Acyl anions enol ether-derived

When the ylid derived from the oxide 7 was treated with benzaldehyde, a 2-methoxy-l,3-diene was formed. This led us to attempt a synthesis of (i) ar-turmerone by a sequence (Scheme 3) in which 7 effectively acts as an acyl anion equivalent. The required aldehyde 8 was synthesized in good yield using initially diphenyl(methoxymethyl)phosphine oxide (3), and then chlorotri-methylsilane-sodium iodide (12) to cleave the enol ether. 

The lithiation of an O-vinyl carbamate with rAr-BuLi followed by transmetallation with zinc bromide provides the convenient acyl anion derivative, which undergoes smooth Pd(0)-catalyzed cross-coupling reactions (Equation (24)).67 This reaction sequence has been extended to lithium enolates. The deprotonation of the aminoester with LDA followed by a transmetallation with zinc bromide in ether furnishes a zinc enolate, which readily adds to the double... 

Acyl Anions Derived from Enol Ethers... 

Amino nitriles are useful for conjugate addition Acyl anion equivalents of the ester d1 synthon - C02R Methods Based on Vinyl (Enol) Ethers and Enamines Lithium derivatives of cyclic vinyl ethers The synthesis of pederin and related anti-tumour agents Lithium derivatives ofallenyl ethers Oxidative Cleavage of Allenes... 

Just as anions of allyl derivatives can be homoenolate equivalents (chapter 13) so anions of vinyl derivatives can be acyl anion equivalents. Vinyl (or enol) ethers can be lithiated reasonably easily, especially when there is no possibility of forming an allyl derivative, as with the simplest compound 81. The most acidic proton is the one marked and the vinyl-lithium derivative 82 reacts with electrophiles to give the enol ether of the product17 84. However, tertiary butyl lithium is needed and compounds with y-CHs usually end up as the chelated allyl-lithium 85. These vinyl-lithium compounds add directly to conjugated systems but the cuprates will do conjugate addition.18... 

Alkylation of enolate anions usually gives C-alkylation and is therefore not suitable for the preparation of enol ethers. The exception is when triethyloxonium tetrafluoroborate is used as the alkylating agent in a dipolar aprotic solvent. 0-Alkylation can be regioselectiveiy achieved if the enolate anion is derived from acetoacetate or a similar compound. On the other hand, 0-acylation of enols or enolate anions is quite common. Enol esters can therefore be prepared readily from the parent carbonyl compounds. For... 

A second major class of acyl anion equivalents are the enol ethers such as methyl 1-propenyl ether (359). When 359 was treated with tert-butyllithium (note the need for a stronger base with the less acidic vinyl hydrogen) and then condensed with benzaldehyde, the product was 260. Lithiation of vinyl derivatives was described in Section 8.5. Facile hydrolysis with aqueous acid liberated the corresponding ketone (361), completing the acyl anion equivalency. Schlosser co-workers found that a mixture of 5ec-butyllithium and potassium tert-butoxide could be used to generate the lithium anion of O-tetrahydropyranyl enol ethers.360 This modification generates a product that is more easily hydrolyzed to the ketone. 

The anions derived from (160) and its enol ether were acylated and alkylated to give 1-benzothiepins, e.g. (161) thermal desulphurization then gave the expected naphthalenes.1-Benzothiepin 1-oxides (162) were prepared by two routes, and were decomposed at 30 or 50 °C to give naphthalenes by loss of SO the 1,1-dioxides that were produced by further oxidation of (162) were much more thermally stable. 

Addition reactions of (l-methoxyalkyl)triphenylphosphonium ylides, derived from the corresponding phosphonium salts (82) and n-BuLi, to aldehydes at — 78 °C followed by quenching the reaction mixture with aqueous NH4CI at the same temperature alforded a-hydroxyketones instead of the expected enol ethers.This is the first example of phosphonium ylides acting as an acyl anion equivalent. Flash vacuum pyrolysis (FVP) in a conventional flow system at 10 (84), prepared in a few steps... 

The concept of silyl enol ether synthesis via / -elimination from a Brook rearrangement-derived carbanion also appeared in Wicha s studies on additions of 1-phenyl-l//-tetrazol-5-yl (PT) sulfonyl anions to acyl silanes. When PT sulfone 34 was deprotonated in the presence of acyl(triphenyl)silane, ketone 36 was isolated in good yield after hydrolysis of the silyl enol ether intermediate 35. The mechanism involved addition of the... 

Potassium enolates derived from acylfulvalenes were trapped with TBDMSCl but not TMSCl or diphenylmethylsilyl chloride. Interestingly, TBDMSCl was found to be compatible with CpK anion at —78 °C. TBDMS enol ethers have also been used as /3-acyl anion equivalents. The TBDMS-silyl enol ethers of diketones (eq 7) and /3-keto esters (eq 8) may be prepared by mixing them with TBDMSCl in THF with imidazole. Alcohols may be protected under acidic conditions as their TBDMS ethers by treatment with /3-silyl enol ethers in polar sovents. 

Enolates formed by organocopper conjugate addition may be acylated cleanly by acid chlorides to give 3-diketones.146 Although O- and C-acylation are both possible, the latter is favored by the use of acid chlorides rather than anhydrides and by the use of diethyl ether as solvent, rather than DME. Good yields of 3-diketones have been obtained by acylation of the anions derived from both acyclic and cyclic unsaturated ketones with cuprates, or in copper-catalyzed Grignard reactions. Some synthetic applications are given in Scheme 54. 

---