title: Organocatalytic asymmetric Mannich reactions with N-Boc and N-Cbz protected alpha-amido sulfones (Boc: tert-butoxycarbonyl, Cbz: benzyloxycarbonyl).
abstract: Different malonates and beta-ketoesters can react with N-tert-butoxycarbonyl- (N-Boc) and N-benzyloxycarbonyl- (N-Cbz) protected alpha-amido sulfones in an organocatalytic asymmetric Mannich-type reaction. The reaction makes use of a simple and easily obtained phase-transfer catalyst and proceeds under very mild and user-friendly conditions. The optimised protocol avoids the preparation and the isolation of the relatively unstable N-Boc and N-Cbz imines that are generated in situ from the bench-stable alpha-amido sulfones. The corresponding Mannich bases are generally obtained in good yields and enantioselectivities, and can be readily transformed into key compounds, such as optically active beta3-amino acids in one easy step. Enantioenriched N-Boc and N-Cbz protected beta-amino acids that are suitable for peptide synthesis are also available from the Mannich adducts through simple manipulations. Control experiments showed the dual role of the enolate-catalyst ion pair in this reaction, as well as the crucial role of the presence of water to achieve high enantioselectivities.
authors: Department of Organic Chemistry A. Mangini, Faculty of Industrial Chemistry, University of Bologna, V. Risorgimento 4, 40136 Bologna, Italy.
authors: Olindo
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authors: Marianacci
authors: Gabriele
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authors: Bernardi
authors: Francesco
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