Enol ether from ketones

Lead tetra-acetate in conjunction with a metal halide provides a high-yielding route to a-halogeno-ketones from enol ethers, and lead chemistry is used for the conversion of pyrazolones (201) into either alk-2-ynoic [Pb(OAc)4-MeOH] or allenic [Pb(OAc)4-MeOH-BF3 EtaO] esters. ... 

Nitration of ketones or enol ethers provides a useful method for the preparation of a-nitro ketones. Direct nitration of ketones with HN03 suffers from the formation of a variety of oxidative by-products. Alternatively, the conversion of ketones into their enolates, enol acetates, or enol ethers, followed by nitration with conventional nitrating agents such as acyl nitrates, gives a-nitro ketones (see Ref. 79, a 1980 review). The nitration of enol acetates of alkylated cyclohexanones with concentrated nitric acid in acetic anhydride at 15-22 °C leads to mixtures of cis- and rrans-substituted 2-nitrocyclohexanones in 75-92% yield. 4-Monoalkylated acetoxy-cyclohexanes give mainly m-compounds, and 3-monoalkylated ones yield fra/w-compounds (Eq. 2.40).80... 

I. From Aldehydes and Ketones with 1-Alkoxy-l-lithiocyclopropanes and from Enol Ethers by Cyclopropanation... 

BINOL-derived titanium complex was found to serve as an efficient catalyst for the Mukaiyama-type aldol reaction of ketone silyl enol ethers with good control of both absolute and relative stereochemistry (Scheme 8C.24) [57]. It is surprising, however, that the aldol products were obtained in the silyl enol ether (ene product) form, with high syn-diastereoselec-tivity from either geometrical isomer of the starting silyl enol ethers. 

Reaction of (284) with an aldehyde, ketone, or enol ether in the presence of acid results in an electrophilic substitution that produces a -ferrocenylalkyl carbocations that may be trapped by nucleophiles (azides, amines, thiols). This chemistry may be used to prepare enantiomerically pure ferrocene derivatives in a maimer that avoids resolution procedures (Scheme 86)." For example, the enol ether from (-)-menthone affords a kinetic carbocation (302) that may be trapped or allowed to rearrange to the more thermodynamically stable cation (303) and then trapped, thus offering a means of controlling the configuration of the stereocenter adjacent to the ferrocene unit. Use of an enantiomerically pure aldehyde derived from Q -pinene (304) affords a 1 1 carbocationic mixture that similarly isomerizes to a single cation. 

TABLE 3. Synthesis of a-benzylated ketones starting from enol ethers... 

Vinylcyclopropanols can be prepared either from the readily available cyclopro-panone hemiacetaP, from 1-hydroxycyclopropanecarboxaldehyde derivativesfrom a,a -dichloroacetone, from the silver Simmons-Smith cyclopropanation of a-ethylenic ketone silyl enol ethers from the dye-sensitized photo-oxygenation of alkylidene-cyclopropanes or from the ring-opening of oxaspiropentanes (cf. Section III.C). Consequently, they become participants of choice in a number of useful chemical transformations (see also Section IV.A). 

Table 15. l-Acyl-2-oxycyclopropanes from Diazo Ketones and Enol Ethers or Enol Acetates or O-Silyl Enols... 

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... 

The Mn(salen) complex 8 has also been appHed towards the synthesis of a-hydroxy carbonyl compounds from enol ethers and ketene acetals [87,88,89]. These substrates are a special class of trisubstituted olefins, previously demonstrated to be excellent substrates for AE. Indeed, the observed sense of stereoinduction in the oxidation of enol ether derivatives adheres to the skewed side-on approach model developed for trisubstituted olefins. In the presence of 7 mol % of 8, silyl enol ether 37 was oxidized under bleach conditions to afford the a-hy-droxy ketone in 87% ee (Scheme 14) [89]. 

A-ip -Toluenesulfonyl)amino]acetophenone (Amination of a Ketone Silyl Enol Ether with [N-(p -tolylsulfonyl)imino] phenyliodinane).172 A solution of l-(trimethylsilyloxy)styrene (0.5 mmol) in dry MeCN (7 mL) was treated with TsN=IPh (0.6 mmol). The mixture was warmed and the solvent was removed after the reagent had dissolved. The residue was purified by chromatography on silica gel followed by crystallization from Et20 to give the title product in 95% yield. No analytical or spectroscopic data were reported. 

Six-membered chiral acetals, derived from aliphatic aldehydes, undergo aldol-type coupling reactions with a-silyl ketones, silyl enol ethers," and with silyl ketene acetals " in the presence of titanium tetrachloride with high diastereoselectivities (equation 41) significant results are reported in Table 20. This procedure, in combination with oxidative destructive elimination of the chiral auxiliary, has been applied... 

ACSA(B)62]. The stannanes are available from enol ethers by a-lithiation and quenching with trialkylstannyl chloride. The coupling reactions have been run on derivatives that had either a chlorine atom in an activated position or a bromine atom in the benzenoid position. Mild acid hydrolysis of the a-pyrimidinylalkenyl ethers yields ketones, the acyl-substituted pyrimidines. In the 4,5-dichloro derivative (130), the masked acyl group is introduced into the electrophilic 4-position (131). In the 2,5-disubstituted pyrimidine (133), having a methyl group in the 5-position and a chlorine atom in the 2-position results in the addition of a masked acyl group in the electrophilic 2-position (134). When the 5-substituent in the latter example is a bromine atom, the chemoselectivity leads to masked acylation in 5-position (135). This reaction sequence constitutes a convenient... 

The readily prepared cobalt-stabilized cations (31) react with ketones, silyl enol ethers, and enol acetates to form, after decomplexation, the acetylenic adducts free from allenic by-products (Scheme 50). Quaternary centres can be generated by the use of suitably substituted cations. 

Horn JS, Paul AG, Rapoport H (1978) Biosynthetic conversion of thebaine to codeinone. Mechanism of ketone formation from enol ether in vivo. J Am Chem Soc 100 1895-1898 Kametani T, Ihara M, Honda T (1970) The alkaloids of Corydalis pallida var. tenuis (Yatabe) and the structures of pallidine and kikemanine. J Chem Soc (C) 1060—1064 Kirby GW, Massey SR, Steinreich P (1972) Biosynthesis of unnatural morphine derivatives in Papaver somniferum. J Chem Soc Perkin Trans 1 1642-1647 Kleinschmidt G, Mothes K (1959) Physiology and biosynthesis of alkaloids in Papaver somniferum. Z Naturforsch 14b 52-56... 

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). 

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