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Stereochemistry and chirality

Chirality is a phenomenon that pervades the universe. How can we know whether a particular object is chiral or achiral (not chiral)  [Pg.186]

Every object has a mirror image. Many objects are achiral. By this we mean that the object and its mirror image are identical, that is, the object and its mirror image are super-posable one on the other. Superposable means that one can, in one s mind s eye, place one object on the other so that all parts of each coincide. Simple geometrical objects such as a sphere or a cube are achiral. So is an object like a water glass. [Pg.186]

The human body is structurally chiral, with the heart lying to the left of center and the liver to the right. Helical seashells are chiral and most are spiral, such as a right-handed screw. Many plants show chirality in the way they wind around supporting structures. Honeysuckle winds as a left-handed helix bindweed winds in a right-handed way. DNA is a chiral molecule. The double helical form of DNA turns in a right-handed way. [Pg.187]

Bindweed (top photo) (Convolvulus sepium) winds in a right-handed fashion, like the right-handed helix of DNA. [Pg.187]

Let us now consider what causes some molecules to be chiral. To begin, we will return to aspects of isomerism. [Pg.188]


Another feature that is crucial in considering rearrangements in monosubstituted allyls is the effect on the chirality and stereochemistry. In crotyl complexes, formation of a a-bond at the unsubstituted terminus provides a path for racemization for the stereogenic center at the substituted terminus (equation 21). Formation of the a-bond at the monosubstituted terminus, however, results in conversion to a different isomer (equation 22). The most stable isomer is the syn isomer (72) and, in the absence of a substituent on the central carbon, the anti isomer (74) will only occur to the extent of 5%. Thus if one considers complexes like (acac)Pd(allyl), some racemize, whereas others only isomerize because there is no path for racemization (equation 23). These concepts have been used effectively by Bosnich in the design of systems for asymmetric allylic alkylation. These concepts also allow the rationalization of why certain substrates give low enantiomeric yields. It should be noted here that the planar rotation found in some of the molybdenum complexes retains the chirality in the allyl moiety. [Pg.4575]

Within the text itself, a particularly important change is that the chapter on chirality and stereochemistry at tetrahedral centers, a topic crucial to understanding biological chemistry, has been moved forward to Chapter 4 from its previous placement in Chapter 9. In addition, the chapter on organo-halides has been moved from Chapter 10 to Chapter 12, thereby placing spectroscopy earlier (Chapters 10 and 11). [Pg.1153]

During the past two decades, chirality and stereochemistry became very important topics in pharmacology and analytical chemistry. The chiral nature of living systems has evident implications on biologically active compounds interacting with them. [Pg.331]

In this chapter, recent development on the synthesis and stereochemistry of optically active chalcogen compounds over the last 10 years will be described, focusing mainly on chiral selenium and tellurium compounds. [Pg.577]

Nakazaki, M., The Synthesis and Stereochemistry of Chiral Organic Molecules with High Symmetry, 15, 199. [Pg.598]

G. Bellucci, C. Chiappe, G. Ingrosso, Kinetics and Stereochemistry of the Microsomal Epoxide Hydrolase-Catalyzed Hydrolysis of cw-Stilbene Oxides , Chirality 1994, 6, 577 - 582. [Pg.677]

The limited extent of intramolecular rearrangements undergone by the chiral oxonium ions 35 and 36 at 720 torr and at 40 °C (Table 22) allows their use for probing the regio- and stereochemistry of the displacement reactions of Scheme 19. In this case, the allylic alcohol, precursor of the chiral oxonium ions 35 and 36, acts as the nucleophile NuH. The relevant results are condensed in Scheme 21. [Pg.254]

We hope that this review of chiral sulfur compounds will be useful to chemists interested in various aspects of chemistry and stereochemistry. The facts and problems discussed provide numerous possibilities for the study of additional stereochemical phenomena at sulfur. As a consequence of the extent of recent research on the application of oiganosulfur compounds in synthesis, further developments in the field of sulfur stereochemistry and especially in the area of asymmetric synthesis may be expected. Looking to the future, it may be said that the static and dynamic stereochemistry of tetra- and pentacoordinate trigonal-bipyramidal sulfur compounds will be and should be the subject of further studies. Similarly, more investigations will be needed to clarify the complex nature of nucleophilic substitution at tri- and tetracoordinate sulfur. Finally, we note that this chapter was intended to be illustrative, not exhaustive therefore, we apologize to the authors whose important work could not be included. [Pg.457]

M. V. Stewart and E. M. Arnett are the authors of the third chapter, Chiral Monolayers at the Air-Water Interface. The chapter brings together the disciplines of surface chemistry and stereochemistry to demonstrate that the properties of stereoisomers may be useful in extending our understanding of the weak yet important inter-molecular forces that operate in surface monolayers. The authors demonstrate that, in a complementary way, the techniques of surface chemistry make possible novel experiments that yield clear and... [Pg.500]

Subsequent monosilylation and Wittig reaction furnished unsymmetrical double diene 170. The synthesis of the other Diels-Alder partner started from bromophenol 173 (prepared in three steps from dimethoxytoluene), which was doubly metalated and reacted with (S,S)-menthylp-toluenesulfinate 173. CAN oxidation delivered quinone 171, which underwent a Diels-Alder reaction with double diene 170 to give compound 175 possessing the correct regio- and stereochemistry. Upon heating in toluene the desired elimination occurred followed by IMDA reaction to adduct 176, which was obtained in an excellent yield and enantioselectivity. Both Diels-Alder reactions proceeded through an endo transition state the enantioselectivity of the first cyclization is due to the chiral auxiliary, which favors an endo approach of 170 to the sterically less congested face (top face) (Scheme 27). [Pg.38]

