem ring cleavage and formation through intimate ion-pair intermediates after nucleophilic addition of aziridines to thioketenes generated from 4-substituted 1,2,3-thiadiazoles in the presence of a base.

Consecutive and Selective Double Methylene Insertion of Lithium Carbenoids to Isothiocyanates: A Direct Assembly of Four-Membered Sulfur-Containing Cycles

A formal CH₂−CH₂ homologation conducted with C1 carbenoids on a carbon electrophile for the obtainment of a four‐membered cycle is reported. The logic proposes the consecutive delivery of two single nucleophilic CH₂ units to an isothiocyanate—as competent electrophilic partner—resulting in the assembling of a rare imino‐thietane cluster. The single synthetic operation procedure documents genuine chemocontrol, as indicated by the tolerance to various reactive elements decorating the starting materials. Significantly, the double homologation protocol is accomplished directly on a carbon electrophile, thus not requiring the installation of heteroatom‐centered manifolds (e.g. boron).

Recent strategies used in the synthesis of saturated four-membered heterocycles

The importance and prevalance of O-, N-, and S-atom containing saturated four-membered ring motifs in biologically active molecules and potential therapeutics continues to drive efforts in their efficient synthetic preparation. In this review, general and recent strategies for the synthesis of these heterocycles are presented. Due to the limited potential bond disconnections, retrosynthetic strategies are broadly limited to cyclizations and cycloadditions. Nonetheless, diverse approaches for accessing cyclization precursors have been developed, ranging from nucleophilic substitution to C-H functionalization. Innovative methods for substrate activation have been developed for cycloadditions under photochemical and thermal conditions. Advances in accessing oxetanes, azetidines, and thietanes remain active areas of research with continued breakthroughs anticipated to enable future applications.