Enones cyclohexenone

Reduction of enones. Cyclohexenone is reduced by LiAlH4 in ether almost exclusively by 1,2-addition to give 2-cyclohexenol. However in the presence of cryptate [2.2.1], which binds Li, 1,4-addition is the predominant reaction. The same effect is observed with LiBH4, but is less pronounced. ... 

A resident stereocenter in the enone part can control the formation of two new stereocenters in one step, guided by a synclinal transition state and an axial cyclization mode. The major product on cyclization of 4-methyl-3-[6-(trimethylsilyl)-4-(Z)-hexenyl]-2-cyclohexenone was formed in a ratio of 7.5 13S. 

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. 

A series of chiral phosphinous amides bearing pendant oxazoline rings (50, Ri=H,Tr R2=H,Tr, 51, Ri=H,Tr R2=H,Tr and 54, Ri=H,Tr R2=H,Tr in Scheme 41) have been used as ligands in the copper-catalyzed 1,4-addition of diethylzinc to enones. Two model substrates have been investigated, the cyclic 2-cyclohexenone and the acyclic trans-chalcone. The addition products are obtained quantitatively in up to 67% ee [171]. 

An unusual reaction was been observed in the reaction of old yellow enzyme with a,(3-unsat-urated ketones. A dismutation took place under aerobic or anaerobic conditions, with the formation from cyclohex-l-keto-2-ene of the corresponding phenol and cyclohexanone, and an analogous reaction from representative cyclodec-3-keto-4-enes—putatively by hydride-ion transfer (Vaz et al. 1995). Reduction of the double bond in a,p-unsaturated ketones has been observed, and the enone reductases from Saccharomyces cerevisiae have been purified and characterized. They are able to carry out reduction of the C=C bonds in aliphatic aldehydes and ketones, and ring double bonds in cyclohexenones (Wanner and Tressel 1998). Reductions of steroid l,4-diene-3-ones can be mediated by the related old yellow enzyme and pentaerythritol tetranitrate reductase, for example, androsta-A -3,17-dione to androsta-A -3,17-dione (Vaz etal. 1995) and prednisone to pregna-A -17a, 20-diol-3,ll,20-trione (Barna et al. 2001) respectively. 

Me2CuLi to enones with sulfonamide hgands 94 MeLi to cyclohexenone with p-amino sulfide ligand 88-9 MeMgl to acychc enone with arene-thiolatocopperfl) complexes 90-1... 

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... 

H)-furanones undergo efficient cycloadditions as oxa-enones 450). The cycloadducts have been successfully utilized as synthetic precursors for 8-valerolactones 473) (4.62) or for 2-cyclohexenones (4.63)474>. 

In 1993, Alexakis et al. reported the first copper-catalyzed asymmetric conjugate addition of diethylzinc to 2-cyclohexenone using phosphorous ligand 28 (32% ee).36 An important breakthrough was achieved by Feringa et al. with chiral phosphoramidite (S,R,R)-29 (Figure 1), which showed excellent selectivity (over 98% ee) for the addition of 2-cyclohexenone.37 Since then, efficient protocols for the conversion of both cyclic and acyclic enones, as well as lactones and nitroalkenes, have been developed featuring excellent stereocontrol. 

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. 

With chiral phosphoramidite (S,R,R) 29, 3-ethylcyclohexanone, 3-ethylcycloheptanone, and 3-ethylcyclooctanone were obtained with >97% ee s.37,39 (R,R,R)-32 also showed excellent enantioselectivity in the addition of Et2Zn to both 2-cyclohexenone (93% ee) and larger ring enone as 2-cyclopenta-decen-l-one (95% ee).40... 

Hayashi et al. proposed a catalytic cycle for the rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclo-hexenone (Scheme 28), which was confirmed by NMR spectroscopic studies.96 The reaction presumably involved three intermediates, phenylrhodium a, oxa-7r-allylrhodium b, and hydroxorhodium c complexes. Complex a reacted with 2-cyclohexenone to give b by insertion of the carbon-carbon double bond of enone into the phenyl-rhodium bond followed by isomerization into the thermodynamically more stable complex. Complex b was converted to c upon addition of water, liberating the phenylation product. Transmetallation of the phenyl group from phenylboronic acid to rhodium took place in the presence of triphenylphosphine to regenerate a. 

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