Ketones, reaction with singlet oxygens

Details for the use of cncsilylation for 1,2-transposition of ketones (8, 289-290) have been published.2 The paper includes the conversion of vinylsilanes to rearranged allylic alcohols. The trimcthylsilyl group directs the cnc reaction with singlet oxygen exclusively to the a-position. A typical reaction is shown in equation... 

Other steroid systems have also been investigated. Methyl groups at the bridgehead position always shield the /3-side and thus promote exclusive a-attack. Additionally, for the triacetoxy derivative 1268, an intermediate in the synthesis of a-ecdysone, and for 3j6-acetoxy-8(14)-cholesten-6-one (14)69, the regioselectivity of the ene reaction with singlet oxygen is high. In both cases only the y-hydroperoxy-a,/J-unsaturated ketone is formed. 

Epoxldation. Dye-sensitized photooxidation of aliphatic olefins results in formation of altylic hydroperoxides from reaction with singlet oxygen. Photo-IMiidation with -diketone sensitizers follows a different course epoxides are the foain products. Of several a-diketones examined, epoxides are obtained in highest yields with biacetyl, which is oxidized at the same time mainly to acetic acid. In some cases yields of epoxides in biacetyl photooxidation are 90% or more. Both cis- and trans-alkenes yield mainly tran.r-epoxides. Both epoxidatioh and ketone destruction are much slower with benzil. The nature of this epoxidation reaction is discussed, although many points of ambiguity remain. This reaction is Of practical importance. ... 

Wasserman et al. have published details of their studies on the synthesis of a-keto-derivatives of ketones, lactones, esters, lactams and amides, by an approach which involves the cleavage of enamino carbonyl systems by reaction with singlet oxygen [equation (39)]. Several methods for the preparation of the enamino compounds are described with different reagents being preferred for different substrates. Thus, with ketones Bredereck s reagent [Bu OCH(NMe2)2 used, but esters and... 

Numerous reactions, involving either transformation or conservation of the phosphazene unit, have been reported. The oxidation of the indanone phosphazene, 12, with singlet oxygen yields the corresponding ketone and triphenylphosphine oxide presumably via... 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

Singh and coworkers have carried ont a large amonnt of work on the synthesis of sprro 1,2,4-trioxanes by employing the ene reaction of singlet oxygen with substituted ally lie alcohols The resulting hydroperoxy alcohols 86a can be condensed with a variety of different carbonyl compounds inclnding cyclic ketones (Scheme 33). Some of these trioxanes 86b have excellent in vivo activity (see Table 1). 

No isolated derivatives of 1,2,3-dioxathietanes have been reported. It is reasonable to assume their intermediacy in the reaction of singlet oxygen with thioketones and sulfines to give ketones (71CC118, 70TL4683), and energy profiles have been calculated (CNDO/B) on the supposed 1,2,3-dioxathietane intermediate (80JCS(P2)188). 

Trioxacycloalkanes. 1.2,4-Trioxanes (1), X = OR4 R4 and R4 = alkylene) are generally prepared by the interaction of aldehydes with zwitterionic intermediates made from reaction of singlet oxygen with olefins. They can also be prepared by catalyzed reaction of ketones or... 

Another modification of a cyclopentene moiety was achieved by the cycloaddition reaction of the 1,3-dithiolane derivative 483 with singlet oxygen O2 to form the endoperoxide 484, which upon treatment with either triethyl-amine, triphenylphosphine, or bromine gave the corresponding hydroxy ketone 485, a mixture of the epoxide 486 and the enonc 487, or a mixture of isomeric adducts 488 and 489, respectively (Scheme 69) <1995JOC1333>. 

Energy transfer, discussed above, is one example of the direct reaction of the excited triplet state. It is also possible that the excited triplet could react directly through electron or hydrogen-atom transfer. The photochemical reactivity of a series of phenols (methyl and methoxy substituents) in the presence of natural DOM and ketone sensitizers can be best explained by the latter processes. Reactions were carried out at pH 8 where phenols existed primarily (>98%) in the undissociated form, which were shown (p. 225) not to be particularly reactive with singlet oxygen. 

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