Five-Membered Acetals

There are essentially three methods which have been used for incorporation of the glycerol unit in macrobicyclic species. In all of these the pervasive problem is the tendency of glycerol to form five-membered acetals involving the primary and secondary hydroxyls rather than the two primary hydroxyls and the tendency of the glycerol equivalent 1,3-dichloro-2-propanol to form epichlorohydrin. 

The above arguments do not apply to the formation of cyclic acetals. In the more stable classes of six-membered acetal rings, the shape of the carbon chain is not greatly distorted from that of the free glycitols, and it is not necessary to postulate that formation of a five-membered acetal ring is intermediate to the formation of six-membered acetals. 

An entirely different arrangement, in which two five-membered acetal rings are fused to a pyranose ring, is found in the 1,2 3,4-di-0-isopropyli-dene derivatives of galactose and arabinose. Altrose is reported to afford a mixture of 1,2 3,4- and 1,2 5,6-di-0-isopropylidene derivatives. The one formula XXXVIII shows the stereochemistry of the... 

Isomerism at the acetal carbon atom is theoretically possible in five-membered acetal rings formed when pyranose or furanose sugars are condensed with aldehydes. For furanose derivatives, the isomerism is exo-endo, and the superior stability of exo isomers ensures that one isomer only will be obtainable, and this of the exo configuration. " 1,2-0-Benzylidene-a-D-glucofuranose " therefore has the complete configuration XLV. If the... 

The five-membered acetal 12 gives the intercyclic product in almost quantitative yield. 

There is some evidence that the formation of five-membered acetal rings may be favored at higher temperatures (above 100°), and some... 

Another aspect of the stereochemistry of cyclic acetals which has received attention is the phenomenon of diastereoisomerism. It had hitherto been presumed that, if the condensation of an aldehyde with a diol gives a six-membered acetal ring, this would have an equatorially disposed alkyl or aryl group. For a five-membered acetal ring, the isomer expected is not so readily defined. The absolute configurations of some cyclic benzylidene acetals have been determined by Foster and coworkers from proton magnetic resonance spectral data. They established that the phenyl substituent in 4,6-0-benzylidene-D-glucopyranose occupies an equatorial position, as in (12). 

In a simile manner, ring-opening polymerizations of five-membered acetals are helped by free-radical stabilizing substituents. Complete ring-opening polymerizations take place with phenyl-substituted compounds ... 

Very high selectivities were reported for aluminum phenolates of type 215 (R = CH3, C2H5) [68] and with the five-membered acetals 217 even trimethylalu-minum worked with excellent regioselectivity. 

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