Diastereomeric forms

Dehydrohalogenation of the diastereomeric forms of l-chloro-l,2-diphenylpropane is stereo-specific. One diastereomer yields ( )-l,2-diphenylpropene, and the other yields the Z isomer. Which diastereomer yields which alkene Why ... 

The cyclohexanediols, -triols, and -tetrols each have three structures (e.g,. 1,2 1,3 and 1,4 for the diols), but the cyclohexanepentols and -hexols and cyclohexanol itself each have only one structure. For these twelve structures a total of fifty diastereomeric forms (28 meso, 22 racemic) is possible. Cyclohexanol and the -diols have long been known,... 

It is of course possible to name individual radialenes according to IUPAC rules [e.g. per(methylene)cycloalkanes 1-4]. However, the descriptiveness of the term radialene may some day pave its way into the official nomenclature. For substituted [ ]radialenes we have proposed1 a pragmatic numbering system, in which an inner ring is numbered first, followed by an outer ring . The numbering of substituents should follow IUPAC rules. Thus, the hydrocarbon 7 is 4,4-diethyl-5,5-dimethyl[3]radialene, the ester 8 should be called 7-methoxycarbonyl-5,5-dimethyl[4]radialene, the nitrile 9 which can exist in four diastereomeric forms is (6Z,7Z)-6-cyano-5,5,7-trimethyl[4]radialene and the difunctionalized [5]radialene 10 is (7 ,6Z)-7-bromo-6-formyl-6-methyl[5]radialene. 

Theoretically, with 6 chiral centers within the molecule (at 6, 9, 2, 5, 11 and 12 positions) 64 diastereomeric forms are possible. However, bromocriptine is sterically well defined at all of these positions, as it is derived from the naturally occurring a-ergocryptine. 

In the presence of the dehydroamino acid reactant, bidentate complexation as an enamide occurs. With a chiral diphosphine ligand, two diastereomeric forms of the enamide complex are observed in equilibrium that differ in the... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Early studies were based on 13 noncongeneric molecular series of oqA-AR antagonists [65]. The considered series included compounds 1, 3, 9-11, 14, 19 and 32 in Scheme 8.1 and compounds 43 and 45 in Scheme 8.2. Compounds 9 and 32 were considered in both the enantiomeric forms, whereas compound 45, corynanthine, was considered also in its diastereomeric form yohimbine [55]. The natural agonist, norepinephrine, was also considered, for comparative analysis [55],... 

Amino acid derivatives can be examined for enantiomeric purity by the same procedures after removal of the protecting groups. Another approach is to couple them directly with another derivative to give protected dipeptides whose diastereomeric forms are usually easy to separate by HPLC (see Section 4.11). An A-protected amino acid is coupled with an amino acid ester, and vice versa. Use of soluble carbodiimide as reagent (see Section 1.16), followed by aqueous washes, gives clean HPLC profiles. It is understood that the derivative that serves as reagent must have been demonstrated to be enantiomerically pure.43 84-89... 

Stereospecific nucleophilic substitution in chiral ion—dipole complexes. Chiral molecules can be discriminated in the Cl source of a CIMS instmment by specific ion-molecule reactions induced by chiral reagent gas. This method has been applied with success to distinguish between enantiomeric and diastereomeric forms of menthols ((lR,2S,5R)-(— )-14, (lS,2R,5S)-(+)-14, and (lR,2R,5S)-(—)-14 in Scheme 11) through the nucleophilic displacement of their hydroxyl group by (5)-2-amino-l-butanol Self-protonation of As... 

Menthyl p-iodobenzenesulfinate 62 exists in two diastereomeric forms having [a]u + 46 and -146 (79,103). Herbrandson and Cusano (103) determined their absolute configurations on the basis of kinetic studies of the hydrogen chloride-catalyzed equilibration (+>62 (-)-62 and ethanolysis of both diastereomeric esters. They found that the equilibration reaction carried out in nitrobenzene at room temperature results in the formation of a mixture containing 59 3% of the dextrorotatory diastereomer. On the other hand, the rate of ethanolysis of the thermodynamically more stable (+)-62 isomer was found to be twice as large as that of the (->isomer. 

