Indium Trifluoromethanesulfonate as a Mild and Chemoselective Catalyst for the Conversion of Carbonyl Compounds into 1,3-Oxathiolanes

Abstract

An efficient method for the preparation of 1,3-oxathiolanes from aldehydes and ketones with 2-mercaptoethanol in the presence of a catalytic amount of indium trifluoromethanesulfonate is reported.

Key words

chemoselectivity - indium trifluoromethanesulfonate - Lewis acids - 1,3-oxathiolanes - protecting groups

References

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21

The experimental procedure for the reaction of cyclohexyl methyl ketone with 2-mercaptoethanol is representative. To a mixture of cyclohexyl methyl ketone (252 mg, 2.0 mmol) and In(OTf)₃ (56 mg, 0.1 mmol) in CH₂Cl₂ (5.5 mL) 2-mercaptoethanol (313 mg, 4.0 mmol) in CH₂Cl₂ (0.5 mL) was added at 25 °C. After the reaction mixture was kept stirring at the same temperature for 5 h, it was quenched by adding sat. NaHCO₃. The resulting mixture was extracted three times with EtOAc. The combined extracts were dried over Na₂SO₄, and concentrated. Silica gel column chromatography (5% EtOAc-hexane) yielded 2-cyclohexyl-2-methyl-1,3-oxathiolane (359 mg, 96%) as a colorless oil. ¹H NMR (500 MHz, CDCl₃) δ 1.01-1.31 (m, 5 H), 1.51 (s, 3 H), 1.64-1.93 (m, 6 H), 2.94 (m, 1 H), 3.03 (m, 1 H), 4.08 (m, 1 H), 4.21 (m, 1 H); ¹³C NMR (125 MHz, CDCl₃) δ 25.8, 26.3, 26.4, 28.6, 28.7, 33.3, 49.5, 70.2, 98.9.

22

By using InBr₃ (5 mol%), benzophenone was converted into 2,2-diphenyl-1,3-oxathiolane under similar reaction conditions in 40% yield. This result clearly shows the strong catalytic activity of In(OTf)₃ in comparison with InBr₃.

23

Trifluoromethanesulfonic acid (TfOH) arising by hydrolysis of In(OTf)₃ is less effective than In(OTf)₃ in our reaction system. For example, benzophenone was treated with HO(CH₂)₂SH and TfOH (15 mol%) at 25 °C for 22 h to afford the product in 50% yield.