Asymmetric derivatives

The photodynamic effect on red blood cells of the /xara-tetraglucosyl-porphyrin 31a and its analogous with the sugar unit in orto position has also been analysed. Considering the possibility of the photosensitiser causing hemolysis, this study became crucial.29 The two symmetric derivatives have proved to be ineffective in these cells. However, the asymmetric derivatives 49B and 50B are phototoxic and changes in hematological parameters were observed.29... 

The Comparison of Selected Molecular Parameters in the Symmetrical Bis(Silyl) Complexes g. 163,164 Asymmetrical Derivatives 89-92 ... 

Synthesis of asymmetric derivatives of disilanes is also possible ... 

Aziridine-2-carboxylic acid, asymmetric derivatives of 87MI5. Aziridines, formation from nitrenes and unsaturated compounds 79ZVK485. 

For the synthesis of the symmetrical derivatives two equivalents of the same aikylamine are used for the asymmetric derivatives one equivalent of each of two different alkylamines is used. 

Assuming—as it is actually the case — that the determination of the perturbed wave function is the time-limiting step, it is clear that variant (1) is the preferred choice and computationally most efficient. Specifically, there are only three magnetic field components, while there are times this many nuclear magnetic moments. This conclusion holds for Hartree-Fock where the coupled-perturbed HF (CPHF) equations are only solved for B (see, e.g., [35]) as well as the correlated approaches discussed in Section III. We thus conclude that the second derivatives needed for the computation of NMR chemical shifts are best calculated using an asymmetric derivative expression. 

In the solid state most diphosphines appear to exist in the trans configuration, although some compounds of this type may exist in the gauche configuration in the gaseous state or in solution. Pyramidal inversion does not occur in diphosphines at room temperatures, and asymmetrical derivatives can be resolved into optically active forms (Chapter 13.2). 

The original Hantzsch 1,4-dihydropyridine synthesis is a one-pot reaction between an aldehyde, 2 equiv of a P-ketoester, and ammonia or an ammonia derivative. This process, known as the 3CP Hantzsch 1,4-dihydropyridine synthesis, is still useful for the synthesis of symmetrical 1,4-dihydropyridines. However, it is less successful for the synthesis of asymmetrical derivatives due to the number of side products formed when using mixed p-ketoesters in the reaction. Like the Hantzsch pyrrole... 

Lee used baker s yeast in his enzymatic approach to the synthesis of 1,4-dihydropyridines. The traditional Hantzsch synthesis was explored whereby 2 equiv of methyl acetoacetate 180 and ammonium acetate were treated with baker s yeast in a phosphate buffer to produce the corresponding dihydropyridine 179 in 67% yield. A modified Hantzsch was also employed in the synthesis of asymmetrical derivative 277. Treatment of 1 equiv of methyl acetoacetate 180 with 1 equiv of crotonitrile 217 gave the corresponding dihydropyridine derivative 277 in 46% yields. The traditional and modified approaches were also conducted using ethyl acetoacetate with similar yields for the reported reaction products. 

Ionic liquids (ILs) are a class of organic salts that are liquid at or near room temperature. The cation parts of ILs are tetravalent ammonium ions with asymmetric substitution tetralkyl ammonium ion, imidazolium ion, or pyrrolidinium ion (O Fig. 23.8). The most widely used ILs have positive ions as asymmetric derivatives of N,N -dialkylimidazolium. As the anion counterpart a great number of negative ions are used including BFp, FFg"", or N(CN)2". 

Asymmetric derivatives can be synthesized by 4-amino-4 -nitrostilbene-2,2 -disulfonic acid however, their preparation is more expensive, and they show little advantage over the symmetrical compounds. The principal effects of structural variations are changes in solubility, substrate affinity, acid fastness, etc. The bistriazinyl brighteners are anployed principally on cellulosics such as cotton or paper. 2-(Stilben-4-yl)-naphthotriazoles (34, 35) are prepared by diazotization of 4-aminostilbene-2-sulfonic acid or 4-amino-2-cyano-4 -chlorostilbene, coupling with an orthocoupling naphthylamine derivative, and finally, oxidation to the triazole. 

The effect of the substituents is as expected from the influence of their electron donating or withdrawing properties. The same trend is observed when one considers the Q band of the visible spectra. From the spechal data, the delocalization of the hole on the two rings is evident, even for the more asymmetric derivatives for example, the electronic spectra of Lu[Pcl(Pc(Q)8l and Lu[Pc(tBu)4][Pc(a)g are different fiom the ones expected for compounds including two non interacting different phthalocyanine moieties... 

Steroidal podands similar to 3 are, in fact, quite well-adapted for enantioselective recognition. The three functionalised sites on the steroid nucleus may be modified to give asymmetrical derivatives of general form 10, capable in principle of the three-point contact required for the classical model of enantioselectivity. Whether or not this idealised situation is ever achieved in practice, the chiral, functionalised steroidal a-face appears to provide an excellent environment for enantioselection. 

A related exchange involving a cyclic tetrasubstituted diborane (III) has been used to synthesize the corresponding asymmetrical derivative (IV) (6S). 

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