Interaction of electrophilic reagents with cyclic acetals and ketals

Abstract

The reactions of some electrophilic reagents with cyclic acetals and ketals and other derivatives of polyols have been investigated. Friedel-Crafts reagents, such as acetic anhydride and methyl iodide in the presence of aluminium chloride, cause ring opening of cyclic acetals and ketals, but do not yield the expected acetates and methyl ethers. These may be formed during the reaction and then react further with the aluminium chloride; alternatively, aluminium chloride Itself may cause ring opening. Treatment with boron trichloride, followed by methanolysis, converts all except the most stable cyclic acetals and ketals into the parent polyols. The results of these reactions suggest that, of the different acetals and ketals, only the methylene acetals retain the alkylidene residue as an a-chloro ether group. Attempts have been made to replace the chlorine and obtain a more stable derivative and the reduction of the product of the reaction of boron trichloride with 2:5-0-methylene-f-mannitol yielded 2-0-methyl-D-mannitol, a new compound. Further information about the mechanism of the reaction of boron trichloride with cyclic methylene acetals has been obtained from the infra-red spectrum of the product. A mixture of a dicarboxylic acid and trifluoro-acetic anhydride reacts with tri-0-methylene-D-glucitol to yield a linear polymer, containing free alcohol groups. Di-O-methylene pentaerythritol was expected to react similarly, but yielded a cross-linked polymer. The reaction of mono-carboxylic acids with this acetal and also with the benzylidene acetals and isopropylidene ketal of pentaerythritol has been investigated and compared with the reactions of the corresponding hexitol derivatives.