124 Adenosine, silyl derivatives

Replacement of the chlorine by the purine or pyrimidine base is sometimes quite tricky and silver or silyl derivatives are often used. Lewis acid catalysis is necessary to help the chloride ion leave in this S l reaction. We shall avoid detailed technical discussion and simply draw the adenosine prod-... 

Examples of amination of oxopurines to aminopurines via silyl derivatives include conversion of inosine into adenosine with catalysis by mercury ions (76LA745), and hypoxan-thines have been converted directly into adenines with phosphoramides at 225-230 °C (69IZV655). 

One of the better processes for the conversion of adenine to adenosine involves the initial conversion of adenine to the corresponding N -benzoyl derivative with benzoyl chloride in pyridine. Then, as shown in Scheme 12.100, silylation with trimethylchlorosilane in hexamethyldisilazane [(CH3)3Si]2NH solvent at reflux yielded a silyl derivative that was heated in 1,2-dichloroethane (CICH2CH2CI) at reflux with trimethylsilyl perchlorate [(0113)3810104] and l-0-acetyl-23,5-tri-0-benzoyl-P-D-ribofuranose for 12 h. The resulting crude, protected adenosine was converted to unprotected material by standing at room temperature with methano-lic ammonia followed by recrystaflization from aqueous methanol. 

High yields of 2,3,5 -tri-O-acetyl- and -O-benzoyl-adenosine have been obtained from the corresponding 1-0-acetyl- -D-ribofuranose derivative by direct reaction with unprotected adenine in acetonitrile in presence of stannic chloride. Ribosylation of the silylated imidazolethione (7) with the glycosyl chloride (8) in presence of stannic chloride or silver perchlorate gave mainly the diribosyl derivative (9) rather than the expected mono-ribosyl nucleoside (10) (2 1 ratio), whereas use of the tetra-acetyl sugar (11) gave mainly (10). ... 

Silyl Ethers Attempted preparation of trimethylsilylmethyl ethers of sugar alcohols by their treatment with Me3SiCH2l and sodium hydride has led only to mixtures of trimethylsilyl ethers and methyl ethers. A series of papers reporting the synthesis and hydrolytic stability of a number of 3 ,5 -O-(dialkylsilanediyl) derivatives of uridine, adenosine and thymidine is discussed in Chapter 20. 

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