Transformations of Conjugated Cyclohexenones

The 7T orbital extends over the entire enone system. As a consequence the C —CO bond is strengthened in the excited states and a-cleavage, the important primary photoprocess in nonconjugated ketones, is not observed. The most important unimolecular reaction types are processes which involve 

Photoketone (118) has served as the key intermediate in a relatively simple transformation of 3-keto-10/ -steroids to 3-keto-lOa-isomers without the assistance of other functional groups suitably situated next to the ring junction—a task that appears difficult to attempt by other methods. Optimal yields of (118) are achieved by catalytic hydrogenation of the unsaturated ketone (175), the photoisomer of 1-dehydrotestosterone acetate (see section III-C). In this way, a 6-step conversion of 1-dehydrotestosterone acetate (174) to IOa-testosterone acetate (127 acetate) is achieved in good yield. °  

Instead, a solvent molecule is incorporated to give the a-benzyl ketone (123) in high yield.  

Slight structural changes of the model compound testosterone may therefore further reduce the inherently low reactivity and favor the competition of 

322 / ORGANIC REACTIONS IN STEROID CHEMISTRY EXPERIMENTAL PROCEDURES 

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Photochemical transformations of conjugated cyclohexenones, 317 Photochemical transformations of non-conjugatea ketones, 292 Photochemistry of cyclobutanones, 293 Photolysis of nitrites, 253... 

I. Transformations of Conjugated Cyclohexenones and Their Photoisomers. 81 IT. Transformations of Cross-Conjugated Cyclohexadienones and Their Photoisomers. 85... 

This cuprate also underwent the conjugate addition with acyclic enones, for example, 2-hexene-4-one, to give 5-methyl-3-tridecanone in 47% yield. We have not attempted further optimization. Reaction of this calcium cuprate with a sterically hindered enone, for example, isophorone, produced <3% of the desired compound in 24h. The isolated yield, however, increased to 84% when the additives BF3 etherate and chlorotrimethylsilane [12] were used. In the aryl case, / -tolylcalcium cuprate also underwent this transformation with 2-cyclohexenone to give 3-(p-methylphenyl)cyclohexanone in reasonable yield (Table 9.3). 

Regiosp>ecific synthesis of enol silyl ethers can also be achieved from enones either by reductive silylation or by 1,4-addition of the conjugated system. Thus, Li/NH reduction of the decalone (27) and silylation give the enol silyl ether (28). Similarly, addition of lithium dimethylcuprate to cyclohexenone followed by silylation gives the enol silyl ether (29). Trimethylsilyl cyanide (30) normally adds 1,2 to conjugated ketones (e.g. carvone, 31). However, in the presence of trialkylaluminum, 1,4-addition bdces place to give the enol silyl ether (32 Scheme 9). The same overall transformation can be accomplished by diethylaluminum cyanide and trimethylchlorosilane. ... 

Alcoholysis of epoxides. FeCl.i is an effective catalyst for the transformation. Oxidative cleavage of trimethylsiloxycyclopropanes. This method of enone generation, in combination with conjugate addition, enolate trapping, and Simmons-Smith reaction, complete the sequence of ring expansion that also incorporates a substituent at the y-position of a lower homolog. Thus 4-alkyl-2-cyclohexenones are readily prepared from cyclopentenone. 

The conjugated cisoid ketones react satisfactorily with B-alkenyls-9-BBN, whereas transoid conjugated ketones, like cyclopentenone and cyclohexenone, afford a complex mixture of products. The reaction proceeds thermally and does not appear to involve radicals. The alkenyl group of borane is transformed with strict retention of configuration. As the reaction occurs only with cisoid enones, and thus suggesting a cyclic transition state (Chart 7.8) [10]. 

Vinyl siloxonium ion 14 also serves as an important intermediate en route to conjugate adducts of unsaturated carbonyl compounds. For example, treatment of cyclohexenone (13) with reactive silyl electrophiles affords y-functionalized silyl enol ethers 15 and 16 suitable for subsequent synthetic transformations (eq 4). The temporary silicon tether (TST) strategy has been updated (2010) by an excellent review focusing upon metal-mediated reactions. The inception of this strategy is attributable to Nishiyama and Itoh who reported the radical cycUzation of acyclic bromomethyl silyl ethers to sUoxanes and their subsequent oxidation to 1,3-diols. Shortly thereafter, the group of Gilbert... 

As diazocarbonyl compounds bearing a substituent other than H on the diazobearing carbon, some cyclic diazoketones were applied to the Pd-mediated polymerization. The a,j3-unsaturated cyclic diazoketone 6 derived from cyclohexenone can be transformed into polymers with a unique structure, where the main chain carbon is one of carbon atoms of a six-membered ring [45] (Scheme 23). In addition, the C=C bond conjugated with C=0 would be directed outward from the main chain, which may render an attack of nucleophiles (1,4-conjugate addition) feasible. As expected, the addition of nucleophiles such as organolithium and sodium salt of diethyl malonate occurred by the reaction with the polymer. 

Hayashi investigated the asymmetric, rhodium-catalyzed conjugate addition reaction of organoboronic acids to a variety of unsaturated electrophilic acceptors [46, 144-147). The first successful example of such a transformation involved the use of a Rh-BINAP catalyst (187, Equation 34) [144. As shown for cyclohexenone, the rhodium-catalyzed addition of phenylboronic acid takes place to afford 188 in 96% ee and >99% yield. 

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