Meso types

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

Meso-type ligands were synthesized by forcing two trimethylsilyl groups to be located at the 3-position of the ligand with the introduction of two bridges (Fig. 10) [74]. Actually, the complexation of this ligand with YC13 yielded... 

Both enantiomers of racemic 2-propenyl acetate can be formed from meso-type 7r-alkyl palladium intermediates by oxidative addition. 7r-Allylpalladium complexes with two alkyl substituents at the 1- and 3-positons are known to... 

The XXX type of Cs-symmetric metallocene is the meso diastereomer of a corresponding pair of enantiomers. For example, meso-(C I3)2SiInd2ZrCl2 is the meso diastereomer of rac-(CH3)2SiInd2ZrCl2 (XXXIV). The meso type of Cs metallocene is referred to as meso Cs ... 

Bilirubin [635-65-4] M 584.7, e450nin 55,600 in CHCI3. Meso-type impurities eliminated by successive Soxhlet extraction with ethyl ether and MeOH. Then crystd from CHCI3, and dried to constant weight at 80° under vacuum. [Gray et al. JCS 2264 1961],... 

On the other hand, the association of two enantiomeric components yields an achiral supermolecule of meso type this may occur through a symmetrical bridging molecule in order to take care of the identical interaction sites at symmetry-related positions, as shown in the three-component supermolecule 187a the corresponding diastereomeric DL pair may also be obtained (see 187b). Thus, achiral components can associate into a chiral supermolecule and chiral components can give an achiral supermolecule. 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

In the thermodynamically favored species, therefore, both ligand strands must bind with their N -termini to the A-configured metal center of the meso-type dinu-clear complex to adopt the sterically more favored right-handed helical conformation. Thus, under thermodynamically controlled conditions the dominating species in solution is isomer I (Figure 1.3.8). The results and considerations discussed show that stereochemical communication between a metal center and an amino acid residue can control the microstructure at the amino acid. In the example presented this leads, in a thermodynamically controlled system, after initial formation of a complex mixture, to only one final dominating species [20, 21]. 

In the above study, diols 458 and 459 were separated by preparative high pressure chromatography. A rapid, efficient, and nondestructive separation of dimers in the dl series from those of the meso type which involves no chromatography has also been devised.406 The procedure involves direct dehydration of the 458/459 mixture, preferably with phosphorus oxychloride in pyridine, and treatment of the unpurified hexaenes 464 and 465 with 0.5 molar equivalent of N-methyltriazoline-dione at low temperature. Under such conditions, only the meso isomer enters into Diels-Alder reaction since only the s-cis conformation of its conjugated diene moiety... 

In the allylic substitution of racemic 2-propenyl acetates or related substrates with the same substituents at 1 and 3 positions, the jt-allylpalladium intermediate containing a meso type 7r-allyl group is formed from both enantiomers of the allylic substrate. Two jt-allyl carbons at the 1- and 3-positions are diastereotopic on coordination of a chiral phosphine ligand to palladium. The asymmetric induction arises from preferential attack by the nucleophile on either of the two diastereotopic TT-allyl carbon atoms (Scheme 2-28). 

So far we have looked largely at substrates which are flat achiral objects that have chiral centres introduced by means of a reagent. Desymmetrisations are slightly different. Molecules which are desymmetrised tend to be of a meso type. That is, they are achiral because they have a mirror plane and the sides of the molecule contain left and right handed portions 219. This is in contrast to the C2 axis present in many catalysts such as the TADDOLate 218. Desymmetrisations are powerful because there may be several chiral centres embedded in an achiral molecule which suddenly become much more useful in the newly formed chiral molecule. There are a large variety of symmetrical substrates that have been enantioselectively desymmetrised48 and we look at a few of the more important classes. 

Racemic, meso, and Cx symmetric divalent organolanthanide complexes can be synthesized by allowing the dipotassium salt of the corresponding ligand to react with Sml2 [51]. Figure 20 shows their structures determined by 1H NMR and X-ray analyses. Table 8 shows the results of the ethylene polymerization with divalent samarium complexes. It is seen that the meso type complex has the... 

A four-center quadrilateral transition state would always lead to the formation of a meso type from a cis compound and a racemic... 

Fig. 10.2 Two achiral stereoisomers of meso-type (pseudo-asymmetric case), where the ligand p represents the mirror image of p in isolation...

Synthesis of a chirai cyciic imine via desymmetrization of its corresponding prochirai cyciic secondary meso-type amine with a recombinant whoie-ceii cata-iyst containing a monoamine oxidase. 

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