130 Purines
A related reaction with an alkyl nitrite produces purinyl radicals, which are effi ciently converted into halopurines by
abstraction of halogen from poly-halogenated solvents.
Ring synthesis – disconnections
Most purine syntheses involve cyclisation reactions of 4,5-diaminopyrimidines or of derivatives of 5-aminoimidazole-
4-carboxylic acid.
Synthesis of purines from 4,5-diaminopyrimidines
(7,8- and 8,9-bonds made)
8-Unsubstituted purines can be prepared simply by heating 4,5-diaminopyrimidines with a formylating agent, usually
formamide. The reaction involves fi rstly N-formylation of the amine and then in situ dehydration–cyclisation of the
resulting formamide. This is the classical Traube synthesis of purines.
8-Substituted purines can be prepared via N-acylation with higher acids. In some simple cases, in situ cyclisation occurs
but usually a two-step sequence is required in which an amide is formed fi rst.