Hydroxyacid

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

As shown in equation 12, the chemistry of this developer s oxidation and decomposition has been found to be less simple than first envisioned. One oxidation product, tetramethyl succinic acid (18), is not found under normal circumstances. Instead, the products are the a-hydroxyacid (20) and the a-ketoacid (22). When silver bromide is the oxidant, only the two-electron oxidation and hydrolysis occur to give (20). When silver chloride is the oxidant, a four-electron oxidation can occur to give (22). In model experiments the hydroxyacid was not converted to the keto acid. Therefore, it seemed that the two-electron intermediate triketone hydrate (19) in the presence of a stronger oxidant would reduce more silver, possibly involving a species such as (21) as a likely reactive intermediate. This mechanism was verified experimentally, using a controlled, constant electrochemical potential. At potentials like that of silver chloride, four electrons were used at lower potentials only two were used (104). 

In addition, several oj-hydroxyacids have been prepared. The systems prepared by Yamazaki (8) have been evaluated for ion transport action. Those prepared at Upjohn have been reported to have Ca activity comparable to the natural antibiotic X-537A (9) and to be more active than crown ethers. The most active of their structures is shown as 10. 

Treatment of a- or P hydroxyacids with sulfur tetrafluoride leads to conver Sion of the carboxylic group into the tiifluoromethyl group, but the hydroxyl group undergoes either fluorination, fluorosulfination, esterification, or dehydration to form esters, ethers, or alkenes The ratio of the products depends on P substitution [209, 210] (equations 103 and 104)... 

Figure A8.18 A racemic mixture of a-hydroxyacids (like L, D-lactate) can be transformed via the corresponding a-ketoacid (pyruvate) to the desired L-amino acid (L-alanine) with cofactor recycling.

Optically active tertiary a-hydroxyacids are useful starting materials for insect pheromones33, prostaglandin analogs34 and other important products. 

Data for the 2-hydroxy acid, other data is for the 4-hydroxyacid. b Added lithium perchlorate. 

Other asymmetric syntheses, based on aldol condensation of chiral a-sulfinyl carbanions with carbonyl compounds, are the formation of / -hydroxyketones from /J-sulfinylhydrazones 166211-214, of /3, /l -dihydroxyketones from 3-(p-tolylsulfinyl-methyl)-A2-methylisoxalinones 167215, of /1-hydroxyacids from 2-(p-tolylsulfinylmethyl)oxazolines 168216 and of /J-hydroxyesters from ethyl-p-tolylsulfinyl-W-methoxyacetamide 169217. 

Hydroperoxides, as optically active oxidizing agents 289-291 Hydrosulphonylation 172 /J-Hydroxyacids 619 a-Hydroxyaldehydes, synthesis of 330 a-Hydroxyalkyl acrylates, chiral 329 j -Hydroxycarboxylic esters, chiral 329 3-Hydroxycycloalkenes, synthesis of 313 Hydroxycyclopentenones, synthesis of 310 -Hydroxyesters 619 synthesis of 616 Hydroxyketones 619, 636 Hydroxymethylation 767 a-Hydroxysulphones, synthesis of 176 / -Hydroxysulphones 638, 639 reactions of 637, 944 electrochemical 1036 synthesis of 636 y-Hydroxysulphones 627 synthesis of 783... 

TABLE 2.5 Melting Point, Tm (°C), and Glass Transition Temperature, 7 (°C), of Poly(w-hydroxyacids) ... 

Racemic a-amino amides and a-hydroxy amides have been hydrolyzed enantio-selectively by amidases. Both L-selective and o-selective amidases are known. For example, a purified L-selective amidase from Ochrobactrum anthropi combines a very broad substrate specificity with a high enantioselectivity on a-hydrogen and a,a-disubstituted a-amino acid amides, a-hydroxyacid amides, and a-N-hydroxya-mino acid amides [102]. A racemase (a-amino-e-caprolactam racemase, EC 5.1.1.15) converts the o-aminopeptidase-catalyzed hydrolysis of a-amino acid amides into a DKR (Figure 6.38) [103]. 

Experiments showed that while nitration of the available hydroxyacid 16) went in position b) conditions could be found " to nitrate (15) at position (a). Disconnection of the methyl group from the ether must therefore be the last step in the analysis. 

Disconnecting the lactone gives hydroxyacid (48), With succinic anhydride In mind as an available 1,4-dl-carbonyl starting material, we can write keto acid (49) as an intermediate and the orientation is then correct for a Frledel-Crafts disconnection as both MeO and He activate this position. 

---