Enantioselective Phase-Transfer Catalyzed α-Sulfanylation of Isoxazolidin-5-ones: An Entry to β2,2-Amino Acid Derivatives

An unprecedented enantioselective alpha-functionalization of C4-substituted N-alkoxycarbonyl isoxazolidin-5-ones, readily available platforms from Meldrum's acid derivatives, by N-sulfanylphthalimide (PhthSR) electrophiles was achieved upon an efficient phase-transfer catalytic approach, mediated by a commercial N-spiro quaternary ammonium catalyst. Two catalytic activities of the in situ formed R4N(+)Phth(-) species were highlighted, the phtalimidate being involved in the anion metathesis event and likely as a Bronsted base. This sequence offers a straightforward access to alpha,alpha-disubstituted isoxazolidinones, which turned out to be useful precursors of alpha-sulfanyl-beta(2,2)-amino acid derivatives.