Hydroxycarboxylic acids, protonation

Perfluoroalkylated epoxides as representatives of alicyclic ethers are successfully oxidized152 directly to a-hydroxycarboxylic acids 5 despite the fact that the epoxides 4 are protonated and subsequently hydrolyzed with difficulty. It has been found that the tandem ring opening/oxi-dation reaction is accomplished by concentrated nitric acid152 in the presence of copper ions in the mixture.181... 

Portanova, R. Lajunen, L.H.J. Tolazzi, M. Piispanen, J. Critical evaluation of stability constants for alpha-hydroxycarboxylic acid complexes with protons and metal ions and the accompanying enthalpy changes. Part II. Aliphatic 2-hydroxycarboxylic acids. Pure Appl. Chem. 2003, 75 (4), 495-540. 

Diketones undergo a rearrangement in the presence of strong base to yield a-hydroxycarboxylic acids. The best yields are obtained when the subject diketones do not have enolizable protons. 

Unlike ethylene glycol, a-hydroxycarboxylic acids, and p-mercaptoethanol, ligands such as ethanolamine, ethylenediamine, amino acids, and 2-aminoethanethiol do not form V2L2 complexes with vanadate [46], However, weakly formed products of VL stoichiometry (—546 ppm, —556 ppm) are observed with a number of a-amino acids [47], Because the amine functionality in amino acids is not suitable for formation of the cyclic [VN]2 core expected for a dimer, then one or the other of these VL compounds may correspond to the monomeric precursor to such a dimeric product. Certainly, with the a-hydroxycarboxylic acids, the monomeric complex can be a major component in solution. Similar VL complexes have not been reported for the amino alcohols. Perusal of the available reports suggests that the amine derivatives were studied under conditions where the nitrogen functionality was protonated, so the results may be somewhat misleading. 

Figure 3.53 shows an addition of a carboxybc acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming ferf-butyl esters. Because the acid shown in Figure 3.53 is a /i-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the alkene to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

Whilst the majority of the discussion thus far has been concerned with metallo-substituted redox molecular sieves, it is important to note that proto-nated zeolite forms can also be employed for selective oxidation with aqueous hydrogen peroxide. An excellent example of this is the study conducted by the Mobil Oil Corporation.52 Their work has shown that a number of protonated zeolites such as H-ZSM-5 or zeolite-/ can be used with hydrogen peroxide to catalyse the oxidation of cyclic ketones to lactones or the co-hydroxycarboxylic acids (Figure 4.12). 

Later, Dale-followed by Kricheldorf-observed the presence of carbon-carbon double bonds at the extremities of polymers formed in the so-performed polymerization of P-lactones [39, 40]. According to experimental results [41, 42], the initiation involved a nucleophilic attack at the carbonyl carbon atom of a monomer by the alkoxide anion of the initiator, cleaving the acyl-oxygen bond to yield the corresponding potassium alkoxide of the respective P-hydroxycarboxylic acid esters (Scheme 9.12). The acidic a-proton abstraction involved the formation of an unsaturated ester due to KOH elimination. Finally, polymerization occurred in the case involving P-lactone in excess, with the KOH acting as initiator. 

Moran and coworkers also combined urea and amide moieties, in this case, in chiral receptors that are recognized chiral hydroxycarboxylates such as lactic or man-delic acid. Receptor 48 combines a hA-chromenylurea with a spirobifluorenone linker in a macrocyclic stmcture and is capable of resolving a racemic mixture of the tetramethylainmonium salt of mandelic acid by thin-layer chromatography. Proton NMR titrations in DMSO-dg gave association constants for lactic acid [(R)-lactic acid K -... 

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