Ylides reactions with aryl aldehydes

Sulfur ylides, derived from benzyl bromides and an optically active alkyl sulfide, undergo base-promoted reactions with aryl aldehydes to produce optically active 1,2-diatyl epoxides.The reaction is illustrated by equation (17) and produces epoxides with optical purities in the range of 28-47% ee. The bicyclic sulfide shown in equation (17) was derived from (-t-)-camphor8ulfonic acid and produces the (RiO-enantiomer of the epoxide in excess. 

Copper-catalyzed thermal reaction of diazo-trifluoropyruvate 301 with aryl aldehydes in the presence of SbBu3 provides alkenes 302, which are transformed to racemic amino acids 304 (see Scheme 9.65) [96]. Antimony ylide 305 is proposed as an intermediate. 

Finally, carbenoid species can be used as the carbon donor in aldehyde epoxidations. Thus, the rhodium carbenoid derived from the cyclic diazoamide 49 and rhodium(II) acetate reacts stereo selectively with aryl aldehydes to provide spiro-indolooxiranes 50 with Z-stereochemistry. The reaction is believed to proceed via the formation of a carbonyl ylide 51, which undergoes stereospecific thermal conrotatory electrocyclization to form the observed epoxide <04SL639>. 

Scheme 4.41 Schematic reaction profiles ofphosphonium, sulfonium andguanidinium ylides with aryl aldehydes...

Scheme 2.22 Three-component reaction of carbonyl ylide with aryl aldehydes.

The 3-oxo-2-pyrazolidinium ylides 315, easily available by reaction of the corresponding pyrazolidin-3-one with aromatic aldehydes, function as 1,3-dipoles in cycloaddition reactions with suitable alkenes and alkynes to provide the corresponding products. When unsymmetrical alkynes are used, mixtures of both possible products 316 and 317 are usually obtained (Equation 45). The regioselectivity of cycloadditions of the reaction with methyl propiolate is influenced by the substituents on the aryl residue using several 2,6-di- and 2,4,6-trisubstituted phenyl derivatives only compound 316 is formed <2001HCA146>. Analogous reactions of 3-thioxo-l,2-pyrazolidinium ylides have also been described <1994H(38)2171>. 

A variety of optically active 4,4-disubstituted allenecarboxylates 245 were provided by HWE reaction of intermediate disubstituted ketene acetates 244 with homochiral HWE reagents 246 developed by Tanaka and co-workers (Scheme 4.63) [99]. a,a-Di-substituted phenyl or 2,6-di-tert-butyl-4-methylphenyl (BHT) acetates 243 were used for the formation of 245 [100]. Addition of ZnCl2 to a solution of the lithiated phos-phonate may cause binding of the rigidly chelated phosphonate anion by Zn2+, where the axially chiral binaphthyl group dictates the orientation of the approach to the electrophile from the less hindered si phase of the reagent. Similarly, the aryl phosphorus methylphosphonium salt 248 was converted to a titanium ylide, which was condensed with aromatic aldehydes to provide allenes 249 with poor ee (Scheme 4.64) [101]. 

A -Cyano- and A -(p-toluenesulfonyl)-A -(trimethylsilylmethyl)-5-methylisothio-ureas 269a and 269b have also been utilized as synthetic equivalents of azomethine ylides. ° Reaction of 269a and 269b with aromatic aldehydes and aryl ketones, in the presence of CsF, gives 2-iminooxazolines 270a-e in modest-to-good yield. These 2-iminooxazolines apparently are stable to isolation and do not isomerize to 2-aminooxazolines (Scheme 8.76). 

The carbonyl ylide generated from metal carbene can also add to C=0 or C=N bonds. The [2 + 3]-cycloaddition of carbonyl ylide with G=0 bond has been used by Hodgson and co-workers in their study toward the synthesis of zaragozic acid as shown in Scheme n 27a,27d Recently, a three-component reaction approach to syn-a-hydroxy-f3-amino ester based on the trapping of the carbonyl ylide by imine has been reported.The reaction of carbonyl ylide with aldehyde or ketone generally gives l,3-dioxolanes. Hu and co-workers have reported a remarkable chemoselective Rh2(OAc)4-catalyzed reaction of phenyl diazoacetate with a mixture of electron-rich and electron-deficient aryl aldehydes. The Rh(ii) carbene intermediate reacts selectively with electron-rich aldehyde 95 to give a carbonyl ylide, which was chemospecifically trapped by the electron-deficient aldehyde 96 to afford 1,3-dioxolane in a one-pot reaction (Equation (12)). 

It has been reported that yield and diastereoselectivity in reactions of aryl-stabilized ammonium ylides with aldehydes are strongly influenced by the nature of the amine and the ylide substituent (Scheme 5)48 Electron-deficient aromatics, which are able to... 

A very interesting sulfur ylide approach to epoxides has been reported <05JOC4166>. In this method, a catalytic amount (10 - 20 mol%) of a Q symmetric thiolane, 40, with a controlled topology is used to generate ylide 41. Reaction with an aryl aldehyde provides epoxide 42 via a catalytic transfer of benzylidene in generally excellent yields with good ee. 

Ylide and Related Reactions. AUyloxymethylenetriphenyl-phosphonium chlorides yield aUyl vinyl ethers by conversion into ylides with r-BuOK and treatment with aryl and eno-lizable acyclic aldehydes and ketones. Q , Unsaturated nitriles are obtained with high Z-stereoselectivity when diphenyl-cyanomethylenephosphonate is converted into its potassium salt by treatment with r-BuOK in THF, followed by addition of aldehydes with bulky substituents. o -Fluoro-Q, -unsaturated thia-zolines are obtained with slight l -stereoselectivity when mixtures of fluorophosphonomethylthioamides, or the corresponding thia-zolines (eq 94), and aldehydes are treated with t-BuOK in THF at 0 °C higher E-selectivity is usually observed when BuLi in THF is used as the base at —78 °C. ... 

Multicomponent reactions, which have been known for over 150 years, are those chemical transformations in which more than two reactants combine in a sequential manner to give highly selective products that retain majority of the atoms of the starting materials [36]. Because of the inherent molecular diversity, efficiency, and atom economy, they are highly sought after by chemists and have been widely used in the synthesis of some heterocyclic compounds. Hu and coworkers reported one unique example wherein the electron-rich aryl aldehydes selectively formed a carbonyl ylide with phenyldiazoacetate, which successively cyclized with electron-deficient aryl aldehydes to give the dioxolane (Scheme 2.22) [37]. 

When compared with phosphonium and sulfonium ylides, the corresponding selenonium and telluronium ylides frequently exhibit better reactivity and nucle-ophilicity, while the use of selenides and tellurides in catalytic ylide reactions is far less reported. In 2001, Metzner and coworkers reported an asymmetric epoxi-dation reaction of aldehydes using a C2-symmetric selenide 42 resembling their sulfide catalyst (Scheme 20.32). High yields (65-97%) and enantioselecti vity (76-94% ee) were obtained with a range of aromatic aldehydes and dnnamalde-hydes. However, no diastereoselectivity (trans cis = 1 1) was observed in these reactions, while the corresponding sulfide gave around 80% diastereoselectivities, which could be rationalized as the formation of a less diastereoselective early transition state with the more reactive selenonium ylides. The reactions of more electron-deficient aryl aldehydes (p-Cl and p-CFs) were less enantioselective (76% ee and 83% ee, respectively) [58]. 

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