Enol ethers from anhydrides

Nitridomanganese Complexes. Stoichiometric amounts of chiral complexes of type 43 react with silyl enol ethers in the presence of trifluoroacetic or /Moluenesulfonic anhydride to give a-(A-trifluroacetyl)amino- and a-(N-p-tosylamino) ketones, respectively (see Eq. 160).350 - 353 With glycals, the 1-hydroxy-2-( A -trifluoroacetyljamino derivatives are formed (see Eq. 83).354 A mechanism involving approach of the enol ether from the least hindered side of the 43-TFA complex has been proposed.353... 

In order to construct the thiepin conjugation, the seven-membered cyclic ketones containing sulfur could be converted to either their enol ethers or enol acetates. The resulting thiepins should have a number of substituents. The first stable 3,5-diacetoxy-4-phenylbenzo[6]thiepin (15) has been obtained by Hofmann et al. from the diketone (14) by acetylation with acetic anhydride in pyridine in good yield 13). By this methodo-... 

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... 

Nitration of the potassium enolates of cycloalkanones with pentyl nitrate81 or nitration of silyl enol ethers with nitronium tetrafluoroborate82 provides a method for the preparation of cyclic a-nitro ketones. Trifluoroacetyl nitrate generated from trifluoroacetic anhydride and ammonium nitrate is a mild and effective nitrating reagent for enol acetates (Eq. 2.41).83... 

It is not customary to attempt the isolation of ketone or aldehyde intermediates (121) the formula serves merely as a reminder that once hydrolysis of the protecting enol ether or acetal occurs, the same type of structure is formed from any given dicarbonyl compound. Cyclization has been carried out in refluxing ethanolic picric acid or acetic anhydride with a few drops of sulfuric acid, but Hansen and Amstutz (63JOC393) offered excellent theoretical reasons for avoiding an excess of acid, and reported that best results (Table 3) can be obtained by refluxing the dry hydrobromide in acetic anhydride containing no sulfuric acid. 

The statine-like moiety in one of the first drugs, saquinovir (23-8), comprises a transition state mimic for the cleavage of phenylalanylprolyl and tyrosylprolyl sequences. Constmction starts with the protection of the amino group of phenylalanine as its phthaloyl derivative (Phth) by reaction with phthalic anhydride this is then converted to acid chloride. The chain is then extended by one carbon using a Friedel-Crafts-like reaction. The required reagent (21-2) is prepared by reaction of the enolate obtained from the /7A-silyl ether (21-3) of glyoxylic acid and lithio... 

Since the excess trimethylsilyl bromide was difficult to remove, an alternative sequence was investigated (Scheme 10). After bromination of the silyl enol ether, the reaction mixture was poured into water to hydrolyze both the trimethylsilyl bromide and the anhydride. On heating this bromoacid as before, an unexpected compound was formed. This can be rationalized as follows The reaction proceeds from the enol form, and the mechanism is formally 1,5 elimination of hydrogen bromide with concomitant loss of carbon dioxide. The second decarboxylation is analogous to the one seen earlier, and would be expected of the a,8-unsaturated ketone. 

Titanium(iv) benzylidenes (Schlock carbenes) 4 react with resin-bound esters 5 to generate the resin-bound enol ethers 6. Treatment of the enol ethers with a mixture of trifiuoroacetic acid (TEA) and trifluoroacetic anhydride (TFAA) leads to cleavage from the resin, removal of the Bu Me2Si group, and subsequent cyclization to give the benzothiophenes 7 (Scheme 2) <2004JOC6145>. 

Methoxyallyl cation (5a Y = OMe), derived from 2-methoxyallyl bromide (4a X = Br, Y = OMe) and silver trifluoroacetate in the presence of sodium carbonate, reacts with benzene, toluene, p-xylene or mesitylene. 2-Ethoxyallyl alcohol (4c X = OH, Y = OEt) in the presence of trifluoroacetic anhydride generates the 2-ethoxyallyl cation (5c Y = OEt). 2-Ethoxyallyl alcohol (4c) is probably more advantageous than the corresponding 2-methoxyallylic species (4a Y = OMe) because dealkylation of the intermediary oxonium ion seems to be less facile, and the ethyl enol ether is more nucleophilic than the methyl enol ether. Treatment of cyclic a-chloroenamines (4f X = Cl, Y = morpholino) and (4g X = Cl,... 

For the synthesis of (69), the enol ether (71) from the indanone (70) was carboxylated with COa-n-butyl-Iithium in THF at —70 C to yield (72). The methyl ester (73) was converted into (75) via the maleic anhydride adduct (74), essentially as described in earlier work. Lithium aluminium hydride reduction followed by oxidation with dicyclohexylcarbodi-imide afforded the aldehyde (76). This was condensed with excess (77) to yield a mixture of the diastereomers (78). Oxidation with chromium trioxide-pyridine in methylene dichloride gave (79), which could be converted into the diketone (80) by treatment with excess benzenesulphonylazide. The diketo-lactam (81) was prepared from (80) as described for the synthesis of the analogous intermediate used in the synthesis of napelline. Reduction of (81) with lithium tri-t butoxyaluminohydride gave the desired dihydroxy-lactam (82). Methylation of (82) with methyl iodide-sodium hydride gave (83). Reduction of this lactam to the amine (84) with lithium aluminium hydride, followed by oxidation with potassium permanganate in acetic acid, gave (69). 

Naked enolates without any complexation can be made from silyl enol ethers using fluoride ion, a very selective nucleophile for silicon 49, and a non-metallic cation, usually a tetra-alkylammonium ion. The commonest reagent is TBAF (TetraButylAmmonium Fluoride Bu4N+ F ). In this way the naked enolate 50 was made. It proved to be acylated with acetic anhydride exclusively at oxygen to give the enol acetate 53 and alkylated with Mel at carbon to give the ketone 51 in 84% isolated yield.18... 

Claisen rearrangement. The quenching of the lithium enolates derived from allylic acetates results in enol phosphates, which show a higher propensity for Claisen rearrangement than ketene silyl ethers. The mixed anhydride products can be used directly to generate carboxylic acid derivatives. 

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