Glycosides cleavage

Description glycosidic cleavage to form an oxonium ion charge retained on nonreducing end positive-ion mode only often referred to as A -type cleavage, because of similarity to one of the cleavages seen in electron impact-mass spectrometry. 

Description glycosidic cleavage with a hydrogen transfer charge retained on reducing end positive- and negative-ion modes often referred to as )S-cleavage. 

Transformations of Methyl 5-0-Benzyl-2-0-methyl-/3-I)-glueofuranosidurono-6,3-lae-tone (86) to Dimethyl (Z,E)-2-Methoxy-5-(phenylmethoxy)-2,4-hexadienedioatevl (87). ( Elimination employing DBU b oxidation with silver oxide-sodium hydroxide followed by diazomethane esterification c acidic glycoside cleavage, oxidation by dimethyl sulfoxide-acetic anhydride with formation of 5-0-benzyl-2-0-methyI-D-glucaro-1,4 6,3-dilactone, elimination by using DBU, followed by short treatment with diazomethane d elimination by DBU with subsequent diazomethane esterification e sodium borohydride in hexamethylphosphoric triamide 1 catalytic oxidation followed by short treatment with diazomethane " dimethyl sulfoxide-sulfur trioxide-pyridine-triethylamine.150)... 

Reaction of the carbonium ion with water could be reduced if overlap occurred with the carboxylate anion of aspartic acid-52 either during or after the glycoside-cleavage step. Since the carboxylate anion would be held adjacent to the carbonium ion in the active site, equilibrium should be far to the side of the acylal. Reaction of acylal with H2O would then very probably be ratedetermining in the forward direction. Evidence has been obtained that the solvent is directly involved in the hydrolysis of the cyclic acylal 2-(p-nitrophenoxy)phthalide where steric factors are similar... 

Fig. 1. Negative-ion MS/MS ESI—mass spectrum of batatin I (230). This technique provides an easily detectable [M — H] peak m/z 2761.61). All the characteristic ions resulting from glycosidic cleavage are clearly observed. The high-mass ion [M/2 - H] m/z 1379.79) corresponds to the ester cleavage of the dimeric structure. From Escalante-Sdnchez and Pereda-Miranda (2007), with permission of the American Chemical Society (7S)...

For many decades the pain relieving properties of willow bark extracts have been used in folk medicine (Fig. 1). Glycoside cleavage and oxidation is necessary for the biosynthesis of salicylic acid from the plant precursor p-D-glucopyranoside, the so-called saligenins. 

Give a mechanistic explanation for glycoside cleavage and formation of the pyrrole derivative. 

Perform glycosidic cleavage Prjmary fragments Perform cross-ring cleavage... 

Perform glycosidic cleavages Secondary fmnmante Perform glycosidic cleavages ... 

The second type is hydroxyl oxidation which can occur at the C6 (B), C2 (C), or C3 (D) position. The third type is a direct attack on the anomeric carbon (Cl) resulting in glycosidic cleavage or oxidation of the Cl position (E). The tendency of these oxidations depends substantially on the nature of the oxidants [222-231) and the hydroxyl configuration [84,219-221]. 

Similarly, addition of diethyl phosphite to alkyl analogs of the above nitrones gives, after glycoside cleavage and hydrolysis, phosphovaline (R = /-Pr) and phosphoalanine (R = CH3)43. 

Serving as the key step for an anthracydine synthesis, addition of the epoxytetrone dienophile (452) to diene (451) gave a mixture of adducts (453) and (454) in ratios of 4 1 (CsHe, 5 C) up to 7 1 (acetone, 5 °C) (Scheme 107). The major isomer (453), isolated by crystallization (74% yield from 451), was then converted into (+)-4-demeAoxydaunomycinone (641) (Scheme 142) via glycoside cleavage with 0.5N... 

Murray BW, Wittmann V, Burkart MD, Hung S-C, Wong C-H. Mechanism of human a-1,3-fucosyltransferase V glycosidic cleavage occurs prior to nucleophihc attack. Biochemistry 1997 36 823-831. 

Also related to the glycosidic cleavage, the ions differing by 2 daltons which appear at the expected incremental intervals of 204 daltons, which corresponds to the incremental mass of a permethylated glycosidic unit. These ion pairs are of considerable importance for sequence interpretation and their origin appears clear. However, the exact structures of these ions have not been clarified. 

Andrews CW, Fraser-Reid B, Bowen JP (1993) Involvement of ncr interactions in glycoside cleavage. In Thatcher GRJ (ed) The anomer-ic effect and associated stereoelectronic effects. ACS Symposium Series 539, American Chemical Society, Washington, DC, p 114... 

Glycosidic cleavages with hydrogen transfer, charge retained on the reducing end (upper) or non-reducing end (lower) sometimes called -cleavages. In the positive mode, the ions may be protonated as shown or sodiated... 

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