There are a large number of chemical transformations in which alkenes act as the reactants and/or products of the reaction. Perhaps the most widely utilized approach to synthesis of unsaturated large rings is catalytic ring-closing metathesis (RCM), but this reaction often proceeds with little control over alkene. Amir Hoveyda and colleagues have developed a method for highly efficient and stereoselective synthesis of macrocyclic alkenes by catalytic RCM. It involves catalytic Z-isomer-selective cross-metathesis reactions of terminal enol ethers, which have not been reported previously, and allylic amides, used previously only in E-isomer-selective processes. The effectiveness of the method is demonstrated by the stereoselective synthesis of anticancer agents epothilone C and nakadomarin A. Many natural products contain a C = C double bond through which various other derivatives can be prepared; the stereochemical identity of the alkene can be critical to the biological activities of such molecules. Catalytic ring-closing metathesis (RCM) is a widely used method for the synthesis of large unsaturated rings1,2; however, cyclizations often proceed without control of alkene stereochemistry2. This shortcoming is particularly costly when the cyclization reaction is performed after a long sequence of other chemical transformations2. Here we outline a reliable, practical and general approach for the efficient and highly stereoselective synthesis of macrocyclic alkenes by catalytic RCM; transformations deliver up to 97% of the Z isomer owing to control induced by a tungsten-based alkylidene. Utility is demonstrated through the stereoselective preparation of epothilone C (refs 3–5) and nakadomarin A (ref. 6), the previously reported syntheses of which have been marred by late-stage, non-selective RCM7,8,9,10,11,12. The tungsten alkylidene can be manipulated in air, delivering the products in useful yields with high stereoselectivity. As a result of efficient RCM and re-incorporation of side products into the catalytic cycle with minimal alkene isomerization, desired cyclizations proceed in preference to alternative pathways, even under relatively high substrate concentration.

A new chemoselective reductive nitro‐Mannich cyclization reaction sequence of nitroalkyl‐tethered lactams has been developed. Relying on the rapid and chemoselective iridium(I)‐catalyzed reduction of lactams to the corresponding enamine, subsequent nitro‐Mannich cyclization of tethered nitroalkyl functionality provides direct access to important alkaloid natural‐product‐like structures in yields up to 81 % and in diastereoselectivities that are typically good to excellent. An in‐depth understanding of the reaction mechanism has been gained through NMR studies and characterization of reaction intermediates. The new methodology has been applied to the total synthesis of (±)‐epi‐epiquinamide in four steps.