Carbonyl groups with 5,5-acetals

Redistilled N, O-persilylated L-proline 438 reacts with pivaldehyde in pentane at ambient temperature to give the 0,N-acetal 439 in 78% yield in the presence of LDA this condenses, a to the carbonyl group, with N-alkyl-2-pyrrole aldehydes [48] (Scheme 5.14). 

TiCl4 promotes reduction of carbonyl groups or acetals with R3SiH or R3SnH (Scheme 24) Other hydride sources are also effective.91 ... 

If you look in older texts, you may still see the terms hemiketal and ketal. At one time, four terms were used for the products of alcohols with carbonyl groups hemiacetal, acetal, hemiketal, and ketal. A hemiketal is now a type of hemiacetal and a ketal is now a type of acetal. Originally, acetals and hemiacetals came from aldehydes and ketals and hemiketals came from ketones. The structures of hemiacetals and acetals contained a C-H bond, but ketals and hemiketals did not. 

The Diels-Alder reaction of 3-methoxyfuran with octyl vinyl ketone took place at room temperature in quantitative yield to afford exclusively the endo cycloadduct (27) (81CC221). Reduction of the carbonyl group with lithium tri-r-butoxyaluminum hydride produced a single alcohol (28). Ozonolysis of the double bond followed by Jones oxidation yielded the lactone ester (29). Hydrolysis of the ester and lead tetraacetate oxidation gave the lactone acetate. This was converted by further hydrolysis and Jones oxidation to the bis-lactone (30), a known intermediate in the synthesis of ( )-isoavenaciolide (Scheme 6). 

Obviously, the condensation of a carbonyl group with a diol produces 1 mol of water and because of the reversibility of the reaction (hydrolysis of the acetal), yields are lowered if this by-product is not removed. For such a purpose, there are essentially two possibilities (1) the continuous removal of water by an azeotropic distillation with a solvent mainly chosen for its boiling point (petroleum ether, benzene, toluene, xylene, for instance) (2) the presence of a desiccant (the most commonly taken is copper(II)sulfate, but sodium sulfate or molecular sieves have been also used) molecules known to be water scavengers, such as ortho-esters or dialkylsulfites, have also been suggested, even if they are seldom used in carbohydrate chemistry. 

Reaction XLIV. (b) Condensation of Alkyl and Aryl Halogen Compounds with the Sodio- and other Metallo-derivatives of Ethyl Aceto-acetate and its Homolognes. (A., 186, 214 201, 143 213, 143.)—Like malonic ester, acetoacetic ester contains two 1 3-carbonyl groups with a methylene group in position 2. It is only to be expected then that it yields with metallic sodium or sodium alcoholate sodio-derivatives from which mono- and di-, alkyl and aryl homologues can be obtained by treatment with a suitable halide, including halogen esters. Acetoacetic acid... 

The amide is ineffective with respect to nucleophilic attack on the carbonyl group of acetic anhydride. 

Fig. 38.—The shift with temperature of the infrared absorption spectrum of the carbonyl group in acetic acid.

Predict the reactions of carbohydrates in acidic and basic solutions, and with oxidizing and reducing agents. Predict the reactions that convert their hydroxyl groups to ethers or esters, and their carbonyl groups to acetals. Problems 23-56,57,58, G3, G4, G5, and 66... 

Classically, this separation of roles was accomplished by utilizing substrates that differed drastically in their ability to form enolate anions. In fact, the various name reactions mentioned above are distinguished not by basic differences in mechanism, but by the nature of the components preferentially utilized as substrates. For example, in the Perkin reaction, the condensation of an aromatic aldehyde with an aliphatic acid anhydride is based on the fact that the electrophilic component, e.g. benzaldehyde, lacks a-hydrogens and therefore is incapable of forming an enolate. At the same time the utilized electrophile, acetic anhydride, contains a carbonyl group with reduced propensity to interact with nucleophiles. Hence, it is incapable of undergoing self-condensa-... 

Step [3] The acetal is then converted back to a ketone carbonyl group with aqueous acid. 

An example is offered by the room temperature cholinium chlorozincate ionic liquid 22, an efficient catalyst for the protection of carbonyl groups with diols under solvent-free conditions (Strategy C). An equimolar mixture of a diol (1,2- to 1,5-diol) and a carbonyl compound is stirred at room temperature for 10-20 h in the presence of 22. Very high yields of acetals are reported and the catalyst is easily recovered and recycled. A typical reaction is reported in Figure 22. 

Dimethylcyclohexenone was treated with vinyl magnesium bromide in the presence of cuprous bromide to afford a ketone 2. After protection of the carbonyl group with ethylene acetal, hydroboration, oxidation, and methylation with diazomethane yielded a methyl ester 6. The methyl ester 6 was treated with LDA/allyl bromide to give 7, followed by ozonolysis to afford an aldehyde 8. 

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