Structure stereochemically well-defined

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

The oxidations and reductions of organochalcogen compounds all involve the lone-pairs of electrons associated with the chalcogen atoms. These lone-pairs are stereochemically active, which provides well-defined geometries for many of the intermediate species described herein. Oxidized organochalcogen compounds also are stabilized by lone-pair donation from neighboring heteroatoms, which again leads to unusual structures with well-defined geometries as described herein. 

Dendrimers based on octahedral metal complexes may be structurally not well defined because of the presence of several stereogenic centers [116]. Recently, the first examples of stereochemically pure tetrametallic Ru dendrimers have been described compound 34 depicts the A A3 isomer [106]. On oxidation in acetonitrile, a three-electron wave due to the peripheral Ru units is followed by the mono-... 

When six coordination sites around octahedral metal ion are occupied by only bidentate ligands, stereoisomers around the metal ion are formed. However, the coordination of symmetric tridentate ligands to a six-coordinate metal ion leads to only one isomer. Furthermore, tridentate bridging ligands connect metals in a linear fashion, resulting in the formation of stereochemically well-defined supramolecular systems. The rigid structure of these systems is suitable for studies of electron or energy transfer events between the donor-acceptor dyads. 

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

The stabilization of well-defined peptide microstructures is an important challenge in bioorganic chemistry. As the presented results show, metal coordination can be a simple but very effective tool for reaching this goal and fixing three dimensional molecular structures. The conformational (and stereochemical) information which is embedded in configurationally stable metal complex units can be transferred to amino acid residues or even to relatively large peptides and can induce helical-, sheet- or as discussed turn-type structures. 

As mentioned in an earlier section, there are no chemical reagents that can establish with any certainty the specific position of attachment of an acyl ester on a diacylphosphatidylcholine or any other phosphoglyceride. The same is true in any attempt to establish the stereochemical conformation of phosphatidylcholine, whether sn-1 or sn-3, by strictly chemical means. The solution to this dilemma is to use synthetic phosphoglycerides of defined structure as well as phospholipase A2 to establish the correct stereospecificity of the latter enzyme. Certainly, this is reminiscent of the chicken or egg argument, yet it does work as will be explained below. 

Having established the three-dimentional structure of carbocations as planar, we can now study the stereochemical progression of Sk 1 reactions as compared to Sk2 reactions. As shown in Scheme 5.6, the stereochemical course of an Sk2 reaction is well defined because nucleophilic displacement of a leaving group proceeds with inversion of stereochemistry. Thus, the stereochemical outcome is defined by the stereochemistry of the starting material. As for SnI reactions, since the step required for initiation of these reactions involves formation of a planar species, incoming nucleophiles have equal access to both sides of the reactive carbocation. As shown in Scheme 5.7, this results in complete elimination of... 

Preparation. A number of methods have been reported for both the racemic and asymmetric preparations of l-amino-2,3-dihydro-lH-inden-2-ol (1), most commonly starting from inexpensive and readily available indene. These methods have been described in detail in recent reviews. The valuable properties of 1 as both a component of a medicinally active compound and as a chirality control element, derive primarily from its rigid and well-defined stereochemical structure. As a result, the compound is most desirable in enantiomerically pure form. One of the most efficient asymmetric syntheses of 1, which may be employed for the synthesis of either enantiomer of the target molecule, involves an asymmetric epoxidation (89% yield, 88% ee) of indene to give epoxide 2 using the well-established Jacobsen catalyst. This is followed by a Ritter reaction using oleum in acetonitrile resulting in conversion to the oxazoline (3) which is subsequently hydrolysed to the amino alcohol. Fractional crystallization with a homochiral diacid permits purification to >99% ee (eq 1). ... 

In aqueous solution or in water (methanol 1 1), tetracationic porphyrins form molecular assemblies with tetraanionic porphyrins, e.g., meso-tetraphenyl-sulfonato- with meso-tetramethylpyridinium porphyrin. Such assemblies are, however, ill-defined with respect to structure and aggregation number because both surfaces of the chromophores behave identically. They tend to polymerize and precipitate. Only when stereochemically fitting porphyrins are combined by charge interactions and polymerization is prevented by alkyl substituents does one obtain well-defined heterodimers. This has been realized with an isomer mixture of four p-pyridinium—ethyl porphyrins (solubility = 10" M) and meso-tetraphenylsulfonato porphyrin (same solubility) (Scheme 6.6.1). In this case, the porphyrins are forced into close proximity by four charge-charge interactions and into a face-to-face position by the arrangement of charges. The inability of... 

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