Favored Ring Structures in Acetals and Ketals

Favored Ring Structures in Acetals and Ketals 1. Preferential Synthesis of Acetals and Ketals 

Fischer,146 in 1894, and Irvine and Paterson,125 in 1914, recognized that the configuration of the hydroxyl groups in the polyhydric alcohol is a 

They found that these rules complied with the structures that were then known for the methylene acetals of sorbitol, mannitol, dulcitol, iditol, 6-desoxy-sorbitol, xylitol and ribitol. From the rules they predicted structures for methylene acetals of allitol and talitol, and several of these predicted acetals have been discovered in later researches. Among the 

Hudson and their coworkers63 71 observed that the Hann-Hudson rules, although primarily based upon structures of known methylene acetals, were compatible with such of the structures of analogous benzylidene acetals as were then known. It was obvious when the rules were formulated that they did not apply at all to isopropylidene ketals. 

Since much more information is now available on this topic than was the case when the above rules were first proposed, we have attempted, on the basis of all the relevant reactions which are reported in this review and which afford derivatives of known structure, to extend the rules to cover all cases of benzylidenation, ethylidenation and methylenation of the polyhydric alcohols.147 The modified rules are as follows — 

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