Ketones enol carbonates

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

In a reaction related to the mixed Claisen condensation nonenolizable esters are used as acylatmg agents for ketone enolates Ketones (via their enolates) are converted to p keto esters by reaction with diethyl carbonate... 

Acylation of ketones (Section 21.4) Diethyl carbonate and diethyl oxalate can be used to acylate ketone enolates to give p-keto esters. 

A unusual NCN/NCC replacement in which the nitrogen is provided by ammonia and the two carbons by a ketone enolate, is observed in the... 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Ketones are converted to (3-ketoesters by acylation with diethyl carbonate or diethyl oxalate, as illustrated by Entries 4 and 5 in Scheme 2.16. Alkyl cyanoformate can be used as the acylating reagent under conditions where a ketone enolate has been formed under kinetic control.227... 

Not all carbon nucleophiles will add to arene chromium tricarbonyl complexes. For example, alkyllithium reagents and simple ketone enolates do not give adducts.325... 

In reactions with azides, ketones are directly converted to 5-hydroxytriazolines. Ketone enolate 247, generated by treatment of norbornanone 246 with LDA at 0°C, adds readily to azides to provide hydroxytriazolines 248 in 67-93% yield. Interestingly, l-azido-3-iodopropane subjected to the reaction with enolate 247 gives tetracyclic triazoline derivative 251 in 94% yield. The reaction starts from an electrophilic attack of the azide on the ketone a-carbon atom. The following nucleophilic attack on the carbonyl group in intermediate 249 results in triazoline 250. The process is completed by nucleophilic substitution of the iodine atom to form the tetrahydrooxazine ring of product 251 (Scheme 35) <2004JOC1720>. 

Acylbenzotriazoles 162 are efficient C-acylation reagents for the regioselective conversion of ketone enolates into P-diketones . Diethyl(l-benzotriazolmethyl)phosphinate (163) was found to be a convenient reagent for the stereoselective preparation of (E)-l-(l-alkenyl)benzotriazoles <00SC1413>. The novel three-carbon synthon 1-(1//-133-... 

Ketone enolate 51 could also serve as a two-carbon component in [3 + 2] annulation when reacted with -heteroatom-substituted a,/ -unsaturated acylsilane 52345 Ppj. tjjg enolate of 3-methyl-2-butanone 51 (R = /-Pr) reacts with... 

Enol carbonates and carbamates. Olofson and Cuomo have extended the regio-ipccilic alkylation of ketones (6, 44) to a regiospeeific preparation of enol carbonates and carbamates. The enol carbonates are prepared by reaction of silyl... 

Nucleophilic attack on a rt-allyl ligand of a metal complex occurs in general at one of the terminal carbons to afford allylated products. The attack, however, may be directed to the central carbon atom of the 7i-allyl group to produce cyclopropyl derivatives by appropriate choice of nucleophile, metal ligand and reaction conditions (equation 33). A variety of nucleophiles (pA"a > 20) including ester and ketone enolates and a-sulfonyl carbanions react with... 

This section describes the additions of stabilized carbon nucleophiles, such as cyanide, malonate, ketone enolates, enamines, etc., to alkenic ir-systems. These reactions are highly useful in organic synthesis since they are all carbon-carbon bond-forming reactions, and therefore have been used extensively in organic chemistry. 

Takaki reports that ketone enolates add to dimethylstyryl sulfonium perchlorate (155) or methyl styryl sulfone (156) in a Robinson-type annulation sequence to afford the corresponding 3-hydroxythiadecalin (157) or 5-dioxide (158), respectively subsequent reductive desulfonation of (158) affords diene (159).131 However, additions to acceptor (155) suffer from competing cyclopropanation which is dependent on the electrophilicity of the carbonyl group and the ring size of the ketone (Scheme 61). As an aside, DeLucchi reports that l,l-bis(benzenesulfonyl)ethylene (160) adds to ketones at the more substituted a-carbon under neutral conditions in refluxing acetonitrile (equation 18).132... 

The intermolecular dimerization of ketone enolates to give 1,4-diketones has been accomplished earlier with cupric6.7 and ferric salts.6 These transition metal salts have also been used to achieve intramolecular carbon-carbon bond formation.7.6.16 However, step C represents the only reported example11 of cyclopropane construction via technology of this type. 

It is also generally true that the greater the number of contributing resonance forms, the greater will be the resonance stabilization. For this reason die enolate of a yS -diketone has much more resonance stabilization than die enolate of a simple ketone (three resonance forms versus two). The electrons are delocalized over five atoms in the former versus three atoms in the latter. In addition, the electron density on the carbon atom is less in the diketone enolate than in a simple methyl ketone enolate. 

Palladium-catalysed asymmetric a-allyl alkylation of acyclic ketones has been reported allyl enol carbonates of a wide range of ketones undergo allyl transfer in high yields and ees at room temperature.197... 

A regio- and enantio-selective palladium-catalysed allylic alkylation of ketones has been reported, using allyl enol carbonate chemistry in which a CO2 unit tethers the allylating agent to the nucleophile.198... 

Equation 76) <1993OM3019>, which react as ester and ketone enolate equivalents, respectively. The latter reaction requires the use of fluoride ion activation (tetrabutylammonium fluoride, TBAF) to actuate the addition. Central carbon alkylation is less common for allylpalladium reactions despite this, nucleophilic alkylation of TMEDA-stabilized 1,3-diphenylallyl palladium complexes proceeds selectively to the central carbon (Equation 77) <1995AGE100>. 

Essentially all other electrophiles react with the enolate carbon of the enolates of aldehydes, ketones, esters, and amides (important examples are listed in Table 13.5). 

Figure 13.30 shows that even sterically hindered ketone enolates can he alkylated. The carbon atom in the /3-position relative to the carhonyl carhon of an ,/i-dialkylated a,/3-unsatu-rated ketone can be converted into a quaternary C atom via 1,4-addition of an Gilman cuprate (for conceivable mechanisms, see Figure 10.46). As can he seen, a subsequent alkylation allows for the construction of another quaternary C atom in the a-position even though it is immediately adjacent to the quaternary center generated initially in the /1-position.

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