Stereochemistry configurations

Figure 9.5. Defining absolute, relative collection, and relative single configuration stereochemistry. The older convention depends on a "chiral" flag on the molecule to specify whether a given structure represents one or several stereoisomers. In the newer convention, collections of stereo centers can be defined, and they can be designated absolute, relative-part-of-a-mixture, or relative-single-configuration.

Using this methodology, the first total synthesis of the natural product (-)-(19R)-ibogamin-19-ol was reported [106]. The crucial ISQ core containing the entire configurational stereochemistry of the final target compound was prepared in 15 steps. The two key steps involved chirality transfer in an Ireland-Claisen rearrangement and an intramolecular nitrone-olefin 1,3-dipolar cycloaddition described earlier. 

The Weinreb group has recently examined the reaction of chiral N-sulfinyl dienophile 23, prepared from (+)-camphor, with 1,3-cyclohex-adiene (Scheme 1-VIII). Whereas the uncatalyzed cycloadcUtion afforded a mixture of diastereomeric adducts, the reaction promoted by TiCU gave a single adduct 24 having the 35,6/ configuration. Stereochemistry at sulfur in this compound could not be determined. As in the phenylmenthol series, one can reasonably consider two reacting dienophile conformations 23A and 23B (Scheme 1-VIU). If conformer 23A is attacked by the diene in an endo manner from the most exposed face, the observed adduct 24 will be formed. Similarly, if conformer 23B reacts with cyclohexadiene via an exo transition state, 24 will result. 

Symbolic representation of double bond and quasi-rigid-center configurational stereochemistry of single and fully defined molecules can be considered a solved problem. Even for highly symmetrical molecules, the performance of algorithms for canonical numbering has been improved to a level that makes it efficient in all practical applications. Perception of symmetry can usually be considered a by-product of canonical numbering. 

The next sections describe briefly the nomenclature of configurational isomers, and how this stereochemistry can be handled by computer. 

Tabic 2-6 gives an overview on the most common file formats for chemical structure information and their respective possibilities of representing or coding the constitution, the configuration, i.c., the stereochemistry, and the 3D structure or conformation (see also Sections 2..3 and 2.4). Except for the Z-matrix, all the other file formats in Table 2-6 which are able to code 3D structure information arc using Cartesian coordinates to represent a compound in 3D space. 

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

The reaction of phenylzinc reagent proceeds with opposite stereochemistry, namely by retention of configuration at the final step via transmetallation. Both the (S)-( )- and (i )-(Z)-allylic acetates 4 and 9 afford the (/ )-( )-phe-nylated product II by overall inversion[23]. 

The regioselective and stereospecific construction of C-20 stereochemistry is explained by the following mechanism. The Pd(0) species attacks the ( )-/3-carbonate 616 from the a-side by inversion to form the Tr-allylpalladium species 620, which has a stable syn structure[392]. Then concerted decarboxylation-hydride transfer as in 621 takes place from the a-side to give the unnatural configuration in 617. On the other hand, the Tr-allylpalladium complex 622... 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

An advantage that sulfonate esters have over alkyl halides is that their prepara tion from alcohols does not involve any of the bonds to carbon The alcohol oxygen becomes the oxygen that connects the alkyl group to the sulfonyl group Thus the configuration of a sulfonate ester is exactly the same as that of the alcohol from which It was prepared If we wish to study the stereochemistry of nucleophilic substitution m an optically active substrate for example we know that a tosylate ester will have the same configuration and the same optical purity as the alcohol from which it was prepared... 

The hydrolysis of sulfonate esters of 2 octanol is stereospecific and proceeds with complete inversion of configuration Write a structural formula that shows the stereochemistry of the 2 octanol formed by hydrolysis of an opti cally pure sample of (S) (+) 1 methylheptyl p toluenesulfonate identify the prod uct as / or S and deduce its specific rotation... 

Wnte an equation clearly showing the stereochemistry of the starting material and the prod uct for the reaction of (S) 1 bromo 2 methylbutane with sodium iodide in acetone What is the configuration R or S) of the product" ... 

A novel technique for dating archaeological samples called ammo acid racemiza tion (AAR) IS based on the stereochemistry of ammo acids Over time the configuration at the a carbon atom of a protein s ammo acids is lost m a reaction that follows first order kinetics When the a carbon is the only chirality center this process corresponds to racemization For an ammo acid with two chirality centers changing the configuration of the a carbon from L to D gives a diastereomer In the case of isoleucme for example the diastereomer is an ammo acid not normally present m proteins called alloisoleucme... 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

If the observed order of priority of the remaining three functions (a > 6 > c) is in a clockwise direction, the absolute configuration is designated R (rectus or right) if counterclockwise, the configuration is S (sinister or left). The concepts of stereochemistry and chiraUty have been extensively discussed and reviewed (27—29). 

Among the modem procedures utilized to estabUsh the chemical stmcture of a molecule, nuclear magnetic resonance (nmr) is the most widely used technique. Mass spectrometry is distinguished by its abiUty to determine molecular formulas on minute amounts, but provides no information on stereochemistry. The third most important technique is x-ray diffraction crystallography, used to estabUsh the relative and absolute configuration of any molecule that forms suitable crystals. Other physical techniques, although useful, provide less information on stmctural problems. 

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