Stereoisomer 5 symmetric

Chlorins, e.g. 14, form adducts with osmium(VIII) oxide, which can be hydrolyzed in aqueous sodium sulfide to bacteriochlorindiols, e g. 2, or isobacteriochlorindiols, e.g. 3. Thus, similar to diimide reductions of chlorins, metal-free tetraphenylchlorin 14 (M = 2H) is selectively oxidized to a corresponding bacteriochlorin 2 whereas the zinc chlorin gives an isobac-teriochlorin 3 on oxidation with osmium(VIII) oxide.40 With less symmetrical chlorins, very complex mixtures of constitutional isomers and stereoisomers are formed by /i-bishydroxyla-tion.17... 

In the previous section, the adaptation of the RIS model was based on the distance between next-nearest neighbor beads. This approach is obviously inadequate for CH3-CHX-CH2-CHX-CH3, because it necessarily abandons the ability to attribute different conformational characteristics to the meso and racemo stereoisomers. Therefore a more robust adaption of the RIS model to the 2nnd lattice is necessary if one wants to investigate the influence of stereochemical composition and stereochemical sequence on vinyl polymers [156]. Here we describe a method that has this capability. Of course, this method retains the ability to treat chains such as PE in which the bonds are subject to symmetric torsion potential energy functions. 

Hufford et al [57] used proton and 13C NMR spectrometric data to establish the novel sulfur-containing microbial metabolite of primaquine. Microbial metabolic studies of primaquine using Streptomyces roseochromogenus produced an A-acety-lated metabolite and a methylene-linked dimeric product, both of which have been previously reported, and a novel sulfur-containing microbial metabolite. The structure of the metabolite as an S-linked dimer was proposed on the basis of spectral and chemical data. The molecular formula C34H44N604S was established from field-desorption mass spectroscopy and analytical data. The 1H- and 13C NMR spectra data established that the novel metabolite was a symmetrical substituted dimer of primaquine A-acetate with a sulfur atom linking the two units at carbon 5. The metabolite is a mixture of stereoisomers, which can equilibrate in solution. This observation was confirmed by microbial synthesis of the metabolite from optically active primaquine. 

Stereoselective synthesis of pseudo C2-symmetrical 1,3-dibenzyldiamino alcohol (S,S) (323) a core unit of HIV protease inhibitors, and the two meso-stereoisomers (323a) and (323b) was achieved by stereocontrolled addition of benzylmagnesium chloride to nitrones (63a) and (63b) (Scheme 2.137). The yield of (S,S)-(323), based on N-Boc-L-phenylalaninal, accounts for 23% (Scheme 2.137) (211). 

Due to re- and si-coordination of prochir-al substrates at a catalyst with C2-sym-metric chiral ligands two diastereomeric catalyst-substrate complexes emerge. In the case of C,-symmetric ligands already four stereoisomer intermediates result. 

A more complicated picture arises with substrates containing more than one stereocentre which could be subject to redox stereoinversion of the type described in the previous examples. With two carbinol stereocentres in a symmetrical substrate there exist a maximum of five stereoisomers (the R,R and S,S enantiomers and meso isomer of the diol and two enantiomers of the intermediate a-hydroxyketone) for the dehydrogenase enzyme(s) to discriminate and transform irreversibly to a single enantiomer. Of course for 1,2-diols the intermediate j -hydroxyketone may be spontaneously equilibrating through an... 

Titanium in a low valence state, as prepared by treatment of solutions of titanium trichloride with potassium [206] or magnesium [207] in tetrahydro-furan or with lithium in dimethoxyethane [206], deoxygenates ketones and effects coupling of two molecules at the carbonyl carbon to form alkenes, usually a mixture of both stereoisomers. If a mixture of acetone with other ketones is treated with titanium trichloride and lithium, the alkene formed by combination of acetone with the other ketone predominates over the symmetrical alkene produced from the other ketone [20(5] Procedure 39, p.215). 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

