Cyclic fragments

Biological function and chemistry of endothelin, vasoactive peptide with macro-cyclic fragments formed by disulfide bridges between cysteine residues 99CCC1211. 

Self-organization of polysiloxanes containing cyclic fragments into mono- and multilayer structures at interphase boundaries 99MI40. 

Thermolytic processes are so far known to play only a minor role in the generation of the PO ion. An isolated case is the thermolysis of the monosodium salt of acetonylphosphonic acid at 150 °C, which leads to acetone and sodium polymetaphosphate 107). A cyclic fragmentation mechanism, as known for P-ketocarboxylic acids, could lead to PO in this case. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

An analysis of the principal methods for construction of compounds with 1,2,5-oxadiazole heterocyclic units was published in CHEC(1984) and CHEC-II(1996) < 1984CHEC(6)393, 1996CHEC-II(4)229>. In this chapter only reactions that lead to the formation of 1,2,5-oxadiazole cyclic fragments are considered. The functionalization or replacement of substituents of heterocyclic ring as well as oxidation or deoxidation of nitrogen atoms are described in Section 5.05.4. 

Cyclic fragments apparently occur during the fragmentation of longer chain amines. 

The cyclic fragment at m/z = 459 probably reflects the network chain length of the crosslinked polymer. 

In another copper-mediated carbene transfer reaction, diazoester 222 has been decomposed in the presence of bis(triethylsilyl- or -germyl)mercury (equation 72) it was assumed that the obtained ketenes 223 result from the insertion of ethoxycarbonyl(trimethylsilyl)carbene into a Hg-element bond followed by a cyclic fragmentation process110. 

Most substituted imidazoles are obtained via the condensation of non-cyclic fragments to form the desired imidazole. The majority of these condensation reactions has recently been reviewed by Grimmett8. The largest number of imidazoles obtained via condensation approaches is only of limited use for the development of histaminergic ligands as they are mainly polysubstituted imidazoles. For the histamine receptors proper substitution of the imidazole ring is usually limited to the 4(5)-position, with the exception of the histamine H]-receptor, where a broader range of substituents on the 2-position is tolerated. 

Amine or cyclic fragments. A class of compounds used as catalysts or curatives in polyurethane elastomer and foam reactions. Amines are characterized by having N, NH, or NH2 groups in the molecule. 

Figure 4. Potential curve of passage of ion H30+ through a cyclic fragment (OglU)-...

Since thirteen of the possible fourteen ring systems, and three of the possible seven benzo fused systems are known, many in several oxidation states and with a wide range of substituents, it is only possible in this section to discuss the more general approaches to synthesis of the rings. Assembly of the rings will be described from non-cyclic fragments... 

Thermogravimetric studies have displayed that the cyclic fragment causes a considerable effect on carbosiloxane copolymer at n= 2 only, and at n=23 no effect of cyclic fragment on the glass transition temperature of the copolymer is observed. Figure 5 shows dependence of Tg on the length of dimethylsiloxane unit for cyclolinear carbosiloxane copolymers. 

It has been found that expansion of the cyclic fragment volume at the same length of dimethylsiloxane unit, i.e. introduction of a single diphenylsiloxane unit, Tg of the copolymer is increased by 10°C. It is also shown that the effect of 1,7- or 1,5-disposition of cyclic hexasiloxane fragment on Tg of the copolymer is negligible, which conform to the previous results on pure siloxane copolymers [22],... 

Thus, the increase of cyclic fragments concentration in the linear chain induces rise of macromolecular chain rigidity and leads to formation of a one-phase system. The increase of cyclic fragments concentration in the macromolecular chain raises thermal oxidative stability of the copolymers. 

Thermogravimetric studies of copolymers have shown that at 300°C mass losses do not exceed 3 - 6%, but are regularly increased with temperature. The main destruction process proceeds in the tem-perature range of 450 - 700°C and final mass losses increase with the volume of the cyclic fragment, respectively. 

Table 13 shows some properties of carbosiloxane co-polymers with cyclic fragments in the back-bone. 

The copolymers were thermogravimetrically studied with the help of Seteram Co. thermoweighing machine B-60 in argon (at the heating rate of 5 deg/min) with simultaneous selection and analysis of gaseous degradation products, and the effect of introduction of bulky cyclic fragments into the back-bone on their thermal stability was traced. 

Thermomechanical studies have indicated that the glass transition temperature of synthesized copo-lymers is decreased as the volume of cyclosiloxane ring in the chain is increased. The results of X-ray structural analysis indicate that the copolymers represent amorphous systems, and increase of cyclic fragment volume leads to an insignificant increase of the interchain distance. 

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