Acetal functional groups

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water (Eq. 8.49).108 When the allylation was mediated by indium in water, the reaction went smoothly at room temperature without any promoter, whereas the use of zinc and tin usually requires acid catalysis, heat, or sonication. The mildness of the reaction conditions makes it possible to use the indium method to allylate a methyl ketone in the presence of an acid-sensitive acetal functional group (Eq. 8.50). Furthermore, the coupling of ethyl 2-(bromomethyl)acrylate with carbonyl compounds proceeds equally well under the same reaction conditions, giving ready access to various hydroxyl acids including, for example, sialic acids. 

Allylation and subsequent protection of the thus formed hydroxyl group furnishes compound 134, which bears the C-ring skeleton of baccatin HI. Removal of the C-9 silyl group, PhLi treatment of the resulting hydroxyl ketone, and in situ acetylation provides compound 135, which has the C-2 benzoate functionality. In the presence of a guanidinium base, equilibrium between the C-9 to C-10 carbonyl-acetate functional groups can be established. Thus, the desired C-9-carbonyl-C-10-acetate moiety 136 can be separated from the mixture. Compound 136 is then converted to aldehyde 137 via ozonolysis for further construction of the C-ring system (Scheme 7-40). 

The SCLPs are defined as unbranched, aliphatic chains containing between nine and 18 carbon atoms, with up to three double bonds in the carbon backbone, and ending in an alcohol, aldehyde, or acetate functional group (Figure 12.5) [42]. Laboratory studies have demonstrated that there is no acute, subchronic, chronic, or developmental mammalian toxicity even with exposure to high doses of SCLPs or chemically similar compounds [39, 43]. 

Note The acetal functional group survives the reaction conditions in Steps 1-2. 

We saw in the previous chapter that aldehydes react with alcohols to form hemi-acetals, and hemiacetals react with another alcohol to afford acetals. Well, the intramolecular reaction of the aldehyde carbon of an aldose monosaccharide with one of the hydroxyl groups in the same molecule affords a cyclic hemiac-etal. An mferaiolecular reaction of this hemiacetal with the hydroxyl group of another sugar molecule provides an acetal functional group. This kind of acetal linkage connects monosaccharides into polysaccharides. Biochemists refer to these acetal (and ketal) bonds as glycoside bonds. 

Circle and label the hemiacetal functional group and the acetal functional group in the following carbohydrates ... 

Acetate functional groups are quite prevalent in insect sex pheromones and have been introduced into a wide variety of alkyl bromides by reaction with sodium acetate in DMF372 or acetic acid371. As is generally the case, the use of PTC allows reaction under milder conditions and significantly improves yields and reduces reaction times. In one example the reaction of alkyl halides with formate anion has given excellent yields of a variety of alkyl formates212. 

The second example here involves (overall) the elimination of a tertiary alcohol—so why couldn t an acid-catalysed El reaction have been used The problem here, nicely solved by the use of the mesylate, is that the molecule contains an acid-sensitive acetal functional group. An acid-catalysed reaction would also have risked eliminating methanol from the other tertiary centre. 

Widely used as polymeric supports for protein immobilization are the polymers containing aldehyde and acetal functional groups (Figure 3.15) [155]. 

Both compounds have a 1,1-diX relationship between the two oxygen atoms attached to the sarr.. carbon atom. You might have seen this in another way by recognizing the acetal functional group In any case, both compounds can be made from a diol (or two molecules of an alcohol) and ir aldehyde or ketone. 

Multistriatin could be used to trap the beetles and so prevent the spread of the disease but there is no prospect of isolating useful amounts from the beetles. It must be synthesised. In analysing the problem we notice that C-6 has two single bonds to oxygen atoms. We therefore recognise an acetal functional group. Acetals (4) can be made by a reliable reaction from carbonyl compounds and alcohols. 

The sequence consists of a protection step of the aldehyde functional group, an oxidation step of the olefin functional group, and a hydrolysis/hydrogenation step of the two acetal functional groups. 

Specific examples of the hydroaminomethylations of olefins with secondary amines are shown in Equations 17.19-17.21. Cyclic and acyclic secondary amines occur in high yield with linear-to-branched ratios exceeding 50 to 1 in most cases when catalyzed by the rhodium complex generated from [Rh(COD)JBF and xantphos. The reaction of pen-tene with piperidine is shown in Equation 17.19. These reactions are also compatible with alcohol (Equation 17.20) and acetal functional groups (Equation 17.21). 

Both anomeric carbons in sncrose are parts of acetal functional groups. 

Levulinate protecting group for alcohols. 60-90% yieldsj removed by mild reduction,useful for nucleosides. Stable to TFA, 80% HOAc. Compatible with olefin, nitro, ester, acetal functional groups. 

The total chemical modification of PVAL leads to a poly(vinyl acetal) in which the majority of the acetal functional groups are of cyclic type ... 

---