Anomalous Zemplén deacylation reactions of alpha- and beta-D-mannopyranoside derivatives.

Reaction of mono-, di-, and trisaccharide derivatives of methyl beta-D- and octyl beta-D-mannopyranosides bearing ester groups at isolated and non-isolated positions on the same molecule, under Zemplén conditions (catalytic amount of sodium methoxide in methanol) gave partially deacylated compounds, in which the O-acyl groups were retained at isolated sites. In the case of one disaccharide, all the benzoyl groups remained intact at the reducing end, while all the acetyl functions were removable from the nonreducing end. In another case, both isolated ester groups at positions 2 and 4 were retained at the reducing end. The isolated 2-O-acyl groups on methyl alpha-D-mannopyranoside compounds were more labile than on the corresponding beta-mannosides under the same conditions. The mechanism of the reaction may be different for ester groups at isolated or non-isolated positions. In the latter case, acyl migration may take place and carry acyl groups into a less hindered position.

beta-Glycosides of 2,3-Diazido-2,3-dideoxy-D-mannose, a Synthetic Precursor of a Rare Bacterial Cell-Wall Building Unit.

2,3-Diazido-2,3-dideoxy-beta-D-mannopyranoside derivatives were synthesized in order to prepare beta-glycosides of 2,3-diacetamido-2,3-dideoxy-D-mannuronic acid, a rare moiety of bacterial oligosaccharides. A direct glycosyl donor, 4,6-di-O-acetyl-2,3-diazido-2,3-dideoxy-alpha-D-mannopyranosyl bromide, was prepared, and its synthetic capacity was tested in glycosylation reactions. An indirect route was also elaborated: 3-azido-3-deoxy-beta-D-glucopyranosides were converted into beta-D-mannopyranosides. The cis vicinal diazido function successfully tolerated the conditions of mild acidic hydrolysis, tritylation, Jones oxidation, TEMPO oxidation, acetolysis, and bromination with TiBr(4).

Long-range surface-plasmon modes in silver and aluminum films.

We report the first observation to our knowledge of a sharp minimum in the attenuated total reflectivity of a thin metal film between index-matching layers. The resonance is due to the excitation of the long-range surface-plasmon mode on both sides of the thin metal films, as originally discussed by Sarid [Phys. Rev. Lett. 47, 1927 (1981)]. The angular widths of the observed resonance in silver and aluminum films are both reduced by over an order of magnitude relative to that associated with the Kretschmann excited mode.