Principle of cyclicity

Based on the principles of cyclic iteration, Weisz proposed the term transportation to define an amplification method in which an equivalent amount of a reagent is transported, by the compound to be amplified, from one place to another. 

Lindh U (ed) (1993) Nucl Instrum Methods B 77 1—547 Livingood J J (1961) Principles of cyclic partide accder-ators. D. Van Nostrand, Toronto/Princeton/ New York... 

Finally, Fd like to stress the dynamic aspects which are of utmost importance for an understanding of biochemical processes. Biochemical processes are essentially open in nature. This means that all enzymes are constantly, stochastically or periodically activated by substrates which are produced by the cellular environment or by precursor enzymes and transformed so that products are picked up by other enzymes within a reaction sequence. Biochemical processes are controlled by their cyclic design, by allosteric feedback and by electrochemical coupling. The discovery of the principle of cyclic processes induced the notion that the dynamic coupling of networks of integrated biochemical processes must be extremely complex and nonlinear. Indeed, today we observe a large variety of coupled dynamic states and time pattern formations in biochemical processes from simple periodic reactions to the most complex chaotic states of biochemical turnover. 

FIGURE 44.5 Principle of cyclic strain bioreactor for ligament and tendon engineering. 

Fig. 7.7 Measuring principle of cyclic three-point-bending.

The principles of cyclic polarization are similar to that of cyclic voltammetry (CV). The cyclic polarization technique is used to qualitatively measure pitting tendencies of a metallic sample in a corrosive solution environment. ASTM Standard G6I (35] describes the experimental procedures typically used in this study. The beginning potential scan is toward the anode from a potential in the vicinity of Ecorr- When the measured cur-... 

Although in this system radicals are generated and the free valence is conserved in the act of interaction of PhCOO- with RH, no chain reaction appears because the cyclic sequence of steps with conservation of the free valence is not realized. Therefore, second, the fulfillment of one more condition is very in ortant principle of cyclicity of radical steps with conservation of free valence. The chain reaction to occur requires such a combination of the reactants that the cycle of transformations with conservation of fiee valence and reproduction of the initial radical (atom) takes place. In the example presented above, it is enough to replace hydrocarbon by secondary alcohol to induce the chain reaction of dibenzoyl peroxide decomposition with the following steps of chain propagation ... 

To obtain the mass spectrum of a compound present in the ion source the mass scan unit requires activation. It remains for the operator to decide at which point this scan is taken and, in GC-MS, which GC peaks need to be scanned. Hites and Biemann (1970) introduced the principle of cyclic or repetitive... 

An alternative approach to the production of thermally stable polyoxy-methylenes was made by chemists of the Celanese Corporation of America and the commercial products were marketed as Celcon. Hostaform and Duracon. The principle of thermal stability in this case is the copolymerisation of formaldehyde with a second monomer which is a cyclic ether of the general form shown in Figure 19.3 (I). 

Because of the great range of structures containing cyclic hydroxamic acid functions it is difficult to give a concise summary of the available synthetic methods. Nevertheless, the vast majority of published syntheses depend on condensation reactions involving only familiar processes of acylation or alkylation of hydroxylamine derivatives. The principles of such syntheses are outlined in a number of typical examples in Section III, A but no attempt has been made to cover all reported cases. 

In principle, the properties and chemical behavior of cyclic sulfoxides and sulfones having a ring size of seven and up are expected to be quite similar to those of the analogous acyclic systems. 

The names for these mechanisms vary throughout the literature. For example, the Sgi mechanism has also been called the Sp2, the Se2 (closed), and the Se2 (cyclic) mechanism. The original designations, Se 1, Se2, and so on, were devised by the Hughes-Ingold school. It has been contended that the SeI mechanism violates the principle of conservation of orbital symmetry (p. 1068), and that the Se2 (back) mechanism partially violates it Slack, D.A. Baird, M.C. J. Am. Chem. Soc., 1976, 98, 5539. 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

This reaction is reversible and suitable p-hydroxy alkenes can be cleaved by heat (17-34). There is evidence that the cleavage reaction occurs by a cyclic mechanism (p. 1351), and, by the principle of microscopic reversibility, the addition mechanism should be cyclic too. Note that this reaction is an oxygen analog of the ene... 

