Boron-Catalyzed Site-Selective Reduction of Carbohydrate Derivatives with Catecholborane

Abstract

Catalytic sp3 C–O bond cleavage using B­(C6F5)3/HBcat is reported. This method is first demonstrated on simple ethers and silyl protected alcohols, which exhibit reactivity parallel to the known B­(C6F5)3/HSiR3 system. In more complex carbohydrate derivatives, however, unique selectivities that are not possible with hydrosilane reductants have been achieved, including regioselective cyclizations. Preliminary computational studies suggest that diboryl oxonium ions are disfavored and that four-coordinate boronium ions may contribute to selectivity.

Modulating Electrostatic Interactions in Ion Pair Intermediates To Alter Site Selectivity in the C−O Deoxygenation of Sugars

“…Tris(pentafluorophenyl)borane (BCF) has emerged as a highly effective catalyst for the site‐selective deoxygenation of biomass‐derived carbohydrates . The choice of the sugar's protecting group and reductant (hydrosilane or hydroborane) can tailor which C−O bonds are cleaved to yield high‐value chiral products .…”

Deoxygenative Divergent Synthesis: En Route to Quinic Acid Chirons

“…12 Gagné and co-workers have recently progressed the field by replacing hydrosilanes by hydroboranes as precursors of H-B(C6F5)3hydride in the C-O bond cleavage with different selectivities than the ones observed in the hydrosilylation. 13 Morandi 14 and Oestreich 15 have expanded the B(C6F5)3catalyzed hydrosilylation of C(sp 3 )-O bonds to 1,2-diols and primary tosylates, respectively. Both methods are effective in cleaving the terminal C-O bond, the former due to the formation of a cyclic siloxane intermediate and the latter due to the higher reactivity of the tosylate compared with the silyl ether.…”

Regioselective Dehydration of Sugar Thioacetals under Mild Conditions

“…De-oxygenation/dehydration of sugars is of particular interest and only a few approaches have been described. For example, Gagné has reported methods for the regioselective reductive cyclisation of protected sugar-derived polyols 1 using silane reagents 12,13 in the presence of Lewis acids such as B(C6F5)3, leading to the formation of a range of chiral THFs and THPs 2 which can be accessed from sugars in a few steps (Scheme 1a). In previous work, we have developed methods for the regioselective dehydration of sugar hydrazones, e.g 3, (Scheme 1b) to give access to a range of chiral THFs (e.g.…”