1- Aryl-3-pyrazolidinones

The spectra of pyrazolidinones have often been described (63PMH(2)161, 67BSF3502, 70CHE1568). In the l-aryl-3-pyrazolidones (79), both the r (CO) and r (NH) bands changed considerably between the crystal (cyclic dimer) and the solution spectra. Typical values for j (CO) range from 1700 to 1730 cm" in solution. For the l-aryl-5-pyrazolidones (80),... 

As was mentioned previously (eq. 4, p. 10), the principal method used for synthesis of 3-pyrazolidinones is the reaction of hydrazines with a,j8-unsaturated acids,446, sie.911.1342.1550 oc, -unsaturated esters581,762,911,1209,1569 and a,j8-unsaturated amides.758 In many cases these reactions do not give l-substituted-3-pyrazolidinones, as shown in eq. 4, but the isomeric 2-substituted-3-pyrazolidinones, as shown for one particular case in eq. 244. Most frequently aryl hydrazines have... 

Cyclization of a,j8-unsaturated acid hydrazides319 or /3-hydroxy-propionic acid hydrazides1177-1327 forms 3-pyrazolidinones. The /9-hydroxy acid derivatives have usually been cyclized with acid at elevated temperatures. If the hydrazides of aryl hydrazines are used, N-l substitution occurs. It has been reported that a,/9-unsaturated acid hydrazines cyclize by treatment of the hydrazide with nitrous acid (eq. 245),335,479,1028,1029 leading to formation of l-nitroso-... 

The oxidation of 3-pyrazolidinones to 2-pyrazolin-5-ones has been mentioned (eq. 22). The 3-pyrazolidinones are also oxidized by Feh-ling s solution1550 and cupric sulfate.581 The oxidation of l-aryl-3-pyrazolidinones to 3-pyrazolin-5-ones has been mentioned.56,888... 

A number of mercury-containing 3-pyrazolidinones have been prepared by Schrauth and Bauerschmidt.1275 Treatment of l-aryl-3-pyrazolin-5-ones having no substituent at C-4 with mercuric acetate in methanol causes addition of acetoxymercuri and methoxy groups at the 3,4-double bond, substitution of acetoxymercuri at C-4 and mercuration in the aryl rings (eq. 248). 2-Pyrazolin-5-ones also undergo... 

The most frequently used procedures for preparation of 5-imino-3-pyrazolidines can all be considered as variations of cyclization of malonic acid derivatives. Various hydrazines react with ethyl /3-amino-/ -ethoxyacrylate to form 2-substituted-5-imino-3-pyrazolidinones (eq. 261). Complex alkyl groups,706 aryl groups693,695,922,1140,1601 and... 

In compounds which have no substituents at N-l or N-2 nuclear acylation also occurs and under some conditions acylation occurs at C-4. The use of 2-alkyl- or aryl-5-imino-3-pyrazolidinones, which exist as the 3-amino-2-pyrazolin-5-one isomer, with an equivalent of acid chloride, either aliphatic or aryl, leads to 3-acylamides (eq. 266),14,67,594,695,1247,1594,1595 although in some cases a side-reaction gives both N- and O-acylation.1595 Under the same conditions 3-hydroxy-5-imino-2-pyrazolinones give O-acylation.1599 The reaction of... 

In 2008, the Scheldt group reported a direct electrophilic amination via homoenolates catalyzed by N-heterocyclic carbenes using l-acyl-2-aryldiazenes as the electrophilic acceptors, which further increased the versatility of the homoenolate chemistry. It is worthwhile to note that only electron-rich substituents on the aryl component of the diazene could result in product formation (up to 84% yield), while electron-poor aryl substituents gave a lowyield (25%). An example of an asymmetric version of this new ami-nation reaction was achieved with the utilization of the chiral triazolium salt developed in their own group, providing the pyrazolidinone product in good yield (61%) and excellent enantioselectivity (90% ee) (Scheme 7.51). 

This methodology has been extended to kinetic resolution of azomethine imines (364) bearing a chiral center at the 5-position of the pyrazolidinone ring (Scheme 17.81) [111]. Modification of the chiral ligand to phosphaferrocene-oxazoline (366) proved optimal for kinetic resolution and allowed the catalyst loading to be lowered to 1 mol%. A number of aldehydes used to prepare azomethine imines are well tolerated, and azomethine imines with aryl, heteroaryl, and branched aliphatic substituents in the 5-position are resolved with high enantioselectivity. Unfortunately, kinetic resolution of C4-substituted azomethine imines is not efficient (s < 2). 

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