5.5- Disubstituted 2,4-dienones

This method provides a convenient synthesis of alkenes with the double bond in a relatively unstable position. Thus reduction of the p-toluenesulfonylhydrazones of a,(3-unsaturated aryl ketones and conjugated dienones gives rise to nonconjugated olefins. Unsaturated ketones with endocyclic double bonds produce olefins with double bonds in the exocyclic position. The reduction of p-toluenesulfonylhydrazones of conjugated alkynones furnishes a simple synthesis of 1,3-disubstituted allenes. ... 

Scheme 2.13 Cu-catalysed 1,4-addition of AlMcj to 4,4-disubstituted cyclohexa-2,5-dienone with thioether ligands.

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

Disubstitution (A and B H), on the other hand, destabilized intermediate conformations 460 and 462, causing then to be nonplanar and so facilitating their closure to a heterocycle (459). 165.167.174.2°8.395. .402 Cyclic forms 459 seem to exist only in cases where their valence isomeric cis dienones possess no stable planar conformation type of 460 or 462,164 but... 

Dlsubstituted 2-cycbpentenones. Cross-conjugated dienones, or the corresponding ethylene kelals, are cyclized almost entirely to 2,3-disubstituted 2-cyclo-pentenones by H3P04-HCOOH (1 1) or HBr-CHjCOOH (1 3). 3,4-Disubstiluted 2-cyclopentenones are formed, if at all, in low yield. 

Irradiation of 4,4-disubstituted cyclohexa-2,5-dienones yields bicydo[3.1.0]hex-3-en-2-ones (4.87) in a process that resembles the first type of rearrangement just described for cydohexenones. The... 

The literature on derivatives of 2/f-pyran is relatively sparse, reflecting the overall instability of this system. The heterocycle undergoes facile electrocyclic ring opening to the dienone (6) and generally only 2,2-disubstituted derivatives are stable under normal conditions. However, it has been suggested that it is steric destabilization of the dienone, rather than a specific substitution pattern, which is essential for the existence of the pyran (71JOC1977). 

Disubstituted cyclohexa-2,5-dienones from BTI oxidation of phenols... 

Recently, copper-catalyzed enantioselective addition reactions in the presence of phosphorus amidite 258 have also been applied to different Michael acceptors. Thus, both symmetrical and unsymmetrical 4,4-disubstituted cyclohexa-2,5-dienones 261 were found to add diethylzinc with moderate to high enantioselectivities and good... 

Bromination of 3-aminopyridine (9.97) and 3-N,/V-dimethylaminopyri-dine (9.96) with 2,4,4,6-tetrabromocyclohexa-2,5-dienone shows a remarkable difference in orientation. Thus, 9.96 is brominated exclusively at the 6-position, whereas 9.97 gives 2- and 6-monosubstitution together with 2,6-disubstitution. This has been attributed to steric hindrance, which follows from the size of the reagent [73JCS(P1)68]. Both 2-amino-and 2-/V,/V-dimethylamino-pyridines are brominated solely in the 5-posi-tion in view of the previous results, some 3-derivative might also have been expected from the former. 

When this reaction is carried out on a p,p disubstituted a,p-unsaturated complex, cyclohexa-2,4-dienones are obtained, evidently because tautomerization to a phenol is... 

Zimmerman 103) applied the well known rearrangement of 4,4-disubstituted cyclohexa-2,5-dienones (e.g. the 4,4-diphenyl derivative tives the 3,4-disubstituted phenols l)). Using acetic anhydride and a small amount of concentrated sulfuric acid, 97.5 % of 3,4-diphenylphenolacetate is formed, which in turn is hydrolyzed to give 3,4-diphenylphenol. This is one of the easiest synthesis of this phenol. 2-Bromo-4,4-diphenylcyclohexa-2,5-dienone rearranges by a similar route to give a 49 % yield of 2-bromo-3,4-diphenylphenolacetate and 47 %2-bromo-4,5-diphenylphenolacetate U4). A [1.2]-phenyl shift of 4-methoxy-2,3,4,6-tetraphenyl-2,5-dienone in acetic anhydride and zinc chloride gives rise to 4-methoxy-2,3,5,6-tetraphenylacetate (m.p. 284 °C, 85% yield)115 . 

Enantioselective copper phosphoramidite-catalyzed conjugate addition of dialkylzinc reagents to several 4,4-disubstituted cyclohexadienones is achieved with diastereomeric ratios ranging from 1/1 to 99/1 with 85% to 99% ee. When the two substituents are equal (eq 2), selective Re versus Si face-selective addition of the zinc reagent affords a single isomer. Sequential catalytic 1,4-addition to the prochiral dienones gave cis or trans bis-adducts with high enantio and diastereoselectivity. ... 

The Woodward-Hoffman rules also predict that in a given cyclization mode a permutation of alkene geometry must be reflected in the configuration of the products. This test is precluded under the normal reaction conditions (acid, light) which would isomerize the dienone double bonds. However, Corey recently reported the formation of a c -disubstituted cyclopentenone from a (Z, )-precursor, derived fiom an allene oxide, which cyclizes via the 2-oxido pentadienylic cation (Section 6.3.8). ... 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

Disubstituted phenols such as 350 undergo PhI(OAc)2-mediated oxidation in the presence of MeOH as a nucleophile resulting in the formation of two possible cyclohexa-dienones (351 and 352) (Scheme 73). The initially formed intermediate 353 is converted to the cyclohexadienones by two plausible routes. In route A, heterolytic dissociation generates a solvated phenoxonium ion 354, which further reacts with MeOH to afford 351 and/or 352. In route B, both 351 and 352 are produced by direct attack of MeOH on the intermediate (353). In the latter case, the reaction will be strongly influenced by steric factors and a homochiral environment using chiral solvents and chiral oxidants to induce some asymmetric induction, particularly in the formation of 352. 

Solutions of the 4,4-disubstituted 3-methoxycyclohexa-2,5-dienones 3 (10 mmol) in benzene (15 mL) were purged with Nj for 10 min and irradiated at 366 nm for 3-8 h. After concentration of the reaction mixture the diastereomers were separated by flash chromatography on alumina (hexane/EtOAc 1 1). 

An observation along the same lines was made by Piers and Cheng5 with respect to the dienone ester (1). For the synthesis at hand, they desired to selectively reduce the disubstituted 1,2-double bond without attack of the trisubstituted 4,5-double bond. 

The y, 5-double bond of a conjugated keto-diene may be selectively hydrogenated over catalysts, offering useful synthetic applications. Hydrogenation of dienone 16 affords 17 as an intermediate in the total synthesis of steroids. The disubstituted double bond is reduced more readily than the tetrasubstituted double bond in a nonpolar solvent to prevent participation of the carbonyl groups ... 

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