Synthesis and Properties of First and Second Generation Chiral Dendrimers with Triply Branched Units: A Spectacular Case of Diastereoselectivity

Chiral triols (which may be considered as derivatives of tris(hydroxymethyl)methane), without ( 3‐5 ) and with aliphatic ( 6 ) or aromatic ( 7 ) elongating units, and the 1st‐ and 2nd‐generation benzylic branched bromides, 17, 18, 23, 24, 29 , and 30 are subjected to Williamson etherification conditions (NaH in THF). This gave the first ‘fully chiral’ dendrimers, with triple branching and with a stereogenic center at each and every branching point (including the central building block: see 33‐42 , 44 , and 46‐49 ). Higher than 2nd‐generation dendrimers of this type could not be prepared. Certain combinations of diastereoisomeric 2nd‐generation branched bromides, 23, 24, 29 , and 30 , and enantiomeric center‐piece triols, 3 and 4 , would smoothly react to give the desired dendrimers ( e.g. , 44 , and 46‐49 ) and others would not, with the reactions stopping at the dendritic alcohols containing only two branches ( e.g. , 45 , and 50‐53 ; see Schemes 4 and 5 ). Considering the distance at which the intermediate diastereoisomeric ‘doubly coupled’ dendritic alcohols differ in their configuration, this diastereodifferentiation or molecular recognition phenomenon (discovered by trying to prepare only 8 out of 2 39 possible diastereoisomers!) is a most surprising result. All compounds were fully characterized, and the 2nd‐generation dendrimers, e.g. , 38 , 40 , and 47 with and without elongation were shown to be monodisperse and without defects, by MALDI‐TOF mass spectroscopy ( cf. Fig. 4 ). A simple, unambiguous nomenclature for identification of the novel dendritic compounds is proposed and applied in the Exper. Part .