Carbonyl compounds enol ether preparation

Other simple alkenois (enols) also rearrange to carbonyl compounds. However, ether and ester derivatives of enols are known and can be prepared by... 

Some synthetic modification for Pd(II)-catalyzed oxidation of silyl enol ethers is also developed. Silyl enol ethers prepared from aldehydes and ketones are converted to the corresponding a,/3-unsaturated carbonyl compound in good yields by 10 mol % of palladium(II) acetate in the presence of 1 atm pressure of O2 in DMSO as solvent (Scheme... 

Preparation of o,/3-Unsaturated Carbonyl Compounds by the Reactions of Silyl Enol Ethers and Enol Acetates with Ally Carbonates... 

Although ethereal solutions of methyl lithium may be prepared by the reaction of lithium wire with either methyl iodide or methyl bromide in ether solution, the molar equivalent of lithium iodide or lithium bromide formed in these reactions remains in solution and forms, in part, a complex with the methyllithium. Certain of the ethereal solutions of methyl 1ithium currently marketed by several suppliers including Alfa Products, Morton/Thiokol, Inc., Aldrich Chemical Company, and Lithium Corporation of America, Inc., have been prepared from methyl bromide and contain a full molar equivalent of lithium bromide. In several applications such as the use of methyllithium to prepare lithium dimethyl cuprate or the use of methyllithium in 1,2-dimethyoxyethane to prepare lithium enolates from enol acetates or triraethyl silyl enol ethers, the presence of this lithium salt interferes with the titration and use of methyllithium. There is also evidence which indicates that the stereochemistry observed during addition of methyllithium to carbonyl compounds may be influenced significantly by the presence of a lithium salt in the reaction solution. For these reasons it is often desirable to have ethereal solutions... 

Although the conversion of an aldehyde or a ketone to its enol tautomer is not generally a preparative procedure, the reactions do have their preparative aspects. If a full mole of base per mole of ketone is used, the enolate ion (10) is formed and can be isolated (see, e.g., 10-105). When enol ethers or esters are hydrolyzed, the enols initially formed immediately tautomerize to the aldehydes or ketones. In addition, the overall processes (forward plus reverse reactions) are often used for equilibration purposes. When an optically active compound in which the chirality is due to an asymmetric carbon a to a carbonyl group (as in 11) is treated with acid or base, racemization results. If there is another asymmetric center in the molecule. 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

The present preparation illustrates a general and convenient method for a two-step deoxygenation of carbonyl compounds to olefins. Related procedures comprise the basic decomposition of p-toluenesulfonylhydrazones, the hydride reduction of enol ethers, enol acetates, enamines, the reduction of enol phosphates (and/or enol phosphorodiamidates) by lithium metal in ethylamine (or liquid ammonia),the reduction of enol phosphates by titanium metal... 

Iron-acyl enolates, such as 2, prepared by x-deprotonation of the corresponding acyl complexes with lithium amides or alkyllithiums, are nearly always generated as fs-enolates which suffer stereoselective alkylation while existing as the crmt-conformer which places the carbon monoxide oxygen anti to the enolate oxygen (see Section 1.1.1.3.4.1.). These enolates react readily with strong electrophiles, such as primary iodoalkanes, primary alkyl sulfonates, 3-bromopropenes, (bromomethyl)benzenes and 3-bromopropynes, a-halo ethers and a-halo carbonyl compounds (Houben-Weyl, Volume 13/9 a, p 413) (see Table 6 for examples). 

Diethyl [(2-tetrahydropyranyloxy)methyl]phosphonate is useful in the Wlttig-Horner synthesis of enol ethers, which are intermediates in one-carbon homologations of carbonyl compounds. This procedure is an adaptation of a general method for making dialkyl hydroxymethylphosphonates. An 0-tetra-hydropyranyl derivative also has been made from dibutyl hydroxymethyl -phosphonate, and diethyl hydroxymethylphosphonate has been O-silylated with tert-butylchiorodimethylsilane and imidazole. Another useful congener in this series has been prepared by an Arbuzov reaction of methoxyethoxymethyl (HEM) chloride and triethyl phosphite. 

The preparation of a-hydroxy carbonyl compounds has been accomplished by the oxidation of enolates using both oxygen6 and MoC PyHMPA-(MoOPh).7 Acyl anion equivalents offer another route to this useful class of compounds. The procedure presented here for the synthesis of 6-hydroxy-3,5,5-tr1methyl-2-cyclohexen-l-one illustrates the use of MCPBA oxidation of an enol silyl ether as a method for obtaining an a-hydroxy enone. The procedure is a scaleup of a published synthesis. ... 

This procedure illustrates a general method for the preparation of crossed aldols. The aldol reaction between various silyl enol ethers and carbonyl compounds proceeds smoothly according to the same procedure (see Table I). Sllyl enol ethers react with aldehydes at -78°C, and with ketones near 0°C. Note that the aldol reaction of sllyl enol ethers with ketones affords good yields of crossed aldols which are generally difficult to prepare using lithium or boron enolates. Lewis acids such as tin tetrachloride and boron trifluoride etherate also promote the reaction however, titanium tetrachloride is generally the most effective catalyst. 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

Mukaiyama found that Lewis acids can induce silyl enol ethers to attack carbonyl compounds, producing aldol-like products.22 The reaction proceeds usually at -78 °C without selfcondensation and other Lewis acids such as TiCl4 or SnCI4 are commonly used. The requisite silyl enol ether 27 was prepared by treatment of ketone 13 with lithium hexamethyl disilazide (LiHMDS) and trapping the kinetic enolate with chlorotrimethylsilane. When the silyl enol ether 27 was mixed with aldehyde 14 in the presence of BF3-OEt2 a condensation occurred via transition state 28 to produce the product 29 with loss of chlorotrimethylsilane. The induced stereochemistry in Mukaiyama reactions using methylketones and a, -chiral aldehydes as substrates... 

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