Epoxide linkage

These data, taken together with mass spectrometric degradation and other spectral evidence (see Table II), clearly favor a seven-membered cyclic peptide formed from anthranilate and phenylalanine as the basic skeleton of cyclopenin (3) and cyclopenol (4). This structure, including the epoxide linkage, was further supported by biosynthetic probing (51,52) (see Section IV,A). 

Confirmation of the structure of hazuntinine (49) came from the CMR spectrum, which showed close similarity with the spectrum of vincadifformine (2) except for C-14 and C-15, and C-10 and C-ll (20). The former two carbons at 51.8 and 57.0 ppm were clearly attached to oxygen from their chemical shift and are therefore the site of the epoxide linkage. The chemical shift differences of C-10 (+23.0ppm) and C-ll (+22 ppm) in comparison with vincadifformine (2) demonstrate the ortho relationship of the two aromatic methoxyl groups. 

Catalysts are called reactive hardeners or curing agents and become bound to the polymer by opening the epoxide linkage. When the hardener is an amine, the reaction is... 

The acetal polymers are probably never supplied for use without any additives being incorporated. As mentioned in Section 19.3.1 antioxidants of the phenol alkane type are present in both homopolymers and copolymers. Acid acceptors are also believed to be widely used to absorb traces of acidic materials which attack the acetal linkage. Epoxides, nitrogen compounds and basic salts have been successfully employed. 

A variety of more complicated compounds having a CH2CH2 linkage to the POSS core have been prepared using methods outlined in Table 29. Thus, epoxides have been made from cyclohexene-terminated POSS (Table 29, entries 1 and 2) and are precursors for the preparation of nanocomposite polymers under ultraviolet irradiation (Figure 43). ... 

The reductive demercuration was marred by the loss of about half of the peroxide due to competing deoxymercuration which afforded 4-cycloocten-l-ol. An additional complication was the formation of a small amount of trans-1,2-epoxy-cw-cyclooct-5-ene. The bicyclic peroxide 50 was readily separated from the unsaturated alcohol by silica chromatography, but complete removal of the epoxide was more difficult. Preservation of the peroxide linkage was markedly higher in the bromodemercuration. The diastereoisomeric dibromoperoxides 51 were separated by HPLC, although only one isomer was fully characterised. 

A heterogeneous olefin epoxidation catalyst containing both V and Ti in the active site was prepared by sequential non-hydrolytic grafting. The silica was exposed first to VO(OiPr)3 vapor followed by Ti(0 Pr)4 vapor. Formation of propene is evidence for the creation of Ti-O-V linkages on the surface. Upon metathesis of the 2-propoxide ligands with BuOOH, the catalyst becomes active for the gas phase epoxidation of cyclohexene. The kinetics of epoxidation are biphasic, indicating the presence of two reactive sites whose activity differs by approximately one order of magnitude. 

The formation of relatively ill-defined catalysts for epoxide/C02 copolymerization catalysts, arising from the treatment of ZnO with acid anhydrides or monoesters of dicarboxylic acids, has been described in a patent disclosure.968 Employing the perfluoroalkyl ester acid (342) renders the catalyst soluble in supercritical C02.969 At 110°C and 2,000 psi this catalyst mixture performs similarly to the zinc bisphenolates, producing a 96 4 ratio of polycarbonate polyether linkages, with a turnover of 440 g polymer/g [Zn] and a broad polydispersity (Mw/Mn>4). Related aluminum complexes have also been studied and (343) was found to be particularly active. However, selectivity is poor, with a ratio of 1 3.6 polycarbonate polyether.970... 

An example of the use of 1,4-butanediol diglycidyl ether for the activation of soluble dex-tran polymers is given in Chapter 25, Section 2.3. One end of the fezs-epoxide reacts with the hydroxylic sugar residues of dextran to form ether linkages, which terminate in epoxy functionalities. The epoxides of the activated derivative then can be used to couple additional mol-ecules-containing nucleophilic groups to the dextran backbone. 

The reaction of the epoxide with a thiol group yields a thioether linkage, whereas reaction with a hydroxyl gives an ether and reaction with an amine results in a secondary amine bond. The relative reactivity of an epoxy group is thiol > amine > hydroxyl, and this is reflected by... 

Porath, 1974). B/s-oxirane compounds also can be used to introduce epoxide groups into soluble dextran polymers in much the same manner (Boldicke et al., 1988 Bocher et al., 1992). The epoxide group reacts with nucleophiles in a ring-opening process to form a stable covalent linkage. The reaction can take place with primary amines, sulfhydryls, or hydroxyl groups to create secondary amine, thioether, or ether bonds, respectively (Chapter 2, Section 1.7). 

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