Cyclohexadienones rearrangement

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. 

In the monocyclic series, the 2,4-cyclohexadienones rearrange to photoproducts of potential synthetic value. However, for efficient 1,2-acyl migration, only a relatively small number of substrates are suitable. These must be highly substituted, for example (48). Whereas on direct excitation in methanol, cleavage to the isomeric ketenes (49) (4> 2 0.42) predominates, the remarkably stereoselective 1,2-acyl shift to the bicyclohexenone (50) is found either in trifluoroethanol or when the dienone is adsorbed on silica gel. The conversion to (50) is followed by a reversible phototransformation to the cross-conjugated dienone (51) and accompanied by aromatization to (52) to a minor extent. Such reactivity has also been verified for tetra- and penta-methylated 2,4-cyclohexadienones. The only photoreaction of the hexamethylated homolog, on the other hand, is ketene formation. ° ... 

The intramolecular chemical titration is conceptually and experimentally simple and convenient, but it requires that a particular dioxetane must be made that chemi-energizes the photochemically active carbonyl product K. This is usually a formidable and challenging synthetic problem. Representative intramolecularly chemienergized photochemical transformations include Norrish Type I cleavage (Eq. 44), Norrish Type II (Eq. 45a, b, c) cleavages, cyclohexadienone rearrangement (Eq. 46), and cyclopentenyl ketone rearrangement (Eq. 47). 

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). 

Reymond, J. L., Chen, Y., Lemer, R. A. Antibody catalysis of cyclohexadienone rearrangements. US 5500358,1996 (Scripps Research Institute, USA). 

Houser and his co-workers report that the irradiation of anisole gives m-cresol, possibly via cyclohexadienone rearrangement of a keto-tautomer of p-cresol. 

Section 24 13 On being heated allyl aryl ethers undergo a Claisen rearrangement to form o allylphenols A cyclohexadienone formed by a concerted six tt electron reorganization is an intermediate... 

In a general reaction known as the cyclohexadienone phenol rearrangement cyclohexa dienones are converted to phenols under conditions of acid catalysis An example is... 

Claisen rearrangement (Section 24 13) Thermal conversion of an allyl phenyl ether to an o allyl phenol The rearrange ment proceeds via a cyclohexadienone intermediate... 

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). 

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

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. 

When photolyzed, 2,5-cyclohexadienones can undergo a number of different reactions, one of which is formally the same as the di-TC-methane rearrangement. 

Cyclohexadienones also undergo photochemical rearrangements, but the products are different, generally involving ring opening. ... 

The initially formed cyclopropyl ketones from the photolysis of cross-conjugated cyclohexadienones are also photolabile. Usually the photochemical rearrangements of cyclopropyl ketones involve cleavage of the cyclopropyl bond which forms part of the cyclopentenone ring followed by either (a) a substituent migration or (b) rearrangement through a spiro intermediate. While the literature in this area is too voluminous to review in... 

---