Fused pyrimidines, five-membered-ring

The nitrogen-containing bases that occur in DNA and RNA fall into two structural categories the purines and the pyrimidines. The former contain a five-membered ring fused to a six-membered ring, while the latter contain a six-membered ring only. The two purines are common to both DNA and RNA adenine (A) and guanine (G). The pyrimidine cytosine (C) occurs in both DNA and RNA. The other pyrimidine is thymine (T) in DNA but uracil (U) in RNA. 

Ring Synthesis of Fused Pyridazines, Pyrimidines, and Pyrazines with Oxygen, Sulfur, Selenium, and Phosphorus Atoms in the Five-Membered Ring... 

There are several bicyclic compounds with five-membered heterocycles fused to pyridine rings, where metalation occurs in the smaller ring due to activation by the heteroatom. The ring-A lithiation of imidazopyridines and pyrimidines was discussed in Section II,E,6, but in addition, the a-lithio derivatives of thieno[2,3-6]pyridine 119 (74JHC355), thieno[3,2-... 

Synthesis of the Fused Five-Membered Ring [1,2,5]Thiadiazolo[3,4-d]pyrimidine 699... 

When 1,2,5-thia- and selena-diazolo[3,4- pyrimidine-5,7-dione W-oxides are treated with sodium hypochlorite, the respective S or Se atom is replaced by O to give 1,2,5-oxadiazoles, as shown in Scheme 82 (precise yields not given) <2000CHE1359>. A more vigorous oxidation leads to ring-opening of the fused five-membered ring (see Section 10.13.5.4). 

Ring synthesis from a fused pyrimidine with a five-membered ring... 

In Scheme 1, all the ring systems belonging to this chapter are depicted. The six-membered rings can be oxazine, thiazine, pyridazine, pyrazine, and pyrimidine, whereas the five-membered fused ring can contain oxygen, sulfur, selenium, phosphoms, and nitrogen atoms in various arrangements. 

A number of pyrimidines fused to five- or six-membered rings have been degraded to substituted pyrimidines and many of these reactions have been summarized elsewhere <1994HC(52)1>. A recent example used Raney nickel to remove a sulfur atom from the imidazo[4, 5 4,5]thieno[3,2-r/]pyrimidinone 966 <20010L3209, 2002JOC3365>. 

The synthesis of fused 1,2,3-triazoles is nearly always achieved by treating 1,2-diamines attached to diazines with nitrous acid (e.g., [l,2,3]triazolo[4,5-f]pyridazine (Section 10.13.9.2.1(iii)) and [l,2,3]triazolo[4,5-rf pyrimidine (Section 10.13.9.2.1(iv)), <1996CHEC-II(7)489>). When a heterocyclic system containing a diazole fused onto 1,2,4-triazine is required, annulation of the five-membered heterocycle is nearly always the most facile route (e.g., imidazo[4,5-r ][l,2,4]triazine (Section 10.13.9.2.l(i)) and pyrazolo[4,3-r ][l,2,4]triazine (Section 10.13.9.2.1(ii))). In support of the latter, the synthesis of the fused six-membered ring of pyrazolo[3,4-r ][l,2,4]triazine from the pyrazo-ledione was reported as low yielding <1996CHEC-II(7)489>. 

Indole contains a pyrrole ring fused with a benzene ring. Compounds derived from indole include the essential amino acid L-tryptophan (Section 27.1C) and the neurotransmitter serotonin. Purine contains a six-membered pyrimidine ring fused with a five-membered imidazole ring. Caffeine is a trimethyl derivative of an oxidized purine. Compounds derived from purine and pyrimidine are building blocks of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA, Chapter 28). 

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