Dienolate

The enone 807 is converted into the dienol triflatc 808 and then the conjugated diene 809 by the hydrogenolysis with tributylammonium for-mate[689,690]. Naphthol can be converted into naphthalene by the hydrogenolysis of its triflate 810[691-693] or sulfonates using dppp or dppf as a ligand[694]. Aryl tetrazoyl ether 811 is cleaved with formic acid using Pd on carbon as a catalyst[695]. 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

Citral is prepared starting from isobutene and formaldehyde to yield the important C intermediate 3-methylbut-3-enol (29). Pd-cataly2ed isomeri2ation affords 3-methylbut-2-enol (30). The second C unit of citral is derived from oxidation of (30) to yield 3-methylbut-2-enal (31). Coupling of these two fragments produces the dienol ether (32) and this is followed by an elegant double Cope rearrangement (21) (Fig. 6). 

Penicillin V—see Penicillin, phenoxymethyl-, 7, 300 Penicilloate, benzyl-, 7, 303 Penicilloate, D-a-benzyl-a-methyl ester, 7, 303 Penillamine, benzyl-, 7, 303 Penillic acid, benzyl-, 7, 303 Penilloaldehyde, benzyl-, 7, 303 Penilloic acid, benzyl-, 7, 303 Penillonic acid, benzyl-methyl ester, 7, 303 1,2,3,4,6-Pentaazaindene nomenclature, 1, 18 Pentadeca-5,10-dienols synthesis, 1, 428 Pentadienol, tetrachloro-2H-pyran synthesis from, 3, 740 Pentadienonitrile, 5-(l,2-benzoselenazol-3-yl)-X-ray diffraction, 6, 334 Penta-2,4-dienonitrile, 5-(dimethylamino)-2-(2-thienyl)-... 

Dienestrol [4,4 -(diethylidene-ethylene)diphenol, Dienol] [84-17-3] M 266.3, m 227-228, 231-233, pKesi 9.8. Crystd from EtOH or dilute EtOH, sublimes at 130°/lmm. The diacetate has m 119-120 (from EtOH) [Hobday and Short J Chem Soc 609 1943]. 

For some aromatic ketones the reactive dienols can also undergo electrocycliziidoa to cyclobutenols. ... 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

The preferential exchange of the 6 -hydrogen under strongly acidic conditions is attributed to the formation of a A -dienol intermediate (29) which then undergoes deuterium attack from the -side at C-6. In contrast, in neutral or alkaline media the exchange proceeds via an enolate ion intermediate in which deuterium addition occurs in the following order C-4 >... 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

Addition of bromine to the dienol acetate (49) gives the 6j5-bromo-A -3-ketone (50). Dehydrobromination of the crude bromo compound in DMF with lithium or calcium carbonate gives the title compound (51). ... 

Another route to 5a compounds (57) proceeds from the dienol ether (58) by selective catalytic hydrogenation of the A -double bond with concomitant shift of the 3,4-double bond to the 2,3-position. If the hydrogenation is carried out in the presence of traces of base, double bond migration is suppressed and the difficultly accessible A -enol ethers of 5a-series (59) are thus obtained. 

C—O bonding and Cl—F fission of the intermediate cw-fluoro chlorate (29a), which in turn undergoes oxidation to the fluoro ketone (25) by a concerted elimination of chlorous acid. A similar transition state (30) approximating an allylic carbonium ion could be involved in the reaction of the dienol derivatives (6) with perchloryl fluoride, which would be expected to give rise to the c/5-adduct (30a). Reaction of the latter with water leads to product and chlorate ion. 

In this process the lOjS-methyl-3-keto-A -steroid is formed directly using a reagent prepared in situ with an approximately 1 1 molar ratio of zinc and methylene iodide. A one-step mechanism proceeding from the dienol (18) formed by Lewis acids present in solution has been proposed. ... 

Analogous acyl migrations occur with dienol acetates of type (238) and (239), yielding products from the two possible recombinations—(240)/(241) and (243)/(244), respectively—besides the products of reductive acetyl elimination (see Experiment b below). 

Diacyl-3,4-dihydroxythiophenes have been formulated as dienol tautomers (77T191). The same holds for 2,5-diaryl- and 5-aryl-2-carboxy derivatives (91JHC1449). ESR investigations of 6-halogeno-benzo[h] thiophene-2,3-semidiones (Scheme 13) have been reported (81JOC751). 

An a priori choice of an optimum acetylene hydrogenation catalyst is not always easy. For instance, hydrogenation of octadeca-3,6-diynol over P-2 nickel gave the corresponding (Z,Z) dienol satisfactorily (37), but when the... 

There are several examples of successful dienol epoxidations (Table 9.2). Catalytic SAE conditions are generally better than stoichiometric for reactive substrates (Entry 1), whilst stoichiometric conditions, on the other hand, are useful for less reactive substrates. Small variations in substrate structure can cause large differences in reactivity and product stability pentadienol could be epoxidized in acceptable yield, whereas hexadienol gave a complex mixture of products (Entries 1, 2). 

Scheme 9.9 a) SAE on dienols. b) Payne rearrangement, resulting in byproducts. 

The epoxidation of divinyl carbinol constitutes a special case of a dienol epoxida-tion, as the starting diene is not conjugated (Scheme 9.10). Desymmetrization by SAE, followed by a Payne rearrangement, furnishes the vinylepoxide in high yield and with excellent enantioselectivity (compare Table 9.2, Entry 1) [43]. 

Small groups on silicon favor the formation of the corresponding silyl dienol ether, formed by a Brook rearrangement of 81, and this reaction pathway is minimized by employment of P(OPh)3. 

In general, the Michael addition of a-substituted amide dienolates to a,/j-unsaturated esters is a method with great future potential for the diastereoselective construction of adjacent tertiary and quaternary stereogenic centers80. 

The asymmetric 1,4-addition of the dienolate of the optically active camphor derived 3-methyl-3-butenoate to 2-cyclopentenone gives a mixture of four diastereomers. The major adduct was applied in the synthesis of (—)-khusimone188. 

Optically active bicyclo[2.2,2]octanes can be obtained via diastercoselective MIMIRC reaction of lithium dienolates and a,/ -unsaturated esters of various chiral alcohols. Good yields (70-90%), high endo selectivities (> 95%) and diastereomeric ratios that depend on the auxiliary alcohol are found in these additions. The highest diastereomeric ratio reached was 18 82 using a camphor derived sulfonamide. The diastereomeric ratio could be improved (up to 9 91) by titanium(IV) chloride catalyzed addition of the corresponding silylenolates with the chiral a,/J-unsaturated esters358. 

Addition of the lithium dienolate of 3-(tm-butyldimethylsilyloxy)-2,6-diinethyl-2-cyclohexenonc to ( + )-(.S )-4-( 3-methyl-3-butenyl)-3-(4-mcthylphenylsulfinyl)-2(5//)-furanonc in THF at — 95 °C for 2 hours gave an 88 12 mixture of diastereomeric adducts. A transition state has been proposed that involves chelation between both reacting partners20. 

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