Stereoselective Olefin Metathesis Reactions for Natural Product Synthesis
Chapter 1. The first examples of highly Z- and enantioselective ring-opening/cross-metathesis reactions are disclosed. Transformations involve meso cyclic olefin substrate and styrenes or enol ethers as olefin cross partners. A stereogenic-at-Mo monoaryloxide monopyrrolide (MAP) complex, prepared and used in situ, is discovered for the efficient formation of Z olefins. Such complex, bearing a relatively smaller adamantylimido and a larger chiral aryloxide ligand, leads to kinetic Z-selectivity due to the size differential. In most cases, the resulting disubstituted Z olefins are formed with excellent stereoselectivity (>95% Z). Chapter 2. The protocols for efficient Z-selective formation of macrocyclic disubstituted alkenes through catalytic ring-closing metathesis (RCM) is described. Stereoselective cyclizations are performed with either Mo- or W-based monoaryloxide monopyrrolide (MAP) complex at 22 oC. Synthetic utility of such broadly applicable transformation is demonstrated by synthesis of several macrocyclic natural products: relatively simpler molecules such as epilachnene (91% Z) and ambrettolide (91% Z), as well as advanced precursors to epothilones C and A (97% Z) and nakadomarin A (94% Z). Several principles of catalytic stereoselective olefin metathesis reactions are summarized based on the studies: 1) Mo-based catalysts are capable of delivering high activity but can be more prone to post-RCM isomerization. 2) W-based catalysts, though furnish lower activity, are less likely to cause the loss of kinetic Z selectivity by isomerization. 3) Reaction time is critical for retaining the stereoselectivity gained from kinetic, which not only applicable with MAP complexes but potentially with other complexes as well. 4) By using W-based catalyst, polycyclic alkenes can be accessed with sequential RCM reactions, without significant erosion of the existing Z olefins in the molecule. Chapter 3. An enantioselective total synthesis of anti-proliferative agent (+)-neopeltolide is presented. The total synthesis is accomplished in 11 steps for the longest linear sequence and 28 steps in total, including 8 catalytic reactions. Particularly, several Mo- or Ru-catalyzed stereoselective olefin metathesis reactions as well as N-hetereocyclic carbene (NHC)-catalyzed enantioselective boron conjugate addition to an acyclic enoate have proven to be effective for convergent construction of the molecule. The most important novelty of the study incorporates the explorations of feasibility of Z-selective cross-metathesis reactions to solve the challenge of installing two Z olefins with excellent selectivity.