Stereoselective effect

Substrate-Induced Stereoselectivity, Effect of Additional Stereogenic Centers... 

It is interesting to note that the absence of the two OH groups at the 7 and 8 positions of 9,10-BaPE (J7), and the 7,8-carbon atoms in 1-OP (14), lead to a partial loss of stereoselective effects in the covalent binding of these molecules to DNA in the adducts derived from these two molecules, site I adducts dominate. In contrast, in the case of the diol epoxide (+) anti-BaPDE, site II complexes account for over 90% of the binding(31). 

The overall results of substituent effects are observed in the products of a reaction, their rates of formation, and their stereochemistries. The purpose of this article is to apply very simple theoretical techniques to correlations and predictions of the rate and stereoselectivity effects of substituents in [2+2] photocycloadditions. The theoretical methods that will be used are perturbational molecular orbital (PMO) theory and its pictorial representation, the interaction diagram. Only an outline of the theory will be given below, since several more detailed descriptions are available. 4,18-34)... 

They further observed the stereoselective effect in several other systems containing other amino acids as the polymer ligands74, and identified certain prerequisites for effective resolution (i) mixed complexes are more stable and it is better to use Cu(II) or Ni(II), (ii) the sum of the dentation of the polymer ligands and the mobile ligands (amino acids) should be equal to the coordination number of the central metal ion. The difference in free energy of formation of the two dia-stereomeric complexes with L- and D-proline was estimated to be 0.4 to 0.5 kcal/mol for the L-proline-Cu resin. It was emphasized that the stereoselectivity of this process was sufficient for the complete resolution of racemic a-amino acids74. ... 

Yamakura, T., Sakimura, K., Shimoji, K. The stereoselective effects of ketamine isomers on heteromeric N-methyl-D-aspartate receptor channels, Anesth. Analg. 2000, 225-229. 

Hatano and Nozawa" have observed stereoselective effects in the hydrolysis of d- and l-phenylalanine esters using poly-L-lysinecopper(II) complexes as catalysts and some aspects of the topic have been reviewed.99... 

Whether for these reasons or not, the reactivity indices excited little interest among organic chemists, and it was only after Woodward and Hoffmann s early papers that perturbational methods were widely invoked again. Fukui and Fujimoto17-90 adopted Fukui s frontier orbital theory to explain the Woodward-Hoffmann rules, and both they91 and Hoffmann and Woodward92 were able to explain stereoselective effects [such as the preference in the Cope rearrangement (1) (2) for the chair-like intermediate (16) rather than the boat-like one (17), and the tendency for Diels-... 

Fukumoto, K., Kobayashi, T., Komamura, K., Kamakura, S., Kitakaze, M., and Ueno, K. 2005. Stereoselective effect of amiodarone on the pharmacokinetics of racemic carvedilol. Drug Metab. Pharmacokinet. 20 423-427. 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

A new probe of solvent accessibility of bound sensitizers has been described and tested for the particular case of a series of Ru" and Os photosensitizers bound to sodium lauryl sulphate micelles. The method depends upon the large solvent deuterium effect on excited-state lifetimes, and a correlation has been established between accessibility of bound complexes and hydrophobicity of the ligands. Luminescence properties of amphiphilic annelide-type complexes of ruthenium in micellar phases have been described. In the case of [4,4 -bis(nonadecyl)-2,2 -bipyridyl]bis-[4,4 -di-(10,13,16-trioxaundecyl)-2,2 -bipyridyl]ruthenium dichloride, intramicellar self-quenching effects have an influence on the excited-state lifetime, and the mechanism of self-quenching has been determined. Deactivation of [Ru(bipy)3] by [Co(EDTA)] has been studied in a micellar environment and found to occur by electron transfer at diffusion-controlled rates a stereoselective effect has been observed. ... 

Many other drugs and xenobiotics demonstrate stereoselectivity in toxicological or pharmacological effects (Tables 2 and 3) (Vermeulen, 1986 Caldwell et al., 1988 JamaU et al., 1989 Vermeulen, 1989 Eichelbaum and Gross, 1990 Williams, 1990). In addition to direct stereoselective effects of... 

