N-Hydroxyamino acid esters

The splitting of nitrones may be carried out by hydrolysis with hydrochloric acid (155,178,182), hydrazinolysis (182,186) or most suitably hydroxylaminolysis (183) (Scheme 44). Esters (191-193, 212) are easily converted into N-hydroxyamino acid esters (76, 77,130, 213) by action of hydroxylamine or its crystalline salts such as hydroxylamine hydrochloride (184), oxalate or toluenosulfonate (183) without disturbing the ester function. The salts of the last type are easily crystallized. 

Apart from the synthetic methods for esters described in sections VIII.3-VIIL7 and VIIL9-VIIL11, N-hydroxyamino acid esters (76, 77,130, 213) may also be obtained by acid-catalysed esterification of N-hydroxyamino acidb (80,100,160) although the best reagent for quantitative esterification is diazomethane (80) (Scheme 55). When this reagent is used N-methyl derivatives are not formed although this had been suspected previously (100). 

Shin et al (377) developed an additional route to N-acylenamino-carboxylic acid esters, starting from N-hydroxyamino acid esters N-acyl-hydroxyamino acid esters and N,0-diacyl-hydroxyamino acid esters, obtained by mono- or diacylation respectively, eliminate water or carboxylic acid respectively on treatment with triethylamine. The acyl-imino compounds probably formed initially isomerize spontaneously to the acylenamino compounds. Kishi et al (274) have described an analogous sequence for the preparation of N-acetyldehydroalanine ester. 

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