The development of general and expedient methodologies for the preparation of orthogonally protected glycoside building blocks is essential for the efficient synthesis of complex oligosaccharides. Herein, we describe a new approach that uses copper(II) triflate as a versatile catalyst for the onepot preparation of orthogonal protected thio-and O-glycosides from the corresponding unprotected counterparts. The conditions are mild, easy to handle and applicable to two and three one-pot tandem transformations, which include arylidene acetalation, esterification, regioselective reductive acetal ring opening, glycosylation and silylation processes.
The enantioselective H-transfer hydrogenation of quinoline by Hantzsch ester is a relevant example of Brønsted acid catalyzed cascade reactions, with phosphoric acid being a privileged catalyst. The generally accepted mechanism points out the hydride transfer step as the rate- and stereodetermining step, however computations based on these models do not totally fit with experimental observations. We hereby present a computational study that enlightens the stereochemical outcome and quantitatively reproduces the experimental enantiomeric excesses in a series of H-transfer hydrogenations. Our calculations suggest that the high stereocontrol usually attained with BINOL-derived phosphoric acids results mostly from the steric constraints generated by an aryl substituent of the catalyst, which hinders the access of the Hantzsch ester to the catalytic site and enforces approach through a specific way. It relies on a new model involving the preferential assembly of one of the stereomeric complexes formed by the chiral phosphoric acid and the two reaction partners. The stereodetermining step thus occurs prior to the H-transfer step.
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