Catalysis for Total Synthesis: A Personal Account

Construction of spirocarbocycles via gold-catalyzed intramolecular dearomatization of naphthols

A highly efficient, gold-catalyzed intramolecular dearomatization reaction of naphthols via 5-endo-dig cyclization is described. This facile and direct approach furnishes spirocarbocycles in excellent yields under mild conditions.

Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-Hydrogenation

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para-hydrogen (p-H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This "geminal hydrogenation" mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter-ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over-reduction, which frequently interfere with regular alkyne trans-hydrogenation.

Formation of ruthenium carbenes by gem-hydrogen transfer to internal alkynes: implications for alkyne trans-hydrogenation

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para-hydrogen (p-H₂) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-reduction competes with a pathway in which both H atoms of H₂ are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter-ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over-reduction, which frequently interfere with regular alkyne trans-hydrogenation.

Total Synthesis of Amphidinolide K, a Macrolide That Stabilizes F-Actin

The total synthesis of (-)-amphidinolide K (1) based on asymmetric addition of allylsilane C1-C8 to enal C9-C22 is reported. The 1,9,18-tris-O-TBDPS ether was converted into the desired 9,18-dihydroxy acid. Its macrolactonization was accomplished by the Shiina method. Compound 1 together with some of its stereoisomers and analogues were subjected to evaluation of the possible disruption of the α,β-tubulin-microtubule and/or G-actin-F-actin equilibria. Compound 1 behaves as a stabilizer of actin filaments (F-actin) in vitro.

Self-Assembly of Disorazole C1 through a One-Pot Alkyne Metathesis Homodimerization Strategy

Alkyne metathesis is increasingly explored as a reliable method to close macrocyclic rings, but there are no prior examples of an alkyne-metathesis-based homodimerization approach to natural products. In this approach to the cytotoxic C2-symmetric marine-derived bis(lactone) disorazole C1, a highly convergent, modular strategy is employed featuring cyclization through an ambitious one-pot alkyne cross-metathesis/ring-closing metathesis self-assembly process.

Self-Assembly of Disorazole C1 through a One-Pot Alkyne Metathesis Homodimerization Strategy

Alkyne metathesis is increasingly explored as a reliable method to close macrocyclic rings, but there are no prior examples of an alkyne-metathesis-based homodimerization approach to natural products. In this approach to the cytotoxic C₂ -symmetric marine-derived bis(lactone) disorazole C₁, a highly convergent, modular strategy is employed featuring cyclization through an ambitious one-pot alkyne cross-metathesis/ring-closing metathesis self-assembly process.

trans-Hydrogenation: application to a concise and scalable synthesis of brefeldin A

The important biochemical probe molecule brefeldin A (1) has served as an inspirational target in the past, but none of the many routes has actually delivered more than just a few milligrams of product, where documented. The approach described herein is clearly more efficient; it hinges upon the first implementation of ruthenium-catalyzed trans-hydrogenation in natural products total synthesis. Because this unorthodox reaction is selective for the triple bond and does not touch the transannular alkene or the lactone site of the cycloalkyne, it outperforms the classical Birch-type reduction that could not be applied at such a late stage. Other key steps en route to 1 comprise an iron-catalyzed reductive formation of a non-terminal alkyne, an asymmetric propiolate carbonyl addition mediated by a bulky amino alcohol, and a macrocyclization by ring-closing alkyne metathesis catalyzed by a molybdenum alkylidyne.

trans-Hydrogenation: Application to a Concise and Scalable Synthesis of Brefeldin A

The important biochemical probe molecule brefeldin A (1) has served as an inspirational target in the past, but none of the many routes has actually delivered more than just a few milligrams of product, where documented. The approach described herein is clearly more efficient; it hinges upon the first implementation of ruthenium-catalyzed trans-hydrogenation in natural products total synthesis. Because this unorthodox reaction is selective for the triple bond and does not touch the transannular alkene or the lactone site of the cycloalkyne, it outperforms the classical Birch-type reduction that could not be applied at such a late stage. Other key steps en route to 1 comprise an iron-catalyzed reductive formation of a non-terminal alkyne, an asymmetric propiolate carbonyl addition mediated by a bulky amino alcohol, and a macrocyclization by ring-closing alkyne metathesis catalyzed by a molybdenum alkylidyne.