Diastereomeric isomers

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Historically, the first multivalent glycoconjugate obtained by metal-assisted self-assembly was described in 1994 by Sakai and Sasaki (Fig. 35).267 The methodology was based on the trimerization of unsymmetrical Bipy-GalNAc (307) induced by the presence of Fe(II) salts, to give the tridentate Fen(bipy-GalNAc)3 cluster 308 as a mixture of four diastereomeric isomers (A-fac, A-fac, A-mer, and A-mer). 

Takeuchi Y, Nagao Y, Toma K, Yoshikawa Y, AMyama T, Nishioka H, Abe H, Harayama T, Yamamoto S (1999) Synthesis and Siderophore Activity of Vibrioferrin and One of its Diastereomeric Isomers. Chem Pharm Bull 47 1284... 

The reaction of 1-iodonaphthalene with chiral-assisted imide enolate ion 3 provides an interesting example of the stereoselective coupling of an aromatic radical with a nucleophile. In this reaction, the diastereomeric isomers of the substitution compound are formed (43-64%), while the seledivity observed is highly dependent on the metal counterion used and its chelation properties (Scheme 10.11). All of the ions studied (Li, Na, K, Cs, Ti(IV)) are seledive however, the highest stereoseledivity... 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

Lithio-fluoro (or -chloro) methyl phenyl sulfoxide reacts with iV-(benzylidene)aniline derivatives (105) to afford the corresponding aziridines (106) in good yields. Two diastereomeric isomers are produced, but the trans isomer is the major product except in the case of iV-benzylidene-p-nitroaniline (Scheme 30). ... 

The reaction of a chiral aldehyde with an achiral enolate leads to the formation of two diastereomeric isomers in a proportion which largely reflects the facial selectivity inherent to the aldehyde. This proportion is often referred to as the Cram/anti-Cram ratio. The ratio is normally small for acyclic chiral aldehydes (ranging 1 1 - 1 5) and is often unpredictable, as exemplified below for the aldol reaction involving an achiral boron enolate (22) and the chiral aldehyde (24 Scheme 22). ... 

Curtis and others showed that cyclic condensation also was possible in the absence of a metal template ion, but acid was necessary for those reactions. Possibly, the protonated amines formed an internal hydrogen-bonding network that acted as a template. Cyclization was observed in yields of up to 80% (Curtis and Hay, 1966 Kolinski and Korybut-Daszkiewicz, 1969 Tait and Busch, 1978a, 1978b). The isomers of the formed cyclic Schiff bases were difficult to separate. Reduction of the unsaturated macrocycles gave cyclic polyamines with up to six chiral centers and up to 20 diastereomeric isomers (Hay and Piplani, 1977 Kolinski and Korybut-Daszkiewicz, 1969). 

For configurationally unstable carbanions , a (thermodynamically driven) equiUbration between diastereomeric isomers is the method of choice, if the difference in free energy is large enough (>2 kcabmol ). In rare cases, a dynamic kinetic resolution is possible by preferential crystallization of one diastereomer. 

To a suspension of 0.5 mmol Ni(II) complex of Schiff base arising from glycine and (5 )-o-[iV-(Af-benzylprolyl)amino]-benzophenone in 2.0 mL DMF was added 0.525 mmol 3-(fran -3 -methylpropenoyl)oxazolidin-2-one the mixture was stirred at ambient temperature for 10 min. Then 10 /u.L DBU (0.07 mmol) was added, and the reaction was monitored by TLC. After the reaction completed ( 15 min), the mixture was poured into 80 mL icy 5% aqueous acetic acid and stirred with a glass bar to initiate crystallization of the product. The crystalline product was filtered off, thoroughly washed with water, and dried in vacuo to afford > 98% of the addition products. The diastereomeric isomers were isolated by silica gel column chromatography (acetone/chloroform, 1/4), in a ratio of 2.4 to 1. 

Chromatography Separation of the diastereomeric isomers was carried out on Cg or Cjg columns by elution with mixtures of methanol/water (50 50) adjusted to pH 3-4, containing either 2% of acetic acid or 0.1% of triethylamine. The column eluate was monitored using UV absorption at 254 nm. 

The chiral recognition properties of crown ethers(4) for racemic Ct-phenylglycine methyl ester perchlorate(5) and 1-phenylethylamine perchlorate(6) were tested by standard extraction experiments.No appreciable amount of crown ethers was detected in aqueous layers. The relative amount of complexed salts to crown ethers in CDCl layers was determined by NMR integrations. When (5) is complexed with (4), the signal of ester methyl protons is moved and separated into two signals for each enantiomei When (6) is complexed with (4), the signal of methyl protons(doublet) is only shifted but not separated. The ratio of enantiomers of complexed salts was determined by HPLC using a chiral column for (5) or by GLC for (6) as diastereomeric isomers of (-)-(L)-N-trifluoroacetylalanine. The results are summarized in Table I and II. 

With intramolecular oxa-Michael precursor 3.47, constructing the six-seven bicyclic structure directly through the oxygen Michael addition reaction was carried out under the alkaline condition. However, only the starting materials were recovered. Thus, MOM group was first removed to obtain the free phenol 3.50 under sulfuric acid. After Wessely oxidative dearomatization, we got a pair of diastereomeric isomers 3.51 with the ratio 3 1, which was considered as the Diels-Alder precursor. The expected IMDA reaction did not happen in a sealed tube. No desired 3.52 or IMDA product 3.53 was separated. Starting materials were partially recycled, but most of the precursors were decomposed (Fig. 3.23). 

Theoretically, eight diastereomeric isomers could be produced by this reaction since three stereogenic centers are generated on cyclobutane ring. The results of the [2+2] cycloaddition are summarized in Scheme 4.4. In the reaction of tert-butyldimethylsiloxy-1-cyclohexene (5a) with acrylate 7g, production of two diastereomers trans-Xla and cis- la was obtained in 51 and 33% yield, respectively. The... 

An interesting example of stereoselective coupling of an aromatic radical with a nucleophile is found in the reaction of 1-iodonaphthalene with an imide ion containing a chiral auxiliary. In this reaction, the diastereomeric isomers of the substitution compound are formed - ... 

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