Photorearrangements cyclohexadienones

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a czs-diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid ... 

Typical examples of circumambulatory rearrangements of bicyclo[3.1. Ojhexenyl cations are shown in Schemes 19 and 20. Swatton and Hart reported the isomerization shown in Scheme 19 in 1967 and proposed that the observed deuterium scrambling could be accounted for on the basis of a cyclopropyl walk reaction153. This circumambulation is comparable to that proposed by Zimmerman and Schuster as part of the sequence of reactions involved in the type A photorearrangement of 2,5-cyclohexadienones. ... 

The photochemical rearrangements of cross-conjugated cyclohexadienones in general, and of 4,4-diphenylcyclohexadienone (81) in particular, have been intensively studied.115 When 81 is irradiated in dioxane-water, first 6,6-di-phenylbicyclo[3.1.0]hex-3-one-2 (82) is obtained which, on further irradiation, forms 83, 84, and 85. The primary photorearrangement product, 82, can also be obtained by photosensitization of 81, but not by irradiation of 81 in piperylene. Therefore 82 is formed from the lowest triplet of 81. The subsequent rearrange-... 

Uppili, S. and Ramamurthy, V. (2002) Enhanced enantio- and diastereo-selectivities via confinement photorearrangement of 2,4-cyclohexadienones included in zeolites. Organic Letters, 4 (1), 87-90. 

Dimethyl-2,4-cyclohexadienone photorearranges to bicyclic [3.1.0] product via oxa-di-rr-methane rearrangement (Scheme 41). This achiral molecule photorearranges in solution to the chiral bicyclic product as a racemic mixture. This molecule within NaY zeolites in the presence of chiral inductors yields products with respectable ee. The best numbers are 50% (ephedrine as a chiral inductor at — 55°C) and 28% (pseudoephedrine at room temperature) [294]. These numbers are significant when one recognizes that in solution only a racemic mixture is obtained. 

The cyclohexadienone (278) is photochemically reactive and on irradiation both Z-f-isomerism of the aryl substituted double and (2-I-2)-cycloaddition affording (279) takes place. Interestingly no evidence for the usual photorearrangement path was reported. 

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). 

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). 

A tandem photorearrangement-cyeloaddition of cyclohexadienones tethered with various alkenes (28) results in the formation of polycyclic frameworks. With allynyl ether-tethered substrates, [3 + 2] cycloaddition occurs, leading to strained alkenes, which can be further employed for the elaboration of molecular complexity. ... 

Sundararajan, U. and Ramamurthy, V., Enhanced enantio- and diastereoselectivities via confinement photorearrangement of 2,4-cyclohexadienones included in zeolites, Org. Lett., 4, 87, 2002. Paquette, L.A., Eizember, R.E, and Cox, O., Photochemical transformations of selected 3-cyclo-heptenone derivatives,/. Am. Chem. Soc., 90, 5153,1968. 

Broka, C.A., Photorearrangements of carbomethoxy-substituted cyclohexadienones in neutral media, J. Org. Chem., 53, 575,1988. 

Schultz, A.G., Photorearrangement reactions of cross-conjugated cyclohexadienones, in CRC Handbook of Organic Photochemistry and Photobiology, W.M. Horspool and P.-S. Song, Eds., CRC Press, Boca Raton, FL, 1995, 685-700. 

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