Aldehydes aldol reactions with

Table 3. Iron-Acyl Complexes 11 from CarbonyUr/ -acyclopentadienylHl-oxo -ftriinethylsilyllethyl] (triphenylphosphane)iron (8) by the Aldol Reaction with Aldehydes...

Scheme 10 Rh-catalyzed reductive aldol reaction with aldehydes as reductants...

Starting from ketone(i )-/(S )-49, the asymmetric aldol reaction with aldehyde in the presence of 45a or 45b affords all four isomers of //-hydroxyl ketone 47, 48, 50, and 51 with high yields and stereoselectivities (Scheme 3-17). 

Diisobutylaluminium phenyl tellurolate, a highly air- and moisture-sensitive reagent, prepared by reaction of diisobutylaluminium hydride with diphenyl ditelluride, undergoes an in situ addition to a,jS-unsaturated carbonyl compounds, leading to the corresponding jS-phenyltelluroaluminium enolate. This intermediate is hydrolysed by aqueous HCl into jS-phenyltelluro compounds or smoothly affords an aldol reaction with aldehydes to give a-hydroxyalkyl-jS-phenyltellurocarbonyl compounds. 

Scheme 19 Asymmetric aldol reactions with aldehyde donors...

Huang, X.-T. Chen, Q.-Y. Ethyl a-Fluoro Silyl Enol Ether Stereoselective Synthesis and Its Aldol Reaction with Aldehydes and Ketones. J. Org. Chem. 2002, 67, 3231-3234. 

Aldol reactions.1 The anion generated (BuLi) from chromium carbene complexes undergoes aldol reactions with aldehydes or ketones activated by a Lewis acid. Best results are obtained with ketones in the presence of BF3 etherate, whereas TiCl4 is the preferred catalyst for aldehydes and acetals. 

Quantitative yields are realized when a-(/V,/V-dibenzylamino) ethyl ketones undergo aldol reactions with aldehydes. The reaction proceeds in a highly diastereoselective manner... 

A new thiol auxiliary (45, R = COEt) participates in boron-mediated anti-aldol reactions with aldehydes with high yield and de.124 Reaction of the product with (g> nucleophiles displaces it (in the form of the thiol, 45 R = H), converting the aldol product under mild conditions into esters, thiolates, phosphonates, alcohols, or acids. 

Amino-y-butyrolactones.5 The (dimethylamino)carbene 1 undergoes highly diastereoselective aldol reactions with aldehydes even in the absence of a Lewis acid to form syn-adducts 2, from which the metal unit can be removed by photolysis. 

P-Hydroxy carboxylic acids (12,3).2 This acetate on double deprotonation with LDA undergoes diastereoselective aldol reactions with aldehydes. The adducts are easily hydrolyzed to optically active P-hydroxycarboxylic acids with release of (R)-(+)-1,1,2-triphenyl-1,2-ethanediol, the precursor to 1. Optically pure acids can be obtained by crystallization of the salt with an optically active amine such as (S)-(—)-1 -pheny lethylamine. 

Boron enolates of thioglycolates. Esters do not form boron enolates because of the low acidity of the a-protons. However, methyl phenylthioacetate (2) forms a boron enolate on treatment with Hunig s base and dibutylboryl triflate, and this enolate undergoes aldol reactions with aldehydes with high syn-diastereoselectiv-ity.1... 

The chlorotitanium enolates derived from AT-acyloxazolidinones 3.17, N-acyloxazolidinethiones 3.18 and iV-acylthiazolidinethiones 3.19 undergo highly selective and efficient aldol reactions with aldehydes. The iST-acyloxazolidinethione 3.18 and N-acylthiazolidinethione 3.19 auxiliaries are also easier to cleave. 

Stereoselective aldot condensations. Mukaiyama et al. have reported that ketones are converted into vinyloxyboranes by reaction with dialkylboron triflates and a tertiary amine and that these enolates undergo regioselective aldol reactions with aldehydes (equation I). Mukaiyama used di-n-butylboryl triflate in combination... 

Because tin(ll) enolates of thioesters are generated upon reaction of tin(ll) thiolates with ketenes, the optically active p-hydroxy thioesters are also easily synthesized by way of the aldol reaction with aldehydes in the presence of tin(Il) trifluoromethanesulfonate and chiral diamine 1 (eq 7)7... 

