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.

Route to α-Aryl Phosphonoacetates: Useful Synthetic Precursors in the Horner-Wadsworth-Emmons Olefination.

A versatile and general catalytic strategy has been developed for the α-arylation of phosphonoacetates utilizing parallel microscale experimentation. These α-substituted phosphonoacetates are widely useful, notably as substrates in the Horner-Wadsworth-Emmons-type olefinations. However, the current routes to these products involve harsh conditions, limiting the variety of functionality. The reported method can be used with a variety of aryl chlorides and aryl bromides, including several heterocyclic examples.

Palladium-Catalyzed Chemoselective Activation of sp3 vs sp2 C-H Bonds: Oxidative Coupling To Form Quaternary Centers.

The oxidative activation of alkyl C-H bonds vs arene C-H bonds with Pd(OAc)2 has been found to be generalizable to a number of nucleophilic substrates allowing the formation of a range of hindered quaternary centers. The substrates share a common mechanistic path wherein Pd(II) initiates an oxidative dimerization. The resultant dimer modifies the palladium catalyst to favor activation of alkyl C-H bonds in contrast to the trends typically observed via a concerted metalation deprotonation mechanism. Notably, insertion occurs at the terminus of the alkyl arene for hindered substrates. Two different oxidant systems were discovered that turn over the process. Parameters have been identified that predict, which substrates are productive in this reaction.

Palladium-Catalyzed α-Arylation of Aryl Nitromethanes.

Catalytic conditions for the α-arylation of aryl nitromethanes have been discovered using parallel microscale experimentation, despite two prior reports of the lack of reactivity of these aryl nitromethane precursors. The method efficiently provides a variety of substituted, isolable diaryl nitromethanes. In addition, it is possible to sequentially append two different aryl groups to nitromethane. Mild oxidation conditions were identified to afford the corresponding benzophenones via the Nef reaction, and reduction conditions were optimized to afford several diaryl methylamines.