Racemization


A minor chemical use for many of the commoner alkaloids is the resolution of racemic compounds (often acids) into their optically active enantiomorphs.  [c.21]

M.p. 114-116 C. Prepared by racemization of hyoscyamine. It and its salts are used to dilate the pupil of the eye. Given internally they reduce the secretion of saliva and relieve spasmodic pains.  [c.46]

The other tetritols were formerly called d-and /-erythritol, but have been renamed L-and D-threitol respectively. A racemic form can also be prepared,  [c.162]

Pfeiffer effect The change in rotation of a solution of an optically active substance on the addition of a racemic mixture of an asymmetric compound.  [c.302]

A dicyclic monoterpene, the ( + )-form of which is found in oil of savin and many other essentia oils. The (—)- and racemic forms occur occasionally in nature.  [c.350]

Racemic acid, ( )-tartaric acid, is a compound of the two active forms. M.p. 273 C (with IHjO), m.p. 205°C (anhydrous). Less soluble in water than (-t-)-tartaric acid. Formed, together with mesotartaric acid, by boiling (4-)-tartaric acid with 30% NaOH solution, or by oxidation of fumaric acid. Potassium hydrogen racemate is very insoluble.  [c.385]

Now consider such a symmetrical system, that of a racemic mixture of the enantiomers plus the inert third component. A pair of mirror-image conjugate phases will not physically separate or even become turbid, since they have exactly the same density and the same refractive index. Unless we find evidence to the contrary, we might conclude that this is a binary mixture with aT,x phase diagram like one of those on the right-hand side of figure A2.5.30. In particular any syimnetrical tliree-phase region will have to shrink symmetrically, so it may disappear at a tricritical point, as shown in two of the four pseudobinary diagrams. The dashed lines in these diagrams are two-phase critical points, and will show the properties of a second-order transition. Indeed, a feature of these diagrams is that with increasing temperature, a first-order transition ends at a tricritical point that is followed by a second-order transition line. (This is even more striking if the phase diagram is shown in field space as a /i, J or p, T diagram.)  [c.659]

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40].  [c.418]

In this preparation, the ( + ) or dextro-rotatory (natural) camphor or the ( ) or racemic (synthetic) camphor can be used. Perform the oxidation in a fume-cupboard.  [c.148]

RESOLUTION OF A RACEMIC COMPOUND  [c.503]

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic  [c.77]

In this preparation, the (+) or dextro-rotatory (natural) camphor or the ( ) or racemic (synthetic) camphor can be used. Perform the oxidation in a fume-cupboard.  [c.148]

The melting-points of the dextro and laevo forms of any optically active compound may, as in this case, be virtually identical with that of the racemic form in many compounds however there is a marked difference in melting-point, and often in solubility, between the ( + ) and ( - ) forms on one hand and the (i) form on the other.  [c.148]

Borneol and isoboineol are respectively the endo and exo forms of the alcohol. Borneol can be prepared by reduction of camphor inactive borneol is also obtained by the acid hydration of pinene or camphene. Borneol has a smell like camphor. The m.p. of the optically active forms is 208-5 C but the racemic form has m.p. 210-5 C. Oxidized to camphor, dehydrated to camphene.  [c.64]

Optically active substances are termed dextrorotatory or laevorolatory according to whether the plane of polarization of the light is rotated to the right or to the left with respect to the direction of incidence of the light. The prefixes i-and /-were once used to indicate which optical isomer was which, but now the correct prefixes are (-1-)- for dextrorotatory, and ( —)- for laevorotatory compounds, with ( )- for racemic compounds. These signs are arbitrary as the rotation can change with wavelength of the light (Cotton effect, optical rotatory dispersion). With carbohydrates and amino-acids the prefixes d- and l- are used to indicate configuration, not direction of rotation. The convention has been made that the configuration of D-glyceric aldehyde shall be represented by formula (I) and L-glyceric aldehyde by formula (2).  [c.287]

Mesotartaric acid crystallizes in plates (IHjO), m.p. 140 C (anhydrous). Very soluble in water. Obtained from the mother-liquors in the preparation of racemic acid or by oxidation of maleic acid. Potassium hydrogen mesotartrale is soluble in water.  [c.385]

Walden inversion A phenomenon discovered in 1895 by Walden. When one of the atoms or groups attached to the asymmetric carbon atom in an optically active compound is replaced by a different atom, the product is sometimes a derivative of the optical isomer of the original compound. It is thus possible to pass from one isomer to the other without the formation and separation of a racemic compound. ( + )-Malic acid, when treated with PCI5 gives (— )-chlorosuccinic acid, which may be converted to ( —)-malic acid by AgjO or back to (-f)-malic acid by KOH. Similarly, ( —)-malic acid is converted to (-l-)-chloro-succinic acid which undergoes similar changes. A Walden inversion occurs at a tetrahedral carbon atom when the entry of the reagent and the departure of the leaving group are synchronous - the so-called bimolecular nucleophilic substitution mechanism. Since the reagent must approach from the side of the molecule opposite to that of the leaving group an inversion of optical configuration results.  [c.424]

An alternative mechanism of substitution is unimolecular and involves ionization of the leaving group to give a carbenium ion which reacts rapidly with the reagent. The lifetime of this carbenium ion determines the stereochemical course of the reaction - inversion if it is short, racemization if it is long. In the examples quoted above the Ag20 reaction invi)lves the formation of a carbenium ion, but the carboxyl group forms a weak bond with the developing centre of positive charge so that the approach of the reagent from this side is blocked and must occur from the side of the leaving group. The original optical configuration is thus retained because of neighbouring group participation in the reaction.  [c.424]

