Pyrimidine bridge

As shown in Eigure 18.17, thiamine is composed of a substituted thiazole ring joined to a substituted pyrimidine by a methylene bridge. It is the precursor of thiamine pyrophosphate (TPP), a coenzyme involved in reactions of carbo-... 

Ribonucleic acid (RNA) is a polymer of purine and pyrimidine ribonucleotides finked together by 3, 5 -phosphodiester bridges analogous to those in DNA (Figure 35—6). Although sharing many features with DNA, RNA possesses several specific differences ... 

Nucleotides can be linked together into oligonucleotides through a phosphate bridge at the 5 position of one ribose unit and the 3 position of another. The purine bases, adenine and guanine, have two heterocyclic rings, while the pyrimidines cytosine, thymine, and uracil have one. The structure of adenosine monophosphate is shown in Figure 11. 

GG8, the radical cation must traverse five A/T base pairs. Electrochemical measurements in solution have shown that the purine bases (A and G) have considerably lower Eox than the pyrimidines (C and T), with the Eox of G estimated to be about 0.25 V below that of A [20]. It is not very likely that the Eox of bases in DNA will be the same as they are in solution, but it is generally assumed that the order of Eox will remain the same. Consequently, the radical cation at Gi of AQ-DNA(l) must traverse a bridge of five A bases to reach GG8. The process whereby the radical cation crosses such bridges has been a major point of debate in consideration of long distance radical cation migration mechanisms in DNA this issue will be discussed fully below. 

Lippert and co-workers have reported a similar series of A-,O-bridged platinum(III) dimers in which the bridging ligands are the pyrimidines, 1-methyluracil, 1-methylthymine, or 1-ethylthy-mine. Chemical oxidation of dimeric platinum(II) complexes gave [Pt2XY(L)2(NH3)4]2+, X,Y = N03", N02, H20, Cl", or Br", L=l-MeU, 1-MeT, 1-EtT. 27 t30 They too found that the HT dimers are more stable than the HH.430... 

Using a triazole-/pyrimidine-containing hybrid ligand, terminally coordinated in a thiocyanate-bridged copper(II) dimer, Haasnoot et al. reported250 the structure of the f3 isomer of complex (288), revealing that the Cu-SCN distance is relatively short compared to other Cu-SCN... 

The answer is a. (Hardman, p 1302.) Cyclophosphamide, an alkylating agent, reacts with purine and pyrimidine bases of DNA to form bridges and dimers. These products interfere with DNA replication. 5-FU, methotrexate, and 6-thioguanine are anti metabolites, and the steroid prednisone has some tumor-suppressive effects. 

The coordination properties of pyrimidine bases seem to be less versatile than those of purine derivatives. Various Pt(II) and Pt(IV) compounds, including cis- and rrans-DDP, preferentially bind to the N3 site in N1-substituted cytosine derivatives (Figure 7), as verified by a variety of methods [7]. Simultaneous binding to N3 and to the exocyclic amino group C(4)-NH2 upon loss of a proton has been observed in a bridged Pt(II) system and in a chelated Pt(IV) system [7]. With 1,3-di-methyluracil, Pt(II) coordination to the C5 atom has been ascertained by X-ray crystallography [22]. 

Thiamine contains a pyrimidine moiety (A) and a thiazole moiety (B), joined by a methylene bridge as shown in (I). Microorganisms which... 

Chupakhin et al. investigated the reactivity of some [l,2,4]triazolo[l,5- ]pyrimidines with resorcine (Scheme 17) <1999IZV1573>. These authors found that 147 reacts with resorcine to give at first the addition product 148 which undergoes an intramolecular cyclization to produce the bridged skeleton 149. 

Chupakhin and co-workers <1999IZV1573> reported the formation of a bridged polycycle starting from tetra-zolo[ 1.5-//]pyrimidine 1 (Scheme 12) this compound was treated with resorcinol in the presence of boron trifluoride etherate at room temperature to yield a 1,3-addition product on the pyrimidine ring 63 with participation of one... 

In summary, the sulfonylureas are new herbicides which exhibit activity at extremely low rates of application and show very low mammalian toxicity. Exceptionally high activity is shown by compounds containing a benzene ring substituted in the ortho position, an unsubstituted sulfonylurea bridge, and a pyrimidine or triazine heterocycle substituted with methyl or methoxy groups. 

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