As for stereoselectivity, relatively little is known as yet, except that the natural R configuration in the cysteine residue is a prerequisite for a cysteine conjugate to be a substrate of p-lyase. This fact has been used as a tool to ascertain the role of p-lyase in the development of nephrotoxicity by cysteine conjugates. No data are presently available with regard to the substrate-stereoselective effects of the noncysteine part of the thioether substrates of p-lyase (Commandeur and Vermeulen, 1990). Such stereoselective effects may be anticipated, however, since, for example, the regioisomeric 1,2- and 2,2-dichlorovinyl-LrCysteine conjugates have also... 

In recent years, considerable progress has been made in the elucidation of stereochemical mechanisms of several important biotransformation enzymes, despite the fact that the multiplicity, a widely varying substrate selectivity as well as secondary metabolism (which in itself can be stereoselective), considerably complicates such studies. In principle, the same factors do complicate the correlation of data on stereoselective effects obtained in vitro with those obtained in vivo, even when it concerns the same species. The prediction of stereoselective effects from one species to another or from one chemical to another is even more difficult. 

I. Fitos and M. Simonyi, Stereoselective effect of phenprocoumon enantiomers on the binding of benzodiazepines to human serum albumin, 2nd Jniernatima Symposium on Chiral Discriminatimi, Rome, 1991. 

The cyclopentyl substituent was also expanded to cyclohexyl and cycloheptyl. The cycloalkyl substituent provides an important lipophilic interaction, which makes a significant contribution to the affinity. Similar substituents in MK0591 that may correspond with this key lipohilic interaction include the t-butylthiol group or the 4-chlorobenzyl substituent. Stereoselective effects were observed, the R enantiomers displaying a greater affinity for human FLAP. Modification of cyclopentyl to a cycloheptyl alkyl substituent and modification of the acid to a... 

In alcoholic medium, NaBH, converts a,P-unsaturated nitriles into cyanoethyl compounds under more severe conditions (refluxing alcohol for several hours). - i jpig enantioselective reduction of the double bond of each ( )- and (Z)-isomer of a,P-unsaturated nitriles derived from ketones has been evaluated using NaBH in EtOH/diglyme in the presence of semicorrin (/oCT catalyst the yields are good (about 75%), but the enantioselectivity remains modest (53-69%). - In contrast, it has been found that copper(l) hydride, a reagent that presumably operates by a mechanism akin to the Michael reaction, 754 cleanly and stereoselectively effects the reduction of a,P-iinsaturatcd to saturated nitriles in the presence of 2-butanol, as proton donnor, in THE. The use of LAH in THF at room temperature has also been reported in the stereoselective reduction of a,P-iinsaturatcd nitriles. 

Amino-acid Complexes. A small, but reproducible, stereoselective effect has been observed in both the free energy and enthalpy changes associated with formation of [Ni(L,L-methioninate)2]. The meso-complex [Ni(o-Met)(L-Met)] is more stable in AH than the optically active NiL2 by 1.0 (0.1) kJ mol" L The stereoselectivity is attributed to terdentate co-ordination and so supports weak co-ordination of the thioether group. Formation constants of nickel(ii) and copper(ii) with N -benzyl-L-histidine and N N -dibenzyl-L-histidine and of the ternary complexes with d- and L-histidine have been measured. Stabilization of ternary complexes is small but significant stereoselectivity is found with ternary nickel complexes, when the meso configuration is preferred in each case with copper, stereoselectivity is small or absent. The i.r. spectra of tra s-[Ni(Gly)2(H20)2] and its 0-, N-, 1- C-, and 2- C-labelled... 

There is a surprisingly large stereodirecting effect for an alkoxy substituent (ca. 4 kcal mol ) in the amidomercuriation cyclization of the 2-(3-butenyl)-l,3-benzoxazin-4-ones (36). Thus, there is competition between the stereoselective effect of the aminal center and that of the allylic alkoxy group. For example, reaction of (36 R = OBn) with mercury(II) acetate and trifluoroacetate, followed by reduction with lithium borohydride, affords only the tricyclic compound (37 R = OBn), whereas similar treatment of (36 R = H) gives a mixture of the cycloadducts (37 R = H) and (38) in the ratio 1 12.4, respectively <90TL6765>. 

Balmer, K. Persson, B.-A. Lagerstrom, P.-O. Stereoselective effects in the separation of enantiomers of omeprazole and other substituted benzimidazoles on different chiral stationary phases. J.ChromatognA, 1994, 660, 269—273... 

---