1.3- diketones photoaddition

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

By photoaddition of other oxycarbinyl functionalized radicals, 1-4 ketols, 1-4 keto-ketals and 1-4 diketones are formed and a review of the main results has been published [59, 60]. Applications concerning the synthesis of natural C-branched sugars such as pillarose are given using this photoaddition methodology [61]. 

De Mayo reaction. Synthesis of 1,5-diketones by photoaddition of enol derivatives of 1,3-diketones to olefins, followed by a retro-aldol reaction. 

The anthracenes (188), which have a 3,5-dialkoxybenzyloxymethyl substituent on the 9-position, undergo quantitative (4jt + 4ji) intramolecular photocycloaddition to yield (189). " The process is thermally reversible, and (189) is readily converted to the diketone (190) on treatment with acid. In the presence of acid, the linked naphthyl and resorcinyl moieties in (191) undergo (2ji -I- 2jt) photoaddition with 300 nm radiation to give the tetrahydrofuran derivatives (192) by the route outlined in Scheme 3." The reaction also occurs in the absence of acid for (191) with R = -0-(CH2)2-0Me, but the quantum efficiency is reduced by 35-fold. The products (192) are labile under 254 nm radiation and undergo a novel photoextrusion of acetaldehyde to yield (193) by the pathway... 

Further examples of the photoaddition of alkenes to the diketonatoboron difluoride (17) have been published. The irradiations are carried out in 1,4-dioxane or acetonitrile as solvent and use 350 nm light. Irradiation times are relatively long (20 h) but result in the formation of the expected adducts. Thus addition of the alkenes (18) affords the 1,5-diketones (19). These arise by ring opening of the initially formed cyclobutane adducts [e.g. (20)]. Similar results are... 

The latest synthesis (Chart 4) utilizes an intramolecular photoaddition-retroaldol reaction (20 — 22) as the key-step in the construction of the tricarbocyclic frame-work. The resulting diketone (22) could be easily transformed into the known ketone (16 Chart 3) which had been earlier (25) converted into longifolene. 

Photoaddition of enol acetate 20 produces the straight adduct 21 in high yield. This regiochemistry is consistent with the general preference for the formation of five-membered rings when possible. Adduct 21 is fragmented under basic conditions to generate diketone 22. 

The regioselectivity of photoaddition of the enol acetate of 40 depends on reaction temperature, and the ratios of 41 to 42 are 11 89, 2 3, and 51 49 at -70 °C, 25 °C, and 65 °C, respectivelyOf note is that the acetylation of 1,3-diketone 40 is not regiospecific, but the two enol acetates interconvert via a photo-Fries process. However, only the enol acetate leading to 41 and 42 participates in the cycloaddition. Fragmentation of adducts 41 and 42 gives diketone 44 and the aldol product 45 via diketone 43. 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

Non-stereospecific photochemical [2+2]-cycloadditions occur in the dimerization of phenyl cyclohexene 23 in the presence of a sensitizer to produce 24 and 25 [17], and in reactions of Z/E-2 butene with cyclohexenone 26 to give 27 and 28 [18] through the formation of intermediate diradicals. The photoaddition of cyclohexene to an enolised form of 1,3 diketone 29 gives 30 in a concerted process via the formation of an unstable cycloadduct [18]. 

Photoinduced reactions of cyclic a-diketones with different alkenes takes place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. Photoaddition of electron deficient silyl ketene acetals to 2-, 3- and 4-acetylpyridine generates oxetanes as major products. The reaction is favoured in non polar solvents. The photoreaction between silyl enol ethers and henzil affords [2 + 2] cycloaddition products, while in the case of 9,10-phenanthrenequinone [4 + 2] cycloacidition predominates. Photocycloaddition of p-henzoquinones to hicyclopropylidene affords spirooxetanes (21) as the primacy products further irradiation leads to rearranged spiro[4.5]deca-6,9-diene-2,8-diones. With 9,10-anthraqui-none, in addition to the spirooxetane, a spiro[indan-l,l -phthalan]-3 -one is also obtained. ... 

Photoaddition of enols from 1,3-diketones with olefins is followed by a retro-aldol reaction to give 1,5-diketones. 

Alkynyltrialkylborates are versatile intermediates to 1,4-dikelones and y-oxo-esters, -nitriles, and -alkynes, as well as substituted ketones. A route from a -unsaturated cycloalkenones to 1,4-diketones is available wherein an aldehyde undergoes photoaddition to the alkene unit, and a variety of 1,4-diones is obtained by thiazolium salt-catalysed addition of aldehydes to but-2-enone. Monoacetalized 1,4-diketones and y-keto-aldehydes are formed on reaction of acyl chlorides with the Grignard reagents (13) and (14), respectively. 

Dibenzoylmethane undergoes a de Mayo-type photoaddition reaction at the carbonyl group of BQ, NQ, and 9,10-anthraquinone to give hydroxyoxetanes that, on further irradiation, afford 1,5-diketones in reasonable yield by a retro-Aldol process. " The reaction is outlined in Figure 87.5 for NQ and the addition is reported to be reversed by 254-nm radiation, as is the conversion of the hydroxyoxetane to the 1,5-diketone with 300-nm radiation. 

Phenanthenequinone (1) and acenaphthenequinone (13) behaved differently and photoaddition of 125 gave the 1,5-diketone 135 as major product beside the 1,5-ketoester 136 (Scheme 46 Table 88.19). The preferential formation of 135 -was rationalized by comparison of the 1,4-biradical intermediates involved. ... 

---