Regioselective Reductive Opening of Benzylidene Acetals with Dichlorophenylborane/Triethylsilane: Previously...

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 Et₃SiH/PhBCl₂ system presents several advantages: silanes are efficient, environmentally benign, and user-friendly hydride donors, while PhBCl₂ 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-O-p-methoxybenzylidene moieties in protected disaccharides. Surprisingly, its use on 4,6-O-benzylidene-containing substrates 1 and 2 led to unreproducible yields due to the unexpected formation of several side products. Their characterizations allowed us to identify different pitfalls potentially affecting the outcome of reductive opening of arylidenes with the Et₃SiH/PhBCl₂ reagent system: alkene hydroboration, azide reduction, and/or Lewis acid-promoted cleavage of the arylidene. With this knowledge, we optimized reproducible and high-yielding reaction conditions that secure and extend the scope of the Et₃SiH/PhBCl₂ system as a reagent for the regioselective opening of arylidenes in complex and multifunctional molecules.