New potential dendritic cores: Selective chemistry of dipentaerythritol

Abstract

Methods were developed to convert dipentaerythritol efficiently into di-O-benzylidene, di-O-cyclohexylidene, di-O-1-ethylpropylidene, and di-O-methylene diacetals. The known di-O-benzylidene acetal, considered to be a single isomer, was shown to be close to a statistical mixture of the cis,cis-, cis,trans-, and trans,trans-isomers. Structures were established by a combination of NMR spectroscopy and X-ray crystallography. Reaction of the di-O-benzylidene acetals with benzyl chloride and potassium hydroxide surprisingly gave good to excellent yields of the monobenzyl ethers. An explanation for this observation was advanced. The dibenzyl derivatives of the di-O-benzylidene acetals were formed by reaction of their dianions with benzyl bromide in DMF. Selective reduction of the di-O-benzylidene acetals with triethylsilane and boron trifluoride etherate gave 2',6'-di-O-benzyldipentaerythritol. A crown ether was formed in modest yield (12%) by reaction of the dianion of the di-O-methylene acetal with α,α'-dibromo-m-xylene in DMF. Key words: dipentaerythritol, pentaerythritol, acetals, dendrimer, selective benzylation, crown ether.