Abstract
A strategy is outlined for the synthesis of loline alkaloids. This second-generation approach features direct access to the requisite vicinal diamino motif by a Cu(OTf)<sub>2</sub>-mediated highly <i>trans</i>-diastereoselective vinylogous Mannich reaction (VMR) of Ellman <i>N</i>-<i>tert</i>-butanesulfinyl imine derived from isopropylidene-protected (<i>S</i>)-glyceraldehyde, with <i>N</i>-Boc-2-<i>tert</i>-(butyldimethylsilyloxy)pyrrole (TBSOP). Fleming’s method was employed for the introduction of a hydroxyl group at C4 of the adduct, which gave the undesired diastereomer in 40% yield. The intramolecular S<sub>N</sub> reaction of the pyrrolizidine derivative to build the etheral bridge proved to be difficult, and produced <i>N</i>-Bus-norloline in only 20% yield. In light of the valuable information gained during this investigation, an improved version of the second-generation approach is being investigated, which is expected to provide a concise and efficient access to the challenging loline alkaloids.
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