Ketones to enones

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

Utilizing selenoxide syn elimination as the key step, a two-step procedure has been developed for the conversion of ketones to enones.22 Cyclobutyl phenyl ketone (52) and 3-phenylcyclobu-tanone (55) were both transformed via the phenyl selenides to the corresponding 1-cyclobutenyl phenyl ketone and 3-phenyIcyclobut-2-en-l-one in 74 and 26% overall yield, respectively.22... 

The photoequilibrium (114) (118) (see section III-A) and the irreversible rearrangement (118) - (120) contribute additional examples of photochemical isomerizations of cyclopropyl ketones to enones. The triplet nature of these isomerizations of (118) has been demonstrated by complete... 

A novel antibody-catalysed intramolecular Michael addition of aldehydes and ketones to enones [(148) -> (149)] has been accomplished. The reaction is enantio- and diastereo-selective with a high ee and cis/trans ratio. Antibody 38C2 is the only catalyst to date capable of generating this selectivity in Michael addition products.177... 

The oxidation of ketones to enones via the reaction of their silyl enol ethers with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) has been suggested originally to proceed via allylic hydride abstraction [195-198]. A recent reinvestigation, however, [199] has established the intermediate formation of a substrate-quinone adduct 96 which was presumably formed from a geminate radical ion pair after electron transfer. Decomposition of the adduct then finally afforded the observed enone product 97. Recently, the critical role of solvent polarity in the formation of 97 from the PET reaction of 93 and chloranil has been identified by time-resolved spectroscopy [200]. 

One step oxidation of alcohols or ketones to enones (see also Saegusa) selective oxidation of benzylic groups (methyl to aldehydes) by o-iodoxybenzoic acid (IBX). 

A particularly difficult situation arises when combining in the same reaction the use of these rather unreactive acceptors such as enones with the incorporation of ketones as Michael donors in which the formation of the intermediate enamine by condensation with the amine catalyst is much more difficult. For this reason, the organocatalytic Michael addition of ketones to enones still remains rather unexplored. An example has been outlined in Scheme 2.22, in which it has been shown that pyrrolidine-sulfonamide 3a could catalyze the Michael reaction between cyclic ketones and enones with remarkably good results, although the reaction scope was exclusively studied for the case of cyclic six-membered ring ketones as nucleophiles and 1,4-diaryl substituted enones as electrophiles. In this system the authors also pointed toward a mechanism involving exclusively enamine-type activation of the nucleophile, with no contribution of any intermediate iminium species which could eventually activate the electrophile. Surprisingly, the use of primary amines as catalysts in this transformation has not been already considered. 

There is also a later report by the same group regarding the use of a related catalyst 127 in the Michael addition of a-nitro ketones to enones (Scheme 6.19). In this case, a variety of peptides were tested as eatalysts but the best results were obtained with pentapeptide 127, in whieh, among many other different changes, the a-amino iso-butyric acid was employed instead of tert-leucine for the p-turn inducer sequence an additional modified arginine and a phenylalanine methyl ester were incorporated at the NH2 and the CO2H ends respectively. This catalyst furnished the final Michael adducts with moderate enantioselectivities, albeit in good yields in many cases. 

With respect to the use of Cinchona alkaloid-derived organocatalysts, the first example of asymmetric Michel addition of ketones to enones appeared in 1979 when Trost illustrated, during the total synthesis of the sesquiterpene ( )-hirsutic acid C [101], a stereoselective (30% ee) quinine (59)-catalyzed intramolecular conjugate addition of an intermediate functionalized cyclohexanone (Scheme 2.32). 

Turning to more specific strategies, Taber and Gunn have discussed the synthetic utility of enones and exemplified a useful homologation of ketones to enones [reaction (2)], via chlorosulphoxides. Danishefsky has provided an... 

Dehydrogenation of steroidal ketones to give the a,/8-unsaturated species can be accomplished efficiently using benzeneseleninic anhydride.Ketone to enone conversion can also be attained by hydride-ion abstraction from the corresponding trialkylsilyl enol ether [equation (26)]. ... 

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