Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Ethylene derivs., cyclic 2 molecules

The correct amount of the resonance energy (R.E.) can hardly be deduced from experimental data, since the energy content of one separate Kekule configuration is not known (p. 198). However, we can derive fairly well the energy of cyclohexatriene, calculated on the basis of the bond energies (Table 18), that is to say, for a cyclic molecule with alternate bonds, which correspond to those in ethane and ethylene, thus... [Pg.203]

The isolated TE domain from the tyrocidine (tyc) NRPS has recently been shown to catalyze the macrocyclization of unnatural substrates to generate a variety of cyclic peptides. In conjunction with standard solid-phase peptide synthesis, Walsh and coworkers demonstrated a broad substrate tolerance for peptidyl-N-acetylcysteamine thioesters by the tyrocidine TE [41,42], Cyclization of peptide analogs, where individual amino acids were replaced with ethylene glycol units, was observed with high efficiency. In addition, hydroxyacid starter units were readily cyclized by the isolated TE domain to form nonribosomal peptide-derived macrolactones. More recently, Walsh and coworkers have demonstrated effective cyclization of PEGA resin-bound peptide/polyketide hybrids by the tyrocidine TE domain [43], Utilization of a pantetheine mimic for covalent attachment of small molecules to the resin, serves as an appropriate recognition domain for the enzyme. As peptide macrocyclizations remain challenging in the absence of enzymatic assistance, this approach promises facile construction of previously unattainable structures. [Pg.527]

Sorbitan derivatives consist of esters of cyclic anhydrides of sorbitol with a fatty acid (C12-C18). They are named sorbitan esters. They are lipophilic surfactants used as water-in-oil emulsifiers. When these sorbitan esters are polyoxy-ethylened they are called polysorbates. A large range of properties may be obtained by varying the number of oxyethylene groups in the molecule. They have mainly oil-in-water emulsifying and solubilizing properties. Some sorbitan derivatives that are USP/NF-, Eur. Ph.-, BP-compIiant and FDA-approved for oral administration are listed below ... [Pg.9]

Cyclic Acetals Formation of an acetal using a diol as the alcohol gives a cyclic acetal Cyclic acetals often have more favorable equilibrium constants, since there is a smaller entropy loss when two molecules (a ketone and a diol) condense than when three molecules (a ketone and two molecules of an alcohol) condense. Ethylene glycol is often used to make cyclic acetals its acetals are called ethylene acetals (or ethylene ketals). Dithiane (Section 18-8) and its alkylated derivatives are examples of cyclic thioacetals (sulfur acetals). [Pg.847]

Propose a mechanism for the formation of the cyclic acetal by treating acetone with ethylene glycol in the presence of an acid catalyst. Make sure that your mechanism is consistent with the fact that the oxygen atom of the water molecule is derived from the carbonyl oxygen of acetone. [Pg.450]

Isodesmic and homodesmotic reaction schemes use small molecules which retain bond types on both sides of the equation to model the relevant structural components of the cyclic conjugated species. o The homodesmotic reactions match bond and hybridization types for C—C and C—H on both sides of the equation. Calculation of the isodesmic stabilization energy (ISE) from the additivity of the enthalpies of formation of CH4, H3C—CH3, and H2C=CH2 is analogous to the RE derived above for benzene in that the reference system has noninteracting ethylenic double bonds and the calculated energy includes both the cyclic and noncyclic 7r-electron delocalization. Isodesmic reactions, like the RE calculation, tend to overestimate the stabilization because of the inclusion of the noncyclic delocalization energy. [Pg.4]


See other pages where Ethylene derivs., cyclic 2 molecules is mentioned: [Pg.295]    [Pg.217]    [Pg.435]    [Pg.20]    [Pg.546]    [Pg.81]    [Pg.465]    [Pg.195]    [Pg.269]    [Pg.81]    [Pg.465]    [Pg.560]    [Pg.546]    [Pg.264]    [Pg.546]    [Pg.195]    [Pg.137]    [Pg.53]    [Pg.357]    [Pg.164]    [Pg.246]    [Pg.867]    [Pg.1150]    [Pg.560]    [Pg.288]    [Pg.149]    [Pg.444]   
See also in sourсe #XX -- [ Pg.28 ]




SEARCH



Cyclic derivatives

Cyclic molecule

Ethylene cyclic

Ethylene molecules

© 2024 chempedia.info