Cyclohexadienones, photochemistry

Pyran-4-ones bear an obvious structural similarity to the all-carbon cyclohexadienones discussed above. However, the original studies of their photochemical behavior revealed only dimerization processes to produce a cage product resulting from two successive head-to-tail [2 + 2]-photocycloadditions (Scheme 29)54. Much later, small amounts of substituted furfural 121 were observed during the irradiation of 11955a. It was speculated that 121 could arise from bicyclic epoxide 120, an intermediate analogous to those formed in cyclohexadienone photochemistry. Subsequent reports noted that further irradiation of... 

Having returned to Braunschweig, I turned to other problems in organic photochemistry. The photochemistry of 2,4-cyclohexadienones -sol believed - was to be developed further at Imperial College. Months later I met Barton again, and he inquired after new results in 2,4-cyclohexadienone photochemistry. My expressed belief- that this topic was his field - was emphatically rebutted and I was implored to carry on those studies regardless . 

Schultz, A.G., Cyclohexadienone photochemistry trapping reactions, in CRC Handbook of Organic Photochemistry and Photobiology, W.M. Horspool and P.-S. Song, Eds., CRC Press, Boca Raton, FL,... 

Samuel, C.J., Type A zwitterion in cyclohexadienone photochemistry chemical trapping and stereochemistry, /. Chem. Soc., Perkin II, 736-740, 1981. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

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) ... 

The mechanistic photochemistry of cross-conjugated 2,5-cyclohexadienones has been comprehensively examined (Scheme 21). The photochemical isomerization of the... 

Still another example of bond order control of photochemical reactivity is found in the photochemistry of 2,5-cyclohexadienones. Here, strong evidence suggests that the n-7t excited triplet is the species responsible for the rearrangement6,12,17 23). It is seen that the critical step is P, (3-bonding in the excited state. The reaction mechanism is depicted in Scheme 2. 

The photochemistry of a,p nsaturated ketones has attracted much attention and is still a field cld of current interest. 1/Numer-ous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition ructions and rearrangements. For example, cyclopentenones / and/readily rearrange to cyclopropyl ketenes upon irra/idiation. Recently, the related cyclohexadienone/butadienyl ketene rearrangement has been shown to be a highly useful tool in the synthesis of natural products and macrocydic lactone /... 

Following the pioneering work of Schultz and others on the solution phase photochemistry of linearly conjugated cyclohexadienones [13], we prepared the achiral benzo derivative 7a (Scheme 3) and studied the solid-state photochemistry of its salts with a series of optically pure amines [14]. Diazomethane workup afforded the expected chiral bicyclic photoproduct 8, whose optical purity was determined by chiral GC. The results are summarized in Table 2. 

For reviews of the photochemistry of linearly conjugated cyclohexadienones, see. 13a. Schultz AG. In . CRC Handbook of Organic Photochemistry and Photobiology Hors-... 

Studies of the low-temperature photochemistry of umbellone (288) established a dual pathway for its conversion to thymol (291). Besides the opening of (288) to ketene (289), its direct conversion to the cyclohexadienone (290) was also postulated. However, this reaction depends on the substitution pattern, and in the case of lumisantonin (292) only traces of cyclohexenone (294) were detected in the photolysis... 

The photochemistry of cyclohexadienones is a complex topic, which has been reviewed and is discussed in detail elsewhere in this series. It is only appropriate here to point out that aromatization to a phenol is a possible outcome of such reactions, and indicate a very few synthetically useful examples. 

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