Umpolung Approach\nto Aldol Products via Isoxazoline <i>N</i>‑Oxides\nas Intermediates

A two-step\numpolung approach to the diastereoselective\nsynthesis\nof aldols was developed, in which a conjugated nitroalkene is used\nas the synthetic equivalent of the enolonium cation, while a sulfur\nylide acts as the equivalent of the α-carbinol anion. The resulting\nisoxazoline <i>N</i>-oxides undergo catalytic reductive\ncleavage to aldols under mild conditions at room temperature and under\n1 atm hydrogen pressure. The efficiency of the method was demonstrated\nby the synthesis of a series of polysubstituted β-hydroxyketones\nthat are difficult to synthesize using the classical aldol reaction.\nThe developed approach allows for the regiodivergent assembly of aldols\nby selecting a nitroalkene isomer with the appropriate position of\nthe <i>C</i>,<i>C</i> double bond.