Thionolactones isomerization

It has been shown that Lewis acid catalyzed isomerization of thionolactones provides access to thiolactones. For example, exposure of the substrate 22 to catalytic amounts of BF3 OEt2 led to efficient conversion to the thiolactone 23. Such transformations were also found to give minor amounts of lactone or dithiolactone side products <06TL6067>. Substituted tetrahydrothiophene derivatives have also been obtained from 1,4-dithiane-2,5-diol and 2-nitroethyl acetate derivatives by a base induced sequence featuring a Michael addition and a Henry reaction <06TL8087>. 

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... 

Dunach and coworkers have reported an unconventional method to access thiolactones. In their work, BF3 OEt2 efficiently catalyzes the isomerization of thionolactones (68) to the corresponding thiolactones (69). This process is quite general in substrate scope and gives the requisite products in good yields (Equation 43)... 

The isomerization of thionolactones to thiolactones was found to occur through the catalysis of In(OTf)3 as reported by Dunach et al in their screening of appropriate catalysts for such reactions [300]. However, BF3 Et20 was found to give better yields and selectivities (Figure 8.140). 

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