Carboxy carbon

Amino-4,6-dimethyl-3-oxo-3//-phenoxazine-l,9-dicarboxylic acid also named actinocin is the chromophor of the red antineoplastic chromopeptide aetinomyein D (formula A). Two cyclopenta-peptide lactone rings (amino acids L-threonine, D-valine, L-proline, sarcosine, and 7V-methyl-L-valine) are attached to the carboxy carbons of actinocin by two amide bonds involving the amino groups of threonine. 

Assume that acetyl CoA containing a l4C isotopic label in the carboxy carbon atom is used as starting material and that the mevalonate pathway i followed. Identify the positions in a-cadinol where the label would appear. 

In parallel with the direct abstraction mechanism, a second important mechanism of epimerization can occur via an intermediate that can be produced from any type of activation. This common intermediate is the oxazol-5(4//)-one, produced by nucleophilic attack of the penultimate carbonyl oxygen atom attached to the amino group on the activated carboxy carbon atom (Scheme 4). 

Double bonds between csrbons numbered from carboxy carbon. ... 

Carbonyl carbon-13 shifts of aldehydes, ketones and carboxylic acids, including all derivatives, occur between 150 and 220 ppm [281]. Within this range, carboxy carbons are shielded (150-180 ppm) relative to carbonyl carbons in aldehydes and ketones (190-220 ppm). This is attributed to an electron releasing effect of the additional hetero... 

Alkyl carbon atoms attached in a position a, / , or y to a carboxy function give rise to tx, [1. and y effects which shift the carboxy carbon resonances by 10, 4 and — 1 ppm, respectively. This trend is illustrated by the 13C shift values collected for formic, acetic, propionic, and butyric acids among others in Table 4.34 [305-309], Further, carboxy carbons of x halo acids and dicarboxylic acids with closely spaced carboxy groups arc shielded relative to those of parent alkanoic acids (Table 4.34). On the other hand, the x, j3 and y carboxy increments Z, = hi(RCOon)- — krh) f°r the carbon shifts of an alkyl chain are... 

Carboxy carbons of methyl benzoates are shielded by electron-withdrawing substituents in the oposition of the benzene ring [320] (Table 4.38). Carbonyl shifts of phthalic acid diesters and phthalimide are larger than those of phthalic anhydride [321]. fi effects of the O-alkyl group in the esters and hydrogen bonding of the imide are the obvious reasons. 

An adapted soil bacterium identified as an Arthrobacter sp. incorporated most of the labeled carbon from dalapon-2-14C into various cellular components (amino acids, lipids, protein, and nucleic acids) while the carboxy carbon was accounted for mostly as 14C02 (2). Products identified on paper chromatograms were the amino acids alanine and glutamic acid (Figure 8). An enzyme preparation obtained from the supernatant fluid of broken cells of the same Arthrobacter sp. liberated Cl ion from dalapon to yield the organic acid pyruvate (31). The... 

Finally, the polar C-O bonds make the carboxy carbon electrophilic, so carboxylic acids react with nucleophiles. Nucleophilic attack occurs at an sp hybridized carbon atom, so it results in the cleavage of the Jt bond, as well. This reaction is also discussed in Chapter 22. 

Fischer-Speier esterification to give a salt of an amino acid ester (by refluxing an alkanol with anhydrous HC1 or hot benzyl alcohol with toluene-/>-sulphonic acid) is straightforward. The fact that the nearby amino group is protonated when the carboxy group reacts to give a-amino acid esters does not slow the reaction down unduly a nearby positive site might have been expected to reduce the electrophilic character of the carboxy carbon atom. Without an acid catalyst, Walkylation can accompany esterification (Equation 4.6). 

Start by identifying the carbon atoms in the product that correspond to those in the starting material. Each has a methyl group (a) and a carboxy carbon (/ ), so use these as points of reference. 

Proton %-H (Su = 7.62) gives a correlation signal via coupling in the HMBC plot d with the carboxy carbon at 6c = 166.18 which also correlates with the proton of threonine at Sh = 7.15. 

R may be assumed to contain 12 to 18 carbon atoms except where otherwise stated. Hydrocarbon chains derived from vegetable oils or animal fats contain even numbers of carbon atoms, including the carboxy carbon in the case of fatty acids, but those obtained from petrochemical sources contain both odd and even numbers. 

Webb (1968) observed that aminopterin-inhibited Aerohacter aerogenes piled up alanine and valine. The interpretation was that both are used for the synthesis of leucine, with the carboxy carbon of valine eliminated as active formate (IX). 

The heteroorientation of a carbon atom is defined as the sum of all congeners of the atom except hydrogen and carbon, which are bonded by an acyclic bond, multiplied by the bond order. The bond order can be 1, 2 or 3. So, for example, a carbon atom attached to carbon, hydrogen, and one hydroxy group has heteroorientation 1 a keto carbon has heteroorientation 2 and a carboxy carbon has heteroorientation 3. 

The NMR chemical shifts of carboxylic acids also are similar to those of the aldehydes and ketones, with moderately deshielded carbons next to the carbonyl group and the typically low-field carbonyl absorptions. However, the amount of deshielding is smaller because the positive polarization of the carboxy carbon is somewhat attenuated by the presence of the extra OH group. 

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