An optically active molecule is a particular type of molecule in which there are two equivalent minima separated by an insuperable barrier in the potential energy surface and for which the molecular structures at these two minima are not identical (as they are in PH ) but are mirror images of one another. The two fonns of the molecule are called the dextrorotatory (D) and laevorotatory (L) fonns and they can be separated. The D and L wavefiinctions are not eigenfiinctions of E and E interconverts them. In the general case eigenstates of the Hamiltonian are eigenstates of E and they have a definite parity. In the laboratory, when one makes an optically active molecule one obtains a racemic 50/50 mixture of the D and L fonns, but in living organisms use is made of only one isomer natural proteins, for example, are composed exclusively of L-amino acids, whereas nucleic acids contain only D-sugars. This fact is unexplained but it has been pointed out (see [9] and references therein) that in the molecular Hamiltonian the weak neutral current interaction tenn would give rise to a small energy difference between the energy levels of the D and L fonns, and  [c.171]

Figure A2.5.30. Left-hand side Eight hypothetical phase diagrams (A through H) for ternary mixtures of d-and /-enantiomers with an optically inactive third component. Note the syimnetry about a line corresponding to a racemic mixture. Right-hand side Four T, x diagrams ((a) tlirough (d)) for pseudobinary mixtures of a racemic mixture of enantiomers with an optically inactive third component. Reproduced from [37] 1984 Phase Transitions and Critical Phenomena ed C Domb and J Lebowitz, vol 9, eh 2, Knobler C M and Scott R L Multicritical points in fluid mixtures. Experimental studies pp 213-14, (Copyright 1984) by pennission of the publisher Academic Press. Figure A2.5.30. Left-hand side Eight hypothetical phase diagrams (A through H) for ternary mixtures of d-and /-enantiomers with an optically inactive third component. Note the syimnetry about a line corresponding to a racemic mixture. Right-hand side Four T, x diagrams ((a) tlirough (d)) for pseudobinary mixtures of a racemic mixture of enantiomers with an optically inactive third component. Reproduced from [37] 1984 Phase Transitions and Critical Phenomena ed C Domb and J Lebowitz, vol 9, eh 2, Knobler C M and Scott R L Multicritical points in fluid mixtures. Experimental studies pp 213-14, (Copyright 1984) by pennission of the publisher Academic Press.
Thus, each stereochemical stnicttirc can be described and recognised with this rotational list if the structure is designated, c.g., in the STEREO block of the SMD format. The compact and extensible representation of the rotational list can include additional information, such as the name specification of the geometiy or whether the configuration is absohtte, relative, or racemic (Eigitre 2-73).  [c.80]

Thus, to name just a few examples, a nucleophilic aliphatic substitution such as the reaction of the bromide 3.5 with sodium iodide (Figure 3-21a) can lead to a range of stereochemical products, from a l l mbrture of 3.6 and 3.7 (racemization) to only 3.7 (inversion) depending on the groups a, b, and c that are bonded to the central carbon atom. The ring closure of the 1,3-butadiene, 3.8, to cyclobutene  [c.196]

It is important to notice that the united-atom simplification cannot be applied to functional hydrogens which are involved in the formation of a hydrogen hond or a salt bridge. This would destroy interactions important for the structural integrity of the protein. Removing the hydrogen at the u-carbon of the peptide backbone is also dangerous, because it prevents racemization of the amino acid.  [c.363]

The melting-points of the dextto and laevo forms of any optically active compound may, as in this case, be virtually identical with that of the racemic fomi in many compounds however there is a marked difference in melting-point, and often in solubility, between the (-)-) and ( -) forms on one hand and the ( ) form on the other.  [c.148]

Acetophenone similarly gives an oxime, CHjCCgHjlCtNOH, of m.p. 59° owing to its lower m.p. and its greater solubility in most liquids, it is not as suitable as the phenylhydrazone for characterising the ketone. Its chief use is for the preparation of 1-phenyl-ethylamine, CHjCCgHslCHNHj, which can be readily obtained by the reduction of the oxime or by the Leuckart reaction (p. 223), and which can then be resolved by d-tartaric acid and /-malic acid into optically active forms. The optically active amine is frequently used in turn for the resolution of racemic acids.  [c.258]

The search reported in File 5-3 there found four energetically distinguishable conformers. Steric energies 2, 3, and 4 are of racemic pairs. They are degenerate. Strictly speaking, energy 4 corresponds to a pair of structural confomiers that are somewhat twisted relative to one another hence, though they are degenerate, they are not exactly a racemic pair (Mencarelli, 1995).  [c.160]


See pages that mention the term Racemization : [c.29]    [c.46]    [c.84]    [c.240]    [c.316]    [c.331]    [c.339]    [c.339]    [c.339]    [c.344]    [c.375]    [c.382]    [c.385]    [c.388]    [c.424]    [c.1073]    [c.1076]    [c.1079]    [c.1286]    [c.2144]    [c.93]    [c.174]   
Carey organic chemistry (0) -- [ c.0 ]

Organic chemistry (0) -- [ c.0 ]

Hydrogenation methods (1985) -- [ c.128 ]