Search results
Results from the WOW.Com Content Network
Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean–Stark apparatus , lest it hydrolyse the product back to the hemiacetal.
Particularly common spiro compounds are ketal (acetal) formed by condensation of cyclic ketones and diols and dithiols. [15] [16] [17] A simple case is the acetal 1,4-dioxaspiro[4.5]decane from cyclohexanone and glycol. Cases of such ketals and dithioketals are common.
The pyranose ring is formed by the reaction of the hydroxyl group on carbon 5 (C-5) of a sugar with the aldehyde at carbon 1. This forms an intramolecular hemiacetal.If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1]
Cyclic hemiacetals often form readily, especially when they are 5- and 6-membered rings. In this case, a hydroxy group reacts with a carbonyl group within the same molecule to undergo an intramolecular cyclization reaction. [6] Formation of a general cyclic hemiacetal Structures of some readily isolable hemiacetals and hemiketals.
Acid catalyzed acetal formation from the corresponding hemiacetal. Acetals, as already pointed out, are stable tetrahedral intermediates so they can be used as protective groups in organic synthesis. Acetals are stable under basic conditions, so they can be used to protect ketones from a base. The acetal group is hydrolyzed under acidic conditions.
However, in order for anomers to exist, the sugar must be in its cyclic form, since in open-chain form, the anomeric carbon atom is planar and thus achiral. More formally stated, then, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide. [1] Anomerization is the process of conversion of one anomer to the other.
The latter reagent in itself is an acetal and therefore the reaction is actually a cross-acetalisation. Kinetic reaction control results from 2-methoxypropene as the reagent. D-ribose in itself is a hemiacetal and in equilibrium with the pyranose 3. In aqueous solution ribose is 75% pyranose and 25% furanose and a different acetal 4 is formed.
Cyclic acetals are very much more stable against acid hydrolysis than acyclic acetals. Consequently acyclic acetals are used practically only when a very mild cleavage is required or when two different protected carbonyl groups must be differentiated in their liberation.