Stereochemistry asymmetric centers

The 1,3-dipolar cycloaddition reaction of nitrones with alkenes gives isoxazolidines is a fundamental reaction in organic chemistry and the available literature on this topic of organic chemistry is vast. In this reaction until three contiguous asymmetric centers can be formed in the isoxazolidine 17 as outlined for the reaction between a nitrone and an 1,2-disubstituted alkene. The relative stereochemistry at C-4 and C-5 is always controlled by the geometric relationship of the substituents on the alkene (Scheme 8.6). 

The ROESY spectrum of podophyllotoxin exhibits a number of crosspeaks (A-D) representing interactions between dipolarly coupled (space coupling) hydrogens, which can be helpful to determine the stereochemistry at different asymmetric centers. For example, based on the assumption that the C-1 proton (8 4.53) is /3-oriented, we can trace out the stereochemistry of other asymmetric centers. Cross-peak B represents dipolar coupling between the C-1 proton (8 4.53) and the C-2 proton (8 2.8), thereby confirming that the C-2 proton is also... 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

The first stereocontrolled synthesis of ( )-hirsutic acid (720) was achieved by Trost In this work, four of the seven asymmetric centers are fixed in the correct relative stereochemistry in bridged bicyclic compound 721 which in turn is formed by... 

The stereochemistry of rosmarinecine, which contains an additional asymmetric center at C-2, was established by comparison with retro-... 

Disubstituted cyclohexadienones undergo Diels-Alder reactions more slowly than the unsubstituted counterparts. Thus 12 does not react with pipcrylene (13) at 180°, but in the presence of SnCl4 the reaction proceeds in 85% yield at 25°. Moreover a complete reversal of face selectivity can be achieved by use of a Lewis acid catalyst. Thus 15 reacts thermally with 13 to give 16, whereas the catalyzed reaction results in 17. Thus the stereochemistry of four asymmetric centers can be controlled.5... 

Treatment of 6-lithio-3,4-dihydro-2H-pyran (660) with the epoxy iodide (661) afforded the epoxydihydropyran (662). Compound (662) cyclized on exposure to basic A1203 to a single hydroxydihydropyran (663) which was oxidized with Collins reagent to the keto aldehyde (664) containing three contiguous asymmetric centers of fixed stereochemistry (Scheme 153). The metallated dihydropyrans have also found use in the construction of keto lactones (78JA7101). 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

It requires a four-membered transition state involving a ring of the past asymmetric center, the gegen ion, and the two atoms of the new monomer. The presence of the established asymmetric center in the cyclic four membered ring determines the stereochemistry of the new asymmetric center. Steric or electronic repulsion of the substituents in the transition state causes isotactic steric control. 

In the hydroformylation reaction, V, the situation is even worse. Here there is no definite stereochemistry between the phosphine ligand and the metal. One of the reactants, carbon monoxide, competes so well with the phosphine for sites on the metal that it is difficult to insure that the chiral agent is present when the new asymmetric center is formed. 

The description of ci-amino acids as D or L is a holdover from an older nomenclature system. In this system (5)-alanine is called L-alanine. The enantiomer would be D- or ( )-serine. The l (laevo, turned to the left D = dextro, turned to the right) designation refers to the ct-carbon in the essential amino acids. In alanine, there is a single a-carbon that is asymmetric. When two asymmetric centers are present as in L-threonine, the stereochemistry of both carbons must be considered. The common form of L-threonine is the 25,3R stereoisomer. 

Subsequently, the application of a quadrant rule developed for compounds bearing an asymmetric center adjacent to the aromatic chromophore by means of considerations of the CD and ORD spectra of products of established absolute configuration to the alkaloids of the 5,10b-ethanophenanthridine series led to conclusions on their absolute stereochemistry at variance with those mentioned above 42). 

The intermediate iodonium ion controls the relative stereochemistry of the cyclization. An asymmetric center present within the substrate, therefore, allows for enantioselectivity (cf. Scheme 22.12).51 The reaction is very susceptible to the steric interactions within the transition slate.48,59 53... 

From these and other studies we have developed a rule of thumb for prediction of stereochemical induction during enolate derivatization for both syn and anti substrates, the relative stereochemistry at the newly created asymmetric center is opposite to that of the sulfoxide moiety when the structure is drawn as shown above. 

The major isomer proved to have the same relative stereochemistry from both the -78 °C quench and room-temperature quench. The low-temperature acetic acid quench may prevent loss of stereochemical integrity at the new asymmetric center,... 

Transfer of hydrogen to NAD or NADP creates a new asymmetric center at C-4 of the pyridine ring. The hydrogen atoms appear on both sides of the plane of the ring the sides have been designated170 the A and B sides of the molecule, and the hydrogen atoms as HA and Hb. The absolute stereochemistry has been elucidated,171,172 and Ha has been found to correspond to the pro-fi hydrogen atom. 

The formation of a cyclobutane introduces a large strain in the cycloadductsu and cis configurations of the bicyclic molecules are selectively obtained. Wheri asymmetric centers are present in the starting material, the stereochemistry oj the products is controlled by the relative rates of cyclization and of cleavage o these biradicals. For intermolecular processes, it is well established that steric... 

However, if one factors in the additional complications of stereochemistry, which occurs if one uses comonomers that contain asymmetric centers, such as CH CXY, the problem rapidly becomes unmanageable. For example, Figure 7-49 shows a schematic representation of six distinguishable diad sequences. 

Because of the three asymmetric centers in the molecule there exist eight different stereoisomers. To identify the stereochemistry of wine lactone syntheses for the enantiomers were developed. On the basis of enantioselective gas chromatography the stereochemistry of wine lactone was in agreement with the 3S,3aS,7aR-enantiomer (72) for this stereoisomer a low odor threshold was determined (0.00002 ng/L air) ... 

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