Cyclohexadienones rearrangements with

Schuster and Brizzolara<61> have provided a very nice study of the photochemistry of 10-hydroxymethyl-A1 9-2-octalone (87). Schuster and Patel<13) previously used radical fragmentation reactions as probes for the nature of the intermediates in the cyclohexadienone rearrangement. This compound (87) was designed so that it could undergo a radical fragmentation reaction in competition with the usual type A rearrangement if the intermediate involved has radical character (n -> n triplet). Photolysis produced (88)—(92) ... 

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

The reaction proceeds in the singlet state and is stereospecific in that the configuration of the handlebars is retained for migrations of up to three bonds. Both the mechanism of the di- r-methane rearrangement and that of the 2,5-cyclohexadienone rearrangement (dealt with in Section 7.5.5), involve a step that may be formulated as a bicycle rearrangement (Zimmerman, 1982). 

A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

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. 

Analogous olefin trapping chemistry employing 2,7-cyclooctadienone has been investigated51. With this substrate, the oxyallyl zwitterion 112 could be intercepted intermolecularly with simple alkenes in a (presumably stepwise) 3 + 2 manner (Scheme 27). Importantly, intermediate 112 differs from 2,5-cyclohexadienone-derived zwitterions in terms of its lack of a facile rearrangement pathway. An interesting side-issue... 

The cycloaddition of picryl azide with phenoxyallene took place at the C1-C2 double bond of the allene exclusively to give the triazoline intermediate 97 [89]. This intermediate underwent a facile Claisen rearrangement to yield cyclohexadienone 98, which rapidly tautomerized to phenol 99. 

Hexamethylbenzene reacts with DMDO via three pathways (i) to an arene oxide, which rapidly rearranges to an oxepin tautomer that then is oxidized to a cw-diepoxide and then to a cis, cis,trans-triepoxide (ii) a methyl group migrates in the first epoxide to give a cyclohexadienone, which then reacts to give a frani -diepoxide (iii) C—H insertion to give the benzyl alcohol and then the corresponding benzoic acid. ... 

An example of 2,4,6-triphenylpyrylium-3-olate (65 R = R = R = Ph, R = H) reacting as a 1,3-dipole was first provided by Suld and Price who obtained a maleic anhydride adduct (C25HigO5). Subsequently, an extensive study of the cycloadditions of this species has been published by Potts, Elliott, and Sorm. With acetylenic dipolarophiles, compound 65 (R = R = R = Ph, R = H) gives 1 1 adducts that have the general structure 74 and that isomerize to 6-benzoyl-2,4-cyclohexadienones (76) upon thermolysis. This thermal rearrangement (74 -> 76) has been interpreted in terms of an intermediate ketene 75. The 2,3-double bond of adduct 74 (R = Ph) is reduced by catalytic hydrogenation. Potential synthetic value of these cycloadducts (74) is demonstrated by the conversion of compound 74 (R = Ph) to l,2,3,4,6-pentaphenylcyclohepta-I,3,5-triene (79 R= Ph) via the alcohol 78 (Scheme 1). ... 

Zimmerman has shown that the rearrangement of 4,4-diphenyl-cyclohexadienone, 33, can proceed by an excited triplet.406 Acetophenone acts as a photosensitizer for the reaction, but dilute naphthalene is an inefficient quencher of the unsensitized reaction. As observed for santonin, rearrangement apparently is fast enough to compete favorably with diffusion-controlled quenching. 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

Takenaka and Lemal subsequently undertook an extensive study of the perfluorobenzene oxide (3)/perfluorooxepin (4) system.13,14 After considerable effort, benzene oxide 3 was successfully prepared by ozonization of tricyclic precursor 5 and photolytic decomposition of the resultant ozonides. Oxepin 4 was not observed directly, but was estimated by NMR computer simulations to be present in small proportion (3 %) at 55"C. The principal reaction of 3 is its rearrangement to cyclohexadienone 2, which occurs at room temperature in polar solvents, by heating in nonpolar solvents, or in the presence of Lewis acids. Photolysis of 3 with benzophenone as triplet sensitizer also produces 2. Furthermore, attempts to trap the oxepin, for example by treatment with bromine in the dark, result in cyclohexadienone 2. 

Fluonnatedphenols are usually oxidized to cyclohexadienone and benzoqui-none derivatives and to products of ring rearrangement and ring degradation Phenols with a tnfluoromethyl group at the nng are hydroxylated by potassium persulfate [57], whereas chloric acid causes oxidation to quinone, mg contraction, and incorporation of chlorine in the product [58] (equation 53)... 

The photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

Cyclobutenones. Cyclobutenones and cyclobutenediones undergo photochemical rearrangement to unsaturated acids in a manner analogous to that of 6,6-disubstituted-2,4-cyclohexadienones. Irradiation of Formula 133 in ether saturated with water gives Formula 134 (49). In... 

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