Of enones

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

Because the Corey synthesis has been extensively used in prostaglandin research, improvements on the various steps in the procedure have been made. These variations include improved procedures for the preparation of norbomenone (24), alternative methods for the resolution of acid (26), stereoselective preparations of (26), improved procedures for the deiodination of iodolactone (27), alternative methods for the synthesis of Corey aldehyde (29) or its equivalent, and improved procedures for the stereoselective reduction of enone (30) (108—168). For example, a catalytic enantioselective Diels-Alder reaction has been used in a highly efficient synthesis of key intermediate (24) in 92% ee (169). 

Evidence for the formation of oxabicyclobutane (6) in the irradiation (S) was negative (72JA1193). The striking difference in photochemical behavior of enones such as (5) and the analogous conjugated dienes, which readily form bicyclobutanes, is attributed to the substantially greater endothermicity of the isomerization in the oxa series. 

This protective group was used to direct the selective cyclopropanation of a variety of enones. Hydrolysis (HCl, MeOH, H2O, it, 94% yield) affords optically active cyclopropyl ketones. 

The enantiosclective synthesis of (-)-bilobalide was achieved based on successful synthesis of the chiral enone A and the highly stereoselective reduction of enone A to the desired a-alcohol B. Further transformation to (-)-bilobalide was accomplished following the route used for racemic bilobalide (Ref. 2). 

Protonation of the a-carbanion (50), which is formed both in the reduction of enones and ketol acetates, probably first affords the neutral enol and is followed by its ketonization. Zimmerman has discussed the stereochemistry of the ketonization of enols and has shown that in eertain cases steric factors may lead to kinetically controlled formation of the thermodynamically less stable ketone isomer. Steroidal unsaturated ketones and ketol acetates that could form epimeric products at the a-carbon atom appear to yield the thermodynamically stable isomers. In most of the cases reported, however, equilibration might have occurred during isolation of the products so that definitive conclusions are not possible. 

A large number of examples have been reported in the literature for the deuteration or tritiation of double bonds and for the hydrogenation of enones labeled at an allylic position. The following list of substrates, in addition to those already discussed, provides a representative cross section ... 

Hydrogenation of acetylenes to olefins, 134 Hydrogenation of dienes, 124 Hydrogenation of dienones, 130 Hydrogenation of enones, 130... 

Analogous to DPNH (144-146), 1,4-dihydropyridines (147) act as reducing agents. For instance, the conversion of aromatic nitro compounds to amines (148) and reduction of enones to ketones (749) has been achieved. 

Use of the imonium group for protection of enones was explored. Stability to peracids, lead tetraacetate, bromine, and acetic anhydride was claimed (727). The usual resistance of enamines (but not their salts) to additions of Grignard reagents was used for selective addition to a 3,17-diketosteroid by formation of the usual 3-monoenamine 728). 

Hydrogenation of enones in MeOH with Pd/C resulted in acetal formation. When ethylene glycol/THF is used as solvent, the related dioxolane is formed in 86% yield. 

A route to pyridines which involves an isolated 1,5-dicarbonyl compound, has been reported. Aldol reaction of enone 57 with methylketone 58 generated 1,5-diketone 59. When this was submitted to the reaction conditions for a Krohnke reaction, thiopyridine 60 was isolated. 

Although Lewis acid-catalyzed-Diels-Alder reacdons of enones are common, there are few repoiTS on the catalysis of Dieis-Alder reacdon of nltroalkenes The reacdon of nltroalkenes with alkenes in the presence of Lewis acids undergoes a different course of reacdon to give cyclic nltronates fsee Secdon 8 3 Knochei repotted an enhanced reacdvity and seiecdvity of the intramolecular Dieis-Alder reacdon using silica gei as Lewis acid in hexane fEq 8 19 ... 

In this synthesis, a tandem vicinal difunctionalization of enone 18 provides an exceedingly simple solution to the task of synthesizing spirocyclic intermediate 16. When 18 is treated with the or-ganocuprate reagent, t-Bu2Cu(CN)Li2, a smooth 1,4- or Michael ad-... 

The key step in a short and efficient synthesis of pleraplysillin-1 (127) is the palladium-catalyzed cross-coupling of vinylstannane 125 with vinyl triflate 126 (see Scheme 33). This synthesis is noteworthy in two respects. First, vinyl triflate 126 is generated regio-specifically from the kinetic enolate arising from a conjugate reduction of enone 124 the conjugate reduction of an enone is, in fact, a... 

A retrosynthetic analysis of fragment 152 can be completed through cleavage of the C16-C17 bond in enone 155, the projected precursor of epoxide 152. This retrosynthetic maneuver furnishes intermediates 156 and 157 as potential building blocks. In the forward sense, acylation of a vinyl metal species derived from 156 with Weinreb amide 157 could accomplish the construction of enone 155. Iodide 153, on the other hand, can be traced retrosynthetically to the commercially available, optically active building block methyl (S)-(+)-3-hydroxy-2-methyIpropionate (154). 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

Under thermodynamically controlled conditions, using triethylamine as base for the addition of enones to 5 and sodium methoxide in methanol as base for the addition of a,/ -unsaturated esters, the diastereomeric ratios of 6 range from 95 5 to 97 3. The excellent diasteroselectivities are retained in the Michael addition of 5 to -substituted enones and esters, however, modest synjami selectivities are found212,213. 

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