Kinetically E-Selective Macrocyclic Ring-Closing Metathesis

Macrocyclic compounds are central to discovery of new drugs but their preparation is often challenging because of the energy barrier required for bringing together and fusing the two ends of an acyclic precursor(1). Ring-closing metathesis (RCM) (2,3,4) is a catalytic process that has allowed access to countless biologically active macrocyclic organic molecules even on large scale (up to 200 kilograms)(5). The potency of a macrocyclic compound can depend on the stereochemistry of its alkene, or one isomer might be needed for subsequent stereoselective modification (e.g., dihydroxylation(6)). Still, while kinetically controlled Z-selective RCM reactions have been reported(7,8,9,10), the only available olefin metathesis approach for accessing macrocyclic E olefins entails selective removal of the Z component of a stereoisomeric mixture by ethenolysis(10), sacrificing substantial quantities of material if E/Z ratios are near unity. Use of ethylene can also cause adventitious olefin isomerization, a particularly serious problem when the E alkene is energetically less favored. Here, we show that dienes containing an E-alkenyl–B(pinacolato) group, widely used in catalytic cross-coupling(11), possess the requisite electronic and steric attributes to allow them to be converted stereoselectively to E macrocyclic alkenes. Reactions are promoted by a molybdenum monoaryloxide pyrrolide complex and afford products in up to 73 percent yield and >98:2 E:Z ratio. Utility is highlighted by application to preparation of the twelve-membered ring antibiotic recifeiolide(12,13) and the eighteen-membered ring Janus kinase 2/Fms-like tyrosine kinase-3 (JAK2/FLT3) inhibitor pacritinib(14,15) the Z isomer of which has lower potency than the E(16). The eighteen-membered ring moiety of pacritinib, a potent in vivo anti-cancer agent in advanced clinical trials for treatment of lymphoma and myelofibrosis, was prepared by an RCM carried out at 20 times higher concentration than when a ruthenium carbene was employed (0.02 vs. 0.001 M; 73% yield, 92% E).