ENON COMPOUNDS

Conjugate addition of organocopper reagents to a molybdenum n-allyl enone compounds 146 was investigated. Similar to CpFeCCOjPPhjCr] -enone) - and Cr(CO)3(l-enone-2-OR-benzene), s-cis(CO/C=C) enone conformer 146a occurs in the crystal structure according to X-ray measurements and is also the major dissolved species in equilibrium with... 

Gondai, T, M. Shimoda, and T. Hirata, 1999. Asymmetric reduction of enone compounds by Chlorella min-iata. Proceedings of the 43rd TFAC, pp. 217-219. 

The synthesis and structure of a series of T -enone compounds of ruthenium(0) was reported213. 

The sulfone-alkylation method of Julia (1968) was a significant advance in retinoid total synthesis and has potential utility for labeled s)mthesis work. But the most useful reactions that have been adapted for the preparation of labeled retinoids are the well-developed procedures for the Wittig (Pommer, 1960) and the Homer-Emmons reactions (Bautagy and Thomas, 1974). These reactions are for synthesis of unsaturated and trans- and cw-a,p-enone compounds from the condensation of aldehydes and ketones with phosphoranes and with phos-phonoacetates, phosphonoacetonitrile, and oxophosphonates. These reactions suggest profitable application to the synthesis of retinoids, including labeled materials. 

An example that highlights the chemo- and stereoselectivity of dehydrogenases was the selective reduction of enone compounds 63 (Scheme 6.24). 

Saa et al. presented remarkable examples for the tri-fluoroacetic acid (TFA)-promoted cyclization of 5, 6, and 7-alkynals (Scheme 24.72). Alkynals 291 having an internal triple bond were converted into the corresponding 5, 6, and 7-membered cyclic enones 293. On the other hand, the cyclization of terminal alkynal 294 afforded the cyclohexenone compound 296. The authors suggested that the observed formation of each enone compound would involve aldol-type reactions of vinyl trifluoroacetate intermediates 292 and 295, formed by the addition of TEA to terminal or internal alkynes, respectively. 

The intramolecular version for synthesizing cyclic and polycyclic compounds offers a powerful synthetic method for naturally occurring macrocyclic and polycyclic compounds, and novel total syntheses of many naturally occurring complex molecules have been achieved by synthetic designs based on this methodology. Cyclization by the coupling of an enone and alkenyl iodide has been applied to the synthesis of a model compound of l6-membered car-bomycin B 162 in 55% yield. A stoichiometric amount of the catalyst was used because the reaction was carried out under high dilution conditions[132]. 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

Triethylammonium formate is another reducing agent for q, /3-unsaturated carbonyl compounds. Pd on carbon is better catalyst than Pd-phosphine complex, and citral (49) is reduced to citronellal (50) smoothly[55]. However, the trisubstituted butenolide 60 is reduced to the saturated lactone with potassium formate using Pd(OAc)2. Triethylammonium formate is not effective. Enones are also reduced with potassium formate[56]. Sodium hypophosphite (61) is used for the reduction of double bonds catalyzed by Pd on charcoal[57]. 

Sets of empirical rules, often referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 7.11) and enones and dienones (Table 7.12) to be predicted. To the respective base values (absorption wavelength of parent compound) are added the increments for the structural features or substituent groups present. When necessary, a solvent correction is also applied (Table 7.13). 

Oxiranes have been isomerized by palladium compounds to allylic alcohols and enones (79JA1623), and to 1,3-diketones (80JA2095). 

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

There are many examples in the literature reporting the successful exchange of activated hydrogens in various enones (see compounds 19-25). Complications are encountered with 3/3-hydroxy-5oc-spirost-8(9)-en-ll-one since the exchange of the 14a-hydrogen is accompanied by inversion at this tertiary center (22). Another unusual case occurs during the attempted deutera-... 

Deuterium exchange of conjugated enones and dienones on pretreated gas chromatography columns has been found useful for the characterization of these compounds by combined gas chromatography-mass spectrometry. ... 

The conjugated enone (177) is treated withp-toluenesulfonic acid in refluxing toluene to form the more stable product (178). The A -17-keto-system is formed by acid catalyzed cleavage of the A -17-ketal (see page 304), but the conditions are not drastic enough to cause equihbration to the more stable A " -compound. (The latter may be ketalized to form the A -17-ketal.) ... 

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