A-Glycosyl 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. 

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... 

OL-Glycosyl esters.1 The anomeric lithium alkoxide of 2,3,4,5-tetra-O-benzyl-D-glucopyranose (2) reacts with 1 to form mainly an a-glycosyl ester (3, a/p = 16 1). This reaction is generally strongly a-selective, even with highly hindered... 

The mechanism of action is probably like that proposed for pancreatic alpha-amylase, which involves protonation of the glycosidic bond by an imidazolium ion of the enzyme, followed by an attack on the anomeric carbon atom by a C02 group of the enzyme, resulting in a glycosyl ester (/3 form of the D-glucose residue) which is then hydrolyzed by attack by water at the anomeric carbon atom.9... 

The mechanism of the action of beta-amylase involves interaction of the starch with imidazole and carboxyl groups of the enzyme, resulting in a glycosyl ester intermediate (fi form of the D-glucose residue) that is hydrolyzed by addition of water to the carboxyl group, thus releasing the maltose in the /3-anomeric configuration. 

The type of intermediate that is formed in the slow inhibition with D-gly-cals was identified, with the aid of the ) -D-glucosidase A3 from Asp. wentii, as an ester of 2-deoxy-D-araA/ o-hexose with an aspartic acid side-chain. The same aspartoyl residue had already been shown, by labeling with con-duritol B epoxide (see Section 111,1), to be essential for -D-glucoside hydrolysis. In addition, this aspartate was found to form a glycosyl -enzyme... 

The hydrolysis of zeaxanthin esters by a carboxyl ester lipase indeed enhanced both the incorporation of zeaxanthin in the micellar phase and uptake of zeaxanthin by Caco-2 cells. As mentioned earher, carotenoids can also be linked to proteins by specific bindings in nature and these carotenoid-protein complexes may slow the digestion process and thus make their assimilation by the human body more difficult than the assimilation of free carotenoids. Anthocyanins are usually found in a glycosylated form that can be acetylated and the linked sugars are mostly glucose, galactose, rhamnose, and arabinose. 

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

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,... 

Scheme 13 Glycosylation reaction of a mannuronate ester donor.

Acting on ester bonds 3.2.2 Hydrolysing A-glycosyl compounds... 

Chiacchio et al. (43,44) investigated the synthesis of isoxazolidinylthymines by the use of various C-functionalized chiral nitrones in order to enforce enantioselec-tion in their cycloaddition reactions with vinyl acetate (Scheme 1.3). They found, as in the work of Merino et al. (40), that asymmetric induction is at best partial with dipoles whose chiral auxiliary does not maintain a fixed geometry and so cannot completely direct the addition to the nitrone. After poor results with menthol ester-and methyl lactate-based nitrones, they were able to prepare and separate isoxazo-lidine 8a and its diastereomer 8b in near quantitative yield using the A-glycosyl... 

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. 

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. 

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... 

---