Oximino acids

An interesting comparison between a catalytic hydrogenation and a zinc and acetic acid reduction is observed in the case of the oximino acid (49). The cw-fused hydroxamic acid (50) was the sole... 

The same oximino acid (49) underwent a cyclodehydration on treatment with dicyclohexyl carbodiimide to form the unsaturated hydroxamic acid (52) in almost quantitative yield. 

Curtius rearrangement of a-oximino acid azides is reported as a new method of synthesis for 3-substituted 5-hydroxyoxadiazoles. In this method, the oxime of an a-ketoester is converted to the acid hydrazide, which in turn is converted to the azide by reaction with nitrous acid 115a). 

Curtius rearrangement of a-oximino acid azides 115a) (see p. 817). 

Nitriles are also formed in excellent yields by the decarboxylation and dehydration of oximino acids with warm acetic anhydride. A good route for obtaining the starting materials consists in the condensation of aldehydes with rhodanine followed by cleavage of the product with alkali and treatment with hydroxylamine. 

The reduction of a-oximino acids to a-amino acids is accomplished by catalytic hydrogenation with a Raney nickel or palladium-charcoal catalyst or by the action of sodium or aluminum amalgam. ... 

Several procedures involving the formation of a-oximino acid intermediates for the synthesis of a-amino acids have been described ... 

Nitriles. a-Oximino acids react with phenyl isocyanate in benzene solution at ambient temperatures to form nitriles (equation 1). ... 

Oxido-a-cholestane, 104 o-Oximino acids, 284 Oximino esters, 89 Oxindoles, 135... 

This method (Scheme 23) was proposed in 1974 by Ahmad (144) who used excess lithium cyanoborohydride as the reducing agent, a-Oximino acids (80) or equimolar quantities of a-keto acid and hydrox-ylamine were reduced at pH 5 in aqueous solution. This procedure was also successfully applied to the synthesis of N-hydroxytyrosine (35) (95) (Scheme 23). 

Another important method for the preparation of amino acids involves direct cleavage of a cyclic ketone. In section 2.1.A, a lactam product (a cyclic amide) was produced from the ketone and then cleaved by hydrolysis, but in this section the ketone moiety is cleaved directly. A cyclic ketone such as cyclopentanone can be oxidatively cleaved with sulfuric acid. When this cleavage was followed by treatment with nitrosyl sulfate, an oximino acid 2.11) was formed. Catalytic hydrogenation of the oximino group gave 4-aminobutanoic acid (2.72). Similar treatment of cycloheptanone gave 6-aminohexanoic acid (2.75) and cyclononanone led to 8-aminooctanoic acid. ... 

However, it seems more likely that the amide (82), and the other bromo-nietabolites of Verongia, are most probably formed directly from an epoxide (85), in which the epoxide ring is derived from the tertiary OH group by nucleophilic attack on the enone system in (84). In addition, since there is considerable evidence, both from in vivo and in vitro studies, that nitriles can be derived in vivo from a-amino-acids by way of a-oximino-acids, it seems most probable that epoxide (86) is the likely precursor of (76). 

Examination of the structure of nocardicin A (1) reveals that the compound contains three a-amino acid units and one a-oximino acid, residue and suggests that the compound is probably derived from a peptide. 

In a full account of preliminarily published results, detailed evidence is presented for the above conclusions, reached through experiments with linen flax seedlings, producing linamarin (62 R = R = Me), and cherry laurel or peach shoots, synthesizing prunasin (62 R = H, R = Ph). A notable incorporation of 2-oximinoisovaIeric acid and 2-oximino-3-phenylpropionic acid into linamarin and prunasin, respectively, may conceivably reflect the role of the oximino-acids as sources of nitriles, formed by a long-known, non-enzymic conversion. Experiments aimed at inducing accumulation of intermediates... 

Moreover, it has been demonstrated that a-oximino acids undergo facile dehydration/decarboxylation to give nitriles in vitro (180) and in vivo (181),... 

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