Recent Methods for the Synthesis of (E)‐Alkene Units in Macrocyclic Natural Products

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Abstract
(E)‐Alkene units are frequently found in macrocyclic natural products. Among the reactions that form the double bond during the cyclization, the classical Horner–Emmons coupling is still frequently used with success. During the last decade, ring‐closing metathesis has emerged as a very powerful tool for the synthesis of large rings, but the E/Z selectivity, which is rarely predictable, depends on many factors which will be discussed in this review. The best solution might be a two‐step procedure involving ring‐closing alkyne metathesis (RCAM) followed by stereoselective reduction of the macrocyclic alkyne unit to the corresponding E double bond.