A-Amino acids. Table

Bond Distances, Bond Angles, and Bond Energies in Ethane, Ethene, and Ethyne (Table 9.1, p. 342) Stmctures of a-Amino Acids (Table 27.1, pp. 1054-1055)... 

In the rearrangement of divinylaziridines 289, the participation of a boat-like transition state 290 explained the stereochemical outcome of the reactions to give the azepinones 291 in 73 to 85% yield. The divinylaziridines 289 were synthesized via ex-chiral pool sequences starting from optically active a-amino acids. Table 16, Eq. (26) [55]. 

In addition to esters of a-amino acids, Table 8.9 also lists four prodrugs of metronidazole where the pro-moieties are longer-chain amino acids, namely... 

Although this reaction is useful for preparing a-amino acids (Table 22.3, fifth entry), it is not a general method for the synthesis of amines. Its major limitation is that the expected primary amine product is itself a nucleophile and competes with ammonia for the alkyl halide. 

A natural (1 counterpart has not yet been obtained for every proteinogenic a-amino acid (Table 1.5.1). /I-A la 1 and (R)- or (S)-fi- Aib 2 seem to be present in all kingdoms, with no obvious preferences. Selected /l-amino acids, for example /1-Lys 4, have been found only within one kingdom (i.e. bacteria), although they might be quite common there. Accordingly, the various natural products that contain /1-Lys substructures have been isolated exclusively from bacteria. On the other hand, alkaloids and peptides linked to / -Phe 8 have been obtained from bacteria, plants, and fungi, whereas the free /l-amino acid / -Phe 8 has not yet been isolated from natural sources. 

Other 2,3-Dihydrooxazoles and -thiazoles from a-Amino Acids. Table 1 shows oxazolines and thiazolines also prepared... 

According to these considerations, a is a measure of the effects produced by the helix sequence ends. Of course, the monomeric units situated at these ends experience a different environment because of the nearness of the nonhelical sequences as they would in the center of the helix. It has been found that a is very small for proteins and poly(a-amino acids) (Table 4-12). So, end effects are not favored by these polymers. Thus, if a helical state of four monomeric units is separated from another helical state of three monomeric units, a helical state of seven monomeric units will tend to form. 

The collective effects of the double-bond character of the peptide bond, the intramolecular hydrogen bonding, the hydrophobic bonding, and the L configuration of the peptide residues induce most poly(L-a-amino acids) and protein sequences to adopt the shape of a right-handed a-helix. But there are also exceptions. Despite the l configuration of the peptide residues, poly(L-j8-benzyl aspartate) forms a left-handed a helix. In addition to a helices and the pleated-sheet structures 03 structures), other secondary structures are also known for poly(a-amino acids) (Table 30-2). 

The macroconformation of these proteins is a direct function of the composition and sequence of their a-amino acids (Table 30-11). 

R is stabilized, such as occurs for a-amino acids (Table A.5). The stable R" also accounts for the lack of the expected (M — CjH4) and (M — 2113). R—CHNH2COOC2H5 must have a mass of 177 — 102 = 75, and must contain CH3SCH2—, so that the molecular structure CH3SCH2CH2CHNH2COOC2HS is indicated. The formation of m/z 56 is rationalized in Equation 8.99. [Methionine ethyl ester]... 

Optically pure d- and L-a-amino acids are of increasing interest as precursors for semi-synthetic antibiotics, new herbicides, insecticides, and physiologically active peptides, and as chiral building blocks for chemical synthesis. For the production of optically active a-amino acids, enzymatic processes have been developed other than fermentation to cover a wide range of products, not only natural L-a-amino acids but also unnatural and D-a-amino acids (Table 1). 

The melting points of the derivatives of a number of amino acids are collected in Table 111,132. Most a-amino acids decompose on heating so that the melting points would be more accurately described as decomposition points the latter vary somewhat with the rate of heating and the figures given are those obtained upon rapid heating. 

This table lists the and pi (pH at the isoelectric point) values of a-amino acids commonly found in proteins along with their abbreviations. The dissociation constants refer to aqueous solutions at 25°C. 

Protein Components. The simplest picture of the proteinaceous components is one of polypeptides, which are composed of a-amino acid residues. It is estimated that wool contains about 170 different types of polypeptides varying in molecular mass from below 10,000 to greater than 50,000 (34). Complete acid hydrolysis of wool yields 18 amino acids, the relative amounts of which vary considerably from one wool to another. Typical figures for two different samples of wool are given in Table 7. 

Table 2. a-Fluorocarboxylic Adds via Deaminative Fluorination of a-Amino Acids... 

First, they compared CSPs 1 and 3 prepared by the two-step solid-phase methodology with their commercially available counterparts (CSPs 2 and 4) obtained by direct reaction of the preformed selector with a silica support. Although no exact data characterizing the surface coverage density for these phases were reported, all of the CSPs separated all four racemates tested equally. These results shown in Table 3-3 subsequently led to the preparation of a series of dipeptide and tripeptide CSPs 5-10 using a similar synthetic approach. Although the majority of these phases exhibited selectivities lower or similar to those of selectors built around a single amino acid (Table 3-3), this study demonstrated that the solid-phase synthesis was a... 

How many of the a-amino acids shown in Table 26.1 contain aromatic rings How i many contain sulfur How many contain alcohols How many contain hydrocarbon. side chains ... 

The 20 a-amino acids shown in Table 23.3 (p. 622) are those most commonly found in natural proteins. They differ from one another in the nature of the R group attached to the alpha carbon. As you can see, R can be—... 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Table 16 Photo-driven reactions of aminocarbenes to produce a amino acids...

The AB polyamides are made from either >-amino acids or cyclic lactams, derivatives of the oj-amino acids (Table 3.1). In these polymers, the amino and acid groups are inherently balanced and the polymer also contains one amino and one acid endgroup. There are a number of different routes available for polymerizing these AB-type polyamides ... 

The procedure in Section C is representative of the synthesis of non-natural a-amino acids featuring the palladium cross coupling reaction of a (1-alaninc organozinc derivative with aromatic electrophiles. This methodology has been successfully extended with modifications to both the electrophile and the catalyst as shown in the Table. 

Silica gel [11] or alumina [11a, 12] alone, or silica and alumina together modified by Lewis-acid treatment [13] and zeolites [14], have been widely used as catalysts in Diels-Alder reactions, and these solids have also been tested as catalysts in asymmetric Diels-Alder reactions [12,13b,14]. Activated silica gel and alumina at 140 °C were used [15] to catalyze the asymmetric cycloaddition of (-)-menthyl-N-acetyl-a, S-dehydroalaninate (3) (R = NHCOMe) with cyclopentadiene in the key step for synthesizing optically active cycloaliphatic a-amino acids. When the reactions were carried out in the absence of solvent, a higher conversion was obtained. Some results are reported in Table 4.5 and compared with those obtained by using silica and alumina modified by treatment with Lewis acids. Silica gel gives a reasonable percentage of conversion after 24 h with complete diastereofacial selectivity in exo addition. 

Table 3-1. i-a-Amino acids present in proteins, (continued)... 

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