Glycosyl imidazole groups

In contrast to the synthesis of purines, the synthesis of purine nucleosides from glycosylated pyrimidinediamines is less common than from glycosylated imidazoles. The synthesis protocol follows that of simple purines. A one-carbon unit is introduced into a 4-(glycosylamino)-pyrimidin-5-amine by selective thioformylalion of the 5-amino group with subsequent cycliza-tion of the 5-thioformamido compound under basic conditions, e.g. the sequence 4 5 -> 6. ... 

Soon thereafter, two groups (Lemieux et al. [51] and Paulsen et al. [55]) independently concluded that the glycosyl imidazoles (Fig. 2.21) would be more suitable substrates for these studies than pyridinium glycosides, since the protonation of an imidazole ring is not expected to significantly change its size, and... 

Imidazole-HCl soluble polysaccharides Most of the polysaccharides of this group didn t elute from the Q-Sepharose column with 10 mM buffer (Table 3). The glycosyl composition (Table 4) shows that these polysaccharides are homogalacturonans and ramnogalacturonans with... 

P212121 Z — 8 Dx= 1.57 R = 0.085 for 1,743 intensities. The two independent molecules have similar conformations. The glycosyl dispositions are anti (90.1°, 91.2°), and the D-ribosyl groups are 3T4 (24.0°, 34.1° 15.6°, 35.5°). The exocyclic, C-4 -C-5 bond orientations are gauche+ (63.1°, 53.8°). The orientation of the methyl groups in both molecules is such that it is directed away from the imidazole moiety of the base, that is, the 0-6-C-7 bond is trans to the C-5-C-6 bond this arrangement constitutes an obstacle to formation of Watson-Crick hydrogen-bonds to the complementary base cytosine. In molecule A, 0-6 and C-7 are displaced from the purine plane by 79 and 87 pm, and, in molecule B, by 49 and 16 pm. The bases are stacked. 

Several methods are available to access glycosyl iodides (Scheme 2.50). Anomeric hemiacetals bearing diverse protecting groups (Bn, Bz, Ac, N3, CMe2) upon treatment with a polymer-bound triphenylphosphine-iodine complex and imidazole can be converted into a-glycosyl iodides [179]. The precipitated by-products,... 

Approximately 150 different amino acid residues have been reported in proteins (1 5). At least half of these could undergo chemical deteriorations under the conditions of stress usually encountered. Many of these deteriorative reactions involve hydrolytic scissions, not only of peptide bonds but of the many different nonprotein substances added covalently to proteins postribosomally. These susceptible side chain groups are indole, phenoxy, thioether, amino, imidazole, sulfhydryl, and derivatives of serine and threonine (such as 0-glycosyl or O-phosphoryl), the disulfides of cystine, and, of course, the amides (such as asparagine and glutamine). With strong acid or alkali, other residues, such as serine and threonine, also are less stable. 

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. 

P2j Z = 2 Dx = 1.536 R = 0.065 for 1,387 intensities. The conformation of the D-ribosyl group is 2Ti (153°), the glycosyl disposition is syn (—110°), and the exocyclic C-4 -C-5 bond torsion-angle is 62°. The nucleotides are stacked in head-to-tail fashion involving the pyrimidine rings, and the sulfur atom lies over the imidazole ring. 

This group was developed as a passive 0-2 protective group that could be removed in the presence of an acid sensitive target molecule after affecting an ot-selective glycosylation. It is introduced with the dichloride (DMF, imidazole, 1 h, 92% yield) and can be removed with BU4NF (THF, 20°C, 99% yield). 

Alkylthio groups are readily removed by the well-known, Raney niclml desulfurization procedure, which is particularly valuable in cases in which the initial presence of the alkylthio group is necessary in order to direct the entering glycosyl moiety to the desired imidazole nitrogen atom (N-9).i < i -> i- - ... 

Full details of the synthesis of adenine and imidazole nucleosides of d-mannofuranose from 2,3 5,6-di-O-isopropylidene-D-mannofuranosylamine have now appeared (see Vol. 11, p. 171). The same group has also synthesized the arabinofuranosyl 5-aminoimidazole-4-carboxylate (14) by condensation of 2,3,5-tri-O-benzyl a-D-arafemo-furanosyl chloride with the parent base followed by hydrogenation. 1,3-Dipolar cycloaddition of glycosyl azides to 1,4-naphthoquinone or 2-methyl-1,4-benzoquinone gave the nucleosides (15), (16), and (17), which were shown to exhibit some cytostatic activity... 

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