Cyclohexadienones conjugation

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

In aqueous acid cross-conjugated cyclohexadienones are principally photoconverted to one or more hydroxy ketones. In the case of a-santonin (1), isophotosantonic lactone (5) is formed in about 50% yield. A series of papers by Kropp and co-workers has aided in understanding this reaction/32-39-411 They have shown that the presence of a 4-methyl group (steroid numbering) results in the preferential formation of the 5-7 fused ring system (isophoto-... 

The initially formed cyclopropyl ketones from the photolysis of cross-conjugated cyclohexadienones are also photolabile. Usually the photochemical rearrangements of cyclopropyl ketones involve cleavage of the cyclopropyl bond which forms part of the cyclopentenone ring followed by either (a) a substituent migration or (b) rearrangement through a spiro intermediate. While the literature in this area is too voluminous to review in... 

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a czs-diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid ... 

The rearrangement of cross-conjugated cyclohexadienones to bicyclo[3.1.0]-hex-3-en-2-ones was one of the first photochemical reactions to be thoroughly studied for mechanistic and synthetic purposes 344). The examples outlined below are the conversion of the lactone a-santonin to lumisantonin 345) (3.29)... 

The mechanistic photochemistry of cross-conjugated 2,5-cyclohexadienones has been comprehensively examined (Scheme 21). The photochemical isomerization of the... 

Cyclohexadienones 61 and 64 are readily available from monoprotected hydro-quinones or para-substituted phenols, respectively. Conjugate additions to these symmetrical dienones result in desymmetrization of the prochiral dienone moieties, providing access to multifunctional chiral synthons in two steps from the aromatic precursors (Scheme 7.17) [72]. 

I 7 Copper-catalyzed Enantioselective Conjugate Addition Reactions of Organozinc Reagents Tab. 7.4. Conjugate additions to 2,5-cyclohexadienone monoacetals and ethers. 

Scheme 7.18. Selective cis or trans double conjugate addition of Et2Zn to cyclohexadienone monoacetal 66.

The thermal isomerization of a spirocyclic enol ether to the ketone [202] (Eq. 176) is probably a homolytic process. However, it is noted that part of the driving force for the reaction must be the bonding of the ethereal oxygen to a designated donor atom of the cross-conjugated cyclohexadienone moiety. 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

C. Rearrangements of Cross-Conjugated Cyclohexadienones and Their Photoisomers... 

The photochemical rearrangements of cross-conjugated cyclohexadienones in general, and of 4,4-diphenylcyclohexadienone (81) in particular, have been intensively studied.115 When 81 is irradiated in dioxane-water, first 6,6-di-phenylbicyclo[3.1.0]hex-3-one-2 (82) is obtained which, on further irradiation, forms 83, 84, and 85. The primary photorearrangement product, 82, can also be obtained by photosensitization of 81, but not by irradiation of 81 in piperylene. Therefore 82 is formed from the lowest triplet of 81. The subsequent rearrange-... 

Another synthesis of a bridged hydrocarbon takes advantage of high electron release from the para-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyclohexadienone system (S. Masamune, 1961, 1964). 

Arenols 4 and their conjugate arenolate bases are both (a) oxygen- and (b) carbon-based nucleophiles, which react with a wide range of electrophilic reagents (Figure 3). Their reactions with soft electrophiles can lead directly to cyclohexadienone derivatives this is the case, for example, with electrophilic halogenation, which effectively occurs at the electron-rich carbon centers (4 —> 5b) [29, 30]. 

Heterocyclic cage compounds were provided when 2-trimethylsiloxyfuran was allowed to react with 4-amino or 4-hydroxy-4-[(p-tolylsulfinyl)-methyl]cyclohexa-2,5-dienone in a one-pot procedure through domino conjugate additions. High stereoselectivity was realized when there is methyl substituent at the 3-position of the cyclohexadienone <02CEJ208>. 

Following the pioneering work of Schultz and others on the solution phase photochemistry of linearly conjugated cyclohexadienones [13], we prepared the achiral benzo derivative 7a (Scheme 3) and studied the solid-state photochemistry of its salts with a series of optically pure amines [14]. Diazomethane workup afforded the expected chiral bicyclic photoproduct 8, whose optical purity was determined by chiral GC. The results are summarized in Table 2. 

For reviews of the photochemistry of linearly conjugated cyclohexadienones, see. 13a. Schultz AG. In . CRC Handbook of Organic Photochemistry and Photobiology Hors-... 

The third set of molecules that bring out the uniqueness of zeolites in the context of asymmetric induction is of 2,4-cyclohexadienones (6,6-dimethyl-2,4-cyclohexadienones 38, and 2,2-dimethyl-1,2-dihydronaphthalenones 39) [279,282,287,289,293-295]. The basic chromophore in these molecules is the conjugated dienone that undergoes oxa-di-rr-methane rearrangement to give a bicyclic product (Scheme 21). According to the accepted mechanism, the chirality is introduced into the system at the first step, yielding the diradical intermediate... 

In recent years the group of C. A. Merlic has reported photochemically induced cyclizations of dienyl carbene complexes of type 39 to produce phenol derivatives 40 [19]. In these very intelligently designed reactions, which are related to the Dotz reaction, the primary, photochemically generated intermediates of type 41 undergo a (formal) electrocyclic ring-closure to form linear, conjugated cyclohexadienones 42, which then tautomerize to the phenols (Scheme 12). 

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