3,4,6-Tri-O-benzyl-.alpha.-D-arabino-hexopyranos-2-ulosyl Bromide: A Versatile Glycosyl Donor for the Efficient Generation of .beta.-D-Mannopyranosidic Linkages
摘要:
An expedient four-step sequence is described for the conversion of acetobromoglucose into the title 2-oxohexosyl (''ulosyl'') bromide 4. Due to its O-benzyl protection, 4 is considerably more reactive than its acylated analogs 1-3: Ag2CO3-promoted glycosidations with 2-propanol, diacetone-galactose, and methyl 2,3-O-isopropylidene-alpha-L-rhamnoside are complete within minutes and, in addition, are endowed with beta-specificity. This renders ulosyl bromide 4 a most propitious, indirect beta-D-mannosyl donor, inasmuch as the borohydride reduction of the beta-D-glycosiduloses formed (14-16 --> 19, 21, and 22) proceeds with manno selectivities of >20:1. Comparative evaluation of the manno/gluco ratios obtained in all 21 beta-D-arabino hexosidulose reductions (Table 1) reveals the 3-O-blocking group to have a pronounced effect on the outcome: >20:1 in cases with a 3-O-benzyl group versus only 2:1 to 3:1 in the presence of 3-O-acyl functions.
3,4,6-Tri-O-benzyl-.alpha.-D-arabino-hexopyranos-2-ulosyl Bromide: A Versatile Glycosyl Donor for the Efficient Generation of .beta.-D-Mannopyranosidic Linkages
摘要:
An expedient four-step sequence is described for the conversion of acetobromoglucose into the title 2-oxohexosyl (''ulosyl'') bromide 4. Due to its O-benzyl protection, 4 is considerably more reactive than its acylated analogs 1-3: Ag2CO3-promoted glycosidations with 2-propanol, diacetone-galactose, and methyl 2,3-O-isopropylidene-alpha-L-rhamnoside are complete within minutes and, in addition, are endowed with beta-specificity. This renders ulosyl bromide 4 a most propitious, indirect beta-D-mannosyl donor, inasmuch as the borohydride reduction of the beta-D-glycosiduloses formed (14-16 --> 19, 21, and 22) proceeds with manno selectivities of >20:1. Comparative evaluation of the manno/gluco ratios obtained in all 21 beta-D-arabino hexosidulose reductions (Table 1) reveals the 3-O-blocking group to have a pronounced effect on the outcome: >20:1 in cases with a 3-O-benzyl group versus only 2:1 to 3:1 in the presence of 3-O-acyl functions.
manno- versus gluco-Selectivity in reductions of 2-keto-β-d-arabino-hexopyranosides
作者:Frieder W. Lichtenthaler、Matthias Lergenmüller、Siegfried Peters、Zsolt Varga
DOI:10.1016/s0957-4166(03)00028-4
日期:2003.3
Tri-sec-butylborohydrides (L- or K-Selectride) reduce the carbonyl group in acylated 2-keto-beta-D-arabino-hexopyranosides to beta-D-mannosides in an essentially stereospecific fashion, whereas borane reduction gives the 2-epimeric beta-D-glucosides with high preference. As preparative yields are in the 70-85% range, the ulosyl donor approach can thus be utilized for the straightforward construction of oligosaccharides containing either beta-D-Man or beta-D-Glc units. (C) 2003 Elsevier Science Ltd. All rights reserved.
Experimental evidence on the hydroxymethyl group conformation in alkyl β-d-mannopyranosides
作者:Carlos Mayato、Rosa Dorta、Jesús Vázquez
DOI:10.1016/j.tetasy.2004.06.019
日期:2004.8
A rotational population Study of the hydroxymethyl group of alkyl beta-D-mannopyranosides was performed by means of CD and NMR spectroscopy. Three different benzyl, acetyl, and p-bromobenzoyl series of alkyl beta-D-mannopyranosides with different chiral and nonchiral aglycons were synthesized and analyzed. Different rotational populations were observed for each series by changing the structure of the aglycon. The results showed a clear correlation between the rotational population of the hydroxymethyl group around the C5-C6 bond and the pK(a) of the bonded alcohol (aglycon). The population of the gt rotamer gradually increased as the pK(a) increased while that of the gg rotamer decreased and the population of the tg rotamer remained almost constant. This is explained by the exo-anomeric effect. For chiral alkyl derivatives, the results also showed a close dependence on the absolute configuration of the aglycon. Comparison of rotational population anomers revealed the dependence or the hydroxymethyl group on the anomeric configuration and a greater dependence on the aglycon structure in the beta anomers. (C) 2004 Elsevier Ltd. All rights reserved.
Stereocontrolled synthesis of disaccharides via the temporary silicon connection