Pseudo-chiral centres

Determine the configuration of the pseudo chirality centre in the 5-HT3 antagonist tropisetron whose structure is shown in the formula below. 

A pseudochirality centre exists on a tetrahedrally coordinated atom when two of the substituents are constitutionally identical but have the opposite chirality sense, i.e. they are enantiomorphic. An R-configured group has priority over an S-configured group. Thus it is possible to determine the configuration of a pseudo chirality centre by application of the sequence rules. To distinguish a pseudo chirality centre from a chirality centre the stereodescriptors r and s are used. 

Four stereoisomers can be produced in the reaction. All reaction products have chirality centres at C2 and C6, the absolute configuration of which can be specified by R and S. Compounds A and B additionally possess a pseudo chirality centre at Cl since there are two constitutionally identical but enantiomorphic groups attached to this position. By applying the CIP rules, Cl in A has the stereodescriptor r, whilst Cl in B has the stereodescriptor s. The groups attached to Cl in C and D are homomorphic, therefore Cl in these compounds is a pro chirality centre. 

The stereogenic centres are defined using the CIP rules by R and S as well as by r and s depending on whether they are chirality or pseudo chirality centres, respectively. 

Memantine is achiral. The molecule has a plane of symmetry in which the enantiomorphic halves of the molecule are reflections of each other. Both the carbon atom attached to the amino group and the tertiary carbon atom are pseudo chirality centres and both are r-c onfigured. Note that the configuration of the pseudo chirality centres remains unaltered on mirror reflection. Although there are four stereogenic centres in the molecule, it is unnecessary to use any stereodescriptor to describe the configuration of the molecule since there are no stereoisomers. 

The compound has two chirality centres and three pseudo chirality centres. There is however, only one (achiral) diastereomer of the compound shown in the question. The two isomers can be distinguished from one another solely on the relative position of the chlorine or bromine atoms which lie in a plane which also happens to be the plane of symmetry of the molecule (this is the only symmetry element present, therefore the symmetry point group is Cs). It is possible in this instance to specify the configuration unequivocally using the descriptors E and Z. However, in systematic nomenclature the complete configuration of all the stereogenic centres is specified. Thus the (so-called) Z isomer is (ls,3r,5 ,6r,7S)-l,6-dibromo-3,6-dichloroadamantane and the isomer is (ls,3r,5 ,6s,7S)-l,6-dibromo-3,6-dichloroadamantane, i.e. the two isomers can be distinguished simply by the descriptor used for position 6. 

We can define the environment of each nucleus in terms of the cis or trans disposition of nearby double bonds, using as many double bonds as necessary to account for the fine structure observed in the spectrum (often the two nearest double bonds are sufficient, rarely more than three). Figure 1 shows an example using the two nearest double bonds in poly(4-methyl cyclopentene).Many ROMP polymers contain chiral or pseudo-chiral centres in the polymer backbone, and the relative placement of these centres will also affect the NMR spectrum observed. These effects are usually defined in terms of the relative stereochemistry of adjacent chiral centres in the backbone as meso or racemic. Figure 2 shows an example using a sample of poly(4-methyl cyclopentene) in which all the double bonds are trans. 

A logical extension of these ideas will lead you to a recognition of the fact that a phenomenon of this type could yield species in solution which appear to behave as if they contain a chiral centre - even when they don t. We have seen pseudo enantiomeric behaviour in compounds of the type shown in Structure 6.23 (when protonated). 

The metal centre. In organic chemistry, chiral centres are usually associated with an asymmetric carbon atom, but this notion is of limited use for metal ions. Most tetrahedral metal ions are extremely labile, although pseudo-tetrahedral complexes such as (C5H5)MLL L" may be resolved into enantiomers. Octahedral centres with... 

Chiral centres and stereogenic centres Pseudo-asymmetric centres Unhelpful drawings Bond rotation Stereospecificity revisited... 

However, (DHQD)2PYR, the pseudo-enantiomer of (DHQ)2PYR (see chapter 25), would prefer to form two R chiral centres. The substrate and reagent are in competition here—the mismatched case. The selectivity is turned over to 75 25 in favour of the C2 symmetric product 91 this time. So the reagent wins but not outright there is still much of the meso diastereomer formed. Now we can return to the original question which concerns the reaction of triene 89. 

