P,*y-Enones

As mentioned earlier (cf. chapter 2.1.1), p,y-enones undergo an 1,3 acyl shift from the Si-state. In sensitized reactions they rearrange to cyclopropyl ketones in the so termed oxadi-pi-methane rearrangement, as illustrated for... 

Other than the aforementioned ynones, P-iodo-p/y-enone 132 was also converted into a 2,5-disubstituted furan, 133, under Pd-catalyzed cyclization conditions using Herrmann s catalyst, palladacycle catalyst 101 [105]. 

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

P,Y-Unsaturated ketones.1 The reaction of acyclic a,p-epoxy ketones with two equivalents of 1 results in the cyclopropanols 2, which are converted into p,Y-enones on treatment with BF3 etherate or HC1. 

Photocycloaddition of allene to cyclohexenone (341) gave the p,y-enone (342), which reacted with vinyl magnesium bromide to produce the tertiary alcohol (343) in 79% yield. When the compound (343) was treated with KH and 18-crown-6 in THF at room temperature for two hours and quenched with aq. NH4C1, the cyclobutene (344) was obtained. The thermal ring opening of the cyclobutene (344) proceeded in toluene in a sealed-tube at 180 °C for twelve hours to give a readily separable 5 1 mixture of the cw-olefin (345), and the frans-olefin (346) respectively in 95 % yield. Moreover, (345) could be converted to a mixture of (346) and (345) in the ratio of 10 1 by irradiation. The compounds (345) and (346) possess the skeleton of the germacranes (347), (348) and (349) 122). 

The preceding discussion reveals a few of the complexities of the photochemistry that is typical of carbonyl and azo compounds. Results on the olefin-carbonyl Paterno-Buchi system,30 and on the photorearrangements of a, (i-enones,31 P, y-enones,32 azo compounds (diazomethane33 and cyclic diazoalkenes104), and acylcyclopropenes34 show similar features. In these examples, one encounters fourfold intersections as well as conical intersections and singlet-triplet crossings. Thus the potential surfaces are more complex than in hydrocarbon photochemistry. 

Rawal and Thadani [160] have taken advantage of alkynes as excellent insertion partners in Pd(II)-catalyzed allylations. Interestingly, the Pd(II) species present in the medium after alkyne insertion into allyl chlorides or bromides can serve for a sequential Wacker-Tsuji oxidation upon introduction of CuCl, oxygen, and water to the reaction medium to give y-halo /-fy-cnones 220 in good yields (Scheme 91). If phenylacetylene is used as an alkyne l-phenyl-pent-2-ene-l,4-dione can be isolated in 77% yield as the result of a subsequent oxidative hydration of the y-halo P,y-enone 220. 

Most p.-y-unsaturated ketones rearrange upon triplet sensitization to cyclopropyl ketones via the so-called oxa-di-TT-methane (ODPM) rearrangement, the result of which is an overall 1,2-acyl shift as schematically shown for (1) -+ (2). p,y-Enone arrangements in which the alkene moiety is further conjugated to either phenyl or C—O sites also undergo this transformation upon direct excitation, i.e. via the singlet excited state. 

As shown for the transformations of the cyclopentenyl methyl ketones (3a-d) to mixtures of (4a-d) and (5a-d) (Scheme 1), the ODPM rearrangement occurs from the lowest electronically excited p,y-enone triplet state. - The ir. ir configuration has been assigned to this state on the basis of CNDO-MO calculations, phosphorescence studies at 77 and mechanistic examinations. The phosphorescence data together with sensitization experiments have revealed a Ti energy range of 289-310 kJ mol (4.18 kJ = 1.0 kcal) for (3a-c) and a much lower Ti of 253 kJ mol for (3d). ... 

In an extension of the previous concept, starting materials which are readily available in enantiomeri-cally pure form have served for the construction of bridged p, y-enones with an additional five-mem-bered ring. In this way the first total synthesis of enantiomerically pure (-)-silphiperfol-6-en-5-one... 

In a final example, use has been made of a mechanistic variant of the ODPM reaction the conversion of a 1-methoxy-substituted bicyclo[2.2.2]octenone (92) to the bicyclo[3.3.0]octadione (94 Scheme 23). The highly substituted p,y-enone (92) is accessible via hydroxymethylation of (66c), a compound which... 

Irradiation of the (-)-bicyclooctenone (180) at 300 nm brings about racemi-zation (a unimolecular first order reaction) with a quantum yield of 0.406. This is an example of a 1,3-acyl migration in a p,y-enone system. 

Suzuki et al. found that a-trimethylsilylvinylcerium reagents add to readily enolizable p, y-enones. The method has been employed in the syndiesis of (-)-eldanolide, as shown in Scheme 16. ... 

Cyclization ofsilyl enol ethers of. -acetylenic ketones or aldehydes. Reaction of these derivatives of ynones or -als with HgCl2 (1.1 equiv) and hexamethyldisi-lazane (0.2 equiv.) as an acid scavenger in CH2CI2 at 30° followed by acidification with aqueous HCl and Nal (2 equiv. each) results in cyclic p,y-enones. Thermal cyclization of these substrates requires high temperatures, which lead to decomposition and rearrangements. ... 

The knowledge and body of p,y-enone photochemistry have continued to grow rapidly during the past decade following the last comprehensive reviews. " One main interest in this area is the oxa-di-ir-meth-ane (ODPM) photoisomerization, a reaction that has become a powerful synthetic tool and has hence served successfully in natural product synthesis. " Based on detailed investigations, a convincingly consistent picture of the mechanistic events of the reaction has evolved and predictions of the reactivity of new substrates can nowadays be made with reasonable assurance. 

High quantum and chemical yields of ODPM rearrangements are obtained with substrates in which the P.y-enone chromophore is part of a conformationally rigid molecular assembly which at the same time guarantees adequate orbital overlap of the C=C and G—0 ir-bonds. Whereas in bicyclic and bridged P,-y-unsaturated ketones these prerequisites are widely met, acyclic 3,-y-unsaturated ketones usually rearrange inefficiently since other channels of energy dissipation from the triplet state predominate. Exceptions to this rule are substrates in which the C=C ir-lrand is part of a styrene or an a,3-enone moiety (Section 2.6.3.1). In this context it should also be noted that 3.7 unsaturated aldehydes, except for one case, are ODPM unreactive (Section 2.6.3.2). 

Representative examples of conversions of bicyclic p.y-enones have previously been cited in the mechanistic context the efficient transformation of (8) into (11) (Scheme 3), the reaction of an analog of (8) lacking the C-7 carbonyl, - and ultimately the preparation of (14) and (15) from (12) (Scheme 4). ... 

P.y-Enones. Reaction of 1,2-bis(methylenc)cyclohexane with activated magnesium provides a magnesium complex (I). This complex reacts with ethyl acetate at —10°... 

---