Pyrazolines rearrangement

In the case of methylenecyclopropenes such as (315), the pyrazoline rearranges to a diazo-compound, which in turn cyclises at the original exocyclic double bond 250) ... 

The photorearrangement of pyrazoles to imidazoles is probably analogous, proceeding via iminoylazirines (82AHC(30)239) indazoles similarly rearrange to benzimidazoles (67HCA2244). 3-Pyrazolin-5-ones (56) are photochemically converted into imidazolones (57) and open-chain products (58) (70AHC(ll)l). The 1,2- and 1,4-disubstituted imidazoles are interconverted photochemically. 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Pyrazblin-5-one, 3-alkyl-(l,2,4-thiadiazol-5-yl)-reactions, 6, 483 2-Pyrazolin-5-one, 3-amino-tautomerism, 5, 215 2-Pyrazolin-5-one, 4,4-diazido-rearrangement, 5, 720 2-Pyrazolin-5-one, 3-hydroxy-tautomerism, 5, 215 2-Pyrazolin-5-one, 3-methyl-1 -phenyl-reactions, 5, 252... 

The addition of diazomethane to unsaturated esters (1), as ethyl acrylate, methyl crotonate and ethyl cinnamate, was investigated by Auwers who showed that the primary addition product is a A -pyrazoline (2) which rearranges spontaneously to the conjugated A -pyrazoline (3). 

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). 

Cyclohexadiene 45 was converted to 46 by what has proven to be a general method for preparation of the cyclohexa-2,4-dien-l-one ring system.2 Fragmentation of the aziridinyl imine in 46 at 110 °C gave an intermediate diazoalkane which underwent an intramolecular 1,3-dipolar cycloaddition to give the pyrazoline 47. At 140 °C, pyrazoline 47 expelled N2 and rearranged to the tricyclic ketone 48. The development of this and related bicyclizations29 illustrated a practical synthetic equivalence of an intramolecular diene-carbene 4-1-1 cycloaddition in the cyclohexa-2,4-dien-l-one series. 

In general, open structures II are energetically preferred over the closed forms I [8, 9], In the ring closed isomers I two unfavorable double bonds within five-membered rings would be required. No monoadduct with such a structure has been observed. Fulleroids such as 1-3 are usually formed via rearrangement of their pyrazoline-, triazoline- or ozonide [6,6]-precursor adducts accompanied by extrusion of N2 or O2 (see Chapter 4) [7,10-12]. 

Isodehydroacetic acid has been prepared by the action of sulfuric acid on acetoacetic ester 3 4 The ethyl ester has been prepared by the action of dry hydrogen chloride on acetoacetic ester 6 6 and by the sodium-catalyzed condensation of ethyl /3-chloroisocrotonate with ethyl acetoacetate3 The methyl ester of isodehydroacetic acid has been prepared by the thermal rearrangement of pyrazolines 7... 

In the conversion of the pyrazoline 33, formed from the / -nitrostyrene 32 and DPD, to the l//-pyrazole 34 with HCI, migration of the phenyl group was believed to be concerted with loss of N02. 87 It is, however, possible that the reaction proceeds via rearrangement of an intermediate 3//-pyrazoIe 35 (Scheme 11). 

A number of 2-acyI-l-chIoroethenes add to DPD in ether, losing HC1 spontaneously from the intermediate pyrazolines, and giving l/f-pyrazoles from rearrangement of the transient 3H isomers.50,60 The acid chloride 36 gives a small amount of a 3//-pyrazoIe 37 from reaction with two moles of methyl diazoacetate (Scheme 12).88... 

In the presence of base, the sulfone 189 rearranges to the methylene-pyrazoline 190, whereas its regioisomer 191 shows no reaction (Scheme 65).57 This is the reverse of the type of thermal rearrangement shown in Scheme 25. 

Camera and co-workers (287) described the 1,3-dipolar cycloaddition of trimethylsilyldiazomethane 171 with 165 (Note Opposite enantiomers are shown here). The intermediate 1-pyrazoline obtained from this reaction rearranged after acidic work up to furnish the 2-pyrazolines 172 with 80-88% de (Scheme 12.52). By further conversion of these products, optically active azaprolines were synthesized. 

The increase in energy in a molecule on absorption of UV light is sufficient to bring about bond cleavage. As a result, fragmentation and rearrangement of the molecule can occur. The effect on heterocycles is discussed in this section and, for simplicity, the transformations are classified, somewhat arbitrarily, on the basis of ring size pyrazolines are treated separately. Heterocyclic dienes and heteroaromatic compounds are also discussed separately, and the section is completed by consideration of the photochemistry of heteroaromatic A-oxides. 

Other analogous photodecompositions of 1-pyrazolines are known,72 73 while the photolysis74 of 3,3a,5 ,6,6a,66-hexahydro-3,6-ethenocycloprop[gf]indazole gives rise to products resulting from bond cleavage and further rearrangement [Eq. (20)]. 

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... 

The 3 + 2-cycloaddition of commercially available Me3SiCHN2 with camphor sultam-derived dipolarophiles produces 3-trimethylsilyl-substitutcd-A1 -pyrazolincs which on acid treatment convert into optically active A2-pyrazolines.60 The nucleophilic addition of ethyl diazoacetate with /V-cthoxycarbonyl-/V-(2,2,2-trichlorocthylidcne)a-mine produces a new diazo intermediate (35), which by 1,3-dipolar cycloaddition followed by a sigmatropic rearrangement of the cycloadduct (36) furnishes a substituted pyrazole (37) (Scheme 13).61. 

V-acylaziridine-2-imides to oxazoline-4-imides, followed by hydrolysis of these latter compounds, has been used586 to afford chiral / -hydroxy-a-amino acid precursors. It has been suggested587 that the observed thermal rearrangement of c/.s-aziridinyl ketone tosylhydrazones (449) to 5-alkylamino-3,5-diphenyl-l-tosyl-2-pyrazolines (450) is... 

The first a-cyclopropyl acyl silanes to be isolated were generated by treatment of a,f-unsaturated acyl silanes with diazomethane, followed by vapour-phase pyrolysis of the intermediate pyrazoline derivatives (vide infra, Section IV.D)141. They suffer acid-induced cleavage or rearrangement under more mild conditions than do their carbon analogues146. 

Detailed investigations of reactions of diazomethane with unsaturated ketones and the appropriate reaction products were made by Levai et al. [25, 30, 31, 32]. It is unambiguously established that for the reactions studied, only pathway a was observed.. This leads to the formation of isomers A, which spontaneously rearrange into the sole reaction products—2-pyrazolines B (Scheme 2.3). 

The formation of pyrazoline derivatives 175 (but not dihydrotetrazolotria-zepines 173, as assumed earlier [53]) is brought about by the condensation of chalcones with diaminotetrazole 172 [128,129]. The structure of the compounds 175 is convincingly verified using X-rays. The mechanism of their formation implies a Dimrothe rearrangement for either the initial diamine or for one of the cyclocondensation intermediates (Scheme 4.51). 

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