Photosantonic acid

In acidic solution the rearrangement of a-santonin to lumisantonin and then ultimately to photosantonic acid is not as efficient as rearrangement to the hydroxy ketone, isophotosantonic lactone (5), shown on page 308. Fisch and Richards(6) found that the photorearrangements of a-santonin could be sensitized with benzophenone or Michler s ketone. Moreover, if the irradiation of a-santonin (3660 A) is carried out in piperylene as solvent, the photoreaction is completely quenched. This suggests that the rearrangements proceed via triplet states. 

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 ... 

An early example of this process is typified by the formation of photosantonic acid 7, isolated in high yield when 6 is irradiated in aqueous solution (Scheme 2)8. Barton and Quinkert have also utilized this ring opening/trapping process in the synthesis... 

Photosantonic Acid. The formation of photosantonic acid (Formula 30) from lumisantonin poses problems of special interest. The ketene-carbene mechanism (Formulas 36-38) previously proposed (26,27) to account for this transformation fails to account for the formation of phenols in the irradiation of umbellulone (Formula 39) and the bicyclic... 

Santonin (165), a well-studied photochemically active compound, undergoes a number of photorearrangements. The [l,4]-shift product lumisantonin (166) is obtained in 64% chemical yield in dioxane, whereas a subsequent rearrangement product, photosantonic acid (167), is formed in protic media (Scheme 6.73).778,779... 

Another difficult structural problem where NMR spectroscopy was used to confirm the correct structure of a complex compound was in the analysis of photosantonic acid [ ]. The structure proposed for this compound contained a conjugated diene group however, the ultraviolet spectrum of the compound did not have an absorption band in the region where conjugated dienes absorb. Using NMR spectroscopy, it was shown that the conjugated diene structure did exist. 

On the other hand, photosantonic acid derivatives 122 arise from subsequent photolysis of lumisantonin (120) and may indicate the participation of the intermediate with a Hnear dienone structure (i.e., 121), which is called mazdasantonin. Mazdasantonin has been obtained by irradiation of lumisantonin (120) in aprotic media such as ether or benzene and this compound is rapidly transformed into photosantonic acid when the irradiation is continued in the presence of water. ... 

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