Lumiketone

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). 

The fact that the conversion of (85) to (86) proceeds with greater than 90% retention of optical activity would seem to favor the concerted mechanism.(B9> However, Schuster and Brizzolara<61) have pointed out that the examination of molecular models clearly indicates that the angular methyl (or other group) would strongly inhibit the rotation that is necessary for formation of the new 1,5 bond in the lumiketone from other than the bottom side of the molecule ... 

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... 

Earlier work by Matsuura et al. reported that there was a difference in the photochemical behaviour of santonin dependent on whether the reactions were carried out in the liquid or solid phase. - More recent work has demonstrated that the dienones (273) and (274) do not exhibit this difference in behaviour. Irradiation of these compounds yields the same products (275) and (276) respectively whether the reactions are in solution or in the crystal. The authors - suggest that this similarity in behaviour is due to loose crystal lattice structures. The solid state irradiation of the dienone (277) results in the formation of the normal products for such systems, namely the corresponding photoketone, photophenol, and lumiketone. The ratio of these three products was sensitive to temperature. In solution no temperature dependence was detected. Interestingly when the dienone is irradiated in the solid with wavelengths > 400 nm a quantitative yield of the lumiketone is obtained. ... 

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

Irradiation of 2,5-cyclohexadienone with visible light >400 nm) results in quantitative formation of two isomeric lumiketones 1 0 and 21 solid state 1 0 and 21 are formed with comparable yields, whereas in solution the isomer 1 0 is the major product. Failure for crystalline to photorearrange into a 2,4-cyclopentadienone structure, like a- santonin ( ) does in the solid state (eq 4), is attributed to insufficient compactness of the crystal packing of (28). 

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... 

The photorearrangement of chiral cyclohexadienones and cyclo-hexenones has been covered in other reviews and is not examined here [173]. However, it is now well established that the formation of lumiketones from rigid enones and dienones (226, 228, and 231), or from monocyclic, chiral cyclohexenones having C-4 as the only asymmetric center, occurs with more than 99% selectivity [174] (Scheme 36). 

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]. 

Irradiation of 3-metiioxyphenol 34 in presence of AlBrs gives lumiketone t)tpe product, 4-meflioxybicyclo[3.1.0]-hex-3-en-2-one 35. This reaction provides a useful method for synthesis of bicyclo[3.1.0]-hex-3-en-2-ones [40]. 

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