Zinc chloride, acetonation catalyst with acetic acid

The stability of an acetal or a ketal may depend on the catalyst employed in its synthesis. For this and other reasons, Fischer and Taube20 recommended the use of zinc chloride, rather than a mineral acid, as the catalyst in acetonation processes. Presumably the instability of the products prepared with the aid of strong acids is due to the difficulty of ensuring complete removal of the catalysts. 

Acetic acid, as acetonation catalyst with zinc chloride, III, 51 glacial, for laboratory crystallization of /3-glucopyranose, V, 136 as intermediate in fat formation, II,... 

The condensation of acetone with D-ribose, with sulfuric acid as the catalyst, has also been examined in detail.17 The major component is 2,3-O-isopropylidene-D-ribofuranose (59%), together with three minor components l,5-anhydro-2,3-0-isopropylidene-D-ribofuranose (9%), 1,2 3,4-di-O-isopropylidene-a-D-ribopyranose (3%), and 1,2-0-isopropylidene-a-D-ribofuranose (6%). The authors observed that the only marked difference when other catalysts (such as copper sulfate or zinc chloride) were used was the absence of the anhydro compounds. Assuming that the 1-hydroxyl group is equatorial, the preponderance of the 2,3-acetal is consistent with the thermodynamic stability of the isomer having the least number of endo substituents and with the unfavorable interactions between the acetal rings in the 1,2 3,4-diacetal, which possesses a cis-syn-cis arrangement of rings.10... 

It should be emphasized that, in order to make an assessment of the stereochemical principles involved in the formation of cyclic acetals of aldoses and aldosides, the reaction products from a given condensation must be analyzed quantitatively, and the structures of the products determined. In addition, the reaction should, ideally, have reached equilibrium under truly reversible conditions. It now seems that the interpretation of some of the previous results must be carefully reconsidered. The valuable investigations of Buchanan and Saunders have revealed the possibility that condensations catalyzed by zinc chloride may not proceed in a truly reversible manner. Dorcheus and Williams also remarked that the catalytic action of zinc chloride will be lost through formation of the hydrated complex, ZnCU (H80)2. This change will result in kinetic control of the reaction. It has been found, for example, that reaction of methyl a-D-altropyranoside (13) with acetone and sulfuric acid affords a 42% yield of the 3,4-0-isopropylidene acetal. Using zinc chloride as the catalyst, both the 3,4- and the 4,6-0-isopropylidene acetal are obtained. Similar results were re-... 

PANIC at 250 MHz, has shown that it is the 1 3 2 6-triacetal and not the 1,2 3,5 A 6-trlacetal as previously claimed. The three products from acetalatlon of D-mannitol with 2-methoxypropene in DMF with tosic acid catalyst have been shown to be 1,2 5,6- 1 2 4 6- and 1,2 3,6-di-0-isopropylidene-D-mannitol. The preparation of 1 2 5,6-di-O-isopropylidene-D-mannitol by three alternative procedures for acetonation of D-mannitol has been investigated using acetylation and capillary g.l.c. to monitor the products. The highest yield was obtained by propanone-zinc chloride, whereas 2 2-dlmethoxypropane -tin(II) chloride and 2-methoxypropene - tosic acid gave more complex mixtures contrary to claims in the literature. A new trlacetal 1 2 3,6 4,5-trl-O-lsopropylidene-D-mannltol vas Isolated and its graded hydrolysis compared with that of 1 2 3 4 5 6-trlacetal. Acetal-... 

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