Diastereoisomers chirality

The biphenyl diamide 171 also displays diastereoselectivity in its ortholithiation-electrophilic quench, giving the C2-symmetric, chiral diastereoisomer of the diamide 172 . 

When we talk about two chiral diastereoisomers, we have no choice but to draw the structure of one enantiomer of each diastereoisonier, because we need to include the stereochemical information to distinguish them, even if we re talking about a racemic mixture of the two enantiomers. To avoid confusion, it s best to write something definite under the structure, such as (meaning racemic) under a structure if it means this diastereoisomer but not this enantiomer of this diaster eoiso mer . 

In 10, there are two remote stereogenic centers at phosphorus and it was isolated as an almost equimolecular mixture of the two sets of diastereoisomers, that is, (RR/SS) and (RS/SR). In each chiral diastereoisomer, the phosphorus atoms are nonequivalent and four distinct 31P resonances were obtained almost in a 1 1 1 1 ratio <1997J(P2)1445>. The kinetics of the thermal decomposition of 11 in dibutyl phthalate was studied. The high rate of decomposition was probably determined by mutual steric influence of the bulky dinitromethylene moieties <2006RJC499>. 

Currently there is a trend toward the synthesis and large-scale production of a single active enantiomer in the pharmaceutical industry [61-63]. In addition, in some cases a racemic drug formulation may contain an enantiomer that will be more potent (pharmacologically active) than the other enantiomer(s). For example, carvedilol, a drug that interacts with adrenoceptors, has one chiral center yielding two enantiomers. The (-)-enantiomer is a potent beta-receptor blocker while the (-i-)-enantiomer is about 100-fold weaker at the beta-receptor. Ketamine is an intravenous anesthetic where the (+)-enantiomer is more potent and less toxic than the (-)-enantiomer. Furthermore, the possibility of in vivo chiral inversion—that is, prochiral chiral, chiral nonchiral, chiral diastereoisomer, and chiral chiral transformations—could create critical issues in the interpretation of the metabolism and pharmacokinetics of the drug. Therefore, selective analytical methods for separations of enantionmers and diastereomers, where applicable, are inherently important. 

Draw the enantiomers of any chiral diastereoisomer by inverting all the stereogenic centres... 

Draw the enantiomers of any chiral diastereoisomer by inverting all the stereogenic centres. This can easily be achieved by reflecting the molecule in the plane of the paper, as if it were a mirror. Everything that was up is now down and vice versa. 

Reaction of 2-[A -(rra -crotyl)-A -benzylamino]-3-formyl-4/f-pyrido[l,2-n]pyrimidin-4-one (269) with chiral primary amines 270 and 271 gave mixtures of diastereoisomers of tetracyclic compounds 273 and tricyclic 275 (96T131]]). The chiral centers in 272 and 274 did not provide any stereocontrol for the formation of diastereomers 273 and 275, respectively. 

Thermal cyclization of 2-vinyl-N,N-dialkylanilines 138 afforded 139 with creation of a new chiral center in 98% purity (89JOC199). In case of pyrrolidine with methyl or methoxymethyl substituent, cyclization with ZnCl2 occurs via an irreversible 1,5-hydrogen shift in boiling acetonitrile (87JA3136) or BuOH (91RTC115) to afford the diastereoisomers 140 (33%), 141 (35%) and 142 (6%) (87JA3136) (Scheme 27). 

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

The reaction of alkenylcarbene complexes and imines in the presence of a Lewis acid generates pyrroline derivatives as a result of a [3C+2S] cyclisation process [76]. This reaction has been extended to an asymmetric version by the use of chiral alkenylcarbene complexes derived from several chiral alcohols. However, the best results are found when (-)-8-phenylmenthol-derived complexes are used and catalytic amounts of Sn(OTf)2 are added to the reaction. In these conditions high levels of trans/cis selectivity are achieved and the hydrolysis of the major tram diastereoisomers allows the preparation of optically pure 2,5-disubstituted-3-pyrrolidinone derivatives (Scheme 29). 

A small library of thiazolidinones 138 has been prepared mixing directly a primary amine (as the HCl salt), an aldehyde and mercaptoacetic acid in EtOH in the presence of Hiinig s base and molecular sieves (120 °C for 30 min) [88]. Working with a chiral amine, a 1 2 mixture of diastereoisomers was obtained (Scheme 49). 

A number of (S)- or (R)-2-aminoamides were independently obtained by running reactions between chiral 2-bromoamides 1 and an achiral primary or secondary amine, in both sets of conditions (ref. 5). Accordingly, either a diastereoisomeric mixture or a single diastereoisomer in high diastereoisomeric excess, resulted respectively when an (S,R)- or (S)-2-bromoamide is treated with a chiral amine, again in the two sets of conditions. 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

Hayashi et al. [18] have synthesized two diastereoisomers of 2,2 -bis[4-(alkyl)oxazol-2-yI]-l,T-binaphthyl,bis(oxazoline) derivatives possessing both binaphthyl axial chirality and carbon centered chirality (structures 9 and 10, Scheme 5). 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

The most direct method is the separation of the enantiomers by HPLC using chiral columns. It has the advantage that there is no risk of contamination from chiral resolving agents. The formation of diastereoisomers... 

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