Semicarbazones special

Nevertheless, derivatives of the 11-ketone have been prepared in special cases. Thus the ethylene ketal of 3a,20) -dihydroxypregnan-l l-one is obtained in 50% yield by prolonged reaction according to the direct procedure. Ketals of A/B aromatic-11-ketones are formed by exchange dioxolanation. 11 -Semicarbazones have not been prepared, but hydrazones... 

Addition of amino compounds to hexafluoroacetone occurs very readily indeed, but subsequent dehydration to form, for example, oximes or semicarbazones from the corresponding adducts does not normally occur. Special procedures usually have to be employed, the most effective being elimination of amines from adducts to form imines that are derived from the ketone, as illustrated in Figure 8.15. 

There are some special synthetic approaches as, for example, the reaction between morpholinoenamines of aldehydes and the semicarbazone of oj-bromoacetophenone to give 31. On the other hand, enamines of acyclic ketones gave 32 or 31. In solution 31 is in fast equilibrium with the open-chain compound 33. If traces of acid are added the equilibrium disappears and 34 is formed (88G187).The tetrahydro compound 36 was prepared from 35 after hydrogenolysis and treatment with TFA (98SL1279). Glyoxylate adduct... 

Hydrazides condense with carbonyl compounds analogously to alkyl- and aryl-hydrazines. Among these compounds, the semicarbazones have achieved special analytical importance because they crystallize readily and have sharp melting points. They are particularly useful for characterizing lower aliphatic aldehydes. 

Another highly specialized hydrazine derivative used for derivatization purposes is semicarbazide (99), which has a urea unit in the molecule. The structure of urea is 108. The reaction of 99 with acetophenone (109 the lUPAC name is 1-phenylethanone) proceeds in an identical manner with all of the other functionalized amines discussed in this section. This reaction generates 110, which is called a semicarbazone. This is an unusual structure based on compounds discussed previously, but it is essentially a urea-imine. ... 

The hydrazone derivatives, oximes, and semicarbazones discussed in this section are rather specialized compounds. Why mention them Organic chemists rely on spectroscopy (see Chapter 14) to identify the structure of organic compounds, but prior to the development of spectroscopy, chemical reactions were used as an identification tool. One such method converts unknown organic compounds to solid derivatives. The color and melting point of several derivatives are cross-referenced to a library of known derivatives to help identify the unknown. The conversion of aldehydes and ketones to phenylhydrazones, 2,4-dinitrophenylhydrazones, oximes, and semicarbazones is one method used for derivatization. 

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