Marine natural products.

cis-Delivery of H2 to the π-system of an unsaturated substrate is the canonical course of metal catalyzed hydrogenation reactions. The semireduction of internal alkynes with the aid of [Cp*Ru]-based catalysts violates this rule and affords E-alkenes by direct trans-hydrogenation. A pathway involving σ-complexes and metallacyclopropenes accounts for this unconventional outcome. Connected to this process is an even more striking reactivity mode, in which both H atoms of H2 are delivered to one and the same C atom. Such gem-hydrogenation of stable carbogenic compunds is a fundamentally new transformation that leads to the formation of discrete metal carbene complexes. Computational studies suggest that the trans- and the gem-pathway have similar barriers, but polar substituents in the vicinity of the reacting triple bond provide opportunities for imposing selectivity and control. Moreover, it is shown that catalytic trans-hydrogenation is by no means a singularity: rather, the underlying principle is also ma...

trans-Hydrogenation, gem-Hydrogenation, and trans-Hydrometalation of Alkynes: An Interim Report on an Unorthodox Reactivity Paradigm

Diarylprolinol silyl ethers are among the most utilized stereoselective organocatalysts for the construction of complex molecules. With their debut in 2005, these catalysts have been applied in numerous method developments primarily leveraging enamine and iminium-ion catalysis. These strategies have extended into the preparation of complex molecules in both academic and industrial settings. This Review intends to give an overview of the application of the diarylprolinol silyl ether catalysts in total synthesis. Furthermore, integration of these catalysts in patent literature is also disclosed highlighting the versatility of the catalytic system.

Prevalence of Diarylprolinol Silyl Ethers as Catalysts in Total Synthesis and Patents

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η2-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H2 molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative p...

Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-Hydrogenation of Internal Alkynes.

Total synthesis allowed the constitution of the cytotoxic marine macrolides of the formosalide family to be confirmed and their previously unknown stereostructure to be assigned with confidence. The underlying blueprint was inherently modular to ensure that each conceivable isomer could be reached. This flexibility derived from the use of strictly catalyst controlled transformations to set the stereocenters, except for the anomeric position, which is under thermodynamic control; as an extra safety measure, all stereogenic centers were set prior to ring closure to preclude any interference of the conformation adopted by the macrolactone rings of the different diastereomers. Late-stage macrocyclization by ring-closing alkyne metathesis was followed by a platinum-catalyzed transannular 6-exo-dig hydroalkoxylation/ketalization to craft the polycyclic frame. The side chain featuring a very labile unsaturation pattern was finally attached to the core by Stille coupling.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/anie.202011472

The Formosalides: Structure Determination by Total Synthesis

Treatment of alkenylstannanes with Cu(OAc)2/Et3N affords the corresponding enol esters or ketones under conditions that proved compatible with many common functionalities; these include groups that would neither survive under the standard Tamao–Fleming conditions for the oxidation of Csp2 −SiR3 bonds nor under the conditions commonly used to oxidize C−B bonds. Chiral centers adjacent to the unveiled carbonyls are not racemized and competing protodestannation is marginal, even if the substrate carries unprotected −OH groups as internal proton sources. Therefore, the procedure is well suited for the preparation of acyloin and aldol derivatives. These enabling virtues are illustrated by a concise approach to the bicyclic lipid paecilonic acid A.

A Method for the Late-Stage Formation of Ketones, Acyloins, and Aldols from Alkenylstannanes: Application to the Total Synthesis of Paecilonic Acid A.

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The Formosalides: Structure Determination by Total Synthesis

A Method for the Late-Stage Formation of Ketones, Acyloins, and Aldols from Alkenylstannanes: Application to the Total Synthesis of Paecilonic Acid A.