Azines pyrazoline

Burger s criss-cross cycloaddition reaction of hexafluoracetone-azine (76S349) is also a synthetic method of the [CNN + CC] class. In turn, the azomethines thus produced, (625) and (626) (79LA133), can react with alkenes and alkynes to yield azapentalene derivatives (627) and (628), or isomerize to A -pyrazolines (629) which subsequently lose HCF3 to afford pyrazoles (630 Scheme 56) (82MI40401). 

A simple preparative route to the previously unknown lH-3-pyrazolines via azomethine imines was developed. Olefins of the type R cH=CHR react with hexafiuoroacetone azine to give azomethine mines, which undergo a sequence of prototropic shifts to form 17/-3-pyrazolines [196, 202], On heating, the latter are transformed into 3-trifluoromethylpyrazoles [196, 206] and, on treatment with bases, into 1,2,5,6-tetrahydropynmidines [206] (equation 45)... 

Simple criss-cross cycloadditions described so far are in fact limited to aromatic aldazines and cyclic or fluorinated ketazines. Other examples are rather rare, including the products of intramolecular criss-cross cycloaddition. The criss-cross cycloadditions of hexafluoroacetone azine are probably the best studied reaction of this type. It has been observed that with azomethine imides 291 derived from hexafluoroacetone azine 290 and C(5)-C(7) cycloalkenes < 1975J(P 1)1902, 1979T389>, a rearrangement to 177-3-pyrazolines 292 competes with the criss-cross adduct 293 formation (Scheme 39). 

Pyrazolines containing aryl substituents with some functional groups can be used in the synthesis of new heterocycles. For example, the reaction of 5-(2-hydrohyphenyl)pyrazolines 111 with noncyclic 112 or cyclic carbonyl compounds under acidic conditions yields dihydropyrazolo[l,5-c]-l,3-benzox-azines 114 and 115, respectively [167, 168, 169] (Scheme 2.30). 

Pyrazolines may be obtained readily from a,j3-unsaturated aldehydes or ketones and aliphatic diazo compounds.44,349 Convenient syntheses have been worked out using Mannich bases,488-494 by the cyclization of aldehydo- and keto-azines.495-499 The conversion of a number of 4- and 5-substituted pyrazolines to the corresponding pyrazoles by frams-elimination was noted above. The 4-amino-pyrazolines with the appropriate configuration (41) readily eliminate... 

Based on this concept, a preparatively simple route to the previously unknown l//-3-pyrazolines was developed. Olefins of type RCH=CHR and hexafluoroacetone azine react to give azomethine imines, which subsequently are transformed in a series of prototropic shifts to give 1//-3-pyrazolines [75JCS(P1)538 79T389]. The latter on heating with AIBN undergo fluoroform elimination with heteroaromatization to yield trifluor-omethyl-substituted pyrazoles [82JFC(I9)437] (Scheme 63). 

Azomethine imines obtained from 1,4-dichloro-l, 4-bis(trifluoromethyl)-azine and cycloalkenes or cyclodienes undergo [1.4] chlorotropy. Subsequent hydrolysis yields l-trifluoroacetyl-3-trifluoromethyl-2-pyrazolines (93CC9) (Scheme 65). 

Using hexafluoroacetone azine in reactions with olefins affords 5,5-bis(tri-fluoromethyl)-l//-3-pyrazolines 25, whose thermolysis yields 3-trifluoro-methylpyr-azoles 26 (82JFC( 19)437) (Scheme 25). 

Cobaltous bromide Pyrazoline and pyrrole ring from azines... 

Two syntheses of the rare l//-3-pyrazoline ring system have been reported/ and both rely on the ability of azines to act as 1,3-dipolar species in [3 + 2] cycloaddition reactions (Scheme 111). Certain annulated pyridazines are con-... 

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