Regioselective protection strategy remains a central challenge in carbohydrate synthesis. Many methods were developed, in which organotin reagents used to be identified as the best reagents for the selective protection. However, the organotin-methods are not enviroment-friendly due to the inherent potential toxicity of organotin reagents. Recently, we reported a non-toxic and environment-friendly catalyst, Fe(dibm)3, which can be used in selective alkylation of carbohydrates, leading to high regioselectivities [1,2]. The disadvantage of this method is that Fe(dibm)3 is not commercially available. After that, we reported the second green catalyst, Fe(acac)3, which is more inexpensive and commercially available, and can be used in selective acylation of carbohydrates with high regioselectivities . Based on these results, we further demonstrated that FeCl3 could be used directly as a catalyst to catalyze the regioselective benzoylation of carbohydrates with the assistance of acetylacetone . This result opens a gate for us to find the best catalytic system of regioselective protection. After tests of various metal salts and ligands, we have found that FeCl3 with the assistance of benzoyltrifluroacetone was the best catalytic system for regioselective acylation of carbohydrates. The catalytic system led to high regioselectivities in both benzoylation, acetylation and sulfonylation. We have also found that FeCl3 with the assistance of dipivaloylmethane was the best catalytic system for regioselective akylation of carbohydrates. FeCl3 with the assistance of dibenzoylmethane showed slightly lower catalytic activity than with the assistance of dipivaloylmethane for akylation. However, dibenzoylmethane is a much more inexpensive reagent than dipivaloylmethane. Both FeCl3 and dibenzoylmethane/dipivaloylmethane are commercially available and easily acquired. Therefore, FeCl3 as a green catalyst proved to have a great potential to be widely used in regioselective protection of carbohydrates with broad substrate scope.
- a) Ren, B.; Ramstrom, O.; Zhang, Q.; Ge, J.; Dong, H. Chem. Eur. J. 2016, 22, 2481.
- Ren, B.; Lv, J.; Zhang, Y.; Tian, J.; Dong, H. ChemCatChem 2017, 9, 950.
- a) Lv, J.; Ge, J.; Luo, T.; Dong, H. Green Chem. 2018, 20, 1987.
- a) Lv, J.; Luo, T.; Zhang, Y.; Pei, Z.; Dong, H. ACS Omega 2018, 3, 17717.