Ketones photoisomerizations

Noyori, R., Inoue, H., and Kato, M., Direct observation of dienol intermediates in photochemical deconjugation of an a,P-unsaturated ketone photoisomerization of l-acetylcyclooctene,B //. Soc. Chem. Jpn, 49, 3673,1976. 

The first, and to this writing still only case of a ketone a-cleavage-recombi-nation sequence in the steroid field was reported by Butenandt, who found that 17-ketones epimerize at C-13. Ultraviolet irradiation of either stereoisomer produces an equilibrium mixture in which the thermodynamically more favored 13a-compound cf. (15)] with cw-fusion of rings C and D predominates at room temperature. As ultraviolet absorption energies and intensities of the two isomeric ketones are practically identical, the equilibrium composition depends largely on the rate of the competing recombination process from (14). For further examples of the photoisomerization at C-13 of 17-ketosteroids, see ref. 8, 12, 15 and 43. 

The predominant, if not exclusive, formation of 5/7-fused hydroxy ketones was observed in the case of 4-alkylated dienones [(204) (205) (R = CH3) 6 1 from (201) (R = CH3)] ° and of prednisone 21-acetate [(206)-> (207)]. It appears therefore likely that intermediates which represent the conjugate acids of the postulated zwitterionic intermediates in the dienone photoisomerizations [c/. (202), (203)] participate both in the acid-catalyzed transformations of (200) and in the dienone photochemistry in protic solvents. 

Photoisomerization of the unsaturated lactone (200) to the 1,5-cyclo-lactone (201) provides the first example of a Type A rearrangement in an j8-unsaturated lactone. An X-ray structure analysis has confirmed the photoisomerization of the /3-y-unsaturated ketone (202) to the cyclopentanone (203) by a 1,3-acyl shift. A similar rearrangement of the 3-oxo-A "° " -compound (204) to the cyclo-butanone (205) is in contrast with earlier reports of the oxo-di- r-methane rearrangement of 3-oxo-A " Lcompounds on direct irradiation. Benzophenone-... 

In a study of the photoisomerization of 4,4-dimethyl-2-cyclohexenone (8) to 6,6-dimethylbicyclo[3.1.0]-hexane-2-one (9) and 3-isopropyl-2-cyclo-pentenone (10), Chapman and Wampfler48 accounted for the pronounced effect of sensitizer concentration by such a self-quenching mechanism. The phenomenon was exhibited by four ketones with () lowest triplet states but not by benzophenone or acetophenone which have (w, n) triplet states.49... 

The key step in their-approach was asymmetric photoisomerization of the a, 3-unsaturated (Z)-ketone precursor 103b in diethyl ( + )-Lg-tartrate. The bromide 103a obtained by N-bromo-succinimide bromination of the (Z)[8.8] precursor 64b, was converted into the a,P-unsaturated (Z)-ketone 103b by the routine synthetic procedures. Irradiation in a hexane solution with a medium pressure Hg lamp effected the photoisomerization of the (Z)-precursor 103b to afford a 1 5.5 mixture of (Z)-( )[8.8] ketones. After these preliminary experiments, a neat solution of 103 b in diethyl ( + )-Lg-tartrate was irradiated for 3 h. Preparative GLC of the resulting 1 7 mixture of (Z) and ( )[8.8] ketones produced a 38 % yield of ( )[8.8] ketone 104 enriched in the (—)-enantiomer, [a]n4 —13° (hexane). 

The QC is thermodynamically unstable relative to NBD, but a catalyst is needed to revert it to NBD. The reverse reaction of QC to NBD catalytically releases heat. The storage capacity of this is 21 kcal/mol. Since unsubstituted NBD has no absorption in visible region, a sensitizer such as aromatic ketones is required for the NBD isomerization by visible light. The efficiency and stability of various storage systems based on the photoisomerization were studied100b). 

Eberbach and coworkers have made an extensive study of the thermal chemistry and photochemistry of ethenyloxiranes (81CB1027 and refs, therein). Thermal isomerization can involve mainly either C—O (Scheme 8) or C—C (Scheme 9) cleavage. The reluctance of (11) to generate an ylide may be due to the fact that allowed, conrotatory opening would give a strained species (12) (cf. OEO-cycloheptene). The photoisomerization of ethenyloxiranes can involve either C—O (Scheme 10) or C—C (Scheme 11) cleavage. Photolysis of (13) led to the synthesis of the ketone tautomer (14) of a phenol (Scheme 12) (80TL463). 

These photoisomerizations can be solvent sensitive. For example, photolysis of the azepinone (42 R1 = aryl, R2 = Bu, R3 = H) in cyclohexane affords the azabicycloheptadiene (43 R1 = Ar, R2 = Bu , R3 = H) in 80% yield. Irradiation in methanol solution, however, affords a mixture of the two isomeric enols of the cyclopentenone (44 60%), and only 5% of the bicycle (43) (81JOC4077). Apparently, in aprotic solvents the azepine behaves photo-lytically as a heterocyclic conjugated triene, whereas in methanol its /3,-y- unsaturated ketone character predominates. 

The final stage in Dussault s total syntheses of marine 3-methoxy-l,2-dioxines of type 3 is based on the cyclization of a,/S-unsaturated y-hydroperoxy ketones 399 as exemplified in Scheme 106287 288. Thus, photoisomerization of a,/8-unsaturated y-hydroperoxyketone... 

Of the individual ketones, the photoisomerization of 2-methyl cyclohexanone, 23, has already been mentioned. 

The demonstration that both double bonds of the cross-conjugated dienone system are not necessary for photoisomerization considerably extends the potential utility of these rearrangements because of the greater availability of a,/3-unsaturated ketones. It is safe to predict that photochemical rearrangements of a,/3-unsaturated ketones will be subject to intense investigation in the immediate future. 

Irradiation of methylcyclopropylketone (Formula 311) causes isomerization to 3-penten-2-one (Formula 312) (124). This reaction appears to be limited to acyl cyclopropanes, since acyl cycloalkanes with larger rings undergo photochemical fission to acyl radicals and cycloalkyl radicals (105). The photoisomerization of the bicyclic ketone (Formula 313) to Formula 314 (125) is analogous to the isomerization of Formula 311. 

Photoisomerization of eucarvol (Formula 371) followed by chromic acid oxidation of the photoalcohol (Formula 372) gives the bicyclic ketone (Formula 373) in good yield (164). This approach to the synthesis... 

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