Convergent Synthesis of the NS5B Inhibitor GSK8175 Enabled by Transition Metal Catalysis.

A convergent eight-stage synthesis of the boron-containing NS5B inhibitor GSK8175 is described. The previous route involves 13 steps in a completely linear sequence, with an overall 10% yield. Key issues include a multiday SNAr arylation of a secondary sulfonamide using HMPA as solvent, multiple functional group interconversions after all of the carbon atoms are installed (including a Sandmeyer halogenation), use of carcinogenic chloromethyl methyl ether to install a protecting group late in the synthesis, and an unreliable Pd-catalyzed Miyaura borylation as the penultimate step. We have devised an orthogonal approach using a Chan-Lam coupling between a halogenated aryl pinacol boronate ester and an aryl methanesulfonamide. This reaction is performed using a cationic Cu(I) precatalyst, which can be easily generated in situ using KPF6 as a halide abstractor. High-throughput screening revealed a new Pd catalyst system to effect the penultimate borylation chemistry using simple monodentate phosphine ligands, with PCyPh2 identified as optimal. Reaction progress analysis of this borylation indicated likely mass-transfer rate limitations under standard conditions using KOAc as the base. We have devised a K2CO3/pivalic acid system as an alternative, which dramatically outperforms the standard conditions. This new synthesis proceeds in eight stages with a 20% overall yield.

Development of enantioselective synthetic routes to the hasubanan and acutumine alkaloids.

We describe a general strategy to prepare the hasubanan and acutumine alkaloids, a large family of botanical natural products that display antitumor, antiviral, and memory-enhancing effects. The absolute stereochemistry of the targets is established by an enantioselective Diels-Alder reaction between 5-(trimethylsilyl)cyclopentadiene (36) and 5-(2-azidoethyl)-2,3-dimethoxybenzoquinone (24). The Diels-Alder adduct 38 is transformed to the tetracyclic imine 39 by a Staudinger reduction-aza-Wittig sequence. The latter serves as a universal precursor to the targets. Key carbon-carbon bond constructions include highly diastereoselective acetylide additions to the N-methyliminium ion derived from 39 and Friedel-Crafts and Hosomi-Sakurai cyclizations to construct the carbocyclic skeleton of the targets. Initially, this strategy was applied to the syntheses of (-)-acutumine (4), (-)-dechloroacutumine (5), and four hasubanan alkaloids (1, 2, 3, and 8). Herein, the synthetic route is adapted to the syntheses of six additional hasubanan alkaloids (12, 13, 14, 15, 18, and 19). The strategic advantage of 5-(trimethylsilyl)cyclopentadiene Diels-Alder adducts is demonstrated by site-selective functionalization of distal carbon-carbon π-bonds in the presence of an otherwise reactive norbornene substructure. Evaluation of the antiproliferative properties of the synthetic metabolites revealed that four hasubanan alkaloids are submicromolar inhibitors of the N87 cell line.