SYNTHESIS AND PROPERTIES OF FIRST AND SECOND GENERATION CHIRAL DENDRIMERS WITH TRIPLY BRANCHED UNITS - A SPECTACULAR CASE OF DIASTEREOSELECTIVITY [Review]

摘要

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 Znd-generation dendrimers of this type could not be prepared. Certain combinations of diastereoisomeric ?nd-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. [References: 102]