Cope rearrangement photochemical initiation

Photochemical rearrangements have also been reported, as shown in equation (53). " Transition metal catalyzed rearrangement [palladium(O)] of a dienylaziridine has been reported in one case, and a radical opening of a dienylaziridine led to pyrroline formation under the conditions of radical initiation with AIBN/PhsSnH (equation 54). For those vinylaziridines that contain additional unsaturation, the corresponding aza equivalent of a divinylcyclopropane Cope rearrangement is the usual pathwayThe subject of heterodivinylcyclopropane Cope rearrangement is covered in detail elsewhere." The... 

Irradiation (quartz filtered) of the oxazolone derivative (199) in acetonitrile results in decarbonylation and the formation of the imine (200). When this imine is formed in the presence of allyl alcohols, trapping (a thermal reaction) results in the formation of the ethers which are also photochemically reactive and are transformed by a Norrish type II hydrogen abstraction process into the isomeric compounds (201). Oxy-Cope rearrangement of (201) yields the second product (202) isolated from the initial irradiation. 

When vinyldiazomethane was added, the < -l-[(Z)-hexa-l,3-dienyl]-2-vinylcyclopropane cis-47) produced underwent a Cope rearrangement and this was used in the synthesis of (+)-ectocarpene (46). Thermal deazetization of the intermediate 4,5-dihydro-3//-pyrazole gave a better yield than photochemical deazetization, and when the initial mixture of products was heated to 125°C, it was entirely converted to the desired product. The reaction of trans-47 presumably involves an initial isomerization to the cis-isomer. 

When two methylenecyclopropane fragments were linked by a Cj-chain to form a 1,5-hexa-diene, intramolecular reactions can be initiated both thermally and photochemically. Heating in toluene gave rise to a Cope rearrangement with formation of l,r-divinylbicyclopropyl. ° When the starting material was irradiated in benzene, it underwent, among other reactions, an intramolecular [2 +2] cycloaddition. The reaction mixture comprised three different products in an overall yield of 73%. 

The Cope rearrangement is an example of valence isomerization, which has been extensively studied in recent years. Valence isomerizations are iso-merizations in which only single bonds are broken and formed whilst double bonds change their site. They have a high degree of stereospecificity, the steric result depending on the number of double bonds and on whether the reaction is initiated thermally or photochemically.160... 

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