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2.10.4.3.1 Ring-Closing Metathesis–Semireduction for the Construction of Z-Alkenes

DOI: 10.1055/sos-SD-222-00346

Lee, D.; Reddy Sabbasani, V.Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (20162655.

Ring-closing metathesis is a powerful tool to form various cyclic frameworks, yet diene ring-closing metathesis provides a mixture of E- and Z-isomers with unpredictable ratios except for small- and medium-sized rings where Z-isomers are preferred. In macrocyclic ring-closing metathesis, usually E-isomers form more favorably due to the equilibrium nature of the metathesis process although complete stereocontrol is yet to be realized. In contrast, achieving Z-selective metathesis has been an outstanding problem in macrocyclic ring-closing metathesis. However, several successful processes including catalyst-based control have been developed recently (see Section 2.10.2.2.2.2). In this regard, ring-closing alkyne metathesis to construct macrocycles containing an alkyne moiety followed by semireduction would be an attractive solution for the stereoselective synthesis of E- or Z-alkene-containing macrocycles (Scheme 127). From ring-closing alkyne metathesis products, Z-configured cycloalkenes can be synthesized by semihydrogenation with Lindlar catalyst. Alternatively, Birch reduction, trans hydrosilylation–desilylation, and trans hydroboration–protodeborylation lead to the corresponding E-alkene-containing macrocycles.

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References