Carbonic acid anhydrides enol carbonates

Geranyl chloride can be prepared from geraniol by the careful use of triphenylphosphine in carbon tetrachloride. Tris(dimethylamino)phosphine reacts with carbon tetrachloride to form the complex (42) which can be used to form the enol esters (43) from acid anhydrides. Similarly, aldehydes form the alkenes (44), and esters or amides of trichloroacetic acid are converted to glycidic esters. ... 

The third procedure illustrated by this preparation involves the reaotion of ketones with trifluoromethanesulfonic anhydride in a solvent such as pentane, methylene chloride, or carbon tetrachloride and in the presence of a base such as pyridine, lutidine, or anhydrous sodium carbonate.7-11,15 This procedure, which presumably involves either acid-catalyzed or base-catalyzed enolization of the ketone followed by acylation of the enol with the acid anhydride, has also been used to prepare other vinyl sulfonate esters such as tosylates12 or methanesulfonates.13... 

Appropriate quenching of a reductively formed lithium enolate with a carboxylic acid anhydride, chloride, methyl chloroformate or diethyl phosphorochloridate yields the corresponding enol esters, enol carbonates or enol phosphates. These derivatives may be transformed into specific alkenes via reductive cleavage of the vinyl oxygen function, as illustrated by the example in Scheme 8. 

If the reaction between an a-halo ketone and zinc is carried out in an qirotic solvent in the presence of an electrophilic reagent, the zinc enolate (6) can be trapped. Products corresponding to reaction at carbon are observed with carbon electrophiles (alkyl halides or aldehydes equation 8), but reaction occurs at oxygen with halosilanes and acid anhydrides (equation 9). " ... 

A phenyl-substituted cyclic silyl enolate is synthesized from 1,1-dimethylsilacyclobutane and benzoyl chloride in the presence of a stoichiometric amount of triethylamine and a catalytic amount of PdCl2(PhCN)2 (eq 8). In the case of aliphatic acid chlorides, the use of diisopropylamine instead of triethylamine gives better results. A platinum complex, Pt(CH2=CH2)(PPh3)2, also mediates the reaction although its performance is inferior to that of palladium complexes. Acid anhydrides or combinations of organic halides and carbon monoxide are alternatives to acid chlorides. In the case of acid anhydrides, the amine is not necessary for the reaction. 

Transition metal-catalyzed silicon-based cross-coupling reaction has emerged as a versatile carbon-carbon bond-forming process with high stereocontrol and excellent functional group tolerance [35], For example, (a-benzoyloxy)alkenylsilanes 105, prepared as a pure -isomer by 0-acylation of a lithium enolate derived from the corresponding acylsilane, reacts with carboxylic acid anhydrides in the presence of [RhCl(CO)2]2, giving rise to a-acyloxy ketones 106, which are then converted into 1,2-diketones by acidic workup (Scheme 5.27) [36]. 

A 20) oiefin Formation via Enol Acetates and Ozonolysis A stock solution of acetylating mixture is prepared by dissolving 0.2 ml of 70-72% perchloric acid in 5 ml of acetic anhydride. To a solution of 5 g of 3a,6a-diacetoxy-5jff-pregnan-20-one in 50 ml of carbon tetrachloride is added 5 ml of the above stock perchloric acid-acetic anhydride solution and the mixture is allowed to stand at room temperature for 1.25 hr. The mixture is... 

The formulated mechanism is supported by the finding that no halogen from the phosphorus trihalide is transferred to the a-carbon of the carboxylic acid. For instance, the reaction of a carboxylic acid with phosphorus tribromide and chlorine yields exclusively an a-chlorinated carboxylic acid. In addition, carboxylic acid derivatives that enolize easily—e.g. acyl halides and anhydrides—do react without a catalyst present. 

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... 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives.134 The acids or their chlorides or anhydrides can be a chlorinated by treatment with CuCl in polar inert solvents (e.g., sulfolane).135 Acyl halides can be a brominated or chlorinated by use of N-bromo- or N-chlorosuccinimide and HBr or HC1.136 The latter is an ionic, not a free-radical halogenation (see 4-2). Direct iodination of carboxylic acids has been achieved with L-Cu(II) acetate in HO Ac.137 Acyl chlorides can be a iodinated with L and a trace of HI.138 Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium N-isopropylcyclohexylamide in THF and treatment of this solution at - 78° with I2138 or with a carbon tetrahalide.139 Carboxylic acids, esters, and amides have been a fluorinated at -78°C with F2 diluted in Ni.,4°... 

Apart from the reaction of diazomethane with benzoyl chloride,5-6 diazoacetophenone has been prepared by the reaction of 2-aminoacetophenone hydrochloride with sodium nitrite,7 from the mixed anhydride of benzoic acid and ethyl carbonate with diazomethane,8 from benzoyl chloride and potassium methyldiazotate,9 by treating the enamine formed from 2-formylacetophenone and N-methylaniline with p-toluenesulfonyl azide,10 and from the reaction of the sodium enolate of 2-formylacetophenone with p-toluenesulfonyl azide.11... 

Acetic Anhydride, Carbon Tetrachloride, and 2-Methylcyclohexanone. During acetylation of the enolized ketone, the 70% perchloric acid must be added last.6... 

The danger we have to face is that acylation is inclined to occur on oxygen rather than on carbon. In the extreme case, naked enolates (those with completely non-coordinating cations) acylate cleanly on oxygen with anhydrides or acid chlorides. 

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