Enolates, boron ketones

With (Z)-amide enolates and (Z)-thioamide enolates a strong preference for sm-adducts is also observed. In general, boron or zirconium (Z)-enolates of ketones and amides display a higher simple diastereoselectivity in favor of syn-products than the corresponding lithium or magnesium enolates6,7. 

Substrate-induced diastereoselectivity is provided by the chlorotitanium enolate of 14a,b47b and the boron enolate of ketone 15S3 to give predominantly. wt-aldols. 

Diketones are produced from nitroalkenes and the lithium enolates of ketones. Equation 132 shows the reaction of the enolate of 2-hexanone with 2-nitropropene in the presence of acetic anhydride. The resulting betaine 409, a greenish-blue liquid, is hydrolysed to the diketone by successive treatment with boron trifluoride and water441. 

Regio- and stereoselective aldol condensations. The enol boronates of ketones, obtained by reaction with 1 and diisopropylethylamine (1 equiv.), react with both aliphatic and aromatic ketones at —78° to —15° to form p-hydroxy ketones with high sy/t-dia-stereoselectivity.2... 

Lithium enolates of ketones react with cyclic epoxides in the presence of boron trifluoride etherate <2003JOC3049>. The reactions between carbonyl enolate ions and epoxides have been reviewed <2000T1149>. The pseudoephedrine... 

In our synthesis, iterative aldol reactions of dipropionate reagent (R)-18 allowed for the control of the C3-C10 stereocenters (Scheme 9-72) [89]. Hence, a tin-mediated, syn aldol reaction followed by an anti reduction of the aldol product afforded 270. Diol protection, benzyl ether deprotection and subsequent oxidation gave aldehyde 271 which reacted with the ( )-boron enolate of ketone (/ )-18 to afford anti aldol adduct 272. While the ketone provides the major bias for this reaction, it is an example of a matched reaction based on Felkin induction from the... 

We have used this methodology in a recent synthesis of the anti-obesity drug tetrahydrolipstatin (283) (Scheme 9-76) [91J. Hence, the ( )-boron enolate of ketone 284 was reacted with aldehyde 285 to afford the desired anti aldol adduct... 

The reaction of the lithium enolate of 2-methyl-1-indanone with the thiophenium salt (35) leading to the 2-trifluoromethyl derivative in 51% yield is an exception. With all other in situ generated enolates of ketones, no trifluoromethylation was observed. To moderate the reactivity of the enolates, a boron Lewis acid (40) was added to form the boron complexes. This made a regio-, diastereo- and enantio-selective trifluoromethylation possible in good to high yields. ... 

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

Boron or tin (II) Z-enolates are generated by reaction with the corresponding triflates with a carbonyl compound in the presence of tertiary amines like r-P NEt or. M-ethylpiperidine (except when using dicyclopentylboron triflate [407]). E-Enolates are prepared by using dicyclohexyl- or other cyclic chloroboranes in the presence of Et3N or Me NEt [407, 685, 686, 1246, 1247, 1248], Because enolization does not take place under such conditions with esters or aliphatic tertiary amides, thiophenyl esters RGH COSPh have been used as ester/amide substitutes. Furthermore, Z-boron enolates of ketones can be prepared by conjugate addition of acid derivatives of dialkylboranes to a-enones [687],... 

Boron ketone enolates 20 are generated from a hindered dialkylboryl triflate (e.g. dicyclohexyl) and diisopropylethylamine at 0 °C whereas the (Z) isomer 21 is prepared by using a less hindered boryl triflate (e.g. dibutyl) at -78 °C (Figure 3.3) [8],... 

Scheme 2.26 Controlled generation of as- and trans-boron enolates from ketones.

The most successful imide systems for diastereoselective aldol addition reactions are, without question, the oxazolidinones 50-52 developed by Evans. These furnish syn aldol adducts with superb selectivity for a broad range of substrates (Scheme 4.5) [6, 13, 45-47). A hallmark of these system is that enolization yields exclusively the Z-enolates, which can be understood on the basis of steric considerations. Two important discoveries in the area proved critical to the unparalleled success enjoyed by Evans auxiliaries for diastereoselective aldol addition reactions. The first of these was the disclosure by Mukaiyama that the combination of di-n-butylboryl trifluorometh-anesulfonate (n-Bu2BOTf) and diisopropyl ethyl amine can be employed for the generation of dialkylboron enolates from ketones [48]. The second key observation was by Roster, who observed that aldol additions of boron enolates proceeded with higher levels of simple induction [49], This phenomenon is attributed to the short B-0 distances in the attendant Zimmer-man-Traxler transition state structure [14, 47]. 

