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

Murer, Peter; Lapierre, Jean‐Marc; Greiveldinger, Guy; Seebàch, Dieter

doi:10.1002/hlca.19970800522

Cited by 42 publications

(39 citation statements)

References 84 publications

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“…The 24 peripheral, the 21 interior, and the single central Me groups (depicted as squares) give rise to distinct sets of signals between 1.1 and 1.3 ppm; the downfield shift observed when going from outside to inside along the three layers is even more pronounced in the case of the central C H protons of the peripheral, the interior, and the core units (1.9-2.4 ppm, depicted as circles). The chemical shifts of the signals of comparable 'H nuclei are not affected by the dendrimer's generation number which is in contrast to our observation made on dendrimers with triply branched units and a non-elongated core unit [14]. 'l) At least 0.9-g amounts were prepared of each generation.…”

Section: Nmr Characterization: Spin-lattice Relaxation Time ( T I )mentioning

confidence: 93%

Synthesis and Properties of Monodisperse Chiral Dendrimers (up to Fourth Generation) with doubly branched building blocks: An intriguing solvent effect

Murer

Seebàch²

1998

Helvetica Chimica Acta

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'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 chirdl 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, lo2' possible stereoisomers), and has been obtained by a convergent growth approach in 32 steps starting from the biopolymer poly[(R)-3-hydroxybutanoic acid] (P(3-HB)). All compounds were shown to be monodisperse by MALDI-TOF mass spectrometry. Spin-lattice relaxation-time (TI) measurements and size-exclusion chromatography show typical features of structurally related achiral dendrimers. The influence of the chirdl building blocks on the shape 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).1. Introduction. -Chirality, a characteristic property of biological systems, plays an important role in several highly selective recognition processes and reactions [3] [4]. Furthermore, in a biomacromolecule the 'chirality' of the monomer is often 'amplified', and a chiral superstructure, e.g., a helix is formed [5]. Similar observations have been made with synthetic polymers [6-81 or lipid aggregates [9-111. Because of their well-defined molecular weight and structure, one can expect that chiral dendrimers3) could be useful as model compounds for studying how 'chirality is expressed' in a macromolecular system. It can be anticipated that introducing chiral branching units into the dendrimer should result in a non-symmetrical, non-spherical overall shape and should provide chiral cavities. These structures might also be useful for asymmetric catalysis, chiral recognition, and resolution processes.In our preceding paper on 'fully chiral' dendrimers with triple branching units, we reported on a diastereoselective effect in the reactions used for coupling dendrons with cores [14]. All chiral building blocks, e.g., core triol 2 [15-171 and branch triol 3 [1][14][18], were prepared from the dioxanone 1 [19][20], which is easily accessible from the biopolymer P(3-HB)4) by aldol addition and reduction.

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Section: Nmr Characterization: Spin-lattice Relaxation Time ( T I )mentioning

confidence: 93%

Synthesis and Properties of Monodisperse Chiral Dendrimers (up to Fourth Generation) with doubly branched building blocks: An intriguing solvent effect

Murer

Seebàch²

1998

Helvetica Chimica Acta

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“…Nomenclature of dendrimers and dendritic branches is used according to our published convention. [34] Cleavage of the alcohol protecting groups TBDMS and TBDPS General procedure I (GP I): A solution of the protected alcohol (1 equiv) in THF was cooled to ice-bath temperature. After addition of 1 ± 2 equiv of tetrabutylammonium fluoride (TBAF) for each protecting group the reaction mixture was stirred for at least 20 h (TLC control).…”

Section: Methodsmentioning

confidence: 99%

Dendritic TADDOLs: Synthesis, Characterization and Use in the Catalytic Enantioselective Addition of Et2Zn to Benzaldehyde

Rheiner¹,

Seebàch²

1999

Chem. Eur. J.

