Hemi-acetal

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

With 11/3-hydroxy steroids, 11/3, 19-ether formation competes with hemi-acetal formation.This is a consequence of steric hindrance of the 11/5-oxygen by the C-18 and C-19 iodomethyl groups which reduces the rate of hypoiodite formation [(3) (5)] even though the conformation of the... 

The reactions of carbonyl compounds with alcohols, R OH, to yield hemi-acetals (22),... 

Ammonia itself yields imines, R2C=NH, with carbonyl compounds but these derivatives are unstable and react with each other to form polymers of varying size. The classical aldehyde ammonias are found to be hydrated cyclic trimers, but from aldehydes carrying powerfully electron-withdrawing substituents it is possible to isolate the simple ammonia adduct [73, cf. (72), and hydrates, p. 208, hemi-acetals,... 

In another variant of such reactions, the hemi-acetal 104, generated in situ by reaction of the corresponding allylic alcohol with acetaldehyde, undergoes cyclization in the presence of HgClOAc to afford the mercurated acetal 105.128 Under these conditions, the reaction leads to the jy -product with a high selectivity (dr > 10 1 Scheme 14). The same strategy has been applied iteratively for the stereoselective synthesis of natural products containing 1,3,5,7,9-pentaether chains.129... 

One of the more difficult problems encountered in obtaining a valid assay of formic acid is that of formate ester formation. The formate is derived from the cyclic, hemi-acetal structure which is an equilibrium form of many free sugars in solution. For example, the oxidation of one of the cyclic forms of D-glucose can readily be seen to give a formate ester (as well as a C-formyl group) on the atom originally denoted as C5. It... 

In the presence of excess water, the ketal functions of the polysaccharides can be hydrolyzed to the corresponding hemi-acetals, liberating thereby the individual sugars or, at least, their oligomers (Fig. 2.7). This hydrolysis is catalyzed by acids... 

One of the consequences of forming a cyclic hemi-acetal or hemiketal is that the nucleophilic hydroxyl adds to the carbonyl group and forms a new hydroxyl. This new group is susceptible to many normal chemical reactions of hydroxyls, e.g. esterification, and this type of reaction effectively freezes the carbohydrate into one anomeric form, since the ringopening and equilibration can now no longer take place. Consider esterification of glucose with acetic anhydride (see Section 7.9.1). P-o-Glucose will be... 

However, the hemiaminal 17 is unstable as a free base and readily undergoes exchange reactions. Since the hydroxy moiety of 17 is more easily displaced than the amine moiety, a highly reactive cyclopropyliminium salt 18 is formed, which then reacts with weak nucleophiles such as ethanol, to give e. g., 19. Otherwise in water solution 17 can also probably eliminate ammonia to form the highly reactive cyclopropanone 20, which is in equilibrium with its hydrate 21 and hemi-acetal 22, Eq. (8) [20]. It has been reported that hydrate 21 is also a potent inhibitor of ALDH [20,21]. 

AU the products resulting from acid-catalysed solvolysis of A-acetoxy-A-butoxybenz-amides in acetonitrile-water mixtures were derived from the A-butoxy-A-hydroxybenz-amide intermediate (103), which is itself an anomeric amide and is the amide equivalent of a hemi-acetal . Decomposition reactions of 103 under acidic conditions are presented in Scheme 20. 

Names of cyclised (hemi-acetalised) aldoses and ketoses contain the infixes pyran or furan to indicate the six- or five-membered heterocyclic structure and a stereodescriptor, a or p, to indicate the configuration of the anomeric or hemi-acetal carbon atom. 

C-Glycosyl compounds are important molecular targets as they occur in Nature and have interesting biological properties (for reviews on C-glycosylation, see [72-75]). Chain extensions of aldonolactones have been employed to create C-C bond formation at the anomeric center. Claisen-type reactions of aldono-1,4-lactones (e.g., 9, 55) with acetone or acetophenone (Scheme 17) generate hemi-acetals of type 56a-c [76]. Similarly, lactone 55b reacts with CHsCN/NaH to give hemiacetal 57. 

Simple sugars in aqueous solution usually form cyclic structures, either hemi-acetals or hemiketals (Figure 4—5). 

Such a radical would disproportionate (4, 31) to aldehyde and hemi-acetal (Reaction 7). 

In 1925, Levene and Ulpts19 proposed the possibility that polymerization was responsible for some of the unusual properties of the 2,5-anhydroaldoses. The formation of dimers, hydrates, and hemi-acetals with the solvent can likewise be held responsible for the characteristic migration behavior observed for the 2,5-anhydroaldoses with aqueous alcoholic solvents on paper chromatograms.68... 

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hemi-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18]. t-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19]. 

Ring opening of substituted furan Furan may be regarded as a cyclic hemi-acetal that has been dehydrated, and is hydrolysed back to a dicarbonyl compound when heated with dilute mineral acid. 

Mechanism. The first step is the typical acid-catalysed addition to the carbonyl group. Then the alcohol nucleophile attacks the carbonyl carbon, and forms a tetrahedral intermediate. Intramolecular proton transfer from nitrogen and oxygen yields a hemiacetal tetrahedral intermediate. The hydroxyl group is protonated, followed by its leaving as water to form hemi-acetal, which reacts further to produce the more stable acetal. 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

The reduction occurs by interception of the open chain form present in the aldehyde/ketone-hemi-acetal/hemiketal equilibrium. Although only a small amount of the open chain form is present at any given time, that small amount is reduced. Then more is produced by opening of the pyranose form, and that additional amount is reduced, and so on until the entire sample has undergone reaction. 

The experimental values for A HO- for the breakdown of hemiorthoesters are collected in Tables 12-14. As these were determined under several different sets of conditions they have to be extrapolated to 25° with water as solvent and collected together with values of HO- for the breakdown of some hemi-acetals in Table 15. 

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