Ketone/ester exchange

Ketone-ester exchange. Carbomethoxylation of ketones may be followed by deacylation in situ when the reaction is carried out at high temperatures. 

As is to be expected, an alkynic ketone undergoes a Michael addition with a carbanion, leading eventually to a pyranone (50JA1022). Using malonic esters, a 3-alkoxycarbonyl derivative results, which is hydrolyzed to the 2-oxopyran-3-carboxylic acid under alkaline conditions, but to the pyranone by sulfuric acid. Rapid ester exchange is observed with the initial products, the alcohol used as solvent determining the nature of the alkyl group in the 3-carboxylic esters (Scheme 90). 

Carbonates are useful for adding the CC Et group to make stable enolates of the kind we met in chapter 13 and will meet again soon. Here is a case where disconnecting one carbon atom reveals an available starting material 24. Only the ketone 24 can form an enolate and the carbonate is more electrophilic than the ketone. The ideal base is ethoxide ion to avoid ester exchange, but stronger bases such as NaH work well too.2... 

The iododestannylation reaction is a commonly nsed reaction dne to the high reactivity of the formed vinyl iodides, which can be nsed for fnrther transformations. The stannylated precursors are easily prepared by hydrostannation of alkynes, and the iodide-tin exchange proceeds with an excellent regio- and stereocontrol. Fnrthermore, the reaction is compatible with the presence of several fnnctionalities snch as ethers ", alcohols , ketones , esters and amines. Fnnctionalized dienyltins were also nsed for the preparation of l-iodo-l,3-dienes, and the reaction was even extended to the preparation of fully stereo-determined iodo tetraenes. ... 

Recent examples, for instance, of the catalytic application of the commercially available macroporous Amberlyst-15 include the Michael addition of pyrroles to a,P-unsaturated ketones (Scheme 10.4) [48]. In this process, the acid ion exchange resin (dry, 10% w/w) allows on to obtain mono and dialkylated pyrroles 5 and 6 in reasonable yields. Similarly, this catalyst (dry, 30% w/w) can catalyze the aza-Michael reaction of amines with a,P-unsaturated ketones, esters and nitriles to afford 7 in 75-95% yields under solvent-free conditions. Interestingly, yields were significantly lower using typical solvents such as DCM (dichloromethane), CH3CN, THF, DMF or EtOH [49], Recycling the catalyst is possible in both cases, but a smooth decrease in the yield is observed for each new run. 

The synthetic utility of the Haloform reaction is revealed further by a synthesis of isotopically pure (99.2%) CDCb by the reaction of trichloroacetophenone with NaOD in D20. The alkoxide-cleavage of trichloromethyl ketones has been employed to synthesize dihydropyridine esters (e.g., 24) in which ester exchange does not occur.59... 

The interaction of monocarbonyl iodonium ylides, generated by the ester exchange of (Z)-(2-acetoxyvinyl)iodonium salts 792 with EtOLi, with organoboranes affords ketones 795, probably via intermediate formation of the hitherto unknown a-boryl ketones 794 (Scheme 3.314) [1090]. 

The reaction of anionic metal complexes 162 with a-chloro ketones, esters, or tertiary amides occurring under chloro-metal exchange is an appropriate method for the preparation of molybdenum and tungsten enolates. The reaction fails with the corresponding a-bromo compounds. In order to obtain tungsten enolates, which are substituted in a-position, a-methanesulfonyloxy-substituted esters are suitable starting materials. The tungsten and molybdenum complexes 163 were fully characterized as C-bound // -enolates (Scheme 2.47) [162]. 

These reversible reactions are cataly2ed by bases or acids, such as 2iac chloride and aluminum isopropoxide, or by anion-exchange resias. Ultrasonic vibrations improve the reaction rate and yield. Reaction of aromatic aldehydes or ketones with nitroparaffins yields either the nitro alcohol or the nitro olefin, depending on the catalyst. Conjugated unsaturated aldehydes or ketones and nitroparaffins (Michael addition) yield nitro-substituted carbonyl compounds rather than nitro alcohols. Condensation with keto esters gives the substituted nitro alcohols (37) keto aldehydes react preferentially at the aldehyde function. 

A -Dien-3-ol esters e.g., acetates) have greater utility as reaction intermediates than as protecting groups. They are prepared from A" -3-ketones by reaction with the acetic anhydride"" or by exchange with isopropenyl acetate. 

Hence, enol esters such as isopropenyl acetate are good acylating agents for alcohols. Isopropenyl acetate can also be used to convert other ketones to the corresponding enol acetates in an exchange reaction ... 

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