Reactions with semicarbazones

Reactions with Amines and Amides. Hydroxybenzaldehydes undergo the normal reactions with aUphatic and aromatic primary amines to form imines and Schiff bases reaction with hydroxylamine gives an oxime, reaction with hydrazines gives hydrazones, and reactions with semicarbazide give semicarbazones. The reaction of 4-hydroxybenzaldehyde with hydroxylamine hydrochloride is a convenient method for the preparation of 4-cyanophenol (52,53). 

A -20-Dien-20-ol acetates are prepared by acid-catalyzed reaction with isopropenyl acetate.A -20-Semicarbazones can be prepared in the usual manner. ... 

With semicarbazones of lower a-keto acids the reaction proceeds with some difficulty or not at all. Thus, the semicarbazones of pyruvic acid cannot be cyclized and that of glyoxylic acid is predominantly hydrolyzed so that the yield of the cyclization product is only 20-25%. ° This reaction was used in work with a different object, for preparing 6-azauracil, for the first time. 

Isomeric 4,5-dihydro[4,5-/]derivative 76 has been prepared from 7,8-dihydro-5(6//)-quinolinone via a semicarbazone and its subsequent reaction with thionyl chloride (95JHC177). 

Senucarbazide hydrochloride, conversion to semicarbazone, 46, 2 reaction with aminoacetone hydrochloride, 46, 2... 

A novel synthesis of 1,2,3-selenadiazoles 338 starts with the Michael addition of 2-nitropropane to a,P-unsaturated ketones 336 under basic conditions <06JHC149>. The resulting adducts are treated with semicarbazide hydrochloride to give semicarbazones 337, which are converted to 1,2,3-selenadiazoles 338 by reaction with selenium dioxide in THF (the choice of the solvent appears to be important in this case). 

Another fresh-water method which holds some promise for seawater analysis is twin cell potential sweep voltammetry, as proposed Afghan et al. [138]. In this method, semicarbazones are formed by reaction with semicarbazide... 

It is also possible to protect the carbonyl moiety of a phenacyl bromide via the reaction with semicarbazide hydrochloride then the semicarbazone can be electrolyt-ically dimerized, ultimately to afford a 2,5-diarylfuran [86]. Alternatively, the elec-trolytically formed dimer can be converted... 

Diazo esters can also be prepared from glycine esters by treatment with nitrous acid [966] or with alkyl nitrites. Further methods include the oxidation of hydrazones, oximes (Forster reaction), and semicarbazones, the base-induced... 

The basic amino group of the 1-position in semicarbazide or thiosemi-carbazide may be used to react by a substitution reaction with activated halides [52], ethers [51], hydroxy [53], phenoxy [54], and amino groups [55] to yield substituted 1-semicarbazides or thiosemicarbazides. In addition, the amino group of the 1-position may add to electron-deficient double bonds [56]. Formaldehyde and other aldehydes may add to all the available free NH groups to give methylol, alkylol, or polymeric products under basic conditions [57]. Aldehydes or ketenes usually give semicarbazone derivatives, and these in turn are used analytically to identify the purity or structure of a known aldehyde [3]. 

Pyrroles and indoles also appear to behave as 2 7r-electron systems in their reactions with with the a-bromo oxime (276) and with the a-bromo semicarbazone (277) under basic... 

Reaction with semicarbazide hydrochloride. Many simple quinones yield crystalline mono-semicarbazones by the following procedure. Dissolve 0.2 g of semicarbazide hydrochloride in a little water, add 0.2 g of the quinone and warm. The mono-semicarbazone is immediately formed as a yellow precipitate. Filter and recrystallise from hot water any bis-semicarbazone will remain undissolved. 

The most widely used method to synthesize 1,2,3-selenadiazoles is the reaction of semicarbazones with selenium dioxide. This reaction has been described in detail <1996CHEC-II(4)743, 1984CHEC(6)333, 2004HOU777>. This method has often been utilized for synthesis of a variety of 1,2,3-selenadiazoles, including carbocyclic and heterocyclic fused systems. 

Ketones and aldehydes also condense with other ammonia derivatives, such as hydroxyl amine and substituted hydrazines, to give imine derivatives. The equilibrium constants for these reactions are usually more favorable than for reactions with simple amines. Hydroxylamine reacts with ketones and aldehydes to form oximes hydrazine and its derivatives react to form hydrazones and semicarbazide reacts to form semicarbazones. The mechanisms of these reactions are similar to the mechanism of imine formation. 

Oximes, hydrazones, semicarbazones, diazines and carbodiimides all undergo reactions involving addition to a carbon-nitrogen double bond (A, D], but these reactions are of limited value, with the exception of the reaction with ketoximes. This last reaction, with an excess of Grignard at elevated temperatures, is a useful route to aziridines, (4), although yields are rarely high. In some cases, reduction of the intermediate azirine (2) leads to an alternative aziridine (5). 

Applications of Lalezari and Hurd-Mori reactions are also highlighted in the synthesis of a new class of 1,2,3-selenadiazoles 243 and 1,2,3-thiadiazoles 244 <07JHC1165>. Reaction of sulfonylacetate 241 with semicarbazide 237 gives semicarbazone 242, and oxidative cyclization of 242 with selenium dioxide in acetic acid at 60-70 °C furnishes selenadiazole 243. Compound 242 also undergoes Hurd-Mori reaction with excess thionyl chloride to give thiadiazole 244. 

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