Symmetry chirality

One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

De Lucchi, O. High Symmetry Chiral Auxiliaries Containing Heteroatoms, Pure Appl. Chem. 1996, 68, 945-650. 

IV. Generation of High-Symmetry Chiral Molecules by Desymmetrization... 

V. High-Symmetry Chiral Molecules Directly Related to Natural Products...206... 

VI. High-Symmetry Chiral Molecules of Spiro Structures. 207... 

VII. High-Symmetry Chiral Twisted 7r-Eiectron Systems 1 ... 

VIII. High-Symmetry Chiral Twisted jr-Electron Systems 2 Aromatics. 216... 

IX. High-Symmetry Chiral Cage-Shaped Molecules.227... 

X. High-Symmetry Chiral Molecules with Hexa- and Octadentate... 

To circumvent this situation, an unambiguous and separate word to define chiral but not asymmetric is needed, and in this article the term high-symmetry chiral is proposed (13) (Table 1). 

III. GENERATION OF HIGH-SYMMETRY CHIRAL MOLECULES BY ARRANGING ASYMMETRIC UNITS... 

Analysis of the nature of Cn axes suggests that one can create high-symmetry chiral figures by proper arrangement of n AS units with the same chirality around appropriate axes. A simple example of this procedure can be found in the planar diketopiperazine (3) (Figure 3) of C2 symmetry, which is constructed by combining two AS units of the same chirality, that is, two molecular fragments of (+)-(S)-alanine. 

Farina (19) has provided a number of examples of the application of Curie s principle in his review entitled High Symmetry Chiral Molecules, and Prelog (20) has extended the same principle to his simplex in relation to his studies of generalized pseudoasymmetry. 

The desymmetrization of an achiral figure to transform it into a high-symmetry chiral one can also be achieved by a twist deformation around the C axis, and the direction of the twist determines the chirality of the figures generated... 

Another striking example is the T symmetry of tetrakis[trimethylsilyl] silane (25), the discussion of which is deferred to the last section, dealing with high-symmetry chiral polyhedral symmetry. An interesting conformational deformation in a crystal lattice has been analyzed by X-ray crystallography coupled with force-field calculations (26). 

As the Table of Contents for this chapter indicates, the sections to follow describe the synthesis and stereochemistry of high-symmetry chiral compounds classified according to their main structural characteristics. It seems pertinent here to give a few criteria applied in the selection of the material covered in this review. Since Farina s article (19) covered the literature up to the beginning of... 

The enormous number of compounds with C2 symmetry has forced the reviewer to abandon the attempt to include them in this review, except for those that happen to be closely related to compounds of higher symmetry (symmetry number >3), which are described in detail. Also excluded from this review are conformational aspects of relatively simple compounds whose high-symmetry chiral conformations exhibit energy barriers between them so low as to prevent their successful isolation. 

V. HIGH-SYMMETRY CHIRAL MOLECULES DIRECTLY RELATED TO NATURAL PRODUCTS... 

The abundance in nature of amino acids and carbohydrates in optically pure modifications, together with their polyfunctionality, recommends them as candidates for convenient AS units for constructing high-symmetry chiral molecules... 

Cyclo[tri-L-prolyl] offers another example of a high-symmetry chiral cyclic peptide, whose conformation of C3 symmetry in the crystal lattice has been revealed by an X-ray study (33). The C4 symmetry of the cyclic tetramer of chiral aziridine 8 was also confirmed by X-ray analysis (34). 

Because of the easy accessibility of (+)-tartaric acid and D-mannitol, it is not surprising to find that many efforts (35) have been made for the preparation of various high-symmetry chiral crown ethers having these natural products as the AS units. 

VII. HIGH-SYMMETRY CHIRAL TWISTED -ELECTRON SYSTEMS 1 CYCLOALKENES AND CYCLOALKYNES... 

Vffl. HIGH-SYMMETRY CHIRAL TWISTED tt-ELECTRON SYSTEMS 2 AROMATICS... 

DC. HIGH-SYMMETRY CHIRAL CAGE-SHAPED MOLECULES... 

Demonstrating the convenience of this nomenclature, D2 twistane (134), C2 ditwistane (137), D3 trishomocubane (138) and Ci homobasketane (143) are now simply called [2]-, [2.2]-, [1.1.1]-, and [2.1.0]triblattanes respectively. It might be noted that only twistane (D2), trishomocubane (D3), and tritwistane (D3) (141) have high-symmetry chiral molecular frameworks with symmetry numbers higher than 2. 

C2 symmetry), a high-symmetry chiral cage-shaped tricyclic hydrocarbon closely related to D2 twistane. Wagner-Meerwein rearrangement (144) of the unsaturated carboxylic acid (-)-151 provided the lactone (-)-168, which in turn was converted into the ketone (+)-170 via a series of intermediates, including the mesylate 169 whose intramolecular alkylation was a crucial step in this approach. Removal of the carbonyl group by Wolff-Kishner reduction completed the synthesis of (+)-brexane (171) (145). 

X. HIGH SYMMETRY CHIRAL MOLECULES WITH HEXA-AND OCTADENTATE CENTRAL ATOMS... 

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