2-Pyrazolines, 1-alkyl-, reaction with

Pyrazolin-5-ones with an exocyclic double bond at the 4-position (369 X = CMe2> react as heterodienes towards alkyl vinyl ethers (77G91). The kinetics of this Diels-Alder reaction giving pyrazolopyrans (370) have been studied. 

The 5-hydrazino-l,2,4-thiadiazoles (152) are stable in acid and base and readily form hydrazone derivatives on reaction with suitable carbonyl compounds. Thus, (152 R = Me, Ph, m-02NC6H4) reacts with /3-keto esters to form 3-alkyl-l-(l,2,4-thiadiazol-5-yl)pyrazolin-5-ones (161) which undergo coupling with p-nitrosodialkylanilines to form azomethine dyes (162) (65AHC(5)119>. 

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the / -tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 pyrazoline adduct to give the corresponding homotropone complexes (equation 149)217,218 jYiQ 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( -2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation com-plex More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes . ... 

A very important reaction in the synthesis of merocyanine dyes is the alkylation of 2-pyrazolin-5-ones with various heterocyclic rings as illustrated in eqs. 33-35.166-238,239 2-Pyrazolin-5-ones other than the... 

Cusmano340 341,345 has converted 5-alkyl- and 5-arylisoxazole-3-carboxylic acids into 5-imino-2-pyrazolines by treatment with phenyl hydrazine. Musante has also used this procedure.1043-1045 This reaction goes by way of /J-ketonitrile intermediates and gives very poor yields.86... 

Pyrazolin-5-ones having 3- and 4-a-hydroxyalkyl substituents are prepared in a number of ways. Two of these, alkylation of the corresponding 2-pyrazolin-5-one at N-2 and reaction of 3-pyrazolin-5-ones with aliphatic aldehydes, have already been mentioned. The... 

Cyclopropene esters give aminobicyclo[2.1.0]pentane derivatives, which can undergo ring-opening reactions to cyclopentenes For example, methyl 3,3-dimethylcyclo-propene-l-carboxylate (37), obtained by photolysis of the corresponding pyrazolinic derivative, reacted with the diquinanic enamine 36 to give a mixture of the alkylation product 38 and the [2 + 2] cycloadduct 39 which, on acid treatment, was transformed into the triquinanic cyclopentene alcohol 40 (equation 6). 

Irradiation of the pyrazoline mixture 12, 13, and ( )-14 yielded methanofullerenes 15 and 16 (7 1) with a closed transannular 6 - 6 bond besides minor amounts of a third constitutional isomer (17), whereas thermolysis of the same mixture gave 17 and inherently chiral ( )-18 (4 l),both constituting core-modified fullerenes [76] (Scheme l).With open transannular 6-5 bonds , the functionalized regions of 17 and ( )-18 resemble methanoannulene sub-structures. [3 + 2]Cycloaddition of alkyl azides to C70 affords triazolines which are isolable when the temperature does not exceed ca. 50 °C. Similar to the reaction with diazomethane, three constitutionally isomeric adducts were observed [77,113], and the regioisomer of type 19 [113] with the substituent of the heterocycle located above the apex of C70, predominated over its regioisomer of type 20 in two in-... 

Question on stability of the pyrazoline intermediates formed in 1,3-dipolar cycloadditions of alkyl diazoacetates with fullerenes was addressed in subsequent publication [15]. The reaction of glycine ethyl ester hydrochloride (or glycine octyl ester hydrochloride) with sodium nitrite was utilized to prepare ethyl diazoacetate in situ under the HSVM conditions. After 30min of vigorous milling at room temperature, 2-pyrazolines 32a and 32b were formed in 48% and 49% yield, respectively (Scheme 7.8). These pyrazolines were formed via isomerizations of 1-pyrazolines 31a,b which are obtained directly by 1,3-dipolar cycloadditions. 

Formation of esters by reaction of diazoalkanes with carboxylic acids is a mild and often quantitative procedure. It is particularly useful for the preparation of methyl and ethyl [4], benzyl [3, 58], and benzhydryl esters [45, 59, 60], although not on a large scale. The reaction is initiated by proton transfer from the carboxyl group and 0-alkylation is a competing reaction with phenolic acids. Diazoalkanes may also add to carbonyl [61] and olefinic linkages [62]. Thus the shikimic acid derivative (16) with a limited amount of diazomethane at low temperature gives the methyl ester (17) but with an excess of the reagent forms the isomeric pyrazolines (18 and 19) [63, 64]. 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

The 4,5-dihydro compounds (34) might be expected to show different properties. Here, as with the pyrazolines discussed in Section IV, A, lone pairs of electrons should be available on both nitrogen atoms for reaction to give salts of type 35 and/or 36. No salts of type 35 have been reported. Indeed, the reaction between the alkyl halide... 

Scheme 54 shows the synthesis reported by Cox et al. of the pyrazoline compound 198 [98]. The Weinreb amide (e.g., 199) was reacted with a terminal alkyne followed by a reaction of the resulting alkyl ketone (200) with an aryl cuprate to produce the pyrazoline 198. Cox et al. employed the use of microwave technology in this reaction. Kidwai and Misra also employed microwave technology to produce pyrazoline compounds [99]. 

An interesting preparation of aliphatic diazoalkanes (R R C = N2 R, R = alkyl) involves the photolysis of 2-alkoxy-2,5-dihydro-1,3.4-oxadiazoles (see Scheme 8.49). When the photolysis is carried out in the presence of an appropriate dipolarophUe, the diazo compounds can be intercepted (prior to their further photolysis) by a [3 + 2] cycloaddition reaction (54). As an example, 2-diazopropane was intercepted with A-phenylmaleimide (54) and norbornenes (55) to give the corresponding A -pyrazolines. 

Spontaneous isomerization of triazolines to diazo compounds can lead to addition of the latter to a second molecule of olefin, especially in the case of acrylic derivatives, resulting in a A pyrazoline, which by proto tropic rearrangement, gives the A2-compound (Scheme 149). Pyrazolines have been observed in the reactions of alkyl,67 aryl,32,282 heterocyclic,283,453 and gly-cosyl288 azides. A A pyrazoline is reported from the addition of phenyl and tosyl azides to 3,3-dimethylcyclopropene in this case the diazoimine formed by a retro-1,3-addition of the primary cyclopropanotriazoline adduct reacts with another olefin molecule.82... 

Oxidative processes are very characteristic for pyrazoline derivatives such as dihydroheteroaromatic compounds. However, oxidation of pyrazolines is not a widely used preparative method for the synthesis of the appropriate pyrazoles owing to numerous side reactions following heteroaromatization, and other pathways are usually applied to obtain pyrazoles [66]. It should be noted that pyrazolines unsubstituted at positions 1 and 3 are very unstable compounds and easily decompose in air, with nitrogen elimination. Introducing alkyls or aryls in these positions leads to an increase of their stability but oxidative destruction is also possible, for example, under the action of nitrous acid [116, 117]. 

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