Pyrazolines application

The reaction is illustrated by the intramolecular cycloaddition of the nitrilimine (374) with the alkenic double bond separated from the dipole by three methylene units. The nitrilimine (374) was generated photochemically from the corresponding tetrazole (373) and the pyrrolidino[l,2-6]pyrazoline (375) was obtained in high yield 82JOC4256). Applications of a variety of these reactions will be found in Chapter 4.36. Other aspects of intramolecular 1,3-dipolar cycloadditions leading to complex, fused systems, especially when the 1,3-dipole and the dipolarophile are substituted into a benzene ring in the ortho positions, have been described (76AG(E)123). 

Phenidone, l-phenyl-3-pyrazolidone (738) (B-76M140404), has been used as a nonstaining, high contrast photographic developer. New optical brighteners containing 2-pyrazolines (739) and pyrazoles (740) have been synthesized recently and their properties and applications reviewed (75AG(E)66s). 

Nylon can also be brightened using an anionic derivative of 1,3-diphenylpyrazoline, such as FBA 11.26, 11.27 or 11.28. Although these pyrazolines give excellent whites when applied to nylon by exhaustion, they are usually less cost-effective than the DAST brighteners. For continuous application by pad-thermosol or pad-acid shock methods the situation is reversed, however, and the pyrazoline FBAs are commercially important in this sector. 

Acrylic fibres can also be brightened during manufacture by gel application during the wet spinning process. Special FBAs have not been developed for this purpose. Products such as the pyrazoline sulphone 11.50 and the benzofuranyl-benzimidazole 11.55 are suitable for this application. 

Dimerization of pyrazolines-2 on the action of oxidants includes the formation and doubling of cation-radicals (Morkovnik and Okhlobystin 1979). The doubling process is usually characterized by the terms head and tail. The term head is applicable to the position bearing a polar fragment, whereas the term tail is adopted for an unsaturated molecular site. The reaction under consideration follows the head-to-head order. This means that the doubling cation-radical has only a single position with the maximal density of an unpaired electron (Scheme 3.11). 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

Some other examples of application of 15N NMR spectroscopy include establishing the protonation site of A2-pyrazolines (87MI301-01) and assigning the structures to isomeric N-7 and N-9 substituted purines (86T5073). 

A detailed analysis of substituent effects on the pKa values of imidazoles and tetrazoles as well as benzimidazoles and naphthimidazoles has been made.308 The ortho effect is shown to parallel that of 2-substituted pyridines and quinolines47 190 and application of the Hammett equation to the tautomerism of these systems is also considered. The equation also satisfies the effect of substituents on the basicity of the nitrogen in the 1-position for 1-pyrazolines.309... 

Nuclear Magnetic Resonance Spectroscopy. The main application of nmr in the field of pyrazolines is to determine the stereochemistry of the substituents and the conformation of the ring. For pyrazolones, nmr is useful in establishing the structure of the various tautomeric forms. 

Pyrazolines are the intermediates in one of the synthetic pathways to cyclo-propanone and this fact has defined the development of pyrazoline chemistry for a long period. This aspect of their synthetic application is well described in the literature, including textbooks on general organic chemistry, and therefore we do not consider it necessary to cover it in this book. 

Although the intense blue fluorescence of l,3-diphenyl-2-pyrazolines had long been known [86], the usefulness of these compounds as optical brighteners was not recognized until 1949 [87], The most valuable brighteners are the 3-(4-chloro-phenyl)-l-(4-phenylsulfonic acid) derivatives because of their ready manufacturing ability, excellent optical properties, and versatility in application. 

Title Pyrazoline Derivatives, Their Preparation, and Application as Medicaments... 

Various pyrazolinones have been reported in patents and publications to have a variety of applications and activities. For example, both 2-pyrazolin-5-ones and 3-pyrazolin-5-ones have been reported to cause abnormal mitosis in plants and to inhibit their growth.1489,1565 Howland667 has reported that a number of 2-pyrazolin-5-ones are effective antioxidants for rubber and linseed oil. The same property is probably responsible for their inhibition of odor formation in synthetic detergents.454 The perchlorate of antipyrine has been patented for use as an explosive.933 Matsumoto953 has found that aminoantipyrine (probably the 4-amino) is an effective catalyst for decarboxylation. 

Matrix irradiation of the l-pyrazoline-3,5-diones (15) resulted in fragmentation and the eventual formation of carbenes (16) by the pathway shown in Scheme 2. Vinylcarbenes are intermediates in the photoelimination of nitrogen from 3H-pyrazoles. An unusual and potentially valuable application of this photochemical decomposition has been reported in 3,3-dimethyl-5-alkynyl-3ff-pyrazole (17) which, on irradiation in the presence of cyclohex-3-en-l-one, affords the cycloadduct (18) as shown in Scheme 3. ... 

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