The principles set forth above account reasonably well for the course of bifunctional condensations under ordinary conditions and for the relative difficulty of ring formation with units of less than five or more than seven members. They do not explain the formation of cyclic monomers from five-atom units to the total exclusion of linear polymers. Thus 7-hydroxy acids condense exclusively to lactones such as I, 7-amino acids give the lactams II, succinic acid yields the cyclic anhydride III, and ethylene carbonate and ethylene formal occur only in the cyclic forms IV and V. 

The highly ordered cyclic TS of the D-A reaction permits design of diastereo-or enantioselective reactions. (See Section 2.4 of Part A to review the principles of diastereoselectivity and enantioselectivity.) One way to achieve this is to install a chiral auxiliary.80 The cycloaddition proceeds to give two diastereomeric products that can be separated and purified. Because of the lower temperature required and the greater stereoselectivity observed in Lewis acid-catalyzed reactions, the best diastereoselectivity is observed in catalyzed reactions. Several chiral auxiliaries that are capable of high levels of diastereoselectivity have been developed. Chiral esters and amides of acrylic acid are particularly useful because the auxiliary can be recovered by hydrolysis of the purified adduct to give the enantiomerically pure carboxylic acid. Early examples involved acryloyl esters of chiral alcohols, including lactates and mandelates. Esters of the lactone of 2,4-dihydroxy-3,3-dimethylbutanoic acid (pantolactone) have also proven useful. 

The paper is organized in the following way In Section 2, the principles of quasi-elastic neutron scattering are introduced, and the method of NSE is shortly outlined. Section 3 deals with the polymer dynamics in dense environments, addressing in particular the influence and origin of entanglements. In Section 4, polymer networks are treated. Section 5 reports on the dynamics of linear homo- and block copolymers, of cyclic and star-shaped polymers in dilute and semi-dilute solutions, respectively. Finally, Section 6 summarizes the conclusions and gives an outlook. 

Alkyl and silyl nitronates are, in principle, /V-alkoxy and /V-silyloxynitrones, and they can react with alkenes in 1,3-dipolar cycloadditions to form /V-alkoxy- or /V-silyloxyisoxaz.olidine (see Scheme 8.25). The alkoxy and silyloxy groups can be eliminated from the adduct on heating or by acid treatment to form 2-isoxazolines. It should be noticed that isoxazolines are also obtained by the reaction of nitrile oxides with alkenes thus, nitronates can be considered as synthetic equivalents of nitrile oxides. Since the pioneering work by Torssell et al. on the development of silyl nitronates, this type of reaction has become a useful synthetic tool. Recent development for generation of cyclic nitronates by hetero Diels-Alder reactions of nitroalkenes is discussed in Section 8.3. 

Six-Membered Ring (endo-Cyclic). All the previous discussion of stereo-attack is based on steric hindrance, but in the case of a six-membered ring (endo-cyclic) enolate, the direction is affected simultaneously by stereo-electronic effects (Scheme 2-4).10 In the transition state, the attacking electrophiles must obey the principle of maximum overlap of participating orbitals by perpendicularly approaching the plane of atoms that constitute the enolate functional group. Electrophile attacks take place on the two diastereotopic... 

A typical manganese-salen complex (27)[89] is capable of catalysing the asymmetric epoxidation of (Z)-alkenes (Scheme 18) using sodium hypochlorite (NaOCl) as the principle oxidant. Cyclic alkenes and a, (3-unsaturated esters are also excellent starting materials for example indene may be transformed into the corresponding epoxide (28) with good enantiomeric excess1901. The epoxidation of such alkenes can be improved by the addition of ammonium acetate to the catalyst system 911. 

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