Ring structures six-membered

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]. 

Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

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. 

The process of lactonization and enolization of 2-keto esters under the influence of alkaline reagents has also been applied to the production of analogs of L-ascorbic acid containing a six-membered ring structure.22 For example, methyl 3,4,6-trimethyl-2-keto-D-gluconate (XLI) is treated... 

Figure 9.7 Six- and five-membered cyclic ethers. The stable ring structures which are adopted by hexoses and pentoses are five- or six-membered and contain an oxygen atom. They are named as derivatives of furan or pyran, which are the simplest organic compounds with similar ring structures, e.g. glucofuranose or glucopyranose for five-or six-membered ring structures of glucose respectively.

Extension of this condensation to mixed ketones (95) provided further confirmation for the above interpretation. Thus, methyl ethyl ketone and p-chlorophenylbiguanide gave successively the triazine derivatives (CXXVIII) and (CXXIX). The latter (CXXIX) was resolvable into its enantiomers by (+)-tartaric acid this observation supports the six-membered ring structure of (CXXIX) and, because of the very gentle conditions of its isomerisation, that of its isomer (CXXVIII). 

When hexafluoroacetone is treated with thietane (2 1) a concerted addition results in the formation of a six-membered ring (Eq. 51). Here the excited state 205 seems to favor the formation of the six-membered ring structure 207 over the corresponding thietane derivative 206. 

Pyrimidine. A heterocyclic six-member ring structure. Cytosine and uracil are the main pyrimidines found in RNA, and cytosine and thymine are the main pyrimidines found in DNA. 

Ab initio study of the potential energy surface for P—C bond rotation in the 2-oxo-and 2-thioxo-2-methyl-l,3,2-diazaphosphorinane and -1,3,2-diazaphosphosolidine anions has established that in each case the lowest energy six-membered ring structure features a near planar carbanion with its substituent parallel to the P=X axis (X = 0, S).78... 

A kinetic study of structural effects on the A-nitrosation of amino acids by nitrite in aqueous solution has established that the dominant term in the rate equation corresponds to nitrosation by dinitrogen trioxide.189 Nitrosation by intramolecular migration of the nitroso group from an initially nitrosated carboxylate group can compete when the transition state has a five- or six-membered ring structure. Nitrosation of A-methyl-4-tolylsulfonylguanidine involves rapid nitrosation of the N-... 

A careful 11 NMR study, together with the results of the accompanying molecular modeling calculations, predicted the most stable conformation of piperazine-l,4-bis(N-methylaceto-hydroxamic acid) 123 (cf. Scheme 44) (97JCS(P2)1977) the hydroxamate moities of the side chains interact with the nearby amino function via hydrogen bonded six-membered ring structures, tilted upward and downward . 

According to Bailey et al., 6 the ozonization of naphthalene in methanol proceeds with participation of the solvent, and yields the crystalline cyclic peroxide (135), which has a six-membered ring structure. This peroxide results from the intramolecular stabilization... 

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