Stepwise Condensations

The linker 36 was prepared on pMeBHA resin using Fmoc chemistry. After the tetrapeptide was synthesized, the Fmoc group was removed by piperidine/NMP (1 4) for 20 min and the Boc group was removed by TFA/CH2C12 (1 1) for 20 min followed by neutralization with DIPEA/NMP (1 19). Fmoc-p-Ala-OH and bromoacetic acid were then condensed stepwise to the peptide-resin. Completion of each reaction was monitored by the ninhydrin test. The peptide-resin was treated with TFA/3-cresol/TfOH (9 1 1 v/v) for 1 h at rt. Subsequently, H20 was added to the mixture with cooling and the mixture was washed with pentane (2 x ) followed by Et20 (2 x ). The aqueous soln was applied to RP-HPLC, and the purified peptide analyzed by FAB-MS m/z 1067.0 Da [M + H]+ (calcd 1067.2 Da). 

Thermodynamics is, similarly, well suited for the description of processes which lead to changes of the molecular structure, as just seen for phase changes. A reaction with unfavorable thermodynamics expressed by a positive AG does not occur. However, with a negative AG, a reaction may still fail kinetically, while another mechanism may succeed. A typical example is the preparation of polypropylene. Although the polymerization of propylene is possible thermodynamically, it was not achieved until the work of Ziegler (139) and Natta (140), who discovered the catalyzed mechanism with favorable kinetics. Thus, much effort has been devoted to understand the kinetics of polymerization (118). Early work concentrated on predicting molecular masses and their distribution. In this section the thermodynamics of polymerization is briefly discussed. Most attention is paid to addition (chain) polymerization, but the theory is also applicable to condensation (stepwise) polymerization. The subject is extensively reviewed (141-145). 

The intramolecular cyclization can be favored by (i) high dilution techniques, (ii) the action of a template which forces the reacting ends together, and (ill) stepwise condensations. 

The resulting amino acid then condenses in a stepwise manner to form the growing polymer chain. As in direct polymerization, cycHc oligomers are also formed hence, caprolactam (qv) can be formed in the reverse of the reaction just shown above. 

BenZotrichloride Method. The central carbon atom of the dye is supphed by the trichloromethyl group from iJ-chlorobenzotrichloride. Both symmetrical and unsymmetrical triphenyhnethane dyes suitable for acryhc fibers are prepared by this method. 4-Chlorobenzotrichloride is condensed with excess chlorobenzene in the presence of a Lewis acid such as aluminium chloride to produce the intermediate aluminium chloride complex of 4,4, 4"-trichlorotriphenylmethyl chloride (18). Stepwise nucleophihc substitution of the chlorine atoms of this intermediate is achieved by successive reactions with different arylamines to give both symmetrical (51) and unsymmetrical dyes (52), eg, N-(2-chlorophenyl)-4-[(4-chlorophenyl) [4-[(3-methylphenyl)imino]-2,5-cyclohexadien-l-yhdene]methyl]benzenaminemonohydrochloride [85356-86-1J (19) from. w-toluidine and o-chloroaniline. 

Synthesis. The method of synthesis for Methylene Blue described in reference 14 is stiU the stepwise method of choice for thiazine dyes. /V,/V-Dimethy1-y-pheny1ene diamine [99-98-9], CgH22N2, reacts with sodium thiosulfate [7772-98-7] to form the thiosulfonic acid which condenses with /V, /V-dimetby1 ani1 ine [121 -69-7], CgH N, in the presence of sodium dichromate [10588-01-9] to the indamine, then with copper sulfate [18939-61 -2] and sodium dichromate to Methylene Blue (26). 

Aromatic compounds that are sufftciendy nucleophilic to condense with benzenediazonium chloride and form azo compounds generally condense with TCNE, eg, the reaction of /V, /V- dim ethyl a n i1 in e proceeds stepwise (21,22). 

Kelkar and McCarthy (1995) proposed another method to use the feedforward experiments to develop a kinetic model in a CSTR. An initial experimental design is augmented in a stepwise manner with additional experiments until a satisfactory model is developed. For augmenting data, experiments are selected in a way to increase the determinant of the correlation matrix. The method is demonstrated on kinetic model development for the aldol condensation of acetone over a mixed oxide catalyst. 

The molecular chains of plastics are formed by condensation or addition polymerization,. V condensation polymer forms by stepwise reacting molecules with each other and eliminating small molecules such as water. Addition polymer forms chains by the linking without elimin.ating small molecules,... 

With current computer technology there are several commercial programs available (as well as personal and private) that perform tray-to-tray stepwise calculations up or dovm a column, using the latest vapor pressure, K-val-ues, and heat data for the components. This then provides an accurate analysis at each tray (liquid and vapor ancJy-sis) and also the heat duty of the bottoms reboiler and overhead total or partial condenser. 

As in the [22]porphyrin(3.1.3.1) series, the cyclohexene moieties being part of the bridges originate from the preparation of the dimers from dimethylpyrrole and a hydronaphthalene diketone by acid-catalyzed condensation. The synthetic approach developed by Franck2b has the advantage over the foregoing method that more stable conventional pyrroles and dipyrryl-methanes can be used to form the macrotetracycle in a stepwise manner. 

Polycondensation At room temperature, 0.4% mass of Sn(II) chloride dihydrate (SnCl2-2H20) and 0.4% mass of p-toluenesulfonic acid monohydrate (p-TSA) are introduced into the mixture. The mixture is heated to 180°C under mechanical stirring. The pressure is reduced stepwise to reach 13 mbar, and file reaction is continued for 20 h. The reaction system becomes gradually viscous, and a small amount of L-lactide is formed and refluxed through the reflux condenser. At file end of the reaction, the flask is cooled down, file product is dissolved in chloroform and subsequently precipitated into diethyl ether. The resulting white fibrous solids are filtered and dried under vacuum (average yield 67%). 

Stepwise condensation polymerizations can be modeled as a second-order reaction of the functional groups. Let a be the concentration of functional groups so that = —ka - The following viscosity relationship... 

The conversion of the cyclic hexamer of fiiran and acetone into naphthafiirophane was reported <96TL6201>. Stepwise synthesis of core-modified, zncso-substituted porphyrins (e.g., 19) was achieved by acid catalyzed condensations <96TL197>. 

IPP and its DMAPP structural isomer are produced from glycolytic products by the methyl erythritol phosphate (MEP) pathway (Figure 5.3.1, Pathway 1). These isoprene units are condensed in a stepwise fashion to form the precursor to all carotenoids, geranylgeranyl di-phosphate (GGPP). GGPP is not solely metabolized to make carotenoids, but is a precursor for many other primary and secondary metab-... 

In principle, polymers equivalent to those obtained from vinyl and divinyl monomers may be synthesized by this method. The product in the above example possesses the same chain structure as polyethylene. The polymerization process, notwithstanding the likelihood of a metal alkyl intermediate, should conform satisfactorily to stepwise condensation. However, the product, and those obtained by Friedel-Crafts condensation as well, lack the recurrent functional groups which generally characterize condensation polymers. 

A second convergent synthesis of haliclamine A (64) was achieved in a stepwise sequence from cyclopropyl(thiophen-2-yl)methanone (76) (Scheme 7) [37]. The protected thiophene 77 was condensed with formyl-piperidine to give 78, suitable for a Wittig olefination with 79. After desulfurization of the product 80, the deprotected alcohol 82 was subjected to homoallylic rearrangement using MesSiBr in the presence of ZnBr2. The re-... 

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