2-Cyclohexenone structure

Gabosines which have been isolated from Streptomyces strains constitute a family of keto carbasugars, most of them possessing a trihydroxylated cyclohexenone structure. Because of their interesting bioactivities, a large number of synthetic approaches to these compounds have been proposed (for example [115, 116]). The shortest route (four to five steps) to gabosines I and G was accomplished starting... 

It was most convenient to isolate the products after acidic conversion to cyclohexenones. Structures of the products were assigned by chemical correlation and circular dichroism and the enantiomeric purities were based on optical rotations. The selectivities obtained, although impressive for the era, are moderate at best, despite significant attempts to optimize the substrates and reaction conditions. Use of substituted cyclohexanones (29) and other aldehydes (30) lead to optically active products but the extent of enantiomeric induction in these products was not determined. This technology was used for the partial asymmetric synthesis of (+ )-mesembrine (12.1) (29) and (+ )-podocarpic acid (12.2) (31). 

The Robinson annulation combines two of the reactions above to create a cyclic product. It begins with the Michael addition of an enolate nucleophile (often a cyclic ketone) onto methyl vinyl ketone (MVK), or a derivative of MVK. The resulting 1,5-dicarbonyl product can undergo an intramolecular aldol reaction with dehydration to give a cyclohexenone structure. If this pattern is present in a target molecule, it is an indication that the TM could be the result of a Robinson annulation. 

Cyclization reactions resulting in cyclohexenone structures by intramolecular aldol condensation of neighbored methyl vinyl ketone units are held responsible for the red color [313,314]. This view is supported by UV [315] and IR spectroscopy [316]. 

The main formation pathway of C9 apocarotenoids with cyclohexanone and cyclohexenone structures is conversion of hydroperoxides derived from P-damascol (Figure 9.29). Hydroperoxides generated by autoxidation of carotenoids are further oxidised, reduced and hydrated to form a variety of different structures. The most important compound of this apocarotenoid... 

The Robinson annulation reaction is one of the most fundamental methods for the construction of cyclohexenone structural motifs, which are frequently... 

The alkylation of 3-methyl-2-cyclohexenone with several dibromides led to the products shown below. Discuss the course of each reaction and suggest an explanation for the dependence of the product structure on the identity of the dihalide. 

Draw a Lewis structure for cyclohexenone that involves charge separation for the most polar bond. Then, draw a Lewis structure that will delocalize one or both charges. Next, examine the actual geometry of cyclohexenone. Are the bond distances consistent with the Lewis structure shown above, or have they altered in accord with your alternative (charge separated) Lewis structure (Structures for cyclohexene and cyclohexanone are available for reference.)... 

Treatment of 2-cyclohexenone with HCN/KCN yields a saturated keto nitrile rather than an unsaturated cyanohydrin. Show the structure of the product, and propose a I. mechanism for the reaction. 

Photoirradiation of both neat and benzene solutions of 2-cyclohexenone (66b) gives a complex mixture of photodimers [40]. However, photoirradiation of a 1 1 complex of 66b with the chiral host (S,S)-(-)-l,4-bis[3-(o-chlorophenyl)-3-hydroxy-3-phenylprop-l-ynyl]benzene (167) in the solid state (Scheme 24) gave (-)-anf/-head-to-head dimer 168 of 46% ee in 75% yield [40]. This reaction was found to proceed in a single crystal-to-single crystal manner. The mechanism of the reaction was studied by X-ray crystal structural analysis [41]. 

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

In 1994, lithium amide 23 was used in the conjugate addition of 2-cyclohexenone to afford optically active adduct with up to 97% ee (Scheme 13).28-29 A dimeric structure was proposed as the intermediate, where the phenyl group in 23 blocked the bottom face and the cyclohexenone substrate approached from the upper face. 

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. 

Moreover, these rare earth heterobimetallic complexes can be utilized for a variety of efficient catalytic asymmetric reactions as shown in Scheme 7 Next we began with the development of an amphoteric asymmetric catalyst assembled from aluminum and an alkali metal.1171 The new asymmetric catalyst could be prepared efficiently from LiAlH4 and 2 mol equiv of (R)-BINOL, and the structure was unequivocally determined by X-ray crystallographic analysis (Scheme 8). This aluminum-lithium-BINOL complex (ALB) was highly effective in the Michael reaction of cyclohexenone 75 with dibenzyl malonate 77, giving 82 with 99% ee and 88 % yield at room temperature. Although LLB and... 

The solvent isotope effect produces an A-ratio (HOH/DOD) of three with isotope-independent A// of 17-18 kJ/mol. This result is more difficult to interpret, because it is unknown how many isotopic sites in the enzyme or water structure contribute to the isotope effect of 2-3. If a single site should be the origin of the effect, then the site could reasonably be a solvent-derived protonic site of the enzyme involved in general-acid catalysis of the hydride transfer, most simply by protonic interaction with the carbonyl oxygen of cyclohexenone or possibly by proton transfer to an olefinic carbon of cyclohexenone. 

When the enol ring is adjacent to a cyclic moiety, then it is possible to achieve very short hydrogen bonds, as in the structure of usnic acid, a natural product found in lichens. A low-temperature X-ray diffraction analysis of this compound showed two enol moieties, one in which a carbon-carbon bond of the enol was part of a cyclohexenone ring, and this had... 

Fig. 1.37. Calculated structure of the transition state in the reaction between CuLIMe2 and 2-cyclohexenone.

Pfaltz introduced phosphite ligands 22, with BINOL and chiral oxazoline units, which gives excellent enantioselectivities [47]. In phosphoramidites 14 and 15 (Scheme 7.9) the structure of the amine moiety is crucial, but substituents at the 3,3 -positions of the BINOL unit had only minor influences on the enantiose-lectivity of the 1,4-addition to cyclohexenone. In contrast, the introduction of the two 3,3 -methyl substituents in ligand 22 increased the ee drastically from 54% to 90%. 

Fig. 7.3. a) X-ray structure of the Cul complex 21 of ligand 14 b) Possible bimetallic intermediate involved 19 in si-face-selective ethyl transfer to 2-cyclohexenone. 

A structural requirement for the asymmetric Birch reduction-alkylation is that a substituent must be present at C(2) of the benzoyl moiety to desymmetrize the developing cyclohexa-1,4-diene ring (Scheme 4). However, for certain synthetic applications, it would be desirable to utilize benzoic acid itself. The chemistry of chiral benzamide 12 (X = SiMes) was investigated to provide access to non-racemic 4,4-disubstituted cyclohex-2-en-l-ones 33 (Scheme 8). 9 Alkylation of the enolate obtained from the Birch reduction of 12 (X = SiMes) gave cyclohexa-1,4-dienes 32a-d with diastereoselectivities greater than 100 1 These dienes were efficiently converted in three steps to the chiral cyclohexenones 33a-d. 

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