Stereochemistry effect

Scheme 8 Relative stereochemistry effects on olefin diastereoselection.

Fragment ion at mjz 325 is similar and is allowed only for raffinose since this is the only trisaccharide in this study that meets both requirements, i.e., the proper configuration at the (glucosyl) anomeric carbon and the right stereochemistry at C-2 of the fructose unit (remote stereochemistry effect). 

An investigation of the regio- and stereochemistry effects of Cp 2Sm(THF)B on the electrophilic trapping of allylic compounds showed that the nature of the electrophiles has a remarkable influence on the regio- and stereochemistry of the electrophilic trapping (Scheme 320).1113... 

Stereochemistry effect of ring junction A/B observed in HV scan spectra for M [M — CHj] transition [203]... 

From our initial structure-activity relationship (SAR) studies, we have discovered that a class of manganese(II)-based complexes, I, incorporating the macrocyclic ligand 1,4,7,10,13-pentaazacyclopentadecane (where R can be any number of defined substituents with known stereochemistry), effectively catalyze the dismu-tation of superoxide. ... 

Reeve MS et al. (1994) Polylactide stereochemistry effect on enz)nnatic degradability. Macromolecules. 27 825-831. 

Reeve M S, McCarthy S P, Downey M J, Gross R A (1994), Polylactide stereochemistry Effect on enzymatic degradability . Macromolecules, 27, 825-831. 

Tian Y, Fendler J FI, Flungerbuhler FI, Guldi D M and Asmus K-D 1999 Effects of hydrophobic-hydrophilic balance and stereochemistry on the supramolecular assembly of functionalized fullerenes Supramol. Sc/. C 7 67-73... 

A proton can be (numerically) represented by a series of topological and physicochemical descriptors, which account for the influence of the neighborhood on its chemical shift. Fast empirical procedures for the calculation of physicochemical descriptors are now easily accessible [45. Geometric descriptors were added in the case of some rigid substructures, as well as for rr-systems, to account for stereochemistry and 3D effects. 

Corey E J and J C Bailar ]t 1959. The Stereochemistry of Complex Inorganic Compounds. XXII. Stereospecific Effects in Complex Ions. Journal of the American Chemical Society 81 2620-2629. 

Stereochemistry (Chapter 7) Chemistry in three dimensions the relationship of physical and chemical properties to the spatial arrangement of the atoms in a molecule Stereoelectron ic effect (Section 5 16) An electronic effect that depends on the spatial arrangement between the or bitals of the electron donor and acceptor Stereoisomers (Section 3 11) Isomers with the same constitu tion but that differ in respect to the arrangement of their atoms in space Stereoisomers may be either enantiomers or diastereomers... 

Cromakalim (137) is a potassium channel activator commonly used as an antihypertensive agent (107). The rationale for the design of cromakalim is based on P-blockers such as propranolol (115) and atenolol (123). Conformational restriction of the propanolamine side chain as observed in the cromakalim chroman nucleus provides compounds with desired antihypertensive activity free of the side effects commonly associated with P-blockers. Enantiomerically pure cromakalim is produced by resolution of the diastereomeric (T)-a-meth5lben2ylcarbamate derivatives. X-ray crystallographic analysis of this diastereomer provides the absolute stereochemistry of cromakalim. Biological activity resides primarily in the (—)-(33, 4R)-enantiomer [94535-50-9] (137) (108). In spontaneously hypertensive rats, the (—)-(33, 4R)-enantiomer, at dosages of 0.3 mg/kg, lowers the systoHc pressure 47%, whereas the (+)-(3R,43)-enantiomer only decreases the systoHc pressure by 14% at a dose of 3.0 mg/kg. 

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

Other stmctural variations in both series are the stereochemistry at C3 and the degree of oxidation on the nucleus and side chains. Cardiac steroids probably exert their inotropic effects by acting as specific, noncompetitive inhibitors of — ATPases, known as sodium pumps, and thus... 

Fig. 1. Effect of ion pairing on stereochemistry of propagation for alkyl vinyl ethers, where L is large substituent S, small substituent.

The course of the photochemically mediated isomerization of vinylazirines is dependent on the stereochemistry of the vinyl group, as is illustrated in Scheme 94a (75JA4682). Under thermal conditions the isomerization proceeds through formation of the butadienylnitrene and subsequent pyrrole formation. Analogous conversions of azirines to indoles have also been effected (Scheme 94b). It is possible that some of the vinyl azide cyclizations discussed in Section 3.03.2.1 proceed via the azirine indeed, such an intermediate has been observed... 

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

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