Heterocyclics purines

Just as proteins are biopolymers made of amino acids, nucleic acids are biopolv-mers made of nucleotides joined together to form a long chain. Each nucleotide is composed of a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar linked through its anomeric carbon to the nitrogen atom of a heterocyclic purine or pyrimidine base. 

Nucleoside (Section 28.1) A nucleic add constituent, consisting of a sugar residue bonded to a heterocyclic purine or pyrimidine base. 

Unlike common five and six membered heterocycles, purines rarely undergo coupling reactions including the insertion of an olefin or carbon monoxide. This behaviour is not well understood since Heck and CO insertion reactions are known to proceed on similar systems. 

The four bases commonly found in RNA are divided into two classes The mono-cyclic compounds cytosine and uracil are called pyrimidine bases because they resemble substituted pyrimidines, and the bicyclic compounds adenine and guanine are called purine bases because they resemble the bicyclic heterocycle purine (Section 16-9C). 

Michael additions of heteroatoms can be coupled to reactions of the specific enolate produced. The synthesis of the modified carbonucleoside (-)-neoplanocin A 92 required some cyclopentane to which the heterocyclic purine could be joined. The decision was taken to close the cyclopentane by an aldol reaction. This required an aldehyde and an ester in a compound such as 94 to combine regio- and stereo-selectively without epimerisation of any chiral centres. The aldehyde is enolis-able so there are serious problems.33... 

Derivatives of the aromatic heterocycle purine are constituents of DNA and RNf Why is purine aromatic How many p electrons does each nitrogen donate to tk aromatic ir system (See Section 28.7.)... 

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