The generation of cis-boron enolates from ketones with dialkylboron chlorides and hindered tertiary amines had been known for some time, from the work of Mukaiyama and Evans [13, 48, 87]. However, the synthesis of the corresponding trans-boron enolates remained problematic. Brown addressed this issue, reporting enolization conditions that provide access to either eno-late diastereomer [88]. The stereochemical outcome of ketone enolization was shown to be dependent both on the electronegative group on boron and on the nature of the alkyl substituents. In this regard, the combination of bulky boron ligands (cyclohexyl), a boron chloride derivative, and an unhindered base (EtjN) proved optimal for the stereoselective synthesis of trans-enolates (166) from a variety of ketones (Scheme 4.16). 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

Ketones, in which one alkyl group R is sterically demanding, only give the trans-enolate on deprotonation with LDA at —12°C (W.A. Kleschick, 1977, see p. 60f.). Ketones also enolize regioseiectively towards the less substituted carbon, and stereoselectively to the trans-enolate, if the enolates are formed by a bulky base and trapped with dialkyl boron triflates, R2BOSO2CF3, at low temperatures (D A. Evans, 1979). Both types of trans-enolates can be applied in stereoselective aldol reactions (see p. 60f.). 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Stereoselectivities of 99% are also obtained by Mukaiyama type aldol reactions (cf. p. 58) of the titanium enolate of Masamune s chired a-silyloxy ketone with aldehydes. An excess of titanium reagent (s 2 mol) must be used to prevent interference by the lithium salt formed, when the titanium enolate is generated via the lithium enolate (C. Siegel, 1989). The mechanism and the stereochemistry are the same as with the boron enolate. 

It was anticipated that two of the three stereochemical relationships required for intermediate 12 could be created through reaction of the boron enolate derived from imide 21 with a-(benzyloxy)ace-taldehyde 24. After conversion of the syn aldol adduct into enone 23, a substrate-stereocontrolled 1,2-reduction of the C-5 ketone car-... 

Ideal starting materials for the preparation of. svn-aldols are ketones that can be readily deprotonated to give (Z)-enolates which are known to give predominantly yyu-adducts. Thus, when (5,)-1-(4-methylphenyl)sulfonyl-2-(l-oxopropyl)pyrrolidine is treated with dibutylboryl triflate in the presence of diisopropylethylamine, predominant generation of the corresponding (Z)-boron enolate occurs. The addition of this unpurified enolate to 2-methylpropanal displays not only simple diastereoselectivity, as indicated by a synjanti ratio of 91 9, but also high induced stereoselectivity, since the ratio of syn- a/.vyn-lb is >97 3. 

In contrast, highly stereoselective aldol reactions are feasible when the boron etiolates of the mandelic acid derived ketones (/ )- and (5,)-l- t,r -butyldimethylsiloxy-l-cyclohexyl-2-butanone react with aldehydes33. When these ketones are treated with dialkylboryl triflate, there is exclusive formation of the (Z)-enolates. Subsequent addition to aldehydes leads to the formation of the iyn-adducts whose ratio is 100 1 in optimized cases. 

Achiral ketones, for example, 3-pentanone, can be converted predominantly into (Z)-boron enolates [(Z)/( )>97 3] by treatment with (- )-diisopinocampheylboron triflate. Subsequent addition to aldehydes, followed by an oxidative workup procedure, delivers /i-hydroxy ketones with a diastcrcomeric ratio of 95 5 to 98 2 (synjanli) and the xpn-products with 66 to 93% ee33. 

The enantiomeric /1-hydroxy ketones are available in an analogous way using the corresponding enantiomeric borinates. The reaction is plagued by low regioselectivity in the formation of the boron enolates, except when R1 is phenyl or isobutyl53,57. 

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