Glycosylation esters

If k2 > kj, the glycosyl-enzyme intermediate will accumulate, and may be trapped by the rapid denaturation of the enzyme in the presence of (saturating) amounts of substrate. With -glucoside Aj from Asp. wentii and 4-nitrophenyl [ C]-2-deoxy-) -D-irra />jo-hexopyranoside, it was possible to identify the intermediate as a glycosyl ester (acylal) of 2-deoxy-D-arabino-hexose bound to the same aspartate residue that had previously been labeled with the active-site-directed inhibitor conduritol B epoxide and with D-glucal." This constituted an important proof that the carboxylate reacting with the epoxide is directly involved in catalysis. 

Pfister, S. et al.. Isolation and structure elucidation of carotenoid-glycosyl esters in gardenia fruits Gardenia jasminoides Ellis) and saffron (Crocus sativus Linne), J. Agric. Food Chem., 44, 2612, 1996. 

Meyer, R, Riesen, R., and Pfander, H., Example 10 carotenoid glycosides and glycosyl esters, in Carotenoids Isolation and Analysis, lA, Britton, G., Liaaen-Jensen, S., Pfander, H., Eds., Birkhauser, Basel, 1995, 277. 

Masazumi M, Yoshiteru I, Susumu I, Junzo S. 3a-Hydroxy-oleanane-type triterpene glycosyl esters from leaves of Acanthopanax spinosus. Phytochemistry 1993 34 1599-1602. 

Figure 13.7 Glycosyl esters of trans and cis crocetin (A) (3 D glycosyl (B) f3 D gentiobiosyl (C) three (3 D glycosyl...

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

Scheme 3.17 Clycosylations with glycosyl ester donors.

Scheme 3.20 Silicon activation of glycosyl ester donors.

Glycosyl esters with remote functionality constitute a relatively new class of O-carbonyl glycosyl donors, which fulfill the prospect of mild and chemoselective activation protocols (Scheme 3.22). For example, Kobayashi and coworkers have developed a 2-pyridine carboxylate glycosyl donor 134 (Y = 2-pyridyl), which is activated by the coordination of metal Lewis acid (El+) to the Lewis basic pyridine nitrogen atom and ester carbonyl oxygen atom [324]. In the event, 2-pyridyl (carbonyl) donor 134 and the monosaccharide acceptor were treated with copper(II) triflate (2.2 equiv) in diethyl ether at —50 °C, providing the disaccharide 136 in 70% (a P,... 

With exception of the alcoholysis reaction the methods described earlier are all applicable to the synthesis of aryl glycosides, but, in particular, reactions of phenols with glycosyl esters in the presence of acidic catalysts are generally useful. 

The purpose of this Chapter is to summarize the present knowledge concerning the isolation, structure, preparation, and chemical reactions of glycosyl esters of nucleoside pyrophosphates. These aspects have been considered only briefly in the articles cited. Another important topic, which has not been treated previously in a comprehensive manner from a chemical viewpoint, is the mechanism of the enzymic reactions of these compounds, and the specificity of their interaction with the corresponding enzymes. These topics will also be considered here. 

II. Isolation, Characterization, and Elucidation of Structure of Natural Glycosyl Esters of Nucleoside Pyrophosphates... 

Dilute perchloric acid or trichloroacetic acid, or ethanol, is usually employed for extraction of the glycosyl esters of nucleoside pyrophosphates from biological materials.19 The high lability of these compounds in acidic media (see Section IV, p. 356) leads to unavoidable losses during extraction with acids. Extraction with ethanol can lead to difficulties, as ethanol may not completely inactivate pyrophosphatases present in the tissue the action of these enzymes may result in partial degradation of the nucleoside pyrophosphate derivatives. Such a situation has been encountered particularly with plant tissues.20... 

The isolation of an individual glycosyl ester of a nucleoside pyrophosphate is a rather difficult task. Ion-exchange column-chromatography and preparative, paper-chromatography have been the main methods used for solving problems in separation. 

The ion-exchange separation usually affords individual fractions of structurally related glycosyl esters of nucleoside pyrophosphates, containing the same nucleotide residue, but differing in the structure of the glycosyl groups. Separation of the esters of N-acetylhexos-amines, uronic acids, and neutral monosaccharides from one another is also usually achieved. 

Preparative, paper-chromatography is frequently used for further fractionation of the resulting mixtures. The high lability of glycosyl esters of nucleoside pyrophosphates seriously limits the choice of solvent systems. Systems used most commonly are neutral or slighdy acidic mixtures of ethanol with ammonium acetate,24,25 or weakly acidic solvents based on 2-methylpropionic acid.26 A solvent system containing morpholinium borate has also been found extremely useful.27... 

The unambiguous characterization of glycosyl esters of nucleoside pyrophosphates is difficult, as no suitable crystalline derivatives are known for them, and the hygroscopic nature of their ammonium and metal salts prevents reliable interpretation of the data of elementary analysis. 

Comparative identification of a glycosyl ester of a nucleoside pyrophosphate with an authentic sample is usually based on the identity of their ultraviolet spectra and by comparison of chromatographic mobilities of the samples and their degradation products. In addition to paper-chromatographic techniques, it may be of value to use paper... 

F. Eisenberg and A. H. Bolden, Anal. Biochem., 29, 284 (1969). The procedure used results in a degradation of the glycosyl esters of nucleoside pyrophosphates, but the pattern obtained seems to be characteristic. 

---