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114

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The versatile chiral ligand for polar metal centers, TADDOL ((R,R)-a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanol), has been incorporated as core building block into dendrimers by way of benzylation of a fourfold phenolic derivative (hexol 2) with Fre Â chettype branches of up to fourth generation. These carry either benzyl (3 ± 7) or octyl groups (33 ± 35) at the periphery, or they contain chiral branching units (18 ± 20, 36), derived from (R)-or (S)-3-hydroxybutanoic acid. The dendritic compounds of molecular weight up to 13 626 have been fully characterized, including by MALDI-TOF mass spectrometry, NMR spectroscopy, and optical activity measurements; one of the branch precursors with four octyl groups crystallized in an intriguing packing pattern. From the spectra and from the specific and molar optical rotations, there was no indication for the formation of chiral secondary structures of up to the third generation. The new TADDOLs were converted to Ti TADDOLates, which were employed as catalysts for the addition of Et 2 Zn to PhCHO. The stereoselectivities and the reaction rates observed with the novel catalysts were compared with those of the simple Ti TADDOLate: up to the second generation there was no detectable decrease of selectivity ( 98:2), and the rates hardly decreased up to the third generation; also, enantiomeric branches caused no change of stereoselectivity within experimental error. Thus, there may be applications for the special properties (such as high molecular weight, good solubility, spacing of central site from cross-linked polymer matrix) of dendritically modified chiral catalyst ligands.

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“…In addition, these linkers convey spectroscopically unique signatures to different regions of the dendrimer architecture; an effect only rarely observed in related architectures. [24][25][26][27][28][29] …”

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Identification of diamine linkers with differing reactivity and their application in the synthesis of melamine dendrimers

Moreno

Simanek

2008

Tetrahedron Letters

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Diamine linkers for the synthesis of dendrimers based on melamine were identified using competition reactions. The relative reactivity of the cyclic monoamines surveyed vary in relative reactivity by 40 times, expanding the previously identified series to an overall relative reactivity range of 320 times. Azetidine is 40 times more reactive than the cyclic, nine-membered ring (C 8 H 17 N), and 320 times more reactive than benzylamine. Reactivity differences are attributed to pKa values and sterics. Incorporating these groups into diamines provides linkers that can be employed in dendrimer synthesis. Specifically, the nucleophilicity of the individual amine groups comprising 3-aminoazetidine, 3-aminopyrrolidine, and 4-aminopiperidine vary by 100 times, 70 times, and 20 times, respectively. These linkers are incorporated into a generation three dendrimer.Our group has invested significant energies in the synthesis of dendrimers based on triazines, also referred to as dendrimers based on melamine deriving from our use of diamine linkers. 1, 2 Our early targets commonly incorporated p-aminobenzylamine because of the significant differences in the reactivity of the amines of this group. That is, during a convergent synthesis, protecting group manipulations and functional group interconversions could be avoided because the benzylic amine would react preferentially (essentially exclusively) with the monochlorotriazine dendron being elaborated. 1,[3][4][5][6][7] The low stability of these aniline derivatives required reasonable, but additional, care on handling. While the use of distilled solvents and inert atmospheres are not uncommon, nor is the requirement for refrigerated storage of intermediates, we recognized that these precautions could impact the broader acceptance of these materials.The low cost and high reactivity of piperazine soon made it a linking diamine of choice for our investigations. However, when using piperazine, dimerization of monochlorotriazines was observed under non-ideal reaction conditions that were usually attributed to concentration, rate of addition, ineffective stirring or lack thereof, temperature of addition and the magnitude of stoichiometric excess. 4,[8][9][10][11][12][13][14][15][16][17][18] Reactions with either p-aminobenzylamine or piperazine could be readily followed by NMR. The shift of the benzylic protons on reaction with the monochlorotriazine dendron or the desymmetrization of methylene groups of the piperazine group were diagnostic. The use of piperazine led to the undesirable formation of dimers, unfortunately. Detecting these dimers by NMR proves difficult due to signal degeneracy between the desired asymmetric monoamine and the symmetric dimer.

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

Cited by 42 publications

References 84 publications

Synthesis and Properties of Monodisperse Chiral Dendrimers (up to Fourth Generation) with doubly branched building blocks: An intriguing solvent effect

Synthesis and Properties of Monodisperse Chiral Dendrimers (up to Fourth Generation) with doubly branched building blocks: An intriguing solvent effect

Dendritic TADDOLs: Synthesis, Characterization and Use in the Catalytic Enantioselective Addition of Et2Zn to Benzaldehyde

Identification of diamine linkers with differing reactivity and their application in the synthesis of melamine dendrimers

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