Lumiketone rearrangement

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Irradiation of 4,4-dialkyl-2-cyclohexenones affords bicyclo[3.1. OJhexanones in a so called lumiketone rearrangement (3.22) 337). The reaction proceeds with inversion of configuration on the disubstituted C-atom (3.23) 338). 

Conformationally constrained 2-cyclohexenones which cannot undergo the lumiketone rearrangement (cf. chapter 3.1.5) are efficiently reduced to cyclohexanones (4.18) 420). 

Synthetic aspects of the di-7i-methane rearrangement have been reviewed.273 An aryl version of the cyclopropyl-7i-methane rearrangement has been reported 274 see (226) —> (227). A model study of the mechanism of the di-7i-methane and lumiketone... 

There are also important rearrangement reactions for cyclic enones. For cyclohexenones two prominent reactions are the lumiketone rearrangement (Path A) and the di-i -methane rearrangement (Path B). The di-Tr-methane rearrangement pathway is restricted to 4-aryl or 4-vinyl cyclohexenones. 4,4-Dialkylcyclohexenones undergo the lumiketone rearrangement, which involves the shift of the C(4)—C(5) bond to C(3) and formation of a new C(2)-C(4) bond. ... 

The lumiketone rearrangement is stereospecific and can be described as a [Tr2 + cr2J cycloaddition. This mechanism requires that inversion of configuration occur at C(4) as the new cr bond is formed at the back lobe of the reacting C(4)—C(5) cr bond. 

The lumiketone rearrangement also proceeds in the case of some 4-alkyl-4-arylcyclohexenones. 

The conversion of the enone (83) into the products shown in Scheme 8 upon irradiation in t-butyl alcohol is quite inefficient ( = 0.017).46 This study was aimed to solve one of the problems which has bedevilled the study of cyclohexenone photochemistry, namely that of the concertedness or non-concertedness of the rearrangement to the lumiketone products. When optically active starting material (83) was used all the products were optically active. The formation of... 

Dialkylcyclohexenone 60 undergoes photo-induced lumiketone rearrangement to form cyclopropane derivative 61 [35]. 

This lumiketone rearrangement also occurs in steroid 65 and 4-alkyl-4-aryl cyclohexenone 66 to produce 67 and 68, respectively [37, 38]. 

The reaction could proceed via the known lumiketone rearrangement of the tautomeric 2,5-cyclohexadienone (78, Fig. 44) (54). This is however not the case. Since the equilibrium between humulone and 78 is shifted in favour of tautomer 3, all the light is absorbed by 3. Photo-enolization of 3 would only lead to the ground state of 78 (55). It may be concluded that the rearrangement proceeds via the known dienol form of humulone (3, Fig. 44). Usually, 2,4-cyclohexadienones are cleaved... 

The reaction can however take a different course when the irradiation is carried out in polar medium (57) or when the substrate is heavily substituted (56,57). In this case a product analogous to that formed upon the lumiketone rearrangement is found... 

Bicyclo[3,l,0]hex-2-en-3-ones related to 6 (lumiketones) are normally prepared by irradiation of dilute solutions of dienones in dioxane or toluene. Best results are obtained with a low-pressure mercury lamp that emits most of its radiation at 253.7 nm. Control of the irradiation time and of the wavelength are important to avoid further rearrangement of the lumiketones to dienones and phenolic products, which are formed on irradiation with light of wavelength greater than 300 nm. °... 

In contrast to 78a, its CIO epimer 80a, which has a cis relationship between the methyl and the cyclopropane ring, rearranges in the normal way to give the lumiketone 81a or the 5/7-fused hydroxy ketone 82a upon irradiation in dioxane"" or aqueous acetic acid, respectively. The related dienone 80b reacted similarly to 80a in dioxane but it was prone to undergo thermal cleavage of the cyclopropane... 

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