Semicarbazones, hydrogenation

Gyclohexanone semicarbazone hydrogenated 3 hrs. with Raney-Ni in ethyl acetate cyclohexyl semicarbazide (Y 95.5%) dissolved in chloroform and treated with 1 mole of 40%-peroxyacetic acid cyclohexylazoformamide (Y 89.7%) dissolved in alcohol and treated with a few drops of NaOH-soln. cyclohexanone semicarbazone (Y ca. 100%). L. Horner and H. Fernekess, B. 94,712 (1961). 

Dissolve 7 g. of pure oleic acid in 30 ml. of dry ethyl chloride (chloroform may be used but is less satisfactory), and ozonise at about —30°. Remove the solvent under reduced pressure, dissolve the residue in 50 ml. of dry methyl alcohol and hydrogenate as for adipic dialdehyde in the presence of 0 5 g. of palladium - calcium carbonate. Warm the resulting solution for 30 minutes with a slight excess of semicarbazide acetate and pour into water. Collect the precipitated semicarbazones and dry the... 

To decompose the acetone semicarbazone, warm 58 g. with 50 ml. of concentrated hydrochloric acid until it just dissolves. Cool in ice the semicarbazide hydrochloride separates as a thick crystaUine mass. Filter at the pump through a sintered glass funnel, and wash with a small quantity of alcohol and then with ether dry in the air. The yield of pure semicarbazide hydrochloride, m.p. 173° (decomp.), is 35 g. A further quantity of product may be obtained either by saturating the mother liquor with hydrogen chloride or by treating it with twice its volume of alcohol and then with ether. 

In a similar system, Melhado and Leonard (64) obtained better results if the enamine (13) were first converted to a semicarbazone (14) before hydrogenation and cyclization to the indole (15). 

Use of nitrous acid to liberate a free keto-acid from its semicarbazone caused formation of hydrogen azide which was co-extracted into ether with the product. Addition of silver nitrate to precipitate the silver salt of the acid also precipitated silver azide, which later exploded on scraping from a sintered disc. The possibility of formation of free hydrogen azide from interaction of nitrous acid and hydrazine or hydroxylamine derivatives is stressed. 

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). 

For the replacement of oxygen by hydrogen in ketones and aldehydes the method of Kishner and Wolff is used as often as is that of Clemmensen. In the former method the hydrazone or semicarbazone of the carbonyl compound is heated for several hours—preferably in the presence of hydrazine hydrate—in a sealed tube or autoclave with sodium ethoxide at about 160°. The explanation of the reaction is that, under the catalytic influence of the ethoxide, the hydrazone is transformed into a diimine which then decomposes in the same way as does phenyldiimine (p. 286) ... 

An example of anomalous behavior in closely related systems was reported some time ago during an investigation of the cychzation products derived from coumarin semicarbazones (Scheme 6.21). Thus, 78a reacted cleanly with chloro-acetic acid to yield 79a, which cyclized to the expected product 80a upon treatment with sodium acetate. On the other hand, 79b did not cyclize under the same reaction conditions. This was attributed to the absence of the enolizable hydrogen (Ph vs. H). The authors did not comment on the possibility of cyclization of 79b to 80b. The closely related urea analogue 81 did not react with chloroacetic acid thereby... 

Semicarbazones are reduced by sodium amalgam in alcohol [69,70], sodium in alcohol [71 ], sodium hydrosulfite [72], or catalytic hydrogenation using either platinum [73, 74], platinum oxide [75, 76], or Raney nickel [71]. The method involving sodium hydrosulfite is simple and is preferable to the other methods (Eq. 27). 

Enantiomerically pure tripeptide aldehydes are typically synthesized by azide or mixed anhydride coupling of dipeptides to a-amino aldehydes or their semicarbazone derivatives. For example, Ac-Leu-Leu-Phe-H was synthesized by the azide coupling of Ac-Leu-Leu-OH with Phe-H semicarbazone, prepared by catalytic hydrogenation of Z-Phe-H semicarbazone. The tripeptide semicarbazone was deprotected with 37% HCHO/HC1 solution (Table 2)J5 C-terminal argininal, ornithinal, and lysinal peptides such as Z-Leu-Leu-Orn(Boc)-H and Z-Leu-Leu-Lys(Boc)-H were prepared by mixed anhydride coupling of Z-Leu-Leu-OH with Orn(Boc)-H semicarbazone or Lys(Boc)-H semicarbazone. 3 Z-Leu-Leu-Arg(N02)-H was prepared by mixed anhydride coupling of Z-Leu-Leu-OH with Arg(N02)-H semicarbazone trifluoroacetate, prepared from the reaction of TFA and Boc-Arg(N02)-H semicarbazone (Table 2) J31... 

Ketones possessing two different a-methylene groups accessible to the reaction afford a mixture of two isomeric 1,2,3-selenadiazoles. Comparison of the reaction products derived from ethyl methyl ketone and phenylacetone semicarbazones shows that the direction of ring closure depends on the acidity of the a-hydrogens as determined by the substituent effect (Scheme 59) (71JOC2836). 

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