The stereochemistry of these steps is of course controlled by the original chiral centre derived from malic acid. The acetate prefers to adopt a pseudo-equatorial position in the cation 105 and the conformational drawings 105a-107a show that a chair conformation for the [3,3] step with equatorial substituents gives the first two new chiral centres and a pseudo-equatorial position for the isopropyl substituent in the ionic cyclisation gives the third. 

The file benz-01. res on the accompanying CD-ROM contains a complete anisotropic model of a diabetes drug, which crystallizes in the monoclinic space group C2 with two molecules in the asymmetric unit. The model contains hydrogen atoms but not yet the solvent. The two independent molecules are related by a pseudo inversion centre, only violated by the chiral carbon atom. This one atom per molecule... 

The method requires two calibration measurements and the use of pseudo-enantiomeric mass-tagged chiral acylating agents (A-COjH and B-CO2H) that differ in a substituent (A or B) remote to the chiral centre. The mass of the molecule is then correlated with the configuration. 

Michael addition of thioacetic acid to a series of a-substituted (V-acryloyloxazolidin-2-ones, followed by enantioselective protonation, catalysed by the cinchonidine-derived thiourea (288a), has been reported to proceed with <97% ee The pseudo-enantiomeric, cinchonine-derived thiourea (289a) can catalyse the Michael addition of dimedone to enone RCH=CHCO( -Py) with <98% ee The isosteviol-derived thiourea (290) represents yet another variation this organocatalyst has been reported to facilitate the Michael addition of a-substituted cyanoacetates NCCH(Ar)C02R and maleimides in toluene at -30 °C (with <93% ee and <98 2 dr) as a method for the construction of quaternary chiral centres " ... 

The stereochemistry of the heteroyohimbine types resembles that of the yohimbines but with an additional chiral centre at C19. It is again probable that only (155) type compounds occur naturally and there are thus again 4 basic skeletal types, normal (e.g. ajmalicine K9.7, pseudo, alio and epiallo, each of which may in addition have the (19i ) or (195) configuration [4]. ORD of heteroyohimbine alkaloids [5]. 

Full details on the synthesis of prcudo-P-D-mannopyranose and pseudo-fi-D-fructopyianose from quinic acid have appeared (see Vol. 24, p.200). Also reported is the synthesis of pseudo-a-D-glucopyranose from quinic acid in which none of the original chiral centres are retained. ... 

The humulone molecule is flexible around the carbon-carbon bond C-1/C-6 as seen on a Dreiding model. Two important conformations carry the hydroxyl function at the chiral centre either in a pseudo-axial or in a pseudo-equatorial position. The octant diagrams for the enantiomers of humulone in the two extreme conformations are given in Fig. 6. 

The stack of BEDT is of the so-called a-phase. The sulphur-sulphur contacts between the stacks (S- S distances s= 3.47 A, shorter than the sum of the van der Waals radii of 3.6 A) and within the stack (S S distances w4.l-4.2A) give the layers a decidedly two-dimensional character. The BEDT donors are related by a two-fold screw axis, and therefore have no real chiral nature (the molecules have pseudo-centre symmetry). There are several hydrogen bonds between the hydrogen atoms of the ethylene groups of BEDT and the carboxylate groups of the counter-ion. The material is a semiconductor, where the conductivity falls as the temperature is lowered, from about 1 S cm 1 at room temperature. 

Several chiral pseudo-tetrahedral complexes of titanium have been reported this year. They have titanium as the sole centre of chirality or possess additional chiral... 

Another alternative synthetic procedure to the formation of chiral tetrahedral architectures with M4L4 stoichiometry, is the use of trigonally symmetric tris-bidentate ligands that occupy the faces of the tetrahedron with four pseudo-octahedral metals at the vertices. The metal centre is thus coordinated to three bidentate ligands and adopts an octahedral geometry with either a A or A configuration. Such an assembly may possess overall T (AAAA or A AAA), C3 (AAA A or A AAA) or S4 (AAA A or A AAA) symmetry. 

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