Acetals acetone exchange

Lypozyme IM-20 (Mucor miehei lipase immobilized on a weak anion-exchange resin) was kindly supplied by Novo Nordisk (Bagsvaerd, Denmark). The gas chromatography (GC) standards (mono-, di-, and trilaurin) were obtained from Sigma (St. Louis, MO). Analytical-grade glycerol, lauric acid (99.9%), n-hexane, ethyl acetate, acetone, and ethanol were purchased from Merck (Darmstadt, Germany). 

Example 6.4 Use of Spare Exchangers in Series to an Existing Acetone-Acetic Acid Exchanger... 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

Preferential formation of a 5-membered cyclic acetal is not a general rule in the steroid series e.g., the 1,3-dioxolane (23) is obtained directly from the parent triol (22) and acetone. 17,21-Acetals are obtained only by acetal exchange. ... 

Propanediol, acetone. This method removes the boronate by exchange. 2-Methylpentane-2,5-diol in acetic acid cleaves a phenyl boronate (85% yield). 

In 1992, Oda et al. reported a one-pot synthesis of optically active cyanohydrin acetates from aldehydes, which were converted to the corresponding racemic cyanohydrins through transhydrocyanation with acetone cyanohydrin, catalyzed by a a strongly basic anion-exchange resin [46]. The racemic cyanohydrins were acetylated by a lipase from P. cepacia (Amano) with isopropenyl acetate as the acyl donor. The reversible nature of the base-catalyzed transhydrocyanation enabled continuous racemization of the unreacted cyanohydrins, thereby effecting a total conversion (Figure 4.21). 

A similar ion-exchange resin method was used by Ling in 1955 (LI) for the examination of combined amino acids in urine. According to this procedure urine was desalted and simultaneously freed from amino acids by using Amberlite IR-112, H+-form resin. The effluent collected from the column was then fractionated on Amberlite IRA, OH--form resin, by successive elution with 0.16 N acetic acid, 0.08 N formic acid, 0.25 N formic acid, 0.08 N hydrochloric acid, and finally with 0.16 N formic acid. The solutions of all acids contained 10% of acetone. The collected fractions were hydrolyzed with hydrochloric acid and the liberated amino acids identified by means of paper chromatography. 

The resulting tertiary alcohol is then dehydrated and the ketone at 11 reduced to an alcohol by means of lithium aluminium hydride to give the intermediate (22-1). The acetal is next removed by exchange with acetone in the presence of a dilute acid. The requisite 1,4-diene functionality is then put in place by dehydrogenation... 

The iodo acetal could be easily obtained (as a mixture of E and Z isomers, 40/60), by a nickel catalyzed iodine-bromine exchange. This synthon reacted smoothly with the C15 tertiary allylic alcohol in the presence of a catalytic amount of palladium acetate and a stoechiometric amount of either a silver or a thallium salt. The C20 hydroxy-acetal was obtained in 38% yield, as a mixture of E and Z isomers (48/52). Finally retinal was obtained by treatment with dilute HBr in refluxing acetone, as a mixture of E and Z isomers (C(9)=C(io) and C(i3)=C(i4)), Fig. (32). 

Examples are the formation of diacetone alcohol from acetone [reaction type (A)] catalysed by barium or strontium hydroxide at 20—30°C [368] or by anion exchange resin at 12.5—37.5°C [387], condensation of benzaldehyde with acetophenone [type (C)] catalysed by anion exchangers at 25—-45°C [370] and condensation of furfural with nitromethane [type (D)] over the same type of catalyst [384]. The vapour phase self-condensation of acetaldehyde over sodium carbonate or acetate at 50°C [388], however, was found to be first order with respect to the reactant. 

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