• Strecker degradation of amino acids promoted by a camphor-derived sulfonamide

      Carvalho, M. Fernanda N. N.; Ferreira, M. Joao; Knittel, Ana S. O.; Oliveira, Maria da C.; Pessoa, Joao C.; Herrmann, Rudolf; Wagner, Gabriele; Universidade de Lisboa; University of Augsburg; University of Chester (Beilstein-Institut, 2016-04-18)
      A camphor-derived sulfonimine with a conjugated carbonyl group, oxoimine 1 (O2SNC10H13O), reacts with amino acids (glycine, L-alanine, L-phenylalanine, L-leucine) to form a compound O2SNC10H13NC10H14NSO2 (2) which was characterized by spectroscopic means (MS and NMR) and supported by DFT calculations. The product, a single diastereoisomer, contains two oxoimine units connected by a –N= bridge, and thus has a structural analogy to the colored product Ruhemann´s purple obtained by the ninhydrin reaction with amino acids. A plausible reaction mechanism that involves zwitterions, a Strecker degradation of an intermediate imine and water-catalyzed tautomerizations was developed by means of DFT calculations on potential transition states.
    • Synthesis of Alkynyl-substituted Camphor Derivatives and their Use in the Preparation of Paclitaxel-related Compounds.

      Carvalho, M. Fernanda N. N.; Herrmann, Rudolf; Wagner, Gabriele; Universidade de Lisboa, University of Augsburg, University of Chester (Beilstein, 2017-06-26)
      Compounds containing two alkyne groups in close vicinity at the rigid skeleton of camphorsulfonamide show unique reactivities when treated with electrophiles or catalytic amounts of platinum(II), the product structures depending not only on the reagents but also on the substituents attached to the triple bonds. Cycloisomerisations with perfect atom economy lead to polycyclic heterocycles that resemble to some extent the AB ring system of paclitaxel. Herein, we present practical synthetic methods for the selective synthesis of precursor dialkynes bearing different substituents (alkyl, aryl) at the triple bonds, based on ketals or an imine as protecting groups. We show for isomeric dialkynes that the reaction cascade induced by Pt(II) includes ring annulation, sulfur reduction and ring enlargement. One isomeric dialkyne additionally allows for the isolation of a pentacyclic compound lacking the ring enlargement step, which we have proposed as a potential intermediate in the catalytic cycle.