Cycloalkenones cycloadditions

Lewis acids can greatly accelerate the cycloaddition. Instructive examples are the AlQs-catalyzed reaction of cycloalkenones with 1,3-butadienes [12]. The catalytic effect is explained by FMO theory considering that the coordination of the carbonyl oxygen by Lewis acid increases the electron-withdrawing effect of the carbonyl group on the carbon-carbon double bond and lowers the LUMO dienophile energy. 

CARBON DIELS-ALDER REACTION 3.2.1 Cycloadditions of Cycloalkenones... 

The presence of the catalyst can also favor multiple Diels-Alder reactions of cycloalkenones. Two typical examples are reported in Schemes 3.6 and 3.7. When (E)-l-methoxy-1,3-butadiene (14) interacted with 2-cyclohexenone in the presence of Yb(fod)3 catalyst, a multiple Diels-Alder reaction occurred [21] and afforded a 1 1.5 mixture of the two tricyclic ketones 15 and 16 (Scheme 3.6). The sequence of events leading to the products includes the elimination of methanol from the primary cycloadduct to afford a bicyclic dienone that underwent a second cycloaddition. Similarly, 4-acetoxy-2-cyclopenten-l-one (17) (Scheme 3.7) has been shown to behave as a conjunctive reagent for a one-pot multiple Diels-Alder reaction with a variety of dienes under AICI3 catalysis, providing a mild and convenient methodology to synthesize hydrofluorenones [22]. The role of the Lewis acid is crucial to facilitate the elimination of acetic acid from the cycloadducts. The results of the reaction of 17 with diene... 

Cycloalkenones and/or their derivatives can also behave as dienic partners in the Diels-Alder cycloaddition. It is well documented [41] that cyclic acetals, for example, can interconvert with ring-opened enol ether forms, in a reversible manner the latter compounds can then be trapped by various dienophiles. Thus dienes 119 and 120 reacted with [60]-fullerene (Ceo) at high pressure, affording highly thermally stable products [42] (Scheme 5.16). Ketones 123 and 124 could be directly obtained by cycloaddition of enol forms 121 and 122 of 2-cyclopen-ten-and 2-cyclohexen-l-one, respectively. 

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

Irradiation of cycloalkenones in the presence of excess olefin results in cycloaddition to the olefin.420,421 The major products are those which would be predicted to arise from stepwise addition with intermediate biradical formation if the 2 position adds first to the double bond. 

Diels-Alder reactions with cycloalkenones.1 (E)-l-Methoxy-l, 3-butadiene undergoes cycloaddition with cyclohexenone at 160° to afford the endo-adduct (2) in 47% yield. Catalysis with the usual Lewis acid results in resins, but Yb(fod), catalysis results in cycloaddition at 110° and leads to endo- and exo-3 in 55% yield. The adducts 3 arise from a Diels-Alder reaction with the dienone 4, which is formed when 2 is heated at 100° with Yb(fod)3. 

Also of interest are cyclohexenones with the tether attached at the 4-position, which have not been as widely applied as the 2- and 3-substituted cycloalkenones. Croft and Jeffries irradiated the dia-stereomeric enones (61) and obtained two cycloaddition products and two products from intramolecular hydrogen atom abstraction (equation 98). All the products contained a trans decalin, however. This re-... 

Cycloalkenones generally perform poorly as dienophiles in Diels-Alder reactions but their reactivity can be enhanced by Lewis acids [105]. SnCU is effective in promoting the Diels-Alder reaction between simple 1,3-butadienes, for example isoprene and piperylene, and cyclopentenone esters (Eq. 67) [106], Cycloaddition does not, however, occur in the presence of SnCU when the diene contains an oxygen-bearing substituent such as an alkoxy or siloxy group. For such compounds, as is generally true for the Diels-Alder reactions of cycloalkenones, other Lewis acids such as zinc chloride are more effective. 

As with the parent cyclopent-2-enone vide supra), activation of the unsaturated ketone by addition of a stoichiometric amount of triethylborane as Lewis acid is required. The cycloaddition takes place diastereospecifically, with exclusive formation of the product resulting from attack of the C4. moiety at the least hindered face of the cycloalkenone. 

In the palladium(0)-catalyzed reaction of (l-methylethylidene)cyclopropane (1, R = Me) with cycloalk-2-enones, mixtures of isomers with respect to the stereochemistry of the ring fusion are formed, the preferences being dependent on the ring size of the cycloalkenone. Analogous reactions with (diphenylmethylene)cyclopropane (1, R = Ph) lead to improved product yields but decreased stereoselectivity. In the case of cyclohex-2-enone (n = 3), palladium(0)-catalyzed disproportionation of the enone to yield cyclohexanone and phenol may also occur. With (diphenylmethylene)cyclopropane, this side reaction is of minor importance because of the high tendency of this substrate to undergo cycloaddition. However, with 2-methylene-l,l-diphenylcyclopropane as substrate, the disproportionation reaction predominates. " " ... 

A stereoselective cyclopentaannulation of a cycloalkenone can be performed with (4/ )-[bis-(methoxycarbonyl)methyl]cyclopent-2-enone (17), a compound which is available in both individual enantiomeric forms. Palladium-catalyzed [3-1-2] cycloaddition affords high yields of the evo-products 18. 

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