Enone system

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 ... 

Conversion of PGA2 to the highly sensitive PGC2 was accomplished by deconjugation of the enone system by formation of the y-extended enolate using rm-alkoxide as base and a-protonation by pH 4 buffer. 

Problems with A -17-ketones are a result of the highly strained /raw-fused enone system. ... 

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. 

The 7T orbital extends over the entire enone system. As a consequence the C —CO bond is strengthened in the excited states and a-cleavage, the important primary photoprocess in nonconjugated ketones, is not observed. The most important unimolecular reaction types are processes which involve... 

The use of enamines as protective groups seems largely to be confined to steroid chemistry, where they serve (in their protonated form) to protect the A-B enone system from bromination and reduction. A large body of literature exists on the preparation and chemistry of enamines, which are easily hydrolyzed with water or aqueous acid. 

A drawback of tlie Z enoates is usually lower reactivity, rehected in prolonged reaction times and bigliet reaction temperatures. Tliis may be overcome by switdi-ing to more reactive enone systems. Tlius, addition of tlie functionalized cyano-G dnian cuprate s7stem 67 to Z enone 66 proceeded smootlily at low temperatures, witli excellent acydic steteocontrol at tlie /J-steteocenter [26, 27]. Stereocontrol upon... 

A similar complementarity has been observed in LAH vs. DIBAL reduction of / -ketosulfoxides in which the keto group is also part of a conjugated enone system (equation 32)69. After reductive cleavage (Li, EtNH2) of the alkyl-sulfur bond, 3-alken-2-ols of high enantiomeric purity are produced59. 

The photochemical extrusion of S02 from a-phenylsulfonyl-substituted enone systems, to give the analogous /J-phenylenones in modest yield, has also been reported60. Where Z, -photoisomerization is possible, however, for example in compounds such as 28 or 29, photoequilibration of the two isomeric sulfones is the dominant process observed61,62. 

Completion of the total synthesis afforded only six further steps, including the installation of the second 2-aminopyrimidine ring via a second domino sequence. This process presumably involves a conjugate addition of guanidine (2-293) to the enone system of2-292, followed by a cyclizing condensation and subsequent aromatization. Under the basic conditions, the ethyl ester moiety is also cleaved and 2-294 is isolated in form of the free acid, in 89 % yield. Finally, decarboxylation and deprotection of the amino functionality yielded the desired natural product 2-295. 

In another interesting turn of event, while we were attempting to add acetate anion in a 1,4-fashion into the enone system of 45, we isolated ene-dione 49b when using KOAc in the presence of 18-crown-6 (Scheme 8.11). While this was not what we had anticipated, it turned out to be a known protocol developed by Komblum and DeLaMare to ring-open endoperoxides through a deprotonation pathway [55]. It is noteworthy that ene-dione 49b did not equilibrate to the ene-trione 49b which could suffer from the aforementioned HDA pathway as we had also suffered from [47c],... 

One of the C—C double bonds can also be part of an enone system as shown in the synthesis of 2-cyclopropyl-2-eyclopentenones. Such a photoproduct was then converted to the sesquiterpene taylorione (3.19) 329). 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

The high enantioselectivity shown in the above reactions can be attributed to two important factors. First, coordination of the Lewis acid with the a-hydroxy ketone moiety of dienophile 17 or 19 leads to the formation of a rigid five-membered chelate 21. This chelate causes the differentiation of the two dia-stereotopic faces of the enone system. Second, arising from the established absolute configuration of 17 and 19, within 21, the Diels-Alder reaction proceeds with the enone fragment at its cisoid position (yyu-planar). 

The enone system has to preferably adopt an s-cis or s-trans conformation in the transition state. Which one is favored may depend on the nature of the Lewis acid. It is generally accepted that Lewis acid complexation dramatically stabilizes the s-trans conformation204. The s-cis conformation, however, may be the more reactive conformation. The dienophile may react selectively in this conformation, if the s-trans and s-cis conformations are in equilibrium. 

The addition of organocuprates to chiral decalin enone systems has been explored in the context of steroid synthesis. For the addition of lithium dimethylcuprate to enones 28, 31, and 34, the major diastereomer obtained can easily be predicted by employment of a qualitative conformational analysis (Scheme 6.6) [11-13]. Thus,... 

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

As for pyrones. the nature of the nucleophile (hard or soft) determines whether reactions occur at the carbonyl group or at the atom of the enone system. Electrophiles attack in I he benzene ring. 

Hydroxylation of 711 at C-2 can be achieved by an intramolecular conjugate addition of the benzamide to the enone system. The reaction takes place in high yield in the presence of a Lewis acid and affords direct hydroxylation with a syn relationship to the amide group via an intermediate 1,3-oxazoline (Scheme 7.223). " ... 

Methylpyridinium 3-oxide and benzyne form a 1 2 adduct [169]. Interestingly, in the second stage an SN2 process is contrapolarizing with respect to the enone system. 

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