Diastereomeric excess/ratio

The reaction mechanism involves silylation of a nitro group oxygen followed by deprotonation to give the intermediate silyl nitronate 137, which undergoes 1,3-dipolar cycloaddition to give the product 136 in excellent yields and with diastereomeric excess ratios often exceeding 99 1 (Scheme 15). 

When dealing with reactions leading to stereoisomeric products we have the additional complication that descriptors such as enantiomeric (diastereomeric) excess and enantiomeric (diastereomeric) ratio are used to describe product purities. The evaluation of RME for a specific stereoisomer, say the R enantiomer, is exactly as above using the connecting relationships for the fraction of each product shown below. 

A reasonably large difference in diastereomeric excess was observed between product 47b with an adjacent methyl ester and 48b with a primary alcohol in the equivalent position [57]. It was noted by the authors that in cases involving a 1,3 system, changing the pendant group from a primary allylic alcohol to a methyl ester caused a reversal of facial selectivity [54, 58]. The same effect was absent in the 1,2 systems 51b and 52b studied. The diastereomeric ratio in the latter case was attributed mainly to catalyst control [58]. 

The reaction protocol was further developed by alterations to the chiral controlling element of the reaction (49). Use of the precursor 183 under the standard ylide generation and cycloaddition conditions gave a greatly improved diastereomeric excess of >95%, an endo/exo ratio 1 15 and an isolated yield of 62%, with A-phenylmaleimide as the dipolarophile. The improvement in the reaction was rationalized by both endo and exo attack of the dipolarophile to the same diastereomerically favored face of the conformationally restricted U-shaped ylide 184 (Scheme 3.52). 

Diastereomeric Excess (de) - This measure for characterization of mixtures of diastereomers should not be used. This volume uses the diastereomer ratio (d.r.) normalized to 100 whenever possible (see Section 1.2.2.2.). 

Since the above cleavage methods proceed without racemization, the enantiomeric excess of the alkylated carbonyl compounds obtained can be safely determined by measuring the diastereomer-ic ratio of the alkylated hydrazones (see Section I.I.I.4.2.3.). The diastereomeric excess of SAMP-hydrazones derived from aldehydes was further determined by gas chromatography41. 

The chemically induced asymmetric photocyclization of the l-(—)-menthyl ester shows a striking temperature dependence 76), a completely reversed ratio is obtained at low temperature. When the chiral group is placed at C(ll) of the benzo[c]phen-anthryl group (77) the effect is of the same order (about 5 % diastereomeric excess) as when the same group is placed at the paraposition of the phenyl group, or when... 

Reduction of the olefinic bond in 12 and Swem oxidation of the free carbinol function provided ketone 53, onto which installation of the methyl group was performed by reaction with methylmagnesium chloride in THF. After protection of the resulting tertiary alcohol as a TBS-ether, fully protected triol 54 was obtained with a useful 80% diastereomeric excess. Acetonide deblocking and oxidative fission of the diol formed led to aldehyde 55, ready for the planned cyclization step. Exposure of aldehyde 55 to TBSOTTDIPEA reagent system smoothly resulted in formation of the desired bicyclic adducts 56 and 57 which were isolated in a 82% combined yield (60 40 ratio). 

In contrast, the intramolecular energy transfer in cyclooctene derivatives 63E-65E, which are tethered to benzoate, isophthalate, and terephthalate moieties through a (/ ,/ )-2,4-pentadiol linker, does not appear to be seriously affected by the steric bulk of 1-alkylation (Scheme 10) [50]. Upon irradiation at — 65°C, 63, 64, and 65 afforded ZJE ratios of 0.8,0.15, and 0.06, with diastereomeric excesses (des) of 33, 37, and 44%, respectively. 

A number of observations (Scheme 29) suggest that alkali metal ions present in zeolites play an important role in the asymmetric induction process, (a) The de was dependent on the nature of the alkali metal ion (e.g., % de in the case of 42 in LiY, NaY, KY, RbY, and CsY are 80, 28, 14, 5, and 5, respectively), (b) The de varied with the water content of NaY used (42 dry 80%, wet 8%). (c) The de upon irradiation of 42 adsorbed on silica gel, a surface that does not contain cations, was only 8%. (d) Diastereomeric excess in the case of 42 decreased from 80% to 10% when the Si/Al ratio of NaY zeolite was changed from 2.4 to 40. The less the aluminum on the framework of the zeolite, the less the number of alkali metal ions. The number of cations per unit cell decreases from 55 to 5... 

Sharpless epoxidation of symmetrical diols can be expected, on purely mathematical grounds, to produce diepoxide products whose enantiomeric purities are dramatically increased over those obtained for formation of a single epoxide56. Hoyle56 recently exploited the double Sharpless epoxidation of a symmetrical diol 12 to produce epoxides 13, 14 and 15 that were required for subsequent conversion to chiral 2,5-linked bistetrahydrofurans. Although the diastereomeric ratios and enantiomeric purities could not be determined, it was possible to calculate that if the enantiofacial selectivity was 19 1 (90% ee) for a single epoxidation, the ratios of isomers would be 361 38 1 for 13/14/15. Thus, in this double enantioselective epoxidation the diastereomeric excess of the chiral diepoxide (13, 15) is expected to be 99.45%. 

When chiral thiocarbonyl S-oxides derived from (S)-proline, prepared from the appropriate l-(trimethylsilylalkylsulfonyl)-2-(alkoxymethyl)pyrrolidine, are subjected to Diels-Alder reaction with 2,3-dimcthyl-l,3-butadiene, the 2-substituted 2-[(2-alkoxymethyl-l-pyrrolidinyl)sul-fonyl]-3,6-dihydro-4.5-dimethyl-2/f-thiopyran 1-oxides 10A and 10B are formed as a mixture of diastereomers. Their ratio depends on the nature of the substituents R1 and R2 as well as on the reaction temperature. The diastereomeric excess values, determined by H NMR in the presence of Yb(tfc)3 or by HPLC, vary from 0 to 41 % 84. 

