Stereoisomerism in Acetals and Ketals

As was recognized by Fischer (see reference 72), it is theoretically possible for the condensation product of an unsymmetrical glycol and either an aldehyde (other than formaldehyde) or an unsymmetrical ketone to exist in two stereoisomeric forms (V and VI) which differ only in 

In 1930, Tanasescu and Macovski 8 drew attention to the theoretical existence of three stereoisomeric forms of a 1,2 3,4-diacetal of erythritol. 

They designated these forms ayn, amphi and anti. The writers are aware of only one occasion on which an acetal of a polyhydric alcohol has actually been isolated in isomeric forms of the type under discussion, namely that on which Ness, Hann and Hudson71 obtained two of the four theoretically possible diastereomers of 1,3 5,7-dibenzylidene-D-perseitol. One isomer, having m. p. 153-155° and [a]J° — 58.2° (in pyridine), resulted from the treatment of D-perseitol with benzaldehyde in ethanol at 25°, 50% sulfuric acid being employed as the catalyst. The other, having m. p. 280 2° and [a] ° — 58.1° (in pyridine), was produced when the condensation was carried out at 0° in aqueous ethanol saturated with hydrogen chloride. The former was converted into the latter by recrystallization from a pyridine-alcohol mixture. The corresponding labile and stable forms of l,3 5,7-dibenzylidene-L-perseitol also have been prepared.71 The facility with which the labile form could be converted into its stable isomer in the presence of basic substances may well explain why other workers have failed to isolate isomers of this type. 

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