Looking back: The asymmetric organocatalytic 1,4‐addition of aldehydes to nitroolefins was studied by ESI‐MS. Analysis of the back reaction starting from quasienantiomeric mass‐labeled 1,4‐adducts (see scheme) provided conclusive evidence for an enamine rather than an enol mechanism, and allowed identification of the enantioselectivity‐determining step.
An efficient protocol for the evaluation of catalysts for the asymmetric Morita-Baylis-Hillman (MBH) reaction was developed. By mass spectrometric back-reaction screening of quasi-enantiomeric MBH products, an efficient bifunctional phosphine catalyst was identified that outperforms literature-known catalysts in the MBH reaction of methyl acrylate with aldehydes. The close match between the selectivities measured for the forward and back reaction and kinetic measurements provided strong evidence that the aldol step and not the subsequent proton transfer is rate- and enantioselectivity-determining.
Mit Blick zurück: Die asymmetrische organokatalytische 1,4‐Addition von Aldehyden an Nitroolefine wurde mit ESI‐MS untersucht. Eine Analyse der Rückreaktion ausgehend von quasienantiomeren massenmarkierten 1,4‐Addukten (siehe Schema) lieferte eindeutige Belege für einen Enamin‐ anstelle eines Enol‐Mechanismus und ermöglichte es, den enantioselektivitätsbestimmenden Schritt zu identifizieren.
Stereoselective syntheses of 3‐hydroxypiperidines have been developed. Key intermediates are N‐protected allylamines that are prepared by an enantioselective iridium‐catalyzed allylic amination. A subsequent catch and release procedure that involves an epoxidation and base‐mediated elimination yields δ‐lactams that are suitably functionalized to prepare biologically active 3‐hydroxypiperidines. In addition, applications of this method to the total syntheses of deoxymannojirimycin, D‐erythro‐sphingosine, and chiral building blocks of interest for medicinal chemistry are described.
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