O-Linked derivatives

Diethyl 4-hydroxyisoxazole-3,5-dicarboxylate (334) was prepared by the reaction of acetonedicarboxylic acid ester with nitrosyl chloride (78JHC1519). Other 4-hydroxyisoxazoles have been prepared by the reaction of 2-hydroxy (or acetoxy)-1,3-diketones with hydroxylamine (34JA2190, 62HC(i7)i, p. 149), and by hydrolysis of 4-isoxazolediazonium salts (62HC(17)1, p. 149). The parent 4-hydroxyisoxazole has not yet been reported. 

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Figure 4. All O-glycosides are cleaved by strong acids, in addition, O-linked derivatives of serine and threonine can undergo base-catalysed p-elimination.

FIGURE 6.27 Regioselectivity of spiro-pyrano link formation upon dimerization of the o-QMs derived from twin-tocopherol 33. 

The hydroxyl groups at C-2 and C-3 are not essential for the catalytic reaction. McNicol and Baker233 showed that the endopectate lyases of Bacillus sphaericus and Bacillus polymyxa degrade the Vi antigen, the bacterial-surface polysaccharide containing a-D-(1 — 4)-linked residues of 2-acetamido-3-0-acetyl-2-deoxy-D-galac-topyranuronate, in the same way as its O-deacetylated derivative and D-galacturonan. 

Fig. 9.8 presents another, more complex type of phosphate prodrugs, namely (phosphoryloxy)methyl carbonates and carbamates (9-26, X = O or NH, resp.) [84], Here, the [(phosphoryloxy)methyl]carbonyl carrier appears quite versatile and of potential interest to prepare prodrugs of alcohols, phenols, and amines. The cascade of reactions leading from prodrug to drug as shown in Fig. 9.8 involves three steps, namely ester hydrolysis, release of formaldehyde, and a final step of carbonate hydrolysis (X = O) or A-decar-boxylation (X = NH). Three model compounds, a secondary alcohol, a primary aliphatic amine, and a primary aromatic amine, were derivatized with the carrier moiety and examined for their rates of breakdown [84], The alcohol, indan-2-ol, yielded a carrier-linked derivative that proved relatively... 

Another direct approach to chiral polymeric stationary phases is the modification of commercially available polysiloxanes which contain reactive side groups. Thus, the diamide phase was linked to a modified XE-60 polysiloxane phase (Table 2). In one case (XE-60-L-Val-(/ or 5)-a-pea)124 another center of stereogenicity (R or S configuration) has been introduced in the amide group. An XE-60-L-Val-(S)-x-pea column was used for the enantiomer separation of racemic. V-rert-butoxycarbonyl amino acids after their methylation with diazomethane (serine and threonine as the O-trimethylsilyl derivatives) (Figure 12)124. 

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