Formation and stability of oxocarbenium ions from glycosides

Abstract

Structural, protecting group and leaving group effects in the formation of oxocarbenium intermediates were studied in the gas phase. It is found that significant stabilization of oxocarbenium cations is achieved by protecting groups that interact with the cationic center via neighboring group participation despite the electron-withdrawing character of these moieties. On the other hand, ethereal protecting groups do not facilitate the formation of oxocarbenium intermediates. The experimental findings are supported by DFT calculations that show the following order of stabilization by the group adjacent to the cationic center: RCO > SiR(3) > R, where R is an alkyl group. This indicates that the SN1-like mechanism that is commonly proposed for this reaction is not always valid. Moderate leaving group effect is also detected in a series of thioaryl glucopyranosides.

O-2 Substituted pyranosyl oxacarbenium ions are C-2–O-2 2-fold rotors with a strong syn preference

“…5 The observed C-2-O-2 bond lengthening is consistent with this type of interaction. The larger trimethylsilyl (14), benzyl (13), and allyl (12) groups all have steric and electronic interactions with the tetrahydropyran ring which modifies the trends. Rotational analysis for equatorial substituted ions 7 and 9 (see Figs.…”

“…13 The small energy difference for the TMS derivative 14 might be indicative of a steric effect counteracting the electronic effect.…”

“…These groups were proton (11), that is, the parent hydroxyl, allyl (12), benzyl (13), and trimethylsilyl (14). The special case of acyl protecting groups will be considered in a separate communication because these groups are capable of neighboring group participation.…”

Carbohydrate analysis of hemicelluloses by gas chromatography–mass spectrometry of acetylated methyl glycosides

“…The electron impact-induced fragmentation of similarly derivatized monosaccharide compounds was studied as early as 1963 by Biemann et al and was later studied in detail by collision-induced fragmentation and theoretical methods [13,15,[20][21][22][23][24]. The interpretation of the spectra is rather difficult due to the fact that the signals for the molecular ion and the first fragments are typically of very low intensity.…”

A novel O -fucosylation strategy preactivated by ( p -Tol) 2 SO/Tf 2 O and its application for the synthesis of Lewis blood group antigen Lewis a

“…in dichloromethane at -60°C for 0.5 h, followed by the addition of 6 (1.2 eq.). 10,29 The resulting mixture was stirred for 2 h, and warmed up to -40°C for another 2 h. The disaccharide 8 was given in 70% yield, in which all hydroxyls were fully protected. During the selective deprotection process, phenylmethylene group of 8 was reduced to give the free 4-OH secondary sugar alcohol 9 quantitively.…”