Heteroatoms, distribution

Current work is focused on the benzoheteropines with the fused pyrrole (or indole), thiophene or furan rings, i.e., ortho-fused 6 + 7 + 5 ring systems with carbons only on the six-membered ring, one heteroatom on the five-membered ring and one or more heteroatoms on the seven-membered ring. The variety of heteroatoms is limited to nitrogen, oxygen and sulfur. Several examples of the related cyclic systems with the other heteroatom distribution or peri-fusion are briefly summarized in Section 4.3. The current first specialized review covers synthetic, reactivity and structural aspects reported from the late 1989 until 2007. 

A linear relationship is often observed between vanadium removal and sulfur removal, whereas the relationship between nickel and sulfur removal is linear but of smaller slope (Massagutov et al., 1967). For asphaltene-containing stocks, this phenomenon is interpreted on the basis of heteroatom distribution within the asphaltene micelles (Beuther and Schmid, 1963). Sulfur and vanadium are concentrated on the exterior, whereas nickel is concentrated in the interior. Conversion of the asphaltene generally leads to simultaneous removal of sulfur and vanadium, whereas nickel removal is more difficult. 

Temperature affects heteroatom distribution among char, liquid, and gas... 

It is evident that all described privileged structures are heterocyclic scaffolds with various ring sizes, heteroatom distributions, and substitution patterns. The synthesis of these scaffolds was considered to be achievable by means of solid-phase chemistry, thus yielding high-dimensional libraries. No emphasis is laid on whether the template itself contains pharmacophoric elements or is just a scaffold with... 

Inhomogeneous Al- (or ether heteroatom) distributions. (I.e., either with most Al in or near the outside surface to increase activity and life, or most Al in the center core to increase reactant or product type shape selectivities.)... 

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. 

The [5,5]-fused bicyclic ring system with one fusion nitrogen atom and three additional heteroatoms in a 2 1 distribution over both ffve-membered rings (2N1) is the origin of a great number of structures. The additional heteroatoms are mainly nitrogen, oxygen, or sulfur, and less commonly phosphorus or silicon atoms. 

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