Stereochemistry stereoisomers, defined

However, soon afterward the original authors [137] as well as one other group, simultaneously revised the structures of punaglandins 3 (97) and 4 (98) based on synthesis of all possible stereoisomers [138]. These synthetic efforts not only established that the stereochemistry at Cl2 was opposite to that proposed, they also defined the complete absolute stereochemistry in punaglan-... 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

One way to gain fast access to complex stmctures are multicomponent reactions (MCRs), of which especially the isocyanide-based MCRs are suitable to introduce peptidic elements, as the isonitrile usually ends up as an amide after the reaction is complete. Here the Ugi-4 component reaction (Ugi CR) is the most suitable one as it introduces two amide bonds to form an M-alkylated dipeptide usually (Fig. 2). The Passerini-3CR produces a typical element of depsipeptides with ester and amide in succession, and the Staudinger-3CR results in p-lactams. The biggest unsolved problem in all these MCRs is, however, that it is stUl close to impossible to obtain products with defined stereochemistry. On the other hand, this resistance, particularly of the Ugi-reaction, to render diastereo- and enantioselective processes allows the easy and unbiased synthesis of libraries with all stereoisomers present, usually in close to equal amounts. 

According to Ernest L. Eliel and Samuel H. Wilen in Stereochemistry of Organic Compounds (1994, p. 102), configurational stereoisomers result from arrangments of atoms in space of a molecule with a defined constitution without regard to arrangements that differ only by rotation about one or... 

Threading of two cyclodextrins onto a symmetrical dumbbell can occur head-to-head, head-to-tail, or tail-to-tail, defining a new class of diastereoisomeric [3]-rotaxanes, as shown schematically in Figure 2.12d. Anderson and co-workers have shown that end-capping of 4,4,-bis(diazonio)azobenzene chloride 43 with 2,6-dimethylphenol 44 in the presence of a-CDX produced the [3]-rotaxane 45 in 12% yield, in addition to the [2]-rotaxane 46 in 9% yield, and free dumbbell 47 in trace amounts (Figure 2.18).42 The stereochemistry of the [3]-rotaxane species is remarkable because the two cyclodextrin beads have their smallest rims facing to each other. Therefore the threading reaction was stereoselective. The reasons for the exclusive formation of the tail-to-tail stereoisomer are not clearly established. 

Permutational isomerism was defined in 1970 [12]. It relies on a conceptual dissection of molecules into a set of ligands L and a skeleton in a fashion that is appropriate for a considered problem [10]. Any distinct molecules that result from a given molecule by permuting its ligands are called permutationally isomeric. The set of all isomers that can be generated in this way is called a family of permutation isomers. Although stereoisomers exist that are not permutation isomers, and there are non-steroisomeric permutational isomers, the notion of permutational isomerism in combination with the concept of chemical identity serves well as a basis for a unified essentially non-geometric treatment of stereochemistry [10, 35]. 

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.

C, 1-phenylseleno- and 1-methylseleno-l-alkenyllithiums in almost quantitative yields (Scheme 101). The stereochemistry of the reaction is not well defined and the only available results are those concerning the stereochemistry of the compounds resulting from further reaction of these organo-metallics with electrophiles. Both stereoisomers are formed if the methylselenoalkenyllithium is hydrolyzed or reac with an aldehyde or a ketone, whereas only one stereoisomer of an a, -un-saturat carbonyl compound is found from l-methylseleno l-alkenyllithiums and and from the... 

A chirality center (or chiral center ) is one type of stereogenic center or, simply, stereocenter. A stereogenic atom is defined as one bonded to several groups of such nature that interchange of any two groups produces a stereoisomer. See E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, Wiley-Interscience New York, 1994, p. 53. 

There are two alkaloids that are racemic mixtures cycleadrine (58) [(+/-) - fangchinoiine] from Cyclea barbaia and C. peltata (Menispermaceae) and (+/-)-tetrandrine (77) from the same two plants, as well as from Isopyrum thalictroides (Ranunculaceae) and Stephania hernandifolia (Menispermaceae). Menisidine (65) is likely a fangchinoiine isomer and menisine (66) is likely a (+)-tetrandrine stereoisomer. These latter two alkaloids were isolated from Stephania tetrandra (Menispermaceae). Oxofangchirine (349) is a benzyltetrahydoisoquinoline-benzylisoquinolone alkaloid from Stephania tetrandra (Menispermaceae). The stereochemistry at C( 1) is not defined. 

