Regioselective Reductive Opening of Benzylidene Acetals with Dichlorophenylborane/Triethylsilane: Previously Unreported Side Reactions and How to Prevent Them.

Arylidene acetals are widely used protecting groups, because of not only the high regioselectivity of their introduction but also the possibility of performing further regioselective reductive opening in the presence of a hydride donor and an acid catalyst. In this context, the EtSiH/PhBCl system presents several advantages: silanes are efficient, environmentally benign, and user-friendly hydride donors, while PhBCl opens the way to unique regioselectivity with regard to all other Brønsted or Lewis acids used with silanes. This system has been extensively used by several groups, and we have demonstrated its high regioselectivity in the reductive opening of 4,6- and 2,4---methoxybenzylidene moieties in protected disaccharides.

Substrate binding mode and catalytic mechanism of human heparan sulfate d-glucuronyl C5 epimerase.

Heparan sulfate (HS) is a linear, complex polysaccharide that modulates the biological activities of proteins through binding sites made by a series of Golgi-localized enzymes. Of these, glucuronyl C5-epimerase (Glce) catalyzes C5-epimerization of the HS component, d-glucuronic acid (GlcA), into l-iduronic acid (IdoA), which provides internal flexibility to the polymer and forges protein-binding sites to ensure polymer function. Here we report crystal structures of human Glce in the unbound state and of an inactive mutant, as assessed by real-time NMR spectroscopy, bound with a (GlcA-GlcNS) substrate or a (IdoA-GlcNS) product.