Synthesis, aggregation, and chiroptical properties of chiral, amphiphilic dendrimers

The syntheses of amphiphilic dendrimers based on 3,5-dibydroxybenzyl alcoho l containing tri- or tetrafunctional chiral central cores and allyl ester t ermini are described. Water solubility is imparted to the dendrimers via a palladium-catalyzed deprotection of the peripheral allyl esters. This metho d affords complete deprotection of the carboxylate surface because, in cont rast to the basic hydrolysis of methyl ester termini, the solubility of par tially hydrolyzed intermediates is maintained throughout the course of the deprotection, thereby avoiding precipitation during the reaction. Chiroptic al analysis indicates that the structure of the dendrimers collapses in wat er, resulting in an increased steric effect upon the central core that is m anifested by lower optical rotatory power. However, contributions to the ch iroptical properties from the dendron branch segments were not evident in w ater or organic media, suggesting that chiral substructures were not develo ping in the branch segments of the dendrimers. Multiangle light scattering studies revealed that the dendrimers experienced significant aggregation in aqueous media that decreased at higher generations. This behavior could be rationalized by a change in conformational preference from a disklike conf ormation at low generations to a more globular conformation at higher gener ations.