Acidic reagents, Table

Aniline — Diphenylamine — Phosphoric Acid Reagent 183 Table V.hRf values of some sugars... 

Note This reagent sequence is less sensitive than ninhydrin alone. However, it possesses the advantage that the colors produced by the individual amino acids vary (Table 1), whereas ninhydrin alone only produces reddish-violet colored zones. 

Care must be exercised in the choice of acid employed in chloramine T — mineral acid reagent since the detection sensitivity and also the color of the fluorescences produced depend to a significant extent on the choice of acid. This is illustrated for the purine derivatives caffeine, theobromine and theophylline in Figure 1 and Table 1. 

Classical examples of this type of reaction are the various dimethylaminobenz-aldehyde reagents (q.v.) and vanillin-acid reagents, of which one, the vanillin-phosphoric acid reagent, is already included in Volume 1 a. The aldol condensation of estrogens is an example for the reaction mechanism (cf. Chapter 2, Table 6). According to Maiowan indole derivatives react in a similar manner [1]. Longo has postulated that catechins yield intensely colored triphenylmethane dyes [2]. 

A study of the reactions of butadiene, isoprene, or allene coordinated to nickel in a metallacycle, with carbonylic compounds, has been reported by Baker (example 11, Table IV). In the presence of phosphines, these metallacycles adopt a cr-allyl structure on one end and a ir-allyl structure on the other, as mentioned in Section II,A,1. The former is mainly attacked by aldehydes or electrophilic reagents in general, the latter by nucleophiles (C—H acids, see Table I, or amines, see Table IX). 

Crystal structures are available for many (N)4Co-amino acid complexes (Table I). Many of the diastereomers (AS, AS) in the bis-en series have been resolved using classic crystallization (usually via bromocamphor sulfonate, arsenyl-, or antimonyl-tartrate salts) or ion exchange methods (Table II). Reversed-phase ion-pair HPLC, using aryl phosphate or aryl/alkyl sulfonate ion pairing reagents in MeOH/ H20 eluent, has allowed diastereomer separations to be carried out on analytical amounts (28) (Table II). 

The nature of the amino acids is an important factor in the choice of a solvent and different solvents will permit better resolution of acidic, basic or neutral components (Table 10.7). In general, increasing the proportion of water in the solvent will increase all RF values and the introduction of small amounts of ammonia will increase the RF of the basic amino acids. Some solvents contain noxious chemicals, e.g. phenol, and this may restrict their routine use. The chemical composition may also limit the range of locating reagents which can be satisfactorily applied. For example, sulphanilic acid reagent cannot be used with phenolic solvents. 

Elaborate organic substrates and dicarbonyl compounds can also be methylenated in high yields with the Tebbe reagent (Table 3.8) [699-702]. Acid chlorides or... 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

Nielsen " ° used the same chemistry to synthesize both 1,3- and 1,4-dinitrocyclohexanes from the corresponding dioximes. The peroxytrifluoroacetic acid reagent has been used in reaction routes to a number of highly energetic polynitropolycycloalkanes as illustrated in Table 1.5 (see also Chapter 2). 

Cleaving the Polypeptide Chain Several methods can be used for fragmenting the polypeptide chain. Enzymes called proteases catalyze the hydrolytic cleavage of peptide bonds. Some proteases cleave only the peptide bond adjacent to particular amino acid residues (Table 3-7) and thus fragment a polypeptide chain in a predictable and reproducible way. A number of chemical reagents also cleave the peptide bond adjacent to specific residues. 

In 1939 it was observed16 that anhydrous liquid hydrogen fluoride is an excellent agent for effecting intramolecular acylation of aryl substituted aliphatic acids. Since that time a number of cyclizations have been performed with this reagent (Table VIII). 

On hydrolysis of 32 (2 equivalents 0.25 N HC1, r.t.) L-Val-OCH3 and the (R)-amino acid methyl esters 34 are liberated. Their ee can be determined H-nmr-spectroscopically using chiral shift reagents (Table 4)16). 

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