Lanthionine is a dicarboxylic diamino acid possessing two stereocenters. However, because of the symmetry of the molecule there exist only three stereoisomers. Thus, with respect to lanthionine stereochemistry, one must differentiate between the 2/ ,6/ -lanthionine (L-lan-thionine), 25,65-lanthionine (D-lanthionine) and 25,67 -lanthionine (meso-lanthionine, d,l-lanthionine) (1) the latter is the naturally occurring stereoisomer of lanthionine isolated from lantibiotics. However, care must be taken when determining and assigning the stereochemistry of lanthionine residues present within peptide sequences. For naturally occurring lanthionine peptides the sulfide amino acid Lan within the peptide is no longer symmetrical and, in the majority of cases, it has been shown to be present as 25,67 -lan-thionine. 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

In 1894, Fischer wrote "It will probably be possible to obtain all riiem-bers of the sugar group by a combination of the cyanohydrin reaction with the reduction of lactones, as soon as we have succeeded in finding the two optically active forms of glyceraldchyde. All observations agree with the isomerisms foreseen by Van t Hoff, above all the disappearance of isomers if the molecule becomes constitutionally symmetric. This includes the transformation of different stereoisomers into one and the same substance if one of several asymmetric centers is abolished." An example of this is... 

Method of Drawing Stereoisomers from Complicated Symmetrical Structures," ]. Chem. Educ., Vol. 77,2000,864-866. 

Mechanistic studies on the reduction of the isomeric bipyridines (1) have been done.31,32 The generation of radical anions and dianions in liquid ammonia was studied and a mechanism was proposed for the electrolysis of 2,2 -bipyridine (1) in basic solution. In addition, the macroscale reduction of the symmetrical isomers of 1 was studied, and the ease of reduction followed the order 2,2 - =4,4 - > 3,3 -(l),33 Various electrode materials were examined. The reduction products of 2,2 - and 3,3 -bipyridine (1) at various cathodes were found to consist of roughly equal amounts of both the threo and erythro stereoisomers. Reduction of anabasine (4) gave the unsymmetrical 2,3 -bipiperidyl (2).34... 

Reduction of the isomeric lutidines was also studied (Scheme 6).34,46,47 In some cases the products were only cis as in reduction of 2,6-lutidine (20), and in other cases the products were mixtures of stereoisomers, as in 2,3-lutidine (21). When alkyl substituents were in the 3,4- or 3,5-positions, piperideine products were predominant (Table II). Reduction of the symmetrical collidine (13) gave various piperidine isomers of which the all-cis (22) was predominant48 However, the 3-olefin 23 was the major product. The theormodynamically less stable piperidines 24 and 25 were found in significant yield. 

The readily available yttrocene derivative (C5Me5)2YMe(THF) has been shown to be an effective catalyst for the hydrosilylation of internal alkynes [85]. A single stereoisomer, i.e. the product of cis addition of phenylsilane to the alkyne, is formed in the reaction with symmetrically substituted alkynes. Comparable reactions with a variety of unsymmetrically substituted internal alkynes resulted in a regioselective hydrosilylation reaction in which the silane moiety is placed at the sterically less hindered carbon atom of the alkyne. Various functional groups such as halides, amines, protected alcohols, and trisubstituted... 

Only one enolate can be generated from aldehydes or their aza analogs, from symmetric ketones or their aza analogs, or from carboxylic esters or carboxylic amides. For the moment we are ignoring the possibility that two stereoisomers, E- and Z-enolates, may occur for each of these enolates. On the other hand, constitutionally isomeric (regioisomeric) enolates may be derived from unsymmetrical ketones and from their aza analogs if they contain acidic H atoms... 

Sometimes there are fewer stereoisomers than predicted by the preceding rule. This occurs when identical chirality centers are symmetrically placed in a compound. As an example, consider the case of tartaric acid. 

The ozonolysis reaction cleaves the carbon-carbon double bond into two carbonyl groups. In working backward from the carbonyl compounds to the alkene that produces them, just connect the two carbons of the carbonyl groups by a double bond. Because 2 equivalents of the aldehyde are formed in this example, the two carbonyl compounds are the same. The original alkene was the symmetrical 3-hexene. Of course, we have no way of determining whether it was the E or the Z stereoisomer on the basis of this information. 

Mew-Stereoisomer (Section 7.5) A compound that contains chirality centers but is not chiral because it has the same chirality centers, but with opposite configurations, placed symmetrically in the compound, so it has an internal plane of symmetry. 

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