Condensation step

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

Both the conjugate addition step and the intramolecular aldol condensation step can be carried out in one synthetic operation without isolat mg any of the intermediates along the way For example consider the reaction... 

The polymer described in the last problem is commercially called poly (phenylene oxide), which is not a proper name for a molecule with this structure. Propose a more correct name. Use the results of the last problem to criticize or defend the following proposition The experimental data for dimer polymerization can be understood if it is assumed that one molecule of water and one molecule of monomer may split out in the condensation step. Steps involving incorporation of the monomer itself (with only water split out) also occur. 

Although the industrial synthesis of vitamin remains largely unchanged from its early beginnings, significant effort has been devoted to improvements in the condensation step, the oxidation of dihydrovitarnin to vitamin K, and in economical approaches to vitamin (vide infra). Also, several chemical and biochemical alternatives to vitamin have been developed. 

The formation of a phenolic resin is often formally separated into two steps, though it probably should be three. If we use a three-step model, the first step is activation of the phenol or aldehyde. The second step is methylolation, and the third is condensation or chain extension. In addition to the clarity provided by the formalism, these steps are also generally separated in practice to provide maximum control of exothermic behavior, with the strategy being to separate the exotherm from each step from that of the others as much as possible. As there are significant differences in the activation step and in the details of the methylolation and condensations steps of novolacs and resoles, we will treat the two types separately. 

The UF-resin itself is formed in the acid condensation step, where the same high molar ratio as in the alkaline methylolation step is used (F/U = 1.8 to 2.5) the methylolureas, urea and the residual free formaldehyde react to form linear and partly branched molecules with medium and even higher molar masses, forming polydispersed UF-resins composed of oligomers and polymers of different molar m.asses. Molar ratios lower than approx. 1.7-1.8 during this acid condensation step might cause resin precipitation. 

The low molar ratio of the final UF-resin is adjusted by the addition of the so-called second urea, which might also be added in several steps [16-18]. Particular care and know-how are needed during this acid condensation step in order to produce resins of good performance, especially at the very low molar ratios usually in use today in the production of particleboard and MDF. This last reaction step generally also includes the vacuum distillation of the resin solution to the usual 63-66% solid content syrup in which form the resin is delivered. The distillation is performed in the manufacturing reactor itself or in a thin layer evaporator. Industrial preparation procedures are usually proprietary and are described in the literature in only a few cases [17-19]. 

Monomeric methylols, which have been formed mainly by the reaction of the post-added urea with the high eontent of free formaldehyde at the still high molar ratio of the acid condensation step... 

The condensation reaction and the increase of the molar mass can also be monitored by GPC [25], With longer duration of the acid condensation step, oligomers of higher molar masses are progressively formed. 

There is some debate in the literature as to the actual mechanism of the Beirut reaction. It is not clear which of the electrophilic nitrogens of BFO is the site of nucleophilic attack or if the reactive species is the dinitroso compound 10. In the case of the unsubstituted benzofurazan oxide (R = H), the product is the same regardless of which nitrogen undergoes the initial condensation step. When R 7 H, the nucleophilic addition step determines the structure of the product and, in fact, isomeric mixtures of quinoxaline-1,4-dioxides are often observed. One report suggests that N-3 of the more stable tautomer is the site of nucleophilic attack in accord with observed reaction products. However, a later study concludes that the product distribution can be best rationalized by invoking the ortho-dinitrosobenzene form 10 as the reactive intermediate. 

Step 2, another priming reaction, involves a further exchange of thioester linkages by another nucleophilic acyl substitution and results in covalent bonding of the acetyl group to a cysteine residue in the synthase complex that will catalyze the upcoming condensation step. 

This approach is only restricted by the accessibility of the tripyrranes 1. To avoid constitutional isomers it is also necessary that at least one of the building blocks should be symmetric. Since trifluoroacetic acid is used for the condensation step, /rvV-butyl ester moieties can be used in the x-positions of the tripyrrane which are cleaved prior to the decarboxylativc condensation. 

The formation of porphyrins from 1,19-dideoxybi)enes-/r can be achieved starting either from the 1-methyl derivatives or from l,19-dideoxybilene-Z>-l,19-dicarboxylic acid esters. In the first case the desired methine bridge of the porphyrin stems from the 1-methyl group whereas in the latter case orthoformates have to be added in the condensation step as a precursor for the methine unit. The 1-methyl- and also 1,19-dimethyl-l,19-dideoxybilene- > salts can be cy-clized to the corresponding porphyrins with copper(II) acetate in methanol.56 However, when the bilenes contain /i-acceptor substituents, the yields of porphyrins obtained by this method are very low.57... 

Condensation (step-growth) processes, 1 Condensing agents, 81 Conjugated polymers, 477, 478, 482 Containers, PET, 21... 

Fig. 4 Structures and formation routes of crosslinks in elastin. In the first step, lysine is catalyti-cally converted to allysine by lysyl oxidase all subsequent condensation steps are spontaneous...

Individual Na and Cl ions must condense into a three-dimensional array of ions. Energy is released in this condensation, because cations and anions attract each other. We need to examine this attraction in detail to determine how much energy is released in the condensation step. 

Unfortunately, the chemistry of the deamlnation/condensation process is poorly understood thus, little can be said about the mechanisms and kinetics of the amine elimination step, the nature of the "silaimine" (7,8) intermediate or the condensation step. It seems reasonable to predict that if one could learn to control the relative rates for these two steps, more control could be exerted over the condensation process and the properties of the precursor polymer. 

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equilibrium nature of this polymerization process is seen in the molecular weight distribution of the final polymer. All polyesters contain a few percent of low molecular weight oligomers, regardless of the polymerization process. 

Reaction of the enatiopure aldehyde 2-800, obtained from the corresponding imine by enantioselective hydrogenation, with Meldrum s acid (2-801) and the enol ether 2-802a (E/Z= 1 1) in the presence of a catalytic amount of ethylene diammonium diacetate for 4h gave 2-805 in 90 % yield with a 1,3 induction of >24 1. As intermediates, the Knoevenagel product 2-803 and the primarily produced cycloadduct 2-804 can be supposed the latter loses C02 and acetone by reaction with water formed during the condensation step (Scheme 2.178). 

Interesting octahydroacridines 2-860 have been prepared by Beifuss and coworkers by combining the condensation step with a rare intramolecular polar [4jt++2 jrj-cydization of a-aryliminium ions 2-859, obtained from anilines 2-857 by reaction with the oo-unsaturated aldehyde 2-858 (Scheme 2.191) [440]. The overall domino process seems to be stereoselective, since the formation of the two di-astereomers 2-860 can be traced to the use of the substrate 2-858 as a diastereom-eric mixture. 

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

For this purpose, perfluorooctanesulfonyl-tagged benzaldehydes were reacted with 1.1 equivalents of a 2-aminopyridine (or 2-aminopyrazine), 1.2 equivalents of an isonitrile, and a catalytic amount of scandium(III) triflate [Sc(OTf)3] under micro-wave irradiation in a mixture of dichloromethane and methanol (Scheme 7.85). A ramp time of 2 min was employed to achieve the pre-set temperature, and then the reaction mixture was maintained at the final temperature for a further 10 min. The fluorous tag constitutes a multifunctional tool in this reaction, protecting the phenol in the condensation step, being the phase tag for purification, and serving as an acti-... 

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