Synthesis and structure–optical rotation relationships of homochiral, monodisperse, tartaric acid-based dendrimers

Abstract

Optically active, homochiral dendrimers of both the zeroth generation 1 and first generation 2 have been synthesized from (2R, 3R)-tartaric acid and phloroglucinol by an iterative, convergent method. These chiral dendrimers comprise 4-tert-butylphenoxy as the surface groups and phloroglucinol as the branching junctures. The chiral element, which is a derivative of L-tartaric acid, serves as the connecting unit between the surface group and the branching juncture, or between two branching junctures. Homochiral (2S,3S)-(–)-2, 3-O-isopropylidene-1, 4-di-O-tosyl-L-threitol 3 was first connected to the surface moiety via mono-O-arylation with 4-tert-butylphenol to generate the optically active mono-O-tosyl ester 4. Excessive O-alkylation of the branching juncture, 5-benzyloxyresorcinol 5, with 2 molar equivalents of the mono-O-tosyl derivative 4, followed by activation of the benzyl-protected phenol moiety at the focal point, led to dendritic ‘wedge’7. Treatment of the phenol 7 with a further molar equivalent of intermediate 4 gave the zeroth generation dendrimer 1 in 36% overall yield from the di-O-tosyl ester 3. By application of the same reaction sequence, the phenol 7 could similarly be transformed into the first generation dendrimer 2 in 13% overall yield from homochiral diester 3. Investigation on the optical rotation properties of these dendritic compounds showed that their specific rotations remained essentially constant and that the molar rotations were roughly proportional to the number of chiral tartrate units in the molecule.