Synthesis and properties of monodisperse chiral dendrimers (up to fourth generation) with doubly branched building blocks: An intriguing solvent effect

摘要

'Fully chiral' dendrimers, containing a stereogenic center at each and every branching point, have been prepared using a chiral core triol with aromatic elongating units (cf. 27) and chiral branch diols (cf. 8, 12, and 24) as building blocks. The biggest dendrimer prepared is of the 4th generation (33:46 building blocks, 93 stereogenic centers, 10(28) possible stereoisomers), and has been obtained by a convergent growth approach in 32 steps starting from the biopolymer poly[(R)-3-hydroxyburanoic acid] (P(3-HB)). Ail compounds were shown to be monodisperse by MALDI-TOF mass spectrometry. Spin-lattice relaxation-time (T-1) measurements and size-exclusion chromatography show typical features of structurally related achiral dendrimers. The influence of the chiral building blocks on the share of the whole dendrimer has been investigated by chiroptical measurements: the specific rotation can be considered as average of all chiroptical properties of its constituent chiral units, independent of the solvent. the concentration, and the temperature. On the other hand, regularity in the circular dichroism (CD) spectra is completely lost with variation of the solvent (cf. Fig. 13). [References: 42]