Diastereoselective β-hydroxy vinylsulfone isomerizations

Vinylic phenylsulfones containing a β-hydroxyl stereocenter undergo a diastereoselective isomerization to the corresponding allylic isomer upon treatment with 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU). Diastereoselectivity of this reaction increases with increasing size of the group attached to the carbinol carbon (up to >20 : 1 dr with a <i>tert</i>-butyl). Isolated yields of the isomerized allylic 1,2-hydroxy phenylsulfone products were comparable among the different vinylsulfones tested (59-66%). The major competing pathway was a C-C bond cleavage process, proposed to occur after the initial isomerization event. The sense of diastereoselection was consistent among all substrates investigated, in favor of the <i>erythro</i>-isomer based on NMR analysis.