Chiral tris methane

The first dendrimers with chiral cores for studies on the influence of the stereo-genic centres of a core unit on the chiroptical properties of the overall molecule were presented by Seebach s group [18]. These workers first synthesised dendrimers based on a chiral tris(hydroxymethyl)methane core unit. To these were attached zeroth- to second-generation Frechet dendrons, either directly or separated from the core by an aliphatic (n-propyl) or an aromatic spacer (p-xylylene) (Fig. 4.62). Remarkably, the dendrimers with aliphatic spacer showed no significant optical activity. This loss of chiral information was attributed to a dilution effecf, resulting from linkage of the achiral dendron to the chiral core unit,... 

Fig. 4.62 Chiral-core dendrimers of various generations (according to Seebach et al.), consisting of a chiral tris(hydroxymethyl) methane core unit and poly (aryl ether) dendrons (GO, Gl, or G2), which are linked to the core either directly (A), via an aliphatic spacer (B), or via an aromatic spacer (C)...

At the beginning of investigations on chiral dendrimers in our own group was the question of how to synthesize chiral, non-racemic derivatives of tris(hydroxymethyl)-methane [82], which we wanted to use as dendrimer center pieces. We have developed efficient diastereoselective syntheses of such triols [83-85] from ( R)-3-hydroxybutanoic acid, readily available from the biopolymer PHB [59,60] (cf. Sect. 2.4). To this end, the acid is converted to the dioxanone 52 [86, 87], from which various alkylation products and different aldol adducts of type 53 were obtained selectively, via the enolate (Fig. 20). These compounds have been reduced to give a variety of enantiopure chiral building blocks for dendrimers, such as the core unit 54, triply branching units 55a and 55b or doubly branching unit 56 [1,88]. 

C. Foces-Foces et al., Complete energy profile of a chiral propeller compound Tris-(2 -methylbenzimidazol-l -yl)methane. Tetrahedron Asymm. 1, 65-86 (1990)... 

J. Elguero et al., Failed attempt to induce chirality using a magnetic field The case of chiral helicity of tris(2-methylbenzimidazol-l-yl) methane. Heterocycl. Commun. 1, 102-103 (1994)... 

Crownpak CR(+). Chiral separation of gemifloxacin was performed in analytical counter-current chromatography using (- -)-(18-crown-6)-tetra-carboxylic acid as CSP. A successful separation of gemifloxacin enantiomers could be achieved using a two-phase solvent system composed of 1-butanol-ethyl-acetate-bis(2-hydroxyethyl)amino tris (hydroxymethyl)methane acetate buffer with a small amount of CgH4 [19]. 

Complementary dendritic hexamers based on a central scaffold made up of linked pentaerythritol and tri(hydroxymethyl)amino methane units have been found to also exhibit liquid-crystalline properties. This star-shaped scaffold was used to create supermolecules containing two different hemispheres, referred to thereafter as Janus supermolecular Hquid crystals (Fig. 76) [323,324]. One of the hemisphere contains three cyanobiphenyl end-groups, whereas the other lobe consists of three chiral phenyl benzoate mesogenic moieties laterally attached. The type of mesophase observed (N ... 

A combination of methods which include MM2 and MNDO calculations, NMR spectroscopy in the solid state (CPMAS) and in solution ( H, C, LSR, chiral solvents, and NOE experiments), DSC and x-ray powder diffraction, has been used to study the conformation of tris(pyrazol-l-yl)methanes (94) and ethanes <89T7805>. It has been shown that tris(3,5-dimethylpyrazol-l-yl)me-thane exists in two isolable isomers, the abb and the bbb (a or b depending on the orientation of the lone pair with regard to the substituent, H or CHj, on the sp carbon), whose interconversion by melting, crystallization, and sublimation has been studied. 

Tri(indolizinyl)methanes (16) and (17) were prepared from HC(OEt)3 and indolizines. Structure (17) is a three-bladed propeller lacking local Cj-axes. Compounds of this type are chiral and eight stereomeric conformations are possible <87CB(120)239>. 

Chirality (handedness) in molecules occurs, for example, when an atom is bonded to four different groups. Let me exemplify this with carbon in Figure 7.7. Thus, if we replace three of the H s of methane by F, Cl, and Br, we get first, say, the molecule we labeled as object. If we place the object in front of a mirror, we will get the mirror image molecule. Now try to superimpose them. No matter what you try, you will fail to superimpose them (unless you will break two bonds and replace their... 

There are several situations where cleavage of a 1,3,2-dioxaborolane to the boronic acid and diol is useful. One of these is for removal of a chiral director and replacement by its enantiomer. The first time we encountered this problem, a pinanediol ester was converted to the boronic acid via destructive cleavage of the pinanediol with boron trichloride (14). More recently, it has proved possible to convert an (R)-DICHED a-benzyloxy boronate (20) to the free bororric acid (23) with the aid of sodium hydroxide and a tris(hydroxymethyl)methane to form water soluble derivative 21 (R = CH2OH or NH(CHi)3S03 ) plus water insoluble (R)-DICHED (22). Treatment of 23 with (S)-DICHED (24) then yielded diastereomer 25 (76%, 97-98% diastereomeric purity). Further chain extension and alkylation led to 26 and 27, and deboronation yielded 28, all of which are stereoisomers that could not be accessed directly with a single chiral director (Scheme 6). 

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