It may be argued that if the actual extent of enantiomeric contamination of a CDA is known accurately, the reagent may be safely used, because the appropriate correction in diastereomeric peak ratios can be made. An objection (5) to this argument is that if the enantiomerically impure CDA is present in excess, differences, if any, between the CDA enantiomers in their reaction rates with the analyte enantiomers (i.e., diastereoselective kinetics) will stUl result in an error in the determination of the enantiomeric ratio. In practice, however, such kinetic differences are usually negligible. A more precise but cumbersome solution to this problem is to separate the four stereoisomeric derivatives using chiral chromatographic conditions, for example, a chiral stationary phase. Under such conditions, four distinct peaks are obtainable as a matter of principle (whether the four stereoisomers are actuaUy resolved depends, of course, on the chromatographic conditions chosen). A review of the literature indicates that small (1-2%) enantiomeric contamination of a CDA may not necessarily render the CDA useless in many applications. It is clear, nevertheless, that the CDA used should be enantiomerically pure whenever possible. This simplifies the analysis and eliminates any uncertainty associated with enantiomeric contamination. There is, in fact, an application in which enantiomerically impure CDAs cannot be used safely the determination of trace enantiomeric impurity in an analyte. If the CDA used is itself enantiomerically contaminated, the accurate determination of the extent of trace enantiomeric contamination of the analyte may be difficult if not impossible. 

If one of the reactants has a stereogenic center, there then is a possibility of forming two pairs of diastereomers or two pairs of enantiomers, four different products. Depending on a variety of factors, we could distance e.e. or d.e. (diastereomeric excess, the predominance of pair of diastereomers over the second). A few authors will describe the ratio of their products. 

Analogously, one can determine the relative amounts of two diastereoiso-mers, of general formula 48, by integration of a pair of comparable signals in a mixture. The data can be expressed as either (1) a diastereomeric ratio or (2) a diastereomeric excess in a similar manner to enantiomeric excess. The diastereomeric excess (de) of one diastereoiso-mer, X, over another, Y, is given in equation (3). Although one can determine the de values from NMR spectroscopy, no information on the particular configurations is obtained. 

A mixture of Z- and -isomers was obtained the isomer ratio was not reported. The stereochemistry of the alkene is on comparative basis the reaction conditions were not optimized. Synthesized from L-menthol. Diastereomeric excess... 

The diastereoselectivity of the cydopropanation reaction of alkenes with diazoacetates has two facets. The configuration of the alkene double bond is normally fully retained ( stereospecific cydopropanation ), but may be lost partially when a bis(camphorquinonedioxima-to)cobalt(II) catalyst is employed. As far as the stereochemical relationship between the alkene substituents and the carbenoid building block is concerned, the sterically less encumbered diastereomer is usually formed preferentially, but the diastereomeric excess is normally not very impressive (for examples, see Tables 7 and 9). Within certain limits, the diastereomeric ratio depends on the catalyst as well as on the nature of the diazoacetic ester and the alkene s substituents. The thermodynamically more stable trans- or anti-) diastereomer is increasingly favored with increasing steric bulk of the substituents at the C-C double bond (e.g. 1-R-sub-stituted-l-trimethylsiloxyethene ) and of the ester residuc, furthermore in the following sequence of the metal catalyst Pd < Rh, Ru < In contrast, comparisons of... 

It was envisaged that chiral PTC should play a crucial role in achieving an efficient chirality transfer hence, an examination was conducted of the alkylation of the dipeptide, Gly-L-Phe derivative 47 (Table 11.5). When a mixture of 47 and tetrabutylammonium bromide (TBAB 2mol%) in toluene was treated with 50% KOH aqueous solution and benzyl bromide at 0°C for 4h, the corresponding benzylation product 48 was obtained in 85% yield with the diastereomeric ratio (DL-48 LL-48) of 54 46 (8% diastereomeric excess (de)) (entry 1). In contrast, the reaction with chiral quaternary ammonium bromide (S,S)-9g possessing a 3,5-bis(3,5-di-tert-butylphenyl)phenyl group under similar conditions reahzed almost complete diastereocontrol (entries 4—6) [45]. 

As shown in Scheme 3, (Z)-enolate (57), prepared by conjugate addition of lithium bis(phenyldi-methylsilyl)cuprate to methyl crotonate or methyl cinnamate, reacts with acetaldehyde or benzaldehyde to give a mixture of two diastereomeric aldols, (58) and (59), with excellent diastereomeric excess favoring (58) (ratios of 85 15 to 94 6). On the other hand, deprotonation of ester (60) by LDA provides the ( )-enolate (61), which reacts with the same two aldehydes to give the aldol (59) as the major product... 

In one of the first such examples, the lithium enolate of (S)-3-methyl-2-pentanone was allowed to react with several aldehydes in the case of propanal, the two products are formed in 15% diastereomeric excess, favoring (179 equation 115). The di- -butylboron enolate of this ketone has been studied and found to give (179) and (180) in a ratio of 63 37 in CH2CI2 and 64 36 in pentane. ... 

The radical cyclization of suitable 2-0-2-propenyl phenylselenoglycosides provides a simple route to C-glycosides. The diastereomeric excess is strongly dependent on the configuration at C-466. While the glucosyl derivative is relatively unselective, the galactosyl isomer yields the two methyl epimers in a ratio of 99 1. 

---