Enones products

The jS-hydroxy aldehydes or ketones formed in aldol reactions can be easily dehydrated to yield a -unsaturated products, or conjugated enones. In fact, it s this loss of water that gives the condensation reaction its name, because water condenses out of the reaction when the enone product forms. 

Problem 23.3 What enone product would you expect from aldol condensation of each of the following compounds ... 

Problem 23,4 Aldol condensation of 3-methylcyclohexanone leads to a mixture of two enone products, not counting double-bond isomers. Draw them. 

We can extend this kind of reasoning even further by imagining that subsequent transformations might be carried out on the aldol products. For example, a saturated ketone might be prepared by catalytic hydrogenation of the enone product. A good example can be found in the industrial preparation of 2-ethyl-... 

Intramolecular aldol cyclization of 2,5-heptanedione with aqueous NaOH yields a mixture of two enone products in the approximate ratio 9 1. Write their structures, and show how each is formed. 

Photochemical enone Products and rates differ in anionic micelles Berenjian et at., 1982... 

Electrochemical oxidation of enol acetates in an undivided cell gives monomeric products in parallel with the reactions of simple alkenes [47, 48]. Thus, in the reaction of menthol enol acetate 23, the a-acetoxyketone product arises from nucleophilic attack of acetate ion on the radical-cation while the enone product... 

In recent work67, it has been demonstrated that simple a-diazo ketones and esters can, in fact, be induced to undergo 1,5-insertion in preparatively useful yields. It was already known51 that in the rhodium-catalyzed insertion process, methyl C-H is electronically less reactive than methylene C-H or methine C-H. It therefore seemed likely that competing -hydride elimination would be least likely with a diazoethyl ketone. Indeed, on cyclization of 2-diazo-3-tetrade-canone, only a trace of the enone product from /J-hydride elimination is observed. The predominant side reaction competing with 1,5-insertion is dimer formation. 

Entry Vinylic telluride LI.1 11 Enone Product Yield, %) References... 

The new ring is a six-membered ring and we have numbered it to convince you. It is, of course, a rather strained bridged compound, but the key point is that dehydration is impossible. No enolate can form at the bridgehead, because bridgehead carbons cannot be planar (see Chapter 19) and the enone product cannot exist for the same reason the carbons marked ( ) in the brown structure would all have to lie in the same plane. The aldol has a perfectly acceptable conformation but that elimination is impossible. The aldol product remains in equilibrium with the alternative aldol products, but only one elimination is possible—and that is irreversible, so eventually all the material ends up as the one enone. 

Predict which enone product would be formed in this intramolecular aidol reaction. 

Diketone 12 must be condensed with a third, doubly functionalized octahydroacridine unit in the last step of the torand synthesis (cf Scheme 6.1). The following protocols (8-10) describe the three-step synthesis of torand precursor 15 from octahydroacridine 5. The reagents involved in these three steps are shown in Scheme 6.11. According to Protocol 8, octahydroacridine is condensed with benzaldehyde in the presence of acetic anhydride,24 as in the second stage of Protocol 2 (cf Scheme 6.3). The crystalline product 13 precipitates from the reaction mixture in high yield and purity. Ozonolysis of 13 (Protocol 9) is conducted by the method described in Protocol 7 for conversion of 11 to diketone 12 (cf. Scheme 6.10) and the same precautions apply. The product diketone 1425 requires no further purification after removal of benzaldehyde by trituration with diethyl ether. The third octahydroacridine unit is then readied for torand cyclization in Protocol 10 by condensing diketone 14 with Bredereck s reagent, r-butoxybis(dimethylamino)methane,26 which is commercially available. The bis[p-(dimethylamino)]enone product 15 is easily purified by precipitation from ether/dichloromethane. 

Strategy As in Problem 23.1, align the two carbonyl compounds so that the location of the new bond is apparent. After drawing the addition product, form the conjugated enone product by dehydration. In parts (b) and (c), a mixture of E,Z isomers may be formed. 

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]. 

The reactions of enol ester radical cations formed in anodic oxidations were pioneered by Shono [220-225] almost two decades ago. A reaction mode was identified that formally corresponds to that of enol cation radicals. Depending on the electrolysis conditions enol acetates were either converted to a-acetoxy ketones (high concentration of acetate) or to enone products (absence of acetate). Similarly, a-methoxy ketones were obtained through electrolysis in methanol-Et4NOTs. Yields for additional reactions not listed here varied between 29% and 90% [222,223]. 

Denmark et al. reported a general protocol for the catalytic epoxidation of alkenes by in r// -generated reactive dioxiranes capable of epoxidizing a variety of alkenes under biphasic conditions <1995JOC1391>. The epoxide diastereoselectivity (Scheme 4) showed pronounced dependence on the solvent used since the ratio of diastereo-mers, as well as the distribution between epoxide and enone products, is dependent on the solvent <1995TL2437, 1999TL8023>. Selected examples are given in Table 2. 

The facile elimination of -heterosubstituents in ketones allows for the ready construction of a,p-enones. Three different heteroatoms have been employed, chlorine, nitrogen and oxygen. The -chloro enones (products of Friedel-Crafts acylation) suffer Nazarov cyclization under standard conditions. -" Jacquier has prepared a series of -amino enones (31) from Mannich condensations." These substrates undergo cyclization in modest yields under standard conditions (equation 23). Takeda has found that the readily available" tetrahydro-4-pyranones (32) produce 2-cyclopentenone-4-carboxylates upon treatment with TMS-I (equation 24). " It is noteworthy that the putative a-carboalkoxy divinyl ketones have been independently cyclized by Marino using TMS-I. ... 

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