Pyridine, 3-hydroxycarboxylation

Complexes with 2-hydroxycarboxylic acids, pyridine-2-carboxylate, and dicarboxylates are described in Sections II,C,6,c, II,C,4,f, and II,C,6,p, respectively. 

Another level of refinement regarding in situ acyl group activations is reached when the activated hydroxycarboxylic acid A is converted into the corresponding IV-acylpyridinium salt B with / ara-(dimethylamino)pyridine, (Figure 6.9). Under these conditions macrocyclizations routinely succeed in yields well above 50%. 

The presence of pyridine catalyses the cyclization step [1907], and the reaction is to be preferred, as a route to substituted 1,3-dioxolan-2,4-diones, to the reaction of phosgene with the free 2-hydroxycarboxylic acids (see Section 10.3.4.1), as there is no formation of R R"C(C1)C(0)C1 as by-product [1907]. 

Annulation of a hydroxycarboxylic acid is effected at ambient temperature with a carbodi-imide as the reagent and pyridine as solvent. 

Acetic anhydride pyridine a-Acylamino-a,j -ethylenecarboxylic acids from a-acylamino-j -hydroxycarboxylic acids... 

Acetic anhydride pyridine Lactones from hydroxycarboxylic acids... 

Oxetan-2-ones possess the structure of -lactones. They are prepared by cyclodehydration of p-hydroxycarboxylic acids with phenylsulfonyl chloride in pyridine ... 

Finally, cyclic anhydrosulfites of a-hydroxycarboxylic acids (l,3-dioxa-2-tho-lane-4-one-2-oxides, see Formula 16.5g) need to mentioned. First syntheses and polymerization experiments were reported by Blaise et al. [75] as early as 1922. Systematic smdies of this highly reactive class of monomers began about 40 years later [76-88]. Tighe et al. concentrated their kinetic studies on thermal and alcohol-initiated polymerizations. They found that increasing size and number of substituents do not stabilize these heterocycles [83]. They concluded that the first step of a thermal polymerization is a decomposition into SO2 and an a -lactone (compare OCAs in Formula 16.5). Another interpretation of thermal and pyridine-catalyzed polymerizations is presented in Chap. 15. 

Acetic anhydride pyridine 4-Alkylidene-5-oxazolones from a-acylamino>/ >hydroxycarboxylic acids... 

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