Semicarbazone reduction

Alloxan forms an oxime (1007) which is the same compound, violuric acid, as that formed by nitrosation of barbituric acid likewise, a hydrazone and semicarbazone. Reduction of alloxan gives first alloxantin, usually formulated as (1008), and then dialuric acid (1004 R = OH) the steps are reversible on oxidation. Vigorous oxidation with nitric acid and alkaline hydrolysis both give imidazole derivatives (parabanic acid and alloxanic acid, respectively) and thence aliphatic products. Alloxan and o-phenylenediamine give the benzopteridine, alloxazine (1009) (61MI21300). 

The mechanism of such a semicarbazone reduction is exemplified in Figure 17.68 by the deoxygenation of the tt,/i-unsaturated ketone A. Since the substrate contains a C=C double... 

Side Note 17.8. Diazene anions have a key role in both the Wolff-Kishner reduction and its alternative, the Reductive Cyanation semicarbazone reduction (Formula D in Figure 17.67 and Formula G in Figure 17.68, respectively) they decompose into elemental nitrogen and an organometallic compound. The italics immediately explain what happens in the second step of the reductive cyanation of a ketone shown in Figure 17.69. 

Fig. 17.69. Two-step sequence for the conversion of a ketone into the homologous nitrile ("reductive cyanation of a carbonyl compound"). In the second step of the reaction the diazene anion G is generated and decomposes in a similar way as the diazene anion D in the Wolff-Kishner reduction of Figure 17.67 and the diazene anion G of the semicarbazone reduction in Figure 17.68.

NH2-C0-NH NH2,CH5N30. Colourless crystalline substance m.p. 96" C. Prepared by the electrolytic reduction of nitrourea in 20% sulphuric acid at 10 "C. Forms crystalline salts with acids. Reacts with aldehydes and ketones to give semicarbazones. Used for the isolation and identification of aldehydes and ketones. 

Wolff - Kishner reduction of aldehydes and ketones. Upon heating the hydrazoiie or semicarbazone of an aldehyde or ketone with potassium hydroxide or with sodium ethoxide solution (sealed tube), the corresponding hydrocarbon is obtained ... 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Aldehydes have also been obtained by lithium trialkoxyaluminum hydride reduction of 5-nitriles or 5-acid chlorides, and, as the thio-semicarbazones, by the McFadyen-Stevens reaction in surprisingly good yields (50-60%) considering the severity of the reaction conditions. ... 

Catalytic reduction of the nitrile 79 in the presence of semicarbazide affords initially the semicarbazone of 80. Hydrolysis-interchange, for example in the presence of pyruvic acid, gives the aldehyde 80. Condensation with the half ester of malonic acid leads to the acrylic ester 81 the double bond is then removed by means of catalytic reduction (82). Base catalyzed reaction of the... 

It yields on reduction dihydrocopaene, a compound boiling at 118° to 121° at 12 mm. By oxidation hy ozone or hy permanganate of potassium it yields a ketonic acid of the formula CijH Oj, which forms a semicarbazone melting at 221°. These derivatives characterise copaene AS a new sesquiterpene. Its characters, however, have only heen very slightly investigated at present. 

The other reactions of the aldehydes, which are extraordinarily reactive substances, need only he mentioned here. Such reactions are reduction to alcohols, formation of hydrazones, oximes, semicarbazones, bisulphite compounds, acetals and cyanohydrins (by addition of hydrogen cyanide). 

The reduction methods involve primarily the reduction of semicarbazones using various catalysts. However, since semicarbazones are not readily available, the reduction of nitroureas or guanidines to semicarbazides may be more... 

On reduction, nitroguanidine is converted first into nitrosoguanidine and then into aminoguanidine i.e. guanylhydrazine. The latter is used for the manufacture of tetrazene (p. 206), and in organic chemistry to form crystalline derivatives from aldehydes and ketones, just as semicarbazide forms semicarbazones. 

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