Reactions of polyfluorinated thietanes

Recent synthesis of thietanes

Thietanes are important aliphatic four-membered thiaheterocycles that are found in the pharmaceutical core and structural motifs of some biological compounds. They are also useful intermediates in organic synthesis. Various synthetic methods of thietanes have been developed, including inter- and intramolecular nucleophilic thioetherifications, photochemical [2 + 2] cycloadditions, ring expansions and contractions, nucleophilic cyclizations, and some miscellaneous methods. The recently developed methods provide some new strategies for the efficient preparation of thietanes and their derivatives. This review focuses on the synthetic methods to construct thietane backbones developed during 1966 to 2019.

Reaction of 2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithiethane with N-vinyl compounds

The reaction of hexafluorothioacetone dimer (2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithiethane, 1) with vinylamides leads to the rapid formation of [2 + 2] cycloadducts: 4-amino-2,2-bis(trifluoromethyl)thietanes. The reaction proceeds in polar solvents (DMF, DMSO) in the absence of a catalyst at elevated temperature producing the corresponding cycloadducts in 47-86% yield. The reaction of N-vinylimidazole unexpectedly led to the formation of the corresponding 1-(hexafluoroisopropyl)-3-vinyl-1,3-dihydro-2H-imidazole-2-thione (5). The structure of this compound, along with the structures of two new thietanes was confirmed by single crystal X-ray diffraction.

Acid catalyzed cyclodimerization of 2,2-bis(trifluoromethyl)-4-alkoxy-oxetanes and -thietanes. Synthesis of 2,2,6,6-tetrakis(trifluoromethyl)-4,8-dialkoxy-1,5-dioxocanes and 3,3,7,7-tetrakis(trifluoromethyl)-9-oxa-2,6-dithia-bicyclo[3.3.1]nonane

Treatment of 2,2-bis(trifluoromethyl)-4-R-oxetanes (R = C₂H₅O, n-C₃H₇O, n-C₄H₉O) with BF₃·OEt₂ in CH₂Cl₂ solvent results in spontaneous electrophilic [4 + 4] cyclodimerization with the formation of the corresponding 2,2,6,6-tetrakis(trifluoromethyl)-4,8-dialkoxy-1,5-dioxocanes, isolated in 31–42% yield. The structures of two products (R = C₂H₅O and n-C₃H₇O) were established by single crystal X-ray diffraction. The corresponding oxetane carrying the bulky t-C₄H₉O group has different reactivity towards BF₃·OEt₂, slowly producing a mixture of two acyclic, unsaturated products.

Clean and spontaneous reaction with alcohols is another interesting transformation of oxetanes described in this paper. The reaction leads to high yield formation of the corresponding acetals (CF₃)₂C(OH)CH₂CH(OR)OR′.

Structurally related 2,2-bis(trifluoromethyl)-4-R-thietanes (R = i-C₃H₇O, t-C₄H₉O and C₂H₅O) have different reactivity towards electrophiles. They are totally inert to the action of BF₃·OEt₂ and rapidly react with a protic acid (H₂SO₄) forming the same product, 3,3,7,7-tetrakis(trifluoromethyl)-9-oxa-2,6-dithia-bicyclo[3.3.1]nonane in 35–50% yield. The structure of this product was established by single crystal X-ray diffraction.