Thermally-induced rearrangement

Pyridazinones may undergo ring contraction to pyrroles, pyrazoles and indoles, the process being induced either by an acid or base. The structure of the final product is strongly dependent on the reaction conditions. For example, 4,5-dichloro-l-phenylpyridazin-6(lFT)-one rearranges thermally to 4-chloro-l-phenylpyrazole-5-carboxylic acid (12S), while in aqueous base the corresponding 4-hydroxy acid (126) is formed (Scheme 40). 

An experimental probe for the presence of radical intermediates resulting from thermally induced homolytic cleavage of the N-0 bond was derived by incorporating an alkene into a model substrate to act as a potential intramolecular radical trap (Scheme 6.25) [11]. In a control experimental, thermal reaction of 73 gave the desired product 74 in 66% isolated yield. On the other hand, thermal rearrangement of the unsaturated compound 75 under our typical conditions gave the desired hydroxypyrimidinone 76 in only 38% isolated yield. When the vinyl ami-doxime mixture 75Z/E was heated in o-xylene at 125 °C in the presence of a... 

The kinetics of the thermally induced rearrangement of 4-ethyl-3,5-diphenyl-4//-l,2,4-triazole 28 to the corresponding 1-ethyl-substituted compound 29 in the presence of 15-crown-5 in octadecane at 330°C has been studied (Equation 5). A mechanism for the rearrangement was proposed that involved an intermediate triazolium triazolate species <2001JHC955>. 

The thermally induced rearrangements in the furoxan series have also been found. In particular, the transformation of 3-R-substituted 4-(3-ethoxycarbonylthioureido)-l,2,5-oxadiazole 2-oxides into derivatives of 5-amino-3-(a-nitroalkyl)-l,2,4-thiadiazole and into (5-amino-l,2,4-thiadiazol-3-yl)nitroformaldehyde arylhydrazones has been reported (Equation 8) <2003MC188>. 

Final remarks. The 14C-KIE and 2H-SKIE data presented in this Section (VLB) clearly indicate the usefulness of isotope effect methodology in studies of mechanistic details of thermally induced Claisen rearrangement, which provides a synthetic route to y,5-unsaturatcd carbonyl compounds. The primary and secondary 14C KIE supplement strongly the deuterium SKIE. Especially easy for interpretation are 14C and 2H isotope... 

Spirocyclopropane isoxazolidines 75, obtained from alkylidenecyclopropane nitrones, underwent thermally induced selective rearrangement to pyrrolo[3,4-A]pyridinones 76 <00TA897>. The same adducts 75 in the presence of a protic acid afforded exclusively p-lactams 77 (57-60% yield) accompanied by ethylene extrusion <00JA8075>. 

No reaction occurs with Mel, or with Me3SiCl. Thermally induced rearrangements of phosphazenes to phosphoramidates (38)80 and (39)81 have also been reported. 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

Thermal aza-Claisen rearrangements were induced upon heating the reactant 99 in decalin to 135-190 °C. Though some product 100 could be obtained. 

Claisen rearrangement org chem A thermally induced sigmatrophic shift In which an allyl phenyl ether is rearranged to yield an orffio-allylphenol. kla son, re-o ranj-mont ... 

The thermally induced Cornforth rearrangement can be rationalized by postulating the dicarbonylnitrile ylid 103 as intermediate. It is especially the natures of the substituents R and R" that determine the direction of the rearrangement. For example, heating of 5-alkoxy-4-(aminocarbonyl)oxazole (102, R = OAlkyl, R" = NR2) readily gives in a good yield rearrangement into the isomeric 5-(substituted amino)-4-(alkoxycarbonyl) oxazole (104, R = OAlkyl, R" = NR2) (Scheme IV.40). 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

These authors also showed that the indolizidine skeleton can be prepared from cyclopropyl dipolarophiles (Scheme 1.16). The cycloaddition of alkyhdenecyclo-propanes 67 with various nitrones (e.g., 68) afforded the expected isoxazolidine adducts 69 and 70, commonly forming the C(5) substituted adducts 70 (97,105-108) predominantly but not exclusively (109-111). Thermally induced rearrangement of the spirocyclopropyl isoxazolidine adduct 70 afforded the piperidinones 71 (107,108). These authors propose reaction via initial N—O bond homolysis of 70 to diradical 72 followed by ring expansion through relief of the cyclopropyl ring strain forming the carbonyl of a second diradical intermediate 73, which cyclizes to afford the isolated piperidinone 71. 

Table 18. Cycloocta-1,5-dienes from Thermally Induced Rearrangement of 1,2-Divinylcyclobutanes...

The isolation of 1H- and 4//-pyrazoIes from thermally induced sigma-tropic rearrangements of initially formed 3//-pyrazoles is discussed in Section IV.A.l. 

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