Condensation procedures

The phenolic hydroxyl group of tyrosine, the imidazole moiety of histidine, and the amide groups of asparagine and glutamine are often not protected in peptide synthesis, since it is usually unnecessary. The protection of the hydroxyl group in serine and threonine (O-acetylation or O-benzylation) is not needed in the azide condensation procedure but may become important when other activation methods are used. 

The first compound of Equation 36 is not made by the condensation procedure, but by the chemical reduction of a nitro group in parathion. Some compounds are rather difficult to obtain by the general method. However, the next three compounds shown here were prepared by this method. 

A typical condensation procedure involves a one-step reaction where the monomer and suitable catalyst/initiator are mixed and heated to the required reaction temperature. To accomplish a satisfactory conversion, the low molar mass condensation products formed throughout the reaction have to be removed. This is most often accomplished by using a flow of inert gas and/or by reducing the pressure in the reaction vessel. The resulting polymer is generally used without any purification or, in some cases, after precipitation of the dissolved reaction product from a nonsolvent. 

The reprecipitation strategy lies in the conversion of the products dissolved in a suitable organic solvent into nanodispersed systems in a different medium by a precipitation/condensation procedure. On the other hand, the ion-association strategy can produce ion-based dye nanoparticles in pure aqueous media by utilizing a water-insoluble ion-pair formation reaction. The following example shows the size-dependent absorption properties for the cation-based pseudoisocyanine (PIC see the chemical structure in Fig. 4) dye nanoparticles. 

Akita and coworkers established a dehydrative condensation procedure, starting from hydroxo metal precursors containing the hydrotris(3,5-diisopropylpyrazolyl) borato ligand, and they were able to obtain dinuclear (/r-peroxo)Pd complexes, as indicated in Scheme 4. With the same procedure peroxo and hydroperoxo species (for Pd and Rh) and alkyl peroxides (for Mn, Co, Ni and Pd) complexes may be obtained. 

Once we have this library, fragment-condensation procedures might link the oligopeptides to one another so as to build longer chains. Prebiotic methods for fragment condensation are not really known. Kent s synthesis comes close to the target (Kent, 1999) but is restricted to particular chemical structures and cannot be seen as a generalized prebiotic method. 

Commerdal absolute ethanol is used without additional drying. The submitters state that the use of absolute ethanol may be avoided by employment of the alternative condensation procedure of Wood and Cadorin3 using 5 mole per cent of sodium hydroxide in aqueous methanol at 15-20° for 6 hours, but the yield of l-(nitromethyl)cydohexanol is appreciably lower... 

MacLachlan and coworkers have formed large SPMs 91-93 using a simple, one-pot, template-free, [3 + 3] Schiff-base condensation procedure (Scheme 6.21). The yields of the macrocycles were 40-68%, and no evidence of acyclic oligomer or polymer formation was observed. This is presumably due to the reversibility of the imine condensation, which allows the most thermodynamically stable product to... 

Franck and co-workers also prepared the next higher homolog in the series by using pyrrylpentadienol instead of 292 in the initial condensation procedure... 

The apparatus and experimental procedures are similar to those used for the tetrafluoride (synthesis 66), except for the use of the heavier-walled vessel H instead of the vessel F. In order to add the required amount of fluorine, it will be necessary either to use a larger vessel G or to repeat the measuring and condensing procedures. The xenon-to-fluorine mol ratios used are about 1 20 [0.689 g. (0.00525 mol) of xenon and 4.18 g. (0.110 mol) of fluorine]. The gas mixture is heated to 300° for 16 hours. The hexafluoride is purified by distillation, the less volatile fraction of lower fluorides being discarded. The purity may be checked by examination of the infrared spectrum, noting the presence of bands at 520 and 612 cm. and the absence of peaks for the difluoride and the tetrafluoride. The spectrum should also be studied in the 900- to 1000-cm. region, where oxyfluorides have fundamentals, for e.xample, XeOF4 at 928 cm. h... 

Transfer Active Ester Condensation Procedure (TAEC)... 

Of the fragment condensation procedures, the azide procedure was employed for each fragment condensation in order to avoid racemization and side reactions (over reaction). 

The peptide synthesis consists of stepwise-condensation and segment-condensation procedures. In both procedures, it is possible to purify the reaction product at each reaction step. Therefore, protected peptides with a high degree of purity can be obtained from a solution method. The choice of the reagents used in the final deprotection step determines the main strategy of peptide synthesis. The final deprotection procedures are mainly (1) catalytic hydrogenolysis in the presence of palladium,(2) sodium treatment in liquid ammonia for 10 to 15 seconds, repetitively over 30 minutes,(3) TFA treatment at room temperature for 1 hour,t l (4) HF treatment at 0°C for 1 hour,t l or (5) hard-soft acid-base procedure.0 ... 

Application of the template encapsulation procedure to the rigid cyclohexanediaminate Ze(3-[Co(chxn)3] cation has yielded a high symmetric cage systems [107]. For the synthesis of the diaminocyclohexane sarcophaginates, the condensation procedure was modified (pH ll-rl2 and elevated temperatures) compared with that for ethylenediamine sarcophaginates. 

A superior route to these compounds using the condensation procedures involves the use of alkoxycyclopropenyl cations (equation 73) . These can be prepared readily by alkylation of cyclopropenones with trialkyloxonium tetrafluoroborates . This process can be extended to a large number of active methylene compounds if the conditions of the reaction are controlled carefully and the tertiary non-nucleophilic base diisopropylethy-lamine is used Otherwise, a large number of side reactions are observed. 

Chiral N acylhydrazone derivatives may also be prepared from various ketones by condensation with N aminooxazolidinone 7a (Table 2.2) [22]. Mixtures of E/Z isomers were usually obtained, although ketone N acylhydrazones 14d and 15d, with highly branched tertiary butyl (tBu) substituents, were formed as single isomers. Others have recently used the amination and condensation procedures to prepare very similar chiral N acylhydrazones from ketones with excellent results [23]. 

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