Efficient Diastereoselective Syntheses of erythro- or threo-α-Alkyl-β-hydroxy Sulfones by Reductions of α-Alkyl-β-keto Sulfones with TiCl4/BH3 or LiEt3BH/CeCl3, Respectively

A stereoselective synthesis of erythro - and threo -α-alkyl-β-hydroxy sulfones by reduction of the corresponding α-alkyl-β-keto sulfones has been developed. The pivotal role in this methodology is played by the chelating or nonchelating properties of the Lewis acid employed. The strong chelating TiCl 4 led to the erythro isomer in high diastereomeric excess in noncoordinating solvents (CH 2 Cl 2 ) at −78 °C using BH 3 /py as reducing agent, while the nonchelating CeCl 3 gave a high excess of the threo isomer in coordinating solvents (THF) at the same temperature using lithium triethylborohydride (LiEt 3 BH) as reducing agent. Moreover, this methodology has been successfully utilized for the synthesis of α-allyl-β-hydroxy sulfones, which are useful synthetic intermediates to obtain 2,5-disubstituted tetrahydrofurans by electrophilic cyclization.