Hydroxy‐Directed Ruthenium‐Catalyzed Alkene/Alkyne Coupling: Increased Scope,Stereochemical Implications,and Mechanistic Rationale

The recognition of the dual binding mode of propargyl and allyl alcohols to [Cp*Ru] fragments fostered the development of a highly regioselective intermolecular Alder‐ene‐type reaction of alkynes with 1,2‐disubstituted alkenes. The increased substrate scope opens new perspectives in stereochemical terms. As the loaded catalyst is chiral‐at‐metal, stereochemical information is efficiently relayed from the propargylic site to the emerging C−C bond. This interpretation is based on the X‐ray structure of the first Cp*Ru complex carrying an intact enyne ligand, and provides valuable insights into bonding and activation of the substrates. Computational data draw a clear picture of the principles governing regio‐ and stereocontrol.     

Structural Snapshots of α‐1,3‐Galactosyltransferase with Native Substrates: Insight into the Catalytic Mechanism of Retaining Glycosyltransferases

Glycosyltransferases (GTs) are a key family of enzymes that catalyze the synthesis of glycosidic bonds in all living organisms. The reaction involves the transfer of a glycosyl moiety and can proceed with retention or inversion of the anomeric configuration. To date, the catalytic mechanism of retaining GTs is a topic of great controversy, particularly for those enzymes containing a putative nucleophilic residue in the active site, for which the occurrence of a double‐displacement mechanism has been suggested. We report native ternary complexes of the retaining glycosyltransferase α‐1,3‐galactosyltransferase (α3GalT) from Bos taurus , which contains such a nucleophile in the active site, in a productive mode for catalysis in the presence of its sugar donor UDP‐Gal, the acceptor substrate lactose, and the divalent cation cofactor. This new experimental evidence supports the occurrence of a front‐side substrate‐assisted S_Ni‐type reaction for α3GalT, and suggests a conserved common catalytic mechanism among retaining GTs.