Ring structure formation

This type of ring-structure formation is referred to as chelation and is manifested by what are called chelating compounds. Sequestering agents... 

By virtue of their fused /3-lactam-thiazolidine ring structure, the penicillins behave as acylating agents of a reactivity comparable to carboxylic acid anhydrides (see Section 5.11.2.1). This reactivity is responsible for many of the properties of the penicillins, e.g. difficult isolation due to hydrolytic instability (B-49MI51102), antibacterial activity due to irreversible transpeptidase inhibition (Section 5.11.5.1), and antigen formation via reaction with protein molecules. 

The steroid ring structure is complex and contains many chiral carbons (for example at positions 5, 8, 9,10,13,14 and 17) thus many optical isomers are possible. (The actual number of optical isomers is given by 2" where n = the number of chiral carbons). From your knowledge of biochemistry you should have realised that only one of these optical isomers is likely to be biologically active. Synthesis of such a complex chemical structure to produce a single isomeric form is extremely difficult, especially when it is realised that many chemical reactions lead to the formation of racemic mixtures. Thus, for complete chemical synthesis, we must anticipate that... 

Cyclic structures can form as a result of side reactions. One of the most common examples is the formation of diketopiperazines during the coupling of the third amino acid onto the peptide chain (Fig. 7). Intramolecular amide bond formation gives rise to a cyclic dipeptide of a six-membered ring structure, causing losses to the sequence and regeneration of the hydroxyl sites on the resin. The nucleophilic group on the resin can lead to fiuther unwanted reactions [14]. 

The formation of a stable hydrogen-bonded ring structure as in poly(itaconic acid) and in poly(maleic acid) has also been shown to affect hydration states (Muto, Komatsu Nakagawa, 1973 Muto, 1974). 

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

A number of metal complexes have been prepared that have a [3.3.0] ring structure and phosphorus at the bridgehead. Schmidpeter et al. have reported the formation of complexes 129 and 130 <1996PS(118)129>. An extensive study of the reaction between activated alkynes and diphosphamanganacyclopropanes resulted in the... 

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