Efficient diastereoselective synthesis 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-alpha-alkyl-beta-hydroxy sulfones by reduction of the corresponding alpha-alkyl-beta-keto sulfones has I,een developed. The pivotal role in this methodology is played by the chelating or nonchelating properties of the Lewis acid employed. The strong chelating TiCl4 led to the erythro isomer in high diastereomeric excess in noncoordinating solvents (CH2Cl2) at -78 degrees C using BH3/py as reducing agent, while the nonchelating CeCl3 gave a high excess of the three isomer in coordinating solvents (THF) at the same temperature using lithium triethylborohydride (LiEt3BH) as reducing agent. Moreover, this methodology has been successfully utilized for the synthesis of alpha-allyl-beta-hydroxy sulfones, which are useful synthetic intermediates to obtain 2,5-disubstituted tetrahydrofurans by electrophilic cyclization.