Scheme 6.27. Auxiliary-controlled stereoselective cuprate addition as the key step for the construction of both diastereomeric forms of [5-"q-leucine 144.

The catalytic cycle for the Rh-catalyzed 1,4-addition of phenylboronic acid to an a,[5-unsaturated ketone could be nicely described by in situ P NMR (see Figure 1.4) [19]. The three S -Binap species RhPh(PPh3)(Binap), 11, Rh(oxaallyl)(Binap), 12 and [Rh(OH)(Binap)]2, 13, have all been detected. Complex 11 affords a modestly complicated spectrum (see spectrum A), due to the ABMX spin system. The oxa-allyl complex, 12, in spectrum B exists in two diastereomeric forms (with overlapping signals between 48 and 49 ppm). The resonances for the bridging hydro-... 

However, there are two possibilities of deciding which of the two diastereomeric forms exists. Addition of a chiral auxiliary, e.g., a chiral lanthanide shift reagent, leads to a change in the spin-system type of the H signals. Examples of this kind of experiment are found in Section 4.1.1.4. 

Benzylic-type cations derived from PAHs have been studied under superacid conditions, where, not surprisingly, they are relatively stable.Lifetimes in water of diastereomeric forms of the benzo[a]pyrene derivative (100) have been deter-... 

Only from 52 two diastereomeric forms could be isolated. 

The modular synthetic approach has allowed the preparation of many QUINAPHOS derivatives in both diastereomeric forms. Testing of the single diastereo-mers in catalysis has been revealed to be of crucial importance. Thus, a pronounced interplay between the axial and central chirality has been observed in all applications of QUINAPHOS in catalysis, whereby the choice of the diastereomer may affect not only the enantioselectivity but also the activity and the chemoselectivity of a catalyzed reaction. 

To obtain information about the steps in which the asymmetric induction actually takes place, 1-butene, cis-butene, and trans-butene were hydroformylated using asymmetric rhodium catalyst. According to the Wilkinson mechanism, all three olefins yield a common intermediate, the sec-butyl-rhodium complex, which, if the asymmetric ligand contains one asymmetric center, must exist in the two diastereomeric forms, IX(S) and IX(R),... 

Chirality is expressed on both the molecular and the supramolecular levels. Like a molecule, a supermolecule may exist in enantiomeric or diastereomeric forms. Supramolecular chirality results both from the properties of the components and from the way in which they associate. 

DOTA-type complexes exist in two diastereomeric forms (m and M) which may have remarkably different water exchange rates as found for [Eu(D0TAM)(H20)]3+ [58,59]. In this 170 and H NMR study performed in acetonitrile-water solvent, it was possible to detect the NMR signals of the coordinated water molecules in both isomers. In a general case, the observation of the bound water signal for Ln(III) poly(amino carboxylates) is not possible due to the fast exchange, and for Gd(III) complexes, to the slow electronic relaxation. 

Optical activity in a molecule can occur when and only when it contains a chiral centre. This means that the molecule can exist in two diastereomeric forms, which are non-superimposable. Six-coordinate chelate complexes of the type M(bidentate ligand)3 (Fig. 3.1(a)) and ds-M(bidentate ligand)2X2 (Fig 3.1(b)) are common examples in metallic complexes. 

The authors calculated the same pathways as for the model system, but starting from both of the catalyst-enamide diastereomeric forms previously determined. Their results show that the mechanism goes through the same reaction pathway as for the model system (pathway A) but with a significant... 

Apart from geometric isomers, most other compounds that show diastereomerismhave two or more chirality centers, usually asymmetric carbon atoms. For example, 2-bromo-3-chlorobutane has two asymmetric carbon atoms, and it exists in two diastereomeric forms (shown next). Make molecular models of these two stereoisomers. 

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