Acetalisation

In the case of the synthesis of cortisone, Oppenhauer oxidation of hydroxyl group at C(14) -after the necessary acetalisation of the unsaturated carbonyl group-leads to intermediate 16a. which, under the basic conditions of Oppenhauer oxidation, spontaneously isomerises to 17a. with the more stable trans-BIC junction. An additional advantage of this sequence is that the carbonyl group activates the C(13) vicinal position, and allows not only the introduction of the methyl group, but facilitates the construction of ring D of the cortisone molecule. 

In many chemical reactions like esterification, acetalisation, ketalisation or etherification, water is produced as an unwanted by-product. As esterifications are... 

PVA subsequently can be acetalised (reaction 6 in Fig. 2) with aldehydes, the most important of them being butyric aldehyde, leading to PVB. However, since PVB only plays a minor role, both according to the total volume produced and the number of applications that make use of PVB, it will not be discussed in much detail in this chapter. 

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. 

The generic term glycosides defines all mixed acetals formed by the acetalisation of the cyclic forms of aldoses and ketoses. Glycosyl groups are monosaccharides that have lost their anomeric -OH group the suffix -yl is used to indicate the change that has occurred at C-1. 

At the outset of our work on this section of the spongistatin structure.15 it was not known whether an internal acetalisation procedure, analogous to that employed for the AR subunit, would lead to the desired spiroacctal 22a rather than the potentially more stable 22b. However, the axial Cm hydroxyl may hydrogen bond to an acetal oxygen, providing structure 22a additional stability not available to 22b, and this might be exploited in our synthetic endeavours. 

Trans-acetalisations are often performed in DMF at elevated temperatures under reduced pressure or a stream of nitrogen. These conditions result in removal of the methanol that is formed in the reaction and, as a. result, the benzylidenation is driven to completion. It has been reported that the use of chloroform as a solvent gives higher yields and, in this case, pyridinium p-toluene sulfonate (PPTS) may be used as a catalyst. 

Many examples of H-Beta acting as a Brpnsted acid catalyst in organic conversions are known or can easily be envisaged. We mention here etherification, esterification,45 acetalisation, hydration46 and hydroalkoxylations of olefins,47 dehydration of alcohols,... 

Fig. 16 Grafting by hydrosUylation of a sugar derivatized with double bond through acetalisation...

Conjugation deactivates the carbonyl group towards acetalisation. 

Chan and co-workers59 showed that chlorotrimethylsilane was both an effective catalyst and dehydrating agent in acetalisation reactions. The reaction is probably initiated by the liberation of HCI from the reaction of the chlorotrimethyl-... 

The formation of acyclic dialkyl acetals is generally restricted to aldehydes and more reactive ketones. The most common methods involve acid-catalysed acet-alisation using an alcohol solvent (usually methanol) or, more usually, by trans-acetalisation, which is preferred because it provides the most efficient means for removing water liberated in the reaction. Scheme 2.58 illustrates acetal formation in the presence of an add-sensitive jV-Boc group catalysed by... 

The Noyori acetalisation reaction conditions (see above) can also be used to prepare acyclic acetals. During their asymmetric synthesis of LoganinT Vande-walle and co-workers129 protected an aldehyde in good yield by reaction with TMSOMe in the presence of TMSOTf [Scheme 2.61]. 

Figure 3.65 Acetalisation and hydrolysis of acetate/aldehyde functionalised imidazolium salts and their palladium(ll) complexes.

Clerici A, Pastori N, Porta O (2001) Mild acetalisation of mono and dicarbonyl compounds catalysed by titanium tetrachloride. Eacile synthesis of (3-keto enol ethers. Tetrahedron 57 217-225... 

Figure 22. A cholinium chlorozincate-catalysed acetalisation reaction.

A method for producing an improved antimicrobial material is disclosed whereby a soft, diy, iodine/acetalised polyviityl alcohol complex sponge material is produced which has a pleasing yellow-gold colouration indicating activation of the antimicrobial elements in the complex. 

Acetalisation increases electron-withdrawing through space effect of 6-OH by constraining C5-C6 to tg conformation, with positive end of C6-06 dipole pointing towards 05 and 01... 

In the examples shown in Schemes 2.19 and 2.20, neither the substrates nor the products were sensitive to elevated temperatures or acid and so dehydration was accomplished by azeotropic distillation. However, acetalisation of acid-sensitive substrates can be a major challenge. For example, a synthesis of the Crambescidin family of guanidine alkaloids was impeded by problems with the acid sensitivity of the ketone 2L1 [Scheme 2.21]. The triethylsilyl group made the desired acetalisation sluggish the firee alcohol reacted faster but it also underwent very easy -elimination. Conditions that accomplished removal of the triethylsilyl group and acetalisation without competing dehydration entailed reaction of 21.1 with the cyclic ortho ester 213 and 1,3-propanediol in the presence of Amberlyst-15 at room temperature in acetonitrile. The required hydro-... 

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