Ketone irradiations

With 21-acetoxy-20-ketones formation of a 20-ketal via an exomethylene 

A consequence of the orientation of the 11-carbonyl function towards the C-19 methyl group which is retained in the excited state is the exclusive functionalization at C-19. Ring cleavage products of the Norrish II type are not observed but the reaction is rather sensitive to conformational changes in the substrate. In a series of experiments conducted under comparable conditions (24 hr irradiation) the yield of cyclobutanols drops 

In order to convert the cyclobutanols obtained in the photoreaction into angularly oxygenated steroids the 18,20 or 11,19 bond must be broken. 

In the case of 18,20-cyclo-20-hydroxy steroids this is achieved by dehydration and oxidative cleavage.  

The lla-hydroxy-11,19-cyclo steroids are very efficiently converted into 19-acetoxy-11-ketones by treatment with lead tetraacetate in benzene.  

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Conversion of pregnanes to androstanes via the Schmidt Rearrangement 17/3-aminoandrost-5-en-3/3-ol, 145 Cyclobutanol formation by ketone irradiations... 

II. Functionalization of the Angular Methyl Groups / 237 General principles of substitution / 237 Lead tetraacetate oxidations / 240 Hypohalite reactions / 246 Photolysis of nitrites (Barton reaction) / 253 A-Chloroamine reactions (Hoffmann-Lofller reaction) / 257 Ketone irradiations / 260 Summary / 264... 

Table V. Ketone Irradiations in the Small Glass Chamber...

Ketones. The individual reactivities of ketones were measured in several irradiation chambers, including those of Battelle (4, 35) and LA-APCD (2, 3). In addition to the data given in Tables I to III for ketones in the steel chamber, we present Table V with experimental findings for ketone irradiations in the small glass chamber. Fairly good agreement exists between the relative chemical reactivities observed for several ketones in the different investigations. 

It is important to point out that the quantum yield for the Norrish II chain scission reaction ( Cs) is highly affected by the mobility of polymer chains. For example, the photolysis CS for a film of the copolymer poly(styrene-co-phenyl vinyl ketone) irradiated at 313 nm in the solid state was shown to be low (0.04—0.09) at temperatures below the copolymer Tg (glass transition temperature) but increased dramatically at,... 

Figure 4 shows electron attachment spectra from several deoxy-genated aqueous alkaline solutions of some conjugate ketones irradiated with a single 0.2-/xsec. pulse of 1.3 Mev. electrons (9, 8) (e.g., Reaction 18). 

Further mechanistic information concerning type I cleavage reactions has been obtained by Yang in a study of alkyl-t-butyl ketones. Irradiation of these ketones in hexane solution results in a mixture of products corresponding to both type I and type II cleavage, for example,... 

Unsaturated Aldehydes and Ketones.—Irradiation of oxygenated solutions of T7 -allylpalladium complexes leads to unsaturated carbonyl compounds in modest yield. Oxidation of (steroidal) 7r-allylpalladium complexes can also be carried out using chromium trioxide in dimethylformamide containing a trace of sulphuric acid. ... 

Ketones in which the double bond is located in the p,y position are likely candidates for a-cleavage because of the stability of the allyl radical that is formed. This is an important process on direct irradiation. Products then arise by recombination of the radicals or by recombination after decarbonylation. 

Bromination with A-bromosuccinimide generally gives the same result as bromination with free bromine or hypobromous acid. The reaction is considered to proceed with a small concentration of free bromine and does not generate an appreciable concentration of acid. Conditions are therefore mild. In addition, A-bromosuccinimide has been used to brominate the allylic position of a, -unsaturated ketones in the presence of free-radical promoters or with irradiation, and thus gives access to dienones by dehydro-halogenation, for exaraple " ... 

The photochemical addition of simple olefins to a,j -unsaturated ketones is a reaction of great current interest. The steroidal A -20-ketone system is especially prone to cycloaddition under mild conditions. Sunder-Plassmann et irradiated 3j5-acetoxypregna-5,16-dien-20-one (67) in the presence... 

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

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. 

TLC analysis of the crude reaction product reveals no additional products. Similarly quantitative results are obtained using benzene, /-butanol and ether as the solvents in the irradiation of (39). The reaction is less sensitive to oxygen than most ketone photolyses resulting in products from a-cleavage processes. 

Irradiations of the unsaturated epoxy ketone (92) in the presence of tri- -butylstannane at 2537 A (in dioxane) or >3100 A (in dioxane or benzene) produce qualitatively similar mixtures containing products (71), (101),... 

According to Quinkert, photoexcited cyclic ketones may be transformed to open-chain unsaturated carboxylic acids in the presence of molecular oxygen. This reaction may compete efficiently with a-cleavage and secondary transformations thereof. Thus, both stereo iso meric 17-ketones (109) and (110) yield as much as 20% of the unsaturated acid (111) when irradiated in benzene under a stream of oxygen. This photolytic autoxidation has been used notably for partial syntheses of naturally occurring unsaturated 3,4-seco-acids from 3-oxo triterpenes (for references, see ref. 72). 

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