1.5- Dienes thermal rearrangement

The Af-chlorosulfonyl-4-alkenyl-/3-lactams (119), formed by cycloaddition of CSI with the corresponding dienes, undergo a thermal rearrangement to give the formal [4 + 2]... 

Another example of a reaction in which the stereochemistry of the process provides some valuable information about the mechanism is the thermal rearrangement of 1,5-dienes and substituted analogs ... 

The most important sigmatropic rearrangements from the synthetic point of view are the [3,3] processes involving carbon-carbon bonds. The thermal rearrangement of 1,5-dienes by [3,3] sigmatropy is called the Cope rearrangement. The reaction establishes equilibrium between the two 1,5-dienes and proceeds in the thermodynamically favored direction. The conversion of 24 to 25 provides an example ... 

The thermal rearrangement of 1,5-dienes 1 to yield the isomeric 1,5-dienes 2, is called the Cope rearrangemenC —not to be confused with the thermolysis of amine oxides, which is also named after Arthur C. Cope. 

Small amounts of cyclopentene derivatives are detected in cyclopropanation reactions of electron-deficient dienes, but they may result from thermal rearrangement of the corresponding vinyl cyclopropanes and not from a direct [4+1] cycloaddition... 

Dienes (17) and (18) may not actually be photoproducts but may arise from thermal rearrangement of m-l,2-dialkenylcyclobutanes.<8,9) As in the photodimerization of butadiene, the product distributions of sensitized... 

In both cyclic and acyclic dienes which can achieve the necessary geometry the [1, 5] shift is commonly observed because the activation energy is lowest for the transition states involving minimal distortion. This is particularly so in cyclopentadienes and indenes. The preference for [1, 5] over [1, 3] shifts is demonstrated by thermal rearrangement of 1-duterioindene. At 200°C, deuterium became scrambled over all three non-benzenoid carbons. 

Several 4-aminocyclopent-l-enes have been prepared in two steps from conjugated dienes via the corresponding 2-alkenylcyclopropylamines and their thermal rearrangement. 

In contrast to earlier literature reports, photo-oxygenation of 17/3-hydroxyoestr-5(10)-en-3-one has been shown not to be stereospecific, since a mixture of 10a- and 10/3-hydroperoxy-3-oxo-A -compounds was obtained. Thermal rearrangement of these products gave the 10a- and 10/3-hydroxy-3-oxo-A -compounds. Photo-oxygenation of 5a-cholesta-1,3-diene led only to cholesta-1,4-... 

Anomalous isomerizations have been noted during the photolytic and thermal rearrangements of 3-acyl-2-methoxy-3//-azepines (2 R -acyl, R2 = OMe) and 3-acyl-3H- azepin-2-ones (69T5217). Irradiation in methanol solution produces mixtures of 3-azabicyclo[4.1.0]hepta-2,4-dienes (28 R1==acyl and H, R2 = OMe, R3 = H) (or -4-ene-2-ones) and 3-phenacylpyridines (or pyridones), albeit in poor yields. Detailed, but tentative, arguments involving azanorcaradiene and/or diradical intermediates are presented to explain the formation of these unusual products. 

An efficient synthesis of hexafluorobut-2-yne25,26 combined the knowledge that hexafluorocy-clobutene thermally rearranges to hexafluorobuta-1,3-diene (vide supra) and that cesium or potassium fluoride isomerize dienes to alkynes (see Section 5.1.2.3.). Thus, when hexafluoro-cyclobutene is passed over cesium or potassium fluoride at 590-600 C, hexafluorobut-2-yne is obtained in yields up to 90%, with 95% recovery of material. 

Although a number of competing pathways have been shown to be involved301 in the thermal cycloisomerization of hexa-l,3-dien-5-ynes (255), isobenzenes (256) have been established302 as intermediates in their thermal rearrangement to arenes. On solvolysis, the diethylphosphate ester of l-(2-ethynylphenyl)-4-trimethylsilyl-4-(trimethylsilyl-oxy)pent-2-yn-l-ol has been found303 to afford 5-(2-ethynylphenyl)-3-trimethylsilyl-... 

It has been suggested307 that the observed thermal rearrangement of the bicyclo[2.2.2]- to the bicyclo[3.2.1]-octadiene skeleton in tricyclic lactams, viz. (265)—>-(266), is consistent with an intramolecular Diels-Alder reaction in which a zwitterion is formed by the migration of one vinyl group from C(l) to C(5) of the lactam moiety (see Scheme 62). It has been shown308 that the 7r-facial selectivity of the Diels-Alder reaction of 1,3-dienes (267) having an attached stereogenic centre can be... 

Matsumoto, M Nasu, S., Takeda, M., Murakami, H. and Watanabe, N. (2000) Singlet oxygenation of 4-(4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran-5-yl)-2-pyridone non-stereospecific 1,4-addition of singlet oxygen to a 1,3-diene system and thermal rearrangement of the resulting 1,4- endoperoxides to stable... 

Thermal rearrangement of endoperoxides of bicyclic dienes gives syn-diepoxides [9b,33a], which are sometimes accompanied by epoxyketones... 

The p-peroxide 84 was obtained by methylene blue-sensitized photooxygenation of diene 83 together with its a-isomer (2 1 9,13-a p) and was separated by silica gel chromatography. Diepoxides are the thermally rearranged products of a.a -unsubstituted or alkylsubstituted furan endoperoxides [59,60d,g]. 

Earlier work on the photolytic or thermal rearrangement of polyhalogenated pyridazines to corresponding pyrazines has been continued,14,161,774,1690 but the fascinating results offer little of preparative value. It has been reported that 300-nm irradiation of 3,4,5,6-tetra-tert-butylpyridazine (66) gave a quantitative yield of the Dewar isomer (3,4,5,6-tetra-tert-butyl-l,2-diazabicyclo [2.2.0]hexa-2,5-diene 67] that subsequently afforded 2,3,5,6-tetra-tert-butylpyrazine (68) in 18% yield on 254-nm irradiation.1464... 

Aziridines, A/-phenyl-, nitration, 58, 259 Aziridine-2-carbohydrazide, condensation with acetone, 59, 42 Azirines, thermal rearrangement to I -dimethylamino-2-aza-1,3-dienes,... 

A fairly general synthetic route involves the PF3-induced displacement of ligands such as dienes and phosphines coordinated to metal allyl complexes (method F), and it has also been possible in certain complexes to interconvert /3-allyl-metal-PF3 complexes by an intramolecular thermal rearrangement (method G). 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reaction of conjugated dienes with singlet oxygen, is a convenient route fw the preparation of bisepoxides. Epoxidation of cholesteryl acetate (176) with air in the presence of a catalytic amount of dioxo(tetra-mesitylporphyrinato)ruthenium(VI) fimiishes in 8S% yield a 99 1 mixture the epoxides (177a) aitd (177b)." >... 

It was later demonstrated that both Ni and Pd° species catalyzed the conversion of l-methyl-2-vinyl-3-prop-2-enylcyclobutanes to ( )-3,7-di-ethylcycloocta-1,5-diene. However, as shown in Scheme 4, [PdCl2(PhCN)2] forms an equal quantity of the (Z)-isomer, the sole product of the thermal rearrangement. A side reaction in the Ni -catalyzed reaction clearly demonstrates the scission of the C-C bond required in the rearrangement, by producing penta-1,3-diene in low yield. 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reacdon of conjugated dienes with singlet oxygen, is a convenient route for the preparadon of bisepoxides. ... 

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