Diastereomer, defined

Scheme 17). A priori, both bicyclic isoxazoline epi-mers could be utilized in this synthesis because the newly formed stereocenter is eventually destroyed. Nevertheless, the two isoxazoline diastereomers were separated, and the subsequent stages of the synthesis were defined using the major isomer 30. 

Defined as the ratio of the major diastereomer to the sum of the other diastereomers. 

Similar methodology has been applied in the syntheses of 2-amino-3-hydroxycarboxylic acids in high diastereomeric and enantiomeric purity. Two separate pathways give either the antt- or. WM-products. The first strategy relies on haloacetate precursors derived either from (S )-valine 17"- oi or from norephedrine 18102, which are converted into the boron enolates103 and subsequently reacted with aldehydes to deliver. ym-adducts99 102. The diastereomeric ratio, defined as the ratio of the desired diastereomer/the sum of all others, is 50 1 for the former and about 95 5 for the latter adducts. 

Crystalline, diastereomerieally pure syn-aIdols are also available from chiral A-acylsultams. lhe outcome of the induction can be controlled by appropriate choice of the counterion in the cnolate boron enolates lead, almost exclusively, to one adduct 27 (d.r. >97 3, major adduct/ sum of all other diastereomers) whereas mediation of the addition by lithium or tin leads to the predominant formation of adducts 28. Unfortunately, the latter reaction is plagued by lower induced stereoselectivity (d.r. 66 34 to 88 12, defined as above). In both cases, however, diastereomerieally pure adducts are available by recrystallizing the crude adducts. Esters can be liberated by treatment of the adducts with lithium hydroxide/hydrogen peroxide, whereby the chiral auxiliary reagent can be recovered106. 

When the related saccharin derived sultam (R)-29 is converted into the (Z)-boron enolate and subsequently treated with aldehydes,. vy -diastereomers 30 result almost exclusively. Thus, the diasteromeric ratios, defined as the ratio of the major product to the sum of all other stereoisomers, surpass 99 1. Hydroperoxide assisted saponification followed by esterification provides carboxylic esters 31 with recovery of sultam 32106a. 

In a few cases, the synthesis was directed towards well-defined oligomers (dimers, trimers, etc.). The synthesis of bis(5,7,3, 4 -tetra-0-benzyl)-EC 4/1,8-dimer from 5,7,3, 4 -tetra-0-benzyl-EC and 5,7,3, 4 -tetra-0-benzyl-4-(2-hydroxyethoxy)-EC was described by Kozikowski et al. [41]. This compound exhibited the ability to inhibit the growth of several breast cancer cell fines through the induction of cell cycle arrest in the Gq/Gi phase. Analogously, procyanidin-B3, a condensed catechin dimer, has been obtained through condensation of benzylated catechin with various 4-0-alkylated flavan-3,4-diol derivatives in the presence of a Lewis acid. This reaction led to protected procyanidin-B3 and its diastereomer. In particular, octa-O-benzylated procyanidin-B3 has been produced with high levels of stereoselectivity and in excellent isolation yields [42]. 

This formed a basis for the study of the H2 addition step in a precatalyst for Ir enantioselective hydrogenation [70]. By NMR, it proved possible to characterize a single diastereomer of the initial addition product at -40°C in THF, the configuration of which was defined by nOe methods. This was converted into a mixture of two diastereomers of the disolvate dihydride with release of cyclooctane at 0°C. In all cases, H trans-N is preferred over H trans-P, as was originally observed by Crabtree. The investigations were completed by DFT computational studies on the initial steps of the reaction sequence as a model for the stereose-... 

It has been well established that either kinetic or thermodynamic principles can be employed in the aldol process to define product stereochemistry. When conditions are chosen such that the condensation process is rendered reversible, the more stable threo metal aldolate complex 3 with diequatorial substituents Rj and R3 is usually the dominant diastereomer observed (2,5,14). 

The ability of a chiral molecule to distinguish between the enantiomers of a second (different) chiral molecule was defined in Sect. II as a diastereomer discrimination. This phenomenon may be observed in a mixed monolayer of two chiral surfactants and may also occur when a chiral substance is dissolved in the aqueous subphase under the monolayer of a second chiral substance. As before, examples of such chiral discrimination would not include those whose difference in monolayer behavior results only from the gross structural differences of diastereomers such as the different force-area characteristics exhibited by mixed monolayers of l-oleoyl-2-stearoyl-3-s -phospha-tidylcholine with epimeric steroids (120). The relevant experiment, that of comparing the monolayer behavior of mixed monolayers of cholesterol with enantiomeric phospholipids, has been reported (121). As might be anticipated from our previous discussion of... 

The degeneracy of the non-chiral complexes can be removed by incorporating chiral centers, usually as resolved amino acids, into the arms at close vicinity to the hydroxamate iron binding sites. Thus, only one of the energetically non-equivalent diastereomers predominates, leading to pure enantiomeric iron(III) complexes with defined hehcity that allows assessing stereospecific recognition by the ferrichrome receptor. 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

For the assessment of diastereoselectivity three measures are commonly used the ratio of the diastereomers (d.r.), as defined above,... 

In the first step, a cyclic intermediate is generated by a suprafacial addition, followed by a SN2-type ring opening (e.g., halogenation or epoxidation ring opening). In this manner, olefin 1 may either be converted into diastereomer 2 or 3, which may be optically active or racemic. Mechanism control thus means that the relative, but not the absolute, configuration of the two vicinal centers is defined. 

The only difference to the simple diastereoselection lies in the fact that the olefmic components now have two reactive sites each, namely the two olefmic sp1 centers, which are all converted into sp3, and hence potentially stereogenic centers. However, the configurations of these stereogenic centers are not independent, but mutually correlated by the principle of suprafacial addition (see Section 2.3.1). Thus, although a maximum of 24 stereoisomers could be formed, this number must be divided in half for each olefmic reactant with a defined (E)) Z) configuration. This results in only four stereoisomers in the form of two enantiomeric pairs of ejeo/enrfo-diastereomers. 

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