Crowns Incorporating the Xylylene Unit

The ortho-xylyl unit may be considered as the synthetic complement of the catechol unit so often used in crown synthesis. This is especially so for the 1,2-bromomethyl compounds which may be used as electrophiles. These units have been incorporated in numerous crowns over the years and the syntheses are all quite similar. 

In 1972, Vogtle and Zuber condensed 1,2-dibromomethylbenzene (xylylene dibromide) with another xylylene unit, 1,2-dihydroxymethylbenzene. The result was formation of the macrocyclic tetraether (40% yield, mp 194°) shown in Eq. (3.19). The 

These xylylene groups are typically available commercially , by halogenation of xylenes , by halomethylation of an aromatic or by conversion of a diester into its corresponding diol followed by halogen-hydroxyl interconversion . 

Reinhoudt, Gray, Smit and Veenstra prepared a number of monomer and dimer crowns based on a variety of substituted xylylene units. They first conducted the reaction of 1,2-dibromomethylbenzene and a polyethylene glycol with sodium hydride or potassium Z-butoxide in toluene solution. Mixtures of the 1 1 and 2 2 (monomer and dimer) products were isolated and some polymer was formed . The reaction was conducted at temperatures from 30—60° and appeared to be complete in a maximum of one hour. The authors noted that the highest yield of 1 1 cyclic product was obtained with disodium tetraethylene glycolate instead of dipotassium hexaethylene gly-colate (see also Chap. 2) . Chloromethylation of 1,3-benzodioxole followed by reaction with disodium tetraethylene glycolate afforded the macrocycle (29% yield) illustrated in Eq. (3.20). 

Recently, Brown, Muenchausen and Sousa have reported the preparation of an interesting group of naphthalene-based crowns which are characterized by these authors 

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