Achievement of ring inversion of myo-inositol derivatives due to silyloxy/silyloxy repulsion enhanced by the trans-substituents on both sides

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1–SN2 Interface

“…The stabilizing effect of axial and pseudoaxial C-O bonds on nascent positive charge at the anomeric center manifests itself in the so-called super-armed donors developed by Bols and coworkers 90,107,108 in parallel with early demonstrations by the Yamada group. 89,109 In these systems repulsive gauche interactions between vicinal diequatorial siloxy groups, 110 perhaps assisted by attractive van der Waals interactions between 1,3-diaxial siloxy groups, 110,111 cause per-silylated glycosyl donors to preferentially adopt inverted chair and twist boat conformations rich in axial and pseudo-axial C-O bonds, 112,113 resulting in correspondingly high reactivity as glycosyl donors (Figure 8).…”

Turning the Nitrogen Atoms of an Ar2P−CH2−N−N−CH2−PAr2 Motif into Uniquely Configured Stereocenters: A Novel Diphosphane Design for Asymmetric Catalysis

“…Moreover, two N‐inversions might ensue establishing chair conformers ax,ax ‐ dia ‐ 46 and ‐ 47 with axial N−CH 2 P motifs. The overall conversion ax,ax‐ 46 → ax,ax ‐ dia ‐ 46 might be exergonic; this is suggested by the preferences for vic ‐ axial C−O bonds in bulky disilylethers of trans ‐ vic ‐dioxygenated cyclohexanes, myo ‐inositol or glucopyranose . Altogether, the ax,ax‐ 46 → ax,ax ‐ dia ‐ 46 conversion under scrutiny would make two ( R )‐configured N‐atoms ( S )‐configured.…”

The 4-(tert-Butyldiphenylsiloxy)-3-fluorobenzyl Group: A New Alcohol Protecting Group, Fully Orthogonal with the p-Methoxybenzyl Group and Removable under Desilylation Conditions

“…The appreciably greater bulk of the silyl ethers leads to the much reduced chelating ability of α-siloxycarbonyl systems, 2 and contributes to the inverted conformations adopted by some polylsilylated six-membered ring systems. 3 In the course of a study targeted at the synthesis of the common core pentasaccharide of the N -linked glycoproteins we uncovered an unanticipated consequence of the steric bulk of silyl ether protecting groups. 4 Thus, while 4,6- O -benylidene protected thiomannopyranosides, and their sulfoxides, carrying benzyl ethers at positions 2 and 3 are known to be superb donors in the direct stereocontrolled formation of β-mannopyranoside linkages, 5 an analagous system 1 carrying a 3- O - tert -butyldimethylsilyl group in place of a benzyl ether was found to favor the formation of α-mannosides.…”