Blocking groups selectively removable

By reversibly blocking position 3 with a diphenylmethyl group (72, 73), Prystas and Sorm also prepared 6-azauridine. The diphenylmethyl group was removed after ribosidization (74) by selective hydrogenolysis. The crystalline 6-azauridine obtained was identical with 6-azauridine prepared enzymatically. ... 

The primary OH group can be selectively blocked by the bulky triphenyl-methyl (trityl) moiety, followed by esterification at the secondary OH groups and removal of the protecting trityl group. Thus 2,3-di-O-acetyl cellulose has been obtained by this procedure. Moreover, regioselectively substituted mixed cellulose esters, acetate/propionate, were prepared by subsequent acy-... 

The procedure requires the initial synthesis of the linear dipeptide precursor, bearing specific chemical blocking groups, capable of being selectively removed from the N- and C-terminals. ... 

Becker and Hurwitz 94) have found that after infection of E. coli B with T-even bacteriophages a novel 3 -deoxynucleotidase activity appears. They purified the enzyme 2000-fold. In addition to its attack on 3 -deoxymononucleotides, the enzyme selectively removes the 3 -phos-phoryl groups from DNA. It does not attack 3 -ribonucleotides, 3 -phosphoryl groups of RNA, or 5 -phosphate esters. Like bacterial 5 -nucleotidases, this enzyme is markedly activated by Mg2+ and Co2+ and is inhibited by EDTA. The enzyme appears to be a phage-induced enzyme the activity rises early after injection with T-even phages and formation of the enzyme is blocked with chloramphenicol. 

In contrast with the usual complete methylation, a selective methyla-tion of certain hydroxyl groups has been accomplished by direct reaction (as differing from methods requiring blocking groups and their subsequent removal) in a limited number of cases. Monomeric sugar derivatives will be considered first, to be followed by polysaccharides, since with the latter there are factors involved which introduce complications. 

When bromide 14, in molar excess, was reacted with the selectively blocked disaccharide 9 in dichloromethane in the presence of mercury(II) cyanide as catalyst and 4A molecular sieve as the acid acceptor, a 90% yield of the fully blocked tetrasaccharide was obtained after column chromatography. Removal of blocking groups was accomplished by the hydrogenolysls of the benzyl ether and benzylidene acetal groups In acetic acid, followed by transesterification. The deblocked tetrasaccharide 19 had NMR parameters in agreement with its structure. This could be confirmed by comparison with chemical shift data for the... 

The peptides 3 were prepared by condensation of phtaloyl,N-carbobenzoxy,N-t-butoxycarbonyl or N-fomyl-amino acids with 1-aminoalkanephosphonic acids as well as with their dialkyl or diphenyl esters /Scheme/. Special attention was payed on theefectiveness of the peptide bond formation and on the methods for selective and total removal of the blocking groups. 

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