Y-Thionolactones

Boron trifluoride and indium(III) triflate were found to catalyse efficiently the isomerization of thionolactones (26) to thiolactones (27) in good yields. When applied to an optically active y-thionolactone [(/ )-28], the isomerization reaction proceeded with complete inversion of configuration to [(S)-29] by using BF3,OEt2 (Scheme 9). The proposed mechanism (Scheme 10) implicates a double S -type process involving two molecules of thionolactone, each complexed with BF3, that progresses to the product via the dimer (30).49... 

A series of y-thionolactones has been synthesized, with good yields, using a new combination of Lawesson s reagent (LR) and hexamethyldisiloxane (HMDO) in solvent-free conditions under the action of MW irradiation (Eq. 31). An important MW-specific effect was observed when the reaction yield was carefully compared with that from traditional heating under similar conditions [189] ... 

A series of y-thionolactones has been prepared in good yields by a combination of Lawesson s reagent and hexamethyldisiloxane between solvent-free conditions with irradiation [150]. 

Thionation of Carboxylate Esters and Lactones. The reaction of esters and lactones with LR represents a well-established method for the synthesis of the corresponding thiocarbony 1 deriva-tives. - In fact, LR is the much superior reagent for the thionation of esters as compared with phosphorus pentasullide. Open-chain thionoesters have been conveniently obtained. Interestingly, natural triglycerides also smoothly react with LR to yield pure tristhionotriglycerides. Even better results are obtained with carboxylate esters if the improved solubilized LR or microwave irradiation are applied. The method has particularly been used for the preparation of cyclic thionoesters. Obviously there are very few structural or other restrictions that inhibit the formation of y-,S-, or macrocyclic thionolactones. Once more, microwave irradiation significantly improves the re-... 

After completion of the pilot studies with y-dodecalactone, thionation of a series of y-lactones was examined and found in most cases to give the corresponding thionolactone with good yield and selectivity. Yields obtained with the one 3-lactone and the one e-lactone examined were less satisfactory (44% and 15%, respectively). 

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