Pyrimidine substituted , complexes

Most of these studies relate to derivatives of one ring system triazolo[l,5-tf]pyrimidine. 13C and 1SN chemical shifts of some 5,7-disubstituted [l,2,4]triazolo[l,5- ]pyrimidines substituted in the pyrimidine ring (and also some of their Au(m) chloride complexes) were published by Sztyk et al. <2002MRC529> the data are shown in Table 1. 

An early approach to suitable dialdehydes made use of a nucleophilic aromatic substitution process which enabled the synthesis of nitro-substituted complexes to be achieved (Scheme 27)159,160 Yery recently, a similar approach has been used to incorporate pyrimidine rings into macrocyclic complex structures (Scheme 28).161... 

The binuclear and tetranuclear jx-carboxylato complexes are remarkably substitution-inert in acidic aqueous solutions (pH 5-0). The dinuclear structure has been established by single-crystal X-ray analyses in two instances.1819 In the case of the p.-pyrimidine-carboxylato complex, a symmetrical p.-caiboxylato bridged group is found.19... 

A series of picolyl-functionalized NHC ligands attached to a cationic halfsandwich Ru core showed transfer hydrogenation chemistry that was dependent on both the N- and backbone substituents [21]. Thus, in the presence of PrOH/KOH at 82°C, conversion of acetophenone (C B S loading of 1 10 100) followed the order 6 > 9 > 7 8. The most active system (6) with R = Me/R = H proved capable of reducing a range of 4-halo-substituted acetophenones, alkyl ketones, as well as the imine, N-benzylideneaniline. Related efforts with an oxazoline-NHC Ru(arene) system [22], [Ru(p-cymene)(picolyl-NHC) Cl]", and the pyrimidine-NHC complex 10 produced less active catalysts [23,24]. 

A mesoporous SBA-15-supported pyrimidine-substituted NHC-Ir " complex 132 was successfully applied to the N-alkylation of amines with alcohols. 

Examination of the pyrazino[2,3-rf]pyrimidine structure of pteridines reveals two principal pathways for the synthesis of this ring system, namely fusion of a pyrazine ring to a pyrimidine derivative, and annelation of a pyrimidine ring to a suitably substituted pyrazine derivative (equation 76). Since pyrimidines are more easily accessible the former pathway is of major importance. Less important methods include degradations of more complex substances and ring transformations of structurally related bicyclic nitrogen heterocycles. 

PNA targeting of duplex DNA is not limited to homopurine sequences. Under special circumstances (high negative superhelical stress) mixed purine-pyrimidine PNA-peptide conjugates can bind by duplex invasion (Fig. 4.7) [31], but such complexes are of limited stability. However, using a set of pseudo-complementary PNAs containing diaminopurine-thiouracil substitutions, very stable double duplex invasion complexes can be formed (Fig. 4.4) and the only sequence requirement is about 50% AT content. Very recently, it was also demonstrated that reasonably stable helix invasion complexes can be obtained with tail-clamp PNA comprising a short (>six bases) homopyrimidine bis-PNA clamp and a mixed sequence tail extension [32] (Fig. 4.7). 

The availability of different metal ion binding sites in 9-substituted purine and pyrimidine nucleobases and their model compounds has been recently reviewed by Lippert [7]. The distribution of metal ions between various donor atoms depends on the basicity of the donor atom, steric factors, interligand interactions, and on the nature of the metal. Under appropriate reaction conditions most of the heteroatoms in purine and pyrimidine moieties are capable of binding Pt(II) or Pt(IV) [7]. In addition, platinum binding also to the carbon atoms (e.g. to C5 in 1,3-dimethyluracil) has been established [22]. However, the strong preference of platinum coordination to the N7 and N1 sites in purine bases and to the N3 site in pyrimidine bases cannot completely be explained by the negative molecular electrostatic potential associated with these sites [23], Other factors, such as kinetics of various binding modes and steric factors, appear to play an important role in the complexation reactions of platinum compounds. 

Platination of the N3 position in 1-substituted uracil and thymine derivatives requires proton abstraction and usually occurs only at high pH, but the Pt-N3 bond, once formed, is thermodynamically stable (log K 9.6) [7]. Platinum binding to N3 increases the basicity of 04, which becomes an additional binding site leading to di- and trinuclear complexes. A list of X-ray structurally characterized species is given by Lippert [7]. Pt complexes of uracil and thymine can form intensely colored adducts (e.g. platinum pyrimidine blues), which show anticar-cinogenic activity analogously to the monomeric species [7]. 

Two independent papers have reported the synthesis of nitrogen-heterocycle complexes of the type [RhCl3(py-X)3] (py-X = 3-Etpy, 3-CNpy, 4-Etpy, or 4-CNpy) and rr(ans-[RhY2L4] (Y = Cl or Br L = several substituted pyridines, isoquinoline, pyrimidine, pyrazole, thiazole, and substituted imidazoles). All the compounds were prepared catalytically by boiling RhCl3.3H20 with ethanolic solutions of L. It is interesting that 2-substituted... 

A similar chemistry to that described for (substituted) pyridine rhenium(V) oxo complexes is also observed for other heterocyclic nitrogen donor ligands such as pyrimidine, pyrazine, substituted imidazoles, benzimidazoles, or benzotriazole. ... 

Reaction of hydroxyalkylbiuret derivatives 16 and diethyl carbonate yields 1-(hydroxyalkyl)-133-triazin-2,4,6-triones 17, which lead to the corresponding 1-(carboxyalkyl)-13,5-triazin-2,4,6-triones 18. Triazinones 18 afford 1 1 complexes with a series of aminoalkyl substituted pyrimidine-2,4,6-triamines, which form hydrogen-bonded molecular ribbons <99CEJ381>. 

The substitution of pteridines at positions adjacent to the pyridine-like nitrogen atoms in either the pyrimidine or the pyrazine is a well-established synthetic procedure and remains an important contributor to the synthesis of complex substituted pteridines. Significant extensions of these methods have been described at both the pyrimidine and pyrazine rings. 

Despite all of the activity in pyrimidine-based synthesis, only one study has emerged of solid-phase versions of these reactions <2003TL1267, 20030BC1909>. This chemistry was based upon condensation of dicarbonyl compounds with resin-bound pyrimidine-5,6-diamines through a 2-alkylthio link and oxidative cleavage as described in Section 10.18.7.2. The value of alkylthio substituents in the synthesis of complex substituted pterins has also been demonstrated in the synthesis of nucleic acid conjugates <2004OBC3588> (see Section 10.18.12.4). 

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