Experimental evidence on the hydroxymethyl group conformation in alkyl β-d-mannopyranosides
摘要:
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.
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.
Metal-catalyzed Stereoselective and Protecting-group-free Synthesis of 1,2-<i>cis</i>-Glycosides Using 4,6-Dimethoxy-1,3,5-triazin-2-yl Glycosides as Glycosyl Donors
4,6-Dimethoxy-1,3,5-triazin-2-yl glycosides, glycosyl donors prepared in one step from free saccharides without protection of the hydroxy groups, were stereoselectively and equivalently converted t...
Enzymatic Synthesis of Alkyl and Hydroxyalkyl β-D-Mannopyranosides
作者:N. Taubken、J. Thiem
DOI:10.1055/s-1992-26148
日期:——
The preparation of anomerically pure alkyl and hydroxyalkyl ß-mannopyranosides 3a-h by using the transfer activity of ß-mannohydrolase [EC 3.2.1.25] is described.
Efficient glycosylation of unprotected sugars using sulfamic acid: A mild eco-friendly catalyst
作者:Goutam Guchhait、Anup Kumar Misra
DOI:10.1016/j.catcom.2011.07.016
日期:2011.10
Sulfamic acid, a mild and environmentally benign catalyst has been successfully used in the Fischer glycosylation of unprotected sugars for the preparation alkyl glycosides. A diverse range of aliphatic alcohols have been used to prepare a series of alkyl glycosides in good to excellent yield. (C) 2011 Elsevier B.V. All rights reserved.
Organocatalyzed Direct Glycosylation of Unprotected and Unactivated Carbohydrates
作者:Sebastian Schmalisch、Rainer Mahrwald
DOI:10.1021/ol402914v
日期:2013.11.15
Organocatalyzed direct glycosylation of unprotected and unactivated carbohydrates is reported. This process is catalyzed by triphenylphosphine and tetrabromomethane at room temperature under neutral conditions. With this operationally simple protocol thermodynamically favored, glycosides were obtained in a very straightforward reaction.