Reactions between adjacent groups

The most common approach to the synthesis of a fused pyrimidine ring involves condensation reactions between adjacent carboalkoxy and amino groups. Therefore, it is not surprising that the majority of pyrrolo[3,2-4]pyrimidines that involve a pyrrole precursor follow this pathway. 

Multiple ring-closure reactions between adjacent urea functions of a suitably functionalized tetra-urea in a heterodimer with a second tetra-urea bearing bulky groups can also create the structural elements of a rotaxane. This strategy is usually called clipping (see Scheme 5.7). 

One instance where correlation with cuprammonium data appears possible is the reaction between adjacent cis glycol groups and acetone to form isopropylidene derivatives. Although data on the kinetics and equilibria of isopropylidene formation are too meager to prove that reactivity with acetone parallels reactivity with cuprammonium, isolated observations support such a supposition. Knauf, Hann, and Hudson12 have noted the ease of formation of the acetone derivatives of D-mannosan, and this is one of the substances most reactive toward cuprammonium.6 Tipson13 has noted that the optically active 2,3-butanediols react with acetone more readily than the meso form, and this is the order in which these substances react with cuprammonium.14... 

In narrow pore silicas water is eliminated by reaction between hydroxyl groups on adjacent sites (Figure 6.34). No reverse reaction is possible and there is an effective loss of surface area due to the cross-linking effect. 

The first type indudes the reaction 4iidi takes jdace ped< nnantly between adjacent groups. Those are, for examfde, the stripi of dilorine Ity Zn from pdyfvinyl chloride), aldole condensation of poly(niethyi inityl ketone) and so The... 

Coleman and Petcavich (1978) [192] used FTIR to propose a route (Scheme XIX) via the initial formation of the Grassie ladder, followed by a tautomeric change to the polycyclic dihydropyridine structure. Oxidation followed, producing carbonyl groups, a prerequisite for the subsequent condensation reaction between adjacent ladder formations to give a final structure proposed by Potter and Scott and more fully reported by... 

There are numerous examples of chemical reactions consequent upon chemical groups which occur repeatedly along a chain. In some cases the reaction occurs randomly between adjacent pairs of groups such as in the reaction between aldehydes and polyvinyl alcohol and of zinc dust with polyvinyl chloride ... 

Scheme 1 Reaction between two adjacent sUanol groups interacting via H-bonding (dashed line) on the silica surface leads to formation of strained siloxane bonds and molecular water...

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

Scheme 3) <05TL2189>. However, in this case the reaction did not afford the expected DA adduct, the product being the porphyrin derivative 10 resulting from the tetradehydrogenation of the corresponding adduct. The porphyrin derivative 11 was also obtained although in minor amount this product must result from a cyclization reaction between the beta-fused quinoxaline ring and the adjacent maso-aryl group. Also, bisadducts 12 and 13 were isolated these are the result of site specific bisaddition to opposite pyrrolic rings followed by oxidative processes.

The link between adjacent units or monomers in a polypeptide is formed in a condensation reaction between the amine group of one amino acid and the carboxylic acid group of another with the elimination of H20 molecules. These links are called peptide bonds. 

If a DNA adduct involves the nitrogen or oxygen atoms involved in base-pairing, and the adducted DNA is not repaired, base substitution can result. Adducts can be small, such as the simple addition of methyl or ethyl groups, or they can be very bulky, owing to reaction with multiringed structures. The most vulnerable base is guanine, which can form adducts at several of its atoms (e.g., N7, C8, O6 and exocyclic N2) (Venitt and Parry, 1984). Adducts can form links between adjacent bases on the same strand (intrastrand cross-links) and can form interstrand crosslinks between each strand of double-stranded DNA. 

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