FIGURE 1.2 Structure and stereochemistry of commercially available cinchona alkaloid CSPs, marketed under trade name CHIRALPAK by chiral technologies europe. QN denotes quinine- and QD quinidine-derived and AX refers to their anion-exchanger capabilities vide infra). [Pg.4]

Molecular asymmetry, chirality and enantiomers The observation of Louis Pasteur (1848) that crystals of certain compounds exist in the form of mirror Images laid the foundation of modem stereochemistry. He demonstrated that aqueous solutions of both types of crystals showed optical rotation, equal in magnitude (for solution of equal concentration) but opposite in direction. He believed that this difference in... [Pg.27]

Starting with two chiral centres, there should, therefore, be four stereoisomers, and this is nicely exemplified by the natural alkaloid (-)-ephedrine, which is employed as a bronchodilator drug and decongestant. Ephedrine is (li ,25)-2-methylamino-l-phenylpropan-l-ol, so has the structure and stereochemistry shown. [Pg.85]

The enantioselective lithiation of anisolechromium tricarbonyl was used by Schmalz and Schellhaas in a route towards the natural product (+)-ptilocaulin . In situ hthi-ation and silylation of 410 with ent-h M gave ewf-411 in an optimized 91% ee (reaction carried ont at — 100°C over 10 min, see Scheme 169). A second, substrate-directed lithiation with BuLi alone, formation of the copper derivative and a quench with crotyl bromide gave 420. The planar chirality and reactivity of the chromium complex was then exploited in a nucleophilic addition of dithiane, which generated ptilocaulin precnrsor 421 (Scheme 172). The stereochemistry of componnd 421 has also been used to direct dearomatizing additions, yielding other classes of enones. ... [Pg.589]

C Chemical Shifts in Aliphatic Molecular Systems, Substituent Effects on Dependence on Constitution and Stereochemistry (Duddeck) Chirality, On Factoring Stereoisomerism and (Hirschmann and... [Pg.301]

In considering the retrosynthetic analysis of juvabione, two factors draw special attention to the bond between C-4 and C-7. First, this bond establishes the stereochemistry of the molecule. The C-4 and C-7 carbons are both chiral, and their relative configuration determines which diastereomeric structure will be obtained. In a stereocontrolled synthesis, it is necessary to establish the desired stereochemistry at C-4 and C-7. The C(4)—C(7) bond also connects the side chain to the cyclohexene ring. Because a cyclohexane derivative would make a logical candidate for one key intermediate, the C(4)—C(7) bond is a potential bond disconnection. [Pg.849]

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... [Pg.452]

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. [Pg.400]

Principally, the double bond in any chiral molecule is dissymmetrically deformed even if this unsaturated center is situated far from its chiral center. However, discussions in this article are limited solely to the synthesis and stereochemistry of the compounds whose double-bond systems 1 play a decisive role in the generation of their chirality by being explicitly twisted in the molecular environment (e.g. ring) surrounding the unsaturated center. [Pg.2]

A hypothetical compound bis-(( )-polymethylene)ethylene 58 of D2 (V) symmetry had been formulated by Cahn, Ingold, and Prelog U) for the sake of illustrating its planar chirality, but what aroused our independent interests in the synthesis and stereochemistry of this type of compound was a close structural relationship between... [Pg.9]

An extremely interesting and novel method has been described (91TL133). The principle involved is the intramolecular Diels-Alder addition of a 2,4-dienoic acid amide with an azodicarbonyl moiety. /V-Sorbyl-proline (27) was condensed with an acylhydrazine to form (28). Oxidation of this with lead tetraacetate (LTA) in boiling benzene resulted in the piperazinedione (30). This must have come about via (29), which could undergo an intramolecular Diels-Alder reaction. The structure and stereochemistry of (30) were confirmed by X-ray crystallography. The two new chiral centers have the R configuration as shown in (Scheme 9). [Pg.199]

Corriu, R. J., Larcher, F., and Royo, G., J. Organomet. Chem. 104, 161 (1976) (Synthesis of chiral ferrocenylsilanes stereochemistry of their reactions). [Pg.145]

The latest findings of amide-based rotaxanes focus on their chemistry and stereochemistry. The optical properties of an axle bearing chiral glucose stoppers are significantly influenced by rotaxanation - Cotton effects are amplified and the maxima of the CD spectra are shifted bathochromically [71]. [Pg.217]

Murahashi alkylation. Murahashi alkylation (8, 346-347) of the optically active allylic alcohol 2 with butyllithium results in almost exclusive (99%) y-al-kylation to give a mixture of (S)-(E)-3 and (R)-(Z)-3 with only slight loss of chirality and with predominant syn-stereochemistry.1 This syn-stereochemistry is opposite... [Pg.191]


See other pages where Stereochemistry and chirality is mentioned: [Pg.9]    [Pg.192]    [Pg.186]    [Pg.187]    [Pg.9]    [Pg.192]    [Pg.186]    [Pg.187]    [Pg.1301]    [Pg.62]    [Pg.153]    [Pg.577]    [Pg.589]    [Pg.211]    [Pg.244]    [Pg.248]    [Pg.627]    [Pg.44]    [Pg.196]    [Pg.540]    [Pg.348]    [Pg.666]    [Pg.333]    [Pg.628]    [Pg.268]    [Pg.29]    [Pg.182]    [Pg.269]    [Pg.194]   
See also in sourсe #XX -- [ Pg.192 , Pg.193 ]

See also in sourсe #XX -- [ Pg.186 , Pg.187 ]




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