Basic Stereochemistry

The common stereochemical configurations adopted by 3-, 4-, 5- and 6-connected phosphorus are illustrated by the compounds (3.1a-f) where the bonding is believed to be covalent in type. 

The pyramidal structure of phosphorus trichloride, PCI3 typifies trivalent phosphorus compounds, while tetrahedral phosphoryl chloride, POCI3, the ions PCIJ and PO together with trigonal bipyramidal phosphorus pentachloride, PCI5, typify the spatial arrangements adopted by pentavalent phosphorus compounds. The hexachloro-phosphate (hexachlorophosphoride) anion. 

based on an octahedral bond configuration, represents a rather smaller group of compounds. There are a few stable 5-connected compounds which have a square pyramidal (pentahedral) rather than a trigonal bipyramidal arrangement of bonds. 

Orthophosphate esters (3.2) derived from (3.1d) (where the organic group, R, can be very complex) are particularly abundant and important in all life species. 

1- and 2-Connected P compounds are of much less common occurrence than 3-, 4- or 5-connected compounds, although many 2-connected varieties have been synthesised over the past two decades. Examples of 2-connected P atoms are provided by molecules such as a-P=0, F2C=PCF3 and cyclic phosphinine, C5H5P. There also exist ionised species such as PCIJ cations and Ph2P anions (3.3). 

Unfortunately, several stereochemical terms are not well-defined in a mathematical sense, see e.g. the definitions of configuration and conformation in [65] and [121]. Nevertheless, such terms are useful in the chemist s everyday life. A few important, informal definitions are given below, based on the definitions given by lUPAC [215]. 

A molecule s constitution describes the nature of atoms and their connectivity (including bond multiplicity), disregarding their spatial array. 

Stereoisomers are isomers of identical constitution that differ in the arrangement of their atoms in space. 

A molecule s configuration describes aspects of steric arrangement that are long-lived on a laboratory time scale, such as substituent arrangement at double bonds or at tetrahedrally-substi-tuted carbon atoms. 

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Macromolecular diffraction data are rarely of sufficient quality and quantity to allow construction of atomic models that would obey basic stereochemistry just based on this optimization of parameters to data. The observation-to-parameter ratio is a key factor in optimization procedures. For the optimization of a crystallographic model. 

B-type. Distortion isomers in which complexes of the same empirical formula can exist in only slightly different stereochemistries, namely, with slightly different degrees of distortion of the same basic stereochemistry, as in a- and /S-[Cu(salicylaldehydrate)2] [21]71, [22]72 which are rhombic coplanar, or the two forms of a- and /3-[Cu(l,3-... 

Some structural modifications of racemic calanolide A have been studied by us. Removal of Cn-methyl or methyl groups at both Cn and Cg positions maintains the basic stereochemistry of (it)-calanolide A as well as evaluates their anti-HIV activity. ( )-Cn-demethyl calanolide A and ( )-calanolide A exhibited similar potencies, whereas the 6,6,11-demethyl calanolide A also had diminished activity. The primary biological indicated that two methyl groups in the Cg position might be necessary for antiviral activity. 

The following is a summary of the representation and description of basic stereochemistry. See also the relevant entries in Chapter 5, especially nnder amino acids and carbohydrates. 

In all five of these hybrid orbital schemes, the use of hybridisation is only to give an improved directional overlap of orbitals to form two electron pair covalent bonds. Hybridisation does not determine the basic stereochemistry. This must still be determined by VSEPR theory and only then can hybridisation schemes be invoked to describe, more effectively, the covalent bonding present. These hybridisation schemes may equally be applied to cations and anions. The NH4 cation and BF4" anion have already been shown to involve a tetrahedral stereochemistry (Figure 6.4, examples 3 and 4) consequently the bonding in both ions may be described as involving sp hybridisation. 

Basic Stereochemistry of the 1,4-Dioxa Group in Polyether Complexes... 

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