Carboxylic acid synthesis problems

Eukaryotic cells face a dilemma in providing suitable amounts of substrate for fatty acid synthesis. Sufficient quantities of acetyl-CoA, malonyl-CoA, and NADPH must be generated in the cytosol for fatty acid synthesis. Malonyl-CoA is made by carboxylation of acetyl-CoA, so the problem reduces to generating sufficient acetyl-CoA and NADPH. 

With its structure known, the synthesis of a peptide can then be undertaken— perhaps to obtain a larger amount for biological evaluation. A simple amide might be formed by treating an amine and a carboxylic acid with dicyclo-hexylcarbodiimide (DCC Section 21.7), but peptide synthesis is a more difficult problem because many different amide bonds must be formed in a specific order rather than at random. 

When the Woodward-Eschenmoser synthesis began, it was known from the work of Bernhauer et al.5 that cobyric acid (4), a naturally occurring substance, could be converted directly into vitamin B12. Thus, the synthetic problem was reduced to the preparation of cobyric acid, a molecule whose seventh side chain terminates in a carboxylic acid group and is different from the other side chains. Two strategically distinct and elegant syntheses of the cobyric acid molecule evolved from the combined efforts of the Woodward and Eschenmoser groups and both will be presented. Although there is naturally some overlap, the two variants differ principally in the way in which the corrin nucleus is assembled. 

How could this problem be solved Only traces of thioesters are formed from free carboxylic acid and thiols in aqueous solution, i.e., the equilibrium reaction 7.15 is shifted to the left. According to de Duve (1991), there are two possibilities for spontaneous thioester synthesis under conditions present on the primeval Earth ... 

SYNTHESIS OF MIXED OXIDES USING POLYBASIC CARBOXYLIC HYDROXY-AND AMINO-ACID ROUTES PROBLEMS AND PROSPECTS... 

We were interested in applications of the high level of stereocontrol associated with the asymmetric Birch reduction-alkylation to problems in acyclic and heterocyclic synthesis. The pivotal disconnection of the six-membered ring is accomplished by utilization of the Baeyer-Villiger oxidation (Scheme 7). Treatment of cyclohexanones 25a and 25b with MCPBA gave caprolactone amides 26a and 26b with complete regiocon-trol. Acid-catalyzed transacylation gave the butyrolactone carboxylic acid 27 from 26a and the bis-lactone 28 from 26b cyclohexanones 31a and 31b afforded the diastereomeric lactones 29 and 30. ... 

A new stereocenter is formed when a synthon 143 with umpoled carbonyl reactivity (d reactivity) is introduced into aldehydes or imines. The enantioselective variant of this type of reaction was a longstanding problem in asymmetric synthesis. The very large majority of a-hetero-snbstitnted carbanions which serve as eqnivalents for synthons like 142 and 143 lead to racemic products with aldehydes or imines. However, enantiomerically pnre acylions and a-hydroxy carboxylic acids or aldehydes (144 and ent-144, respectively) as well as a-amino acids and aldehydes (145 and ent-145) are accessible either by nsing chiral d reagents or by reacting the components in the presence of chiral additives (Scheme 18). 

The A-hydroxycarboxamide group is a key fragment of many siderophores so that a convenient synthesis of this group is crucial for further progress. A variety of methods have been attempted for the preparation of hydroxamic acids starting from carboxylic acids. Although some of these methods are quite efficient for the preparation of substituted hydroxamic acids, the preparation of the parent compound is still a problem and yields are often moderately unacceptable, in part due to the low solubility of the parent hydroxylamine hydrochloride in organic solvents. 

For a,a-dialkylamino acids enantiomerization is not a problem. The preparation of 4,4-dimethyl-2-[(9-fluorenylmethyl)oxy]-5(4F/)-oxazolone, an intermediate used in the synthesis of ( )-mirabazole C has been described. Recently, two new 2-aIkoxy-5(4F/)-oxazolones derived from Toac (2,2,6,6-tetramethyl-4-amino-l-oxy-piperidine-4-carboxylic acid) that incorporate Z or 9-fluorenylmethoxycarbonyl (Fmoc) protection at C-2 have been described. The Toac analogues were synthesized as part of a study of the crystal structure and ab initio calculations for these interesting systems. 

Thus, in the case of 3-unsubstituted tetramic acids (X), the synthesis would start from acetic acid and an a-amino carboxylic acid. (In practice the acid will be replaced by suitable carboxylic acid derivatives.) The problem in these syntheses is the low reactivity of the methyl group of acetic acid. 

Tertiary aliphatic alcohol linkers have only occasionally been used in solid-phase organic synthesis [73], This might be because of the vigorous conditions required for their acylation. Esterification of resin-bound linker 4 with /V-Fmoc-prolinc [72,74] could not be achieved with the symmetric anhydride in the presence of DMAP (20 h), but required the use of /V-Fmoc-prolyl chloride (10-40% pyridine in DCM, 25 °C, 10-20 h [72]). A further problem with these linkers is that they can undergo elimination, a side reaction that cannot occur with benzyl or trityl linkers. Hence, for most applications in which a nucleophile-resistant linker for carboxylic acids is needed, 2-chlorotri-tyl- or 4-acyltrityl esters will probably be a better choice than ferf-alkyl esters. 

This synthesis also gives a small glimpse at the chemistry of heterocyclic compounds. Most active compounds in today s pharmaceuticals or agrochemicals include heterocycles, as well as most vitamins and natural products. The chemistry of heterocycles is thus very important and lectures or textbooks should be consulted.6 Formation of amide bonds also plays a large role in this problem. It was demonstrated that the strong amide bond can be formed from an amine and a carboxylic acid only after the acid has been activated. This can be done by transformation into the carboxylic halide or imidazolide or by application of an activating agent developed for peptide synthesis. 

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