Santonin photochemical rearrangement

A second and more recent example, the photochemical rearrangement of 4,4-diphenylcyclohexadienone (VIII), was provided by the present author and co-workers (4, 5,14). This compound (VIII) when photolyzed in aqueous dioxane with light of wavelength above 310 mp. was found (4, 5) to afford the bicyclic ketone IX, 2,3-diphenylphenol (X) and an acid whose structure was shown (14) to correspond to XI. Additionally, 3,4-diphenylphenol (XII) was shown (14) to be a minor by-product. Strikingly and reminiscent of the dependence of product distribution on solvent in santonin photolysis, it was found (14) that approximately equal quantities of 3,4-diphenylphenol and 2,3-diphenylphenol (X) were formed when the photolysis was run in 50% aqueous acetic acid. [Control experiments (14) demonstrated that neither 4,4-diphenylcyclohexadienone nor bicyclic ketone IX were reactive in the dark under the aqueous dioxane or aqueous acetic acid reaction conditions, in the presence or absence of acid XI.] Furthermore, the bicyclic ketone IX has been demonstrated to afford 2,3-diphenylphenol (X) and the photoacid XI on photolysis in aqueous dioxane, and consequently this ketone may be formulated as a reaction intermediate in the formation of X and XI from 4,4-diphenylcyclohexadienone (VIII) (4, 5, 14). 

Formula 46) in the irradiation of the dienone (Formula 56) has also been shown to involve Formula 45 as an intermediate (35). The difference in the nature of the photochemical rearrangements of prednisone acetate (Formula 47) and santonin (Formula 28) appears to be associated with the absence of a 4-methyl group in prednisone acetate (see Secs. IIA,4 and IIA,7). 

In the l960s, Kropp showed that fused bicyclic dienones structurally related to santonin could potentially serve as synthetically useful precursors to either spirocyclic skeletons or hydroazulenones. One of these cases, 91, was successfully used by Marshall and Johnson as the starting point in an elegant synthesis of the spirocyclic sesquiterpene )6-vetivone (Scheme 23). More recently, a variety of bicyclic cyclohexadienones have been studied by Caine and coworkers. For example, bicyclic dienone 92 could be photochemically rearranged to the oxygenated bicyclo[4.3.0]nonenone system 93, along with other rearrangement products, via acetic acid solvolysis of the cyclopropyl ketone intermediate. ... 

Transformation of santonin (1) into guaianolides has been carried out by two different methods, namely photochemical rearrangement of the dienone system present in the A ring of santonin, and solvolytic rearrangement of a proper 1-sulfonate derivative. 

A early partial synthesis of the guaianolide nucleus was based on photochemical rearrangement of a-santonin (44) to give isophotosantonic lactone (45) [17] (Scheme 98.5). The stereochemistry of this reaction is remarkable. Irradiation of the santonin isomer, in which the C6-06 bond is 3 disposed gives the C6-06, C7-C11 cis isomer of 45 [18]. 

The first photochemical reaction of a cyclohexadienone chromophore in santonin was noted. a-Santonin undergoes photochemical rearrangement in an non-nucleophilic media to give the cyclopropyl ketone lumisantonin, which is itself photochemically labile and yields the linearly conjugated dienone mazdasantonin. 

Furthermore, santonin has been readily transformed into 8-oxyfunctionalized trienones 96a and 96b, which after photochemical rearrangement and lactonization provided a convenient method for the preparation of, among others, the 8,12-guaianohdes 8-ept-pseudoivalin (141), (-i-)-zedolactone A (142), and podoandin (143) (Scheme 24). ... 

Kropp, P.J., Photochemical rearrangements of cross-conjugated cyclohexadienones. V. A model for the santonin series, /. Org. Chem., 29, 3110,1964. 

The rearrangement of cross-conjugated cyclohexadienones to bicyclo[3.1.0]-hex-3-en-2-ones was one of the first photochemical reactions to be thoroughly studied for mechanistic and synthetic purposes 344). The examples outlined below are the conversion of the lactone a-santonin to lumisantonin 345) (3.29)... 

A particularly intriguing rearrangement is the Type-A transformation of 2,5-cyclohexadienones, a reaction typified by the photochemical conversion of Santonin (1) to... 

There are probably no photochemical transformations which are more intriguing to the organic chemist than the deep-seated skeletal rearrangements of dienones (7-11). An interesting and thoroughly studied example is that of the dienone santonin (III), whose complex transformations are outlined in Chart I. Lumisantonin (8, 9) (IV) results from the irradiation of santonin in dioxane. Isophotosantonic lactone (8) (V) is formed on photolysis of santonin instead in aqueous acetic... 

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... 

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