Trichlorotitanium enolates are directly prepared from a ketone, TiCU, and a tertiary amine [122,123] and undergo aldol reactions with aldehydes [124-129], ketones [129], and imines [130,131], Intramolecular condensation with esters is also known [132-137], Although these reactions, based on a titanium enolate [16], which often results in high diastereoselectivity in aldol and related reactions [122], will not be discussed in detail in this article, the success of the alkylation of this titanium enolate with SNl-active electrophiles should be discussed owing to the high Lewis acidity of the metal center [123], Equation (37) shows stereoselective alkylation with an orthoacetate, which is usually inert to alkali metal enolates [138], Aminoalkylation of trichlorotitanium enolates with (a-chloroalkyl)amine has been performed analogously [139,140],... 

Enol silyl ethers undergo aldol reactions with aldehydes, acetals and their equivalents with the aid of a Lewis acid catalyst. These reactions are discussed in Volumes 2-4. Enol silyl ethers prepared by hydro-silylation of a, 3-unsaturated ketones with a rhodium catalyst can be used for aldol reactions with aldehydes or ketones in situ under neutral conditions (equation 60)." ... 

Fukuzawa et al. foimd that reduction of a-halo ketones followed by aldol reaction with aldehydes or ketones is prtMnoted by Ceb, CeCls/Nal or CeCb/SnCh. These reactions are carried out at room... 

Another highly fluorinated rare earth y9-diketonate, 21 (Ln = Pr, Eu, Ho dppm = di(perfluoro-2-propoxypropionyl)methanate), showed itself to be a delicate reagent, not only for accomplishing high chemoselectivity in Mukaijama aldol reactions with aldehydes and ketene silyl acetals (KSA) [123], for it was attributed with a mode of stereomodulating catalysis in the aldol reaction of alkoxy aldehydes with KSA (eqs. (12) and (13)). 

The (3-keto stannyl enolates will give aldol reactions with aldehydes (Scheme 14-5),121 and are more reactive than the simple enolates in that they also show Michael additions with unsaturated carbonyl compounds (equation 14-67).122... 

Denmark et al. have reported that trichlorosilyl enolates also undergo non-catalyzed aldol reaction with aldehydes (Scheme 10.30) [95]. The sense of diastereoselectivity depends on the geometry of the enolates - ( )-enolate 33 adds to aldehydes with high. syn selectivity, whereas low anti selectivity is observed for (Z)-enolate 35 [96], The stereochemical outcomes can be rationalized by boat-like transition structures arranged by the Lewis acidity of the silicon atom, in which the configuration around silicon is trigonal bipyramidal with aldehyde binding in the apical position. In the transition structure from 35 there are severe steric interactions caused by the enolic Z substituent, which is attributable to the low anti selectivity. 

The first two examples are based on esters, one giving the anti aldol and the other the syn. Lithium enolates of simple esters like EtC02Me give clean aldol reactions with aldehydes but the products 17 are more or less 50 50 mixtures of syn- and anti- diastereoisomers.1 Evidently the lithium enolate 16 is also formed as a 50 50 E Zmixture and this is hardly surprising as OMe and OLi must be about the same size. 

The scope and diastereoselectivity of reactions with various electrophiles are shown below. The only weak point is the poor diastereoselectivity in aldol reactions with aldehydes. These methods have been widely used as they are robust and reliable. Nevertheless the methods we have so far described for chiral enolates are less significant than the Evans chiral oxazolidinones in the next section. 

Reduction of chlorodifluoromethylketones (32) with tetrakis(N,N-dimethyl-amino)ethene (35) produced carbanion (33) via two-electron reduction, which underwent aldol reaction with aldehydes to form 34 [28]. On the other hand, one-electron reduction of 32 with thiolates produced 36 via radical 35 [29] (Scheme 2.41). 

In some cases titanium enolates give as good, if not better, stereoselectivity as the corresponding boron enolate aldol reactions (Volume 2, Chapter 1.7). For example, the tri(isopropoxy)titanium enolate of the chiral ethyl ketone (27) has been found to undergo aldol reactions with aldehydes with very high dia-... 

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