Three possible stereoisomers of lactide (LA) exist d-, l- and me o-lactide (Figure 1). A racemic mixture of d- and L-lactide is referred to as rac-lactide (rac-LA)."° The stereochemistry of these monomers, when incorporated into a polymer chain, creates material with a certain stereocomplexity or tacticity. " The tacticity of a given PLA sample may be defined by two parameters P [probability of forming adjacent stereocenters with the same chirality or a meso (m) linkage] and P [probability of... 

The next statement defines a t r igger function that will be used whenever data is inserted or updated in this table. This function performs three important functions. First, it modifies the SMILES to be inserted into the smi column so that it contains the result of the isosmiles function. The isosmiles function is similar to the cansmiles function, except that it retains any stereochemistry that might be contained within the SMILES. If two stereoisomers are entered into this table, each will have a unique isosmiles value, but the same cansmiles value. In this way, they can be kept distinct, but their identical canonical SMILES shows them to be stereoisomers. The trigger function also computes the fingerprint and inserts it into the table when the SMILES is inserted or updated. 

The structures and designations of D- and L-glyceraldehyde are defined hy convention. In fact, the D- and L-terminology is generally applied only to carbohydrates and amino acids. For organic molecules the D- and L- convention has been replaced by a new system that provides the absolute configuration of a chiral carbon. This system, called the (R) and (S) system, is described in Appendix D, Stereochemistry and Stereoisomers Revisited. 

An olefin of defined stereochemistry about the double bond can readily be converted into a threo or erythro diol by direct cij-hydroxylation or by epoxidation followed by hydrolytic opening of the epoxide obtained ( franj-hydroxylation ). cjj-Hydroxylation of a cis olefin leads to an erythro diol and formation of an epoxide and its hydrolysis gives a threo stereoisomer. Conceivably, from a trans olefin both stereoisomeric diols may be obtained by cis- or frans -hydroxylation applied in the reversed way. 

Chapter 5 Using the Mislow and Siegel definition (/ Am. Chem. Soc. 1984, I06y 3319), I introduce the popular (but often incorrectly defined) term stereocenter and explain the differences between this term and the lUPAC terms chirality center and asymmetric carbon atom (or chiral carbon atom). The term stereocenter is much broader than the more precise term asymmetric carbon atom, and it assumes that one already knows the stereochemical properties of the molecule (to know which bonds will give rise to stereoisomers upon their interchange). Therefore, I have continued to encourage students to identify the (immediately apparent) asymmetric carbon atoms to use as tools in examining a molecule to determine its stereochemistry. 

The most potent and biologically relevant hydroxylated metabolites derived from ARA, EPA, and DHA are formed via a sequence of enzymatic transformations and have stereochemicaUy defined structures with distinct R/S and Z/E configurations. Since these molecules are typically formed in very small quantities, the elucidation of their complete structure and stereochemistry is typically determined via a direct comparison and matching with stereochemicaUy pure materials and related stereoisomers prepared unambiguously via total synthesis [1]. These synthetic molecules also enable the investigation of the biological roles and mechanism of action of these hpid mediators. 

The action of a chiral Homer-Wittig type reagent on a racemic aldehyde has been mentioned in Section 2.3.7. In the E/Z mixture of products arising from olefination of 109, the -stereoisomer is mainly formed from one enantiomer of the aldehyde 109 while the Z-isomer is derived from the other enantiomer. Here, each enantiomer gives a defined stereochemistry for the double bond that is created (69). A new stereogenic unit is formed, which is conceptually similar to the formation of a new asymmetric centre in a racemic substrate. 

Indeed, the concept of stereochemistry is almost as old as organic chemistry itself. In 1848, Louis Pasteur (France 1822-1895) found that tartaric acid (9) existed in two forms that we now know differ only in their ability to rotate plane-polarized light in different directions (they are examples of stereoisomers). Pasteur observed that the crystals had a different morphology, defined here as their external structure, and he was able to separate these two forms by peering through a microscope and using a pair of tweezers to separate them physically. Because of this difference, the two forms of tartaric acid are considered to be different compounds, now called enantiomers (see Chapter 9). 

The sfereochemisfry of the more highly substituted alkenes is difficult to define using the cis and trans designations. Therefore, a more systematic manner of indicating stereochemistry in these systems has been developed that uses the E and Z nomenclature system. Draw the structures of the E and Z stereoisomers of l,4-diphenyl-2-butene-l,4-dione used in this experiment. In this case, which is cis and which is trans ... 

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