2-cyclohexenone 2-cyclopentenone

SSA is an efficient catalyst for thia-Michael addition to electron-deficient and sterically unhindered conjugated enones under solvent-free and mild conditions [95]. The reactions of thiols with enones such as cyclohexenone, cyclopentenone and ethyl vinyl ketone afforded the corresponding adducts in excellent yields. However, when chalcones were used as an enone, the corresponding adducts were obtained in very poor yields (Scheme 5.16). 

Clearly each case must be analyzed separately, but these transition structures serve as a starting point for such analyses. Note also that the structures of Scheme 5.29 all have enones in an 5-cis conformation, which is not available to cyclic acceptors such as cyclohexenone, cyclopentenone, and unsaturated lactones. 

Additions to 2-Alkenones. The reagent undergoes 1,4-addltlon to cyclohexenone, cyclopentenone, and 3-penten-2-one In 50-80% chemical yields in diethyl ether the use of THF greatly diminishes formation of 1,4-adducts. A comparative study indicates that mixed homocuprates, especially lithium (3,3-dlethoxy-l-propen-2-yl)(3,3-dimethyl 1 bulynyl)cuprate, are the reagents of choice in 1,4-additions (eq 2). ... 

Although 2-substituted 2-cyclopentenones are in a base-catalyzed equilibrium with their 5-substituted 2-cyclopentenone isomers (Problem 22.38). the analogous isomerization is not observed for 2-subslitutcd 2-cyclohexenones. Explain. 

The addition of the anions of racemic 1-(diphenyl)- and l-(diethoxyphosphinyl)-2-butenes to 2-cyclopentenone or 2-cyclohexenone gives y-l,4-add ucls. () A1 ly 1 systems give exclusively. vy/t-adducts while (Z)-allyl systems give exclusively //-adducts. 2-Cycloheptenone gives diastereomeric mixtures of 1,4-adducts and 1,2-addition products1. 

Addition of 2-butenyl sulfone anions to 2-cyclopentenone and 2-cyclohexenone at low temperatures ( — 85 °C) gives mixtures of y-1, 4- and a-1,2-addition products. When these reactions are warmed to 1 2CC, then y-l,4-addition products predominate7,8. The lithium salts of the a-1,2-adducts rearrange to 1,4-adducts at 0°C. 

The reactions of the lithium enolates of substituted 2-cyclohexenones and 2-cyclopentenones with ( )-l-nitropropene give a mixture of syn- and ami-products3. The lithium enolate of 3,5,5-trimethyl-2-cyclohexenone gives a mixture of the syn- and //-3.5,5-trimethyl-6-(l-methyl-2-nitroethyl)-2-cyclohexcnoncs in modest diastereoselection when the reaction mixture is quenched with acetic acid after. 30 minutes at —78 =C. When the reaction mixture is heated to reflux, tricyclic products are obtained resulting from intramolecular Michael addition of the intermediate nitronate ion to the enone moiety. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

More recent investigations of the photodimerizations of cyclopentenone and cyclohexenone<134) have revealed that both reactions result from the lowest triplet states of the enones (Et > 70 kcal/mole), which is probably 77- 77 in character/1331 Both photodimerizations occur via reversibly... 

Most investigations of the photocycloaddition of simple a,0-unsaturated carbonyls have involved the reactions of the ring compounds cyclopentenone and cyclohexenone. The photoaddition of 2-cyelohexenone to a number of different olefins has been reported by Corey and co-workers(94) ... 

Epoxides are reactive substrates, which can easily be isomerized to give aldehydes or ketones. Kulawiec and coworkers have combined a Pd-catalyzed isomerization of mono and diepoxide 6/1-348 or 6/1-349 and 6/1-352 or 6/1-353, followed by an aldol condensation to give either cyclopentenones or cyclohexenones 6/1-350, 6/1-351, 6/1-354 and 6/1-355, respectively (Scheme 6/1.89) [165]. 

Silyl enol ethers of alkenyl methyl ketones can be efficiently cyclized to cyclopentenones and cyclohexenones by treating them with stoichiometric amounts of palladium acetate244 an example indicating the elaboration of this approach to the synthesis of a reduced benzoxepinone derivative, and the suggested244 mechanism of the reaction, are depicted in Scheme 174. 

Complicating aspects of the reactions include the solvent dependence of the dimerization of cyclopentenone, the head-to-head ratio increasing with polarity of solvent ".ioo)( and the plethora of products from the reaction cyclohexenone-isobutylene, where several olefinic products are not shown in Eq. 30. 97> The complicated features of these reactions are so well-described in a recent review article 94> that no furhter outline will be provided here. Biradical mechanisms can account for a great... 

A remarkable number of chiral phosphorus ligands (phosphoramidites, phosphites, and phosphines with modular structures) have been introduced into the copper-catalyzed conjugate addition of R2Zn reagents, and high enantio-selectivities (>90%) are now possible for all three different classes of substrates 2-cyclohexenones and larger ring enones, 2-cyclopentenones, and acyclic enones. 

Acid catalyzed the rearrangement of the 2-alkyl-2-vinylcyclobutanones (244) to yield either cyclopentenones (245), (246) or cyclohexenones (247) via 1,2-acyl migration or 1,3-acyl migration respectively (Table 15)82). 

Cyclohexenones preferentially produce the bicyclic compound (7) (Scheme 6.27, n = 3), whereas the reaction between cyclopentenone and f-butyl 2-chloro-2-phenylethanoate produces (7) and (8) (n = 2) in a ca. 2 1 ratio [49]. In a similar reaction, 3-acylcoumarins react with phenacyl bromide under mildly basic conditions to produce 4,5-benzo-3-oxa-2-oxobicyclo[4.1.0]heptanes [50]. 

The intrinsic basicities of cyclopentenone and cyclohexenone (59), and their lactone analogues (60), have been accessed via measurement of their gas-phase proton affinities, and compared with the saturated carbonyl compounds in both cases. The results... 

---