Hydrazones formation

Phenylhydrazine is usually dissolved in acetic acid for hydrazone formation if a salt of phenylhydrazine with an inorganic acid is used, it must be mixed with an excess of sodium acetate (see preparation of osazones, p. 137). 

Regioselectivity becomes important, if unsymmetric difunctional nitrogen components are used. In such cases two different reactions of the nitrogen nucleophile with the open-chain educt may be possible, one of which must be faster than the other. Hydrazone formation, for example, occurs more readily than hydrazinoLysis of an ester. In the second example, on the other hand, the amide is formed very rapidly from the acyl chloride, and only one cyclization product is observed. 

It is made readily by hydrazone formation between ketone 102 and 2,6-dichlorophenylhy-drazone [36],... 

The method of choice for the reduction of keto groups to methylenes remains the WolfT-Kishner reaction (2) or its modification discovered by Huang-Minlon (5). Originally, the hydrazone formation was carried out in a separate step, followed by... 

When present in macro quantities, aldehydes and ketones can be determined by conversion to the 2,4-dinitrophenylhydrazone which can be collected and weighed. When present in smaller quantities (10 3M or less), although hydrazone formation takes place, it does not separate from methanol solution, but if alkali is added an intense red coloration develops the reagent itself only produces a slight yellow colour. Measurement of the absorbance of the red solution thus provides a method for quantitative determination. 

Heterocyclics 252, 260, 299,416 n-Hexadecanol esters 63 Hexaporphyrin 102 Hexitols 426 Hexobarbital 254,255 Hexoses 161,202 Hexuronic acid 158 Histamine 294,296, 355 Homogentisic acid 166,167 Horizontal chamber 127 Hotplates 93 ff -, temperature distribution 95 Hydrazines 269,284 Hydrazone formation 71 ff -with 2,4-dinitrophenylhydrazine 71, 72, 274... 

This silyl hydrazone formation-oxidation sequence was originally developed as a practical alternative to the synthesis and oxidation of unsubstituted hydrazones by Myers and Furrow [31]. The formation of hydrazones directly from hydrazine and ketones is invariably complicated by azine formation. In contrast, silyl hydrazones can be formed cleanly from /V,/V -bis(7< rt-butyldimethylsilyl)hydrazine and aldehydes and ketones with nearly complete exclusion of azine formation. The resulting silylhydrazones undergo many of the reactions of conventional hydrazones (Wolff-Kishner reduction, oxidation to diazo intermediate, formation of geminal and vinyl iodides) with equal or greater efficiency. It is also noteworthy that application of hydrazine in this setting may also have led to cleavage of the acetate substituents. 

React for 2 hours at room temperature. While hydrazone formation does not require the addition of a reductant to create a linkage, including sodium cyanoborohydride in the reaction considerably increases the yield and stability of bonds formed. If the presence... 

Derivatives of hydrazine, especially the hydrazide compounds formed from carboxylate groups, can react specifically with aldehyde or ketone functional groups in target molecules. Reaction with either group creates a hydrazone linkage (Reaction 44)—a type of Schiff base. This bond is relatively stable if it is formed with a ketone, but somewhat labile if the reaction is with an aldehyde group. However, the reaction rate of hydrazine derivatives with aldehydes typically is faster than the rate with ketones. Hydrazone formation with aldehydes, however, results in much more stable bonds than the easily reversible Schiff base interaction of an amine with an aldehyde. To further stabilize the bond between a hydrazide and an aldehyde, the hydrazone may be reacted with sodium cyanoborohydride to reduce the double bond and form a secure covalent linkage. 

Cascade Blue hydrazide is soluble in aqueous solution, and it should be stable for awhile if protected from light. A concentrated stock solution of the reagent may be prepared in water and an aliquot added to a buffered reaction medium to facilitate the transfer of small quantities. For aqueous reactions, a pH range of 5-9 will result in efficient hydrazone formation. 

Hydrazinium salts, 73 567-568 2-(4-Hydrazinocarbonylphenyl)-4,5-diphenylimidazole (HCPI), chemiluminescence reagent, 5 851 Hydrazone formation, microwaves in, 76 565... 

With this tandem hydroformylation/hydrazone formation/Fischer indolization 3-substituted indoles such as valuable intermediates for the synthesis of pharmaceuticals as well as pharmaceuticals can be obtained in a very... 

Others have reported the use of fluorescent dansyl derivatives for the identification of 17a-oestradiol from serum samples (O Figure 2-2) (Nozaki et al., 1988). The derivatization of the keto functional group of particular steroids (ketosteroids) can also be achieved by hydrazone formation (O Figure 2-3) the most... 

One can in principle combine different exchange reactions in the same system in order to further increase the structural diversity accessed by the library. However, as this compounds the problem of selectivity (i.e., one now has two or more reactions that must exclusively involve one pair of functional groups), there are very few examples thus far of the practical implementation of this concept. An early, highly intriguing example was described by Lehn and coworkers in 2001 [68]. In this system, imine exchange (acyl hydrazone formation) and reversible metal coordination were employed in library generation. 

Hydrazone formation of pyridazine-3-hydrazines with aldoses, dialdofuranoses, and dialdopyranoses was studied by Stanovnik and co-workers. The respective hydrazones could be cyclized with Bt2 in MeOH or Pb(OAc)4 to j-triazolo[4,3-3]pyridazin-3-yl substituted polyols <1997JHC1115, 1998JHC513>. Similarly, 4-[(dimethylamino)-methylene]-l,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-one was reacted with pyridazine-3-hydrazines and the resulting mixtures were subsequently treated with Pb(OAc)4. Besides j-triazolo[4,3-3]pyridazine formation also diazenes were obtained. This can be rationalized by the enehydrazine-hydrazone mixtures observed in the first reaction. For phthalazin-l-hydrazines only diazenes were obtained after oxidation <2005TA2927>. Also cyclizations of... 

Since this postsynthetic, chemoselective lipidation procedure bypasses all the difficulties encountered during preparative HPLC purification of lipopeptides, the a-hydrazinoacetyl peptides were further developed into a strategy of lipidation by chemoselective hydrazone formation between lipophilic glyoxaldehyde vectors and prepurified hydrazino peptides (Scheme 10).11241 The hydrazino handle can be located at the N-termini or even at preselected lysine side-chain positions. Various lipophilic vectors were proposed containing one or more fatty acids coupled via amides to spacers which in turn are linked to glyoxylic acid via an amide bond.1124-1271... 

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