Stereospecific, defined

As seen in the structure, moxalactam disodium has three asymmetric centers. Two centers in the ring system, C6 and C7, are stereospecifically defined during the biosynthesis of the penicillin used to produce the compound. A third asymmetric center exists adjacent to the amide carbon on the side chain of the antibiotic. The configuration of this center is free to equilibrate and thus a pair of diasterioisomers is possible for moxalactam disodium. The rate of interconversion between the isomers increases with increasing acidity, with the maximum rate occurring at a pH of about 2.5. For solutions with pH lower than 2.5, the rate of interconversion is decreased only slightly from the maximum rate. 

D-/yAc>-hexoside from D-galactal, a reaction which defines the stereospecificity of glycosyl transfer to the acceptor oxygen atom. 

An interesting mechanistic issue was raised by Firestone on the aqueous Diels-Alder reaction between 2-methylfuran and maleic acid in water, which is found to be 99.9% stereospecific.80 By adding heavy atom (defined as any below the first complete row of the periodic table) salts to the aqueous media, it was found that addition of heavy but not light atom salts reduced the degree of stereospecificity significantly in the retrodiene reaction. The results suggest that a large portion of the Diels-Alder reaction occurs via diradical intermediates (Scheme 12.2). 

Concerted cycloaddition reactions provide the most powerful way to stereospecific creations of new chiral centers in organic molecules. In a manner similar to the Diels-Alder reaction, a pair of diastereoisomers, the endo and exo isomers, can be formed (Eq. 8.45). The endo selectivity in the Diels-Alder arises from secondary 7I-orbital interactions, but this interaction is small in 1,3-dipolar cycloaddition. If alkenes, or 1,3-dipoles, contain a chiral center(s), the approach toward one of the faces of the alkene or the 1,3-dipole can be discriminated. Such selectivity is defined as diastereomeric excess (de). 

Although structurally-diverse as evidenced above, the insecticidal pyrethroids still conform to a unique, operationally-defined, structure-activity relationship based on the physical characteristics and three-dimensional shape of the entire molecule conforming to those originally evidenced in the natural pyrethrins [13]. From this relationship, it becomes apparent that there is no single molecular aspect or reactive moiety that serves as a true toxophore for the pyrethroids and that their actions at target sites are dependent upon the entire stereospecific structure of these insecticides [1]. 

Most polysilanes are synthesized by the Wurtz-type coupling reaction, which is non-stereospecific, and hence the configuration is usually undefined, although in a few cases, stereochemically defined polysilanes have been synthesized (ROP of all-anti l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclotetrasilane99 101 and masked disilene polymerization).61... 

Recently organosilicon compounds are being used for the synthesis of olefines by elimination reactions both in acidic and basic conditions. Thus P hydroxysilanes give defines. These reactions have been shown to be highly stereospecific. The acid catalysed elimination taking place by an anti pathway and the base induced elimination taking place by a syn pathway. 

To rationalize the stereospecificity of PLE toward a large variety of monocarboxylic and dicarboxylic esters, Tamm and co-workers have proposed the general formula displayed in Fig. 7.5 [5 5] [67]. Here, no representation of the active site is implied, but the model does rationalize numerous data and allows some qualitative predictions. A qualitative topographical model of the active site of PLE has been proposed by Jones and co-workers [68] [69], As shown in Fig. 7.6, substrate binding is defined by a carboxylate group that interacts with the catalytic serine residue, and by one or two hydrophobic groups that bind to sites 1 and/or 2. 

One may consider a series of physical states ranging from the crystalline, where molecular aggregation and orientation are large, to the dilute gaseous state, where there are no significant orientational limits. States of intermediate order are represented by micelles, liquid crystals, monolayers, ion pairs, and dipole-dipole complexes. In the crystalline state, the differences between pure enantiomers, racemic modifications, and diastereomeric complexes are clearly defined both structurally and energetically (32,33). At the other extreme, stereospecific interactions between diastereomerically related solvents and solutes, ion pairs, and other partially oriented systems are much less clearly resolved. 

The comprehensive manner by which VOA intensities relate to the details of molecular stereochemistry can be appreciated by recognizing that the set of 3N-6 vibrational degrees of freedom is defined in the same space that specifies the parameters of molecular conformation. No other form of molecular spectroscopy is so closely related to molecular stereochemistry. It is literally trae that VOA spectra arise from stereospecific vibrational oscillations of a chiral molecule. A challenge facing VOA spectroscopy at the present is how to fully extract this stractural and conformational information from the spectra. 

The degeneracy of the non-chiral complexes can be removed by incorporating chiral centers, usually as resolved amino acids, into the arms at close vicinity to the hydroxamate iron binding sites. Thus, only one of the energetically non-equivalent diastereomers predominates, leading to pure enantiomeric iron(III) complexes with defined hehcity that allows assessing stereospecific recognition by the ferrichrome receptor. 

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... 

Define chiral, enantomer, diastereomer, epimer, anomer (see Chapter 4), prochiral (see Chapter 9). What is meant by the statement that biochemical reactions are stereochemically specific Why is such stereospecificity to be expected in organisms (which are constructed from asymmetric units) See Chapter 9 for further discussion. 

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