Purification, general procedures acids

The described two-step procedure is uncomplicated and can be carried out in 1 day to give in good yield a product that does not require any further purification. This procedure has been used for the preparation of 3-methyl-, 9-methyl-, and 6-methoxy-l,2,3,4-tetrahydro-j3-carboline and has been modified to obtain 9-phenyl-l,2,3,4-tetrahydro-j3-carboline. The method is generally applicable to the preparation of other 1-unsubstituted tetrahydro-/3-carbolines providing the 1-carboxylic acid precursor is not insoluble in the hot acid used to effect decarboxylation. 

A salient feature of the oxazolidinone auxiliaries is the fact that they are easily synthesized from inexpensive, commercially available starting materials. The L-amino acids valine and phenylalanine provide access to oxazolidi-nones 114 and 115, respectively, while oxazolidinone 116 is conveniently derived from norephedrine. Moreover, their derivatives are typically crystalline, allowing for ease of purification and handling. The general procedure for the preparation of these chiral oxazolidinones is illustrated with the synthesis of the N-propionyl oxazolidinone 127 derived from phenylalanine (130, Scheme 3.20) [86]. 

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). 

The increased solubility of substituted phthalocyanines (vide infra) enables more common purifications as used for other organic compounds. Usually the purification is done by chromatography either on alumina or silica gel, but recrystallization and extraction procedures can also be used. In some cases, the methods used for unsubstituted phthalocyanines can also be practiced, although the increased molecular weight accompanied by a reduced thermal stability makes sublimation more difficult.97 98 However, for substituted phthalocyanines, the stability towards acid may be reduced97 and, therefore, purification by treatment with sulfuric acid cannot generally be recommended. 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

---