Disymmetric

The mem-dichlorobenzene complex reacts with protected 0-aryltyrosines to give aryl ethers. Both chlorine atoms can be sequentially substituted to give symmetrical or disymmetrical triaryl diethers (Scheme XVI). The building up of such diaryl ethers from phenolic compounds which have amino groups in their side chains... 

Thiazolines (2,3-dihydrothiazoles) were also prepared under microwave irradiation. Hamelin and coworkers have described the alumina-supported solvent-free synthesis of various 4-iminothiazolines by condensation of disymmetric thioureas and a-chloro ketone (Scheme 10). The experiments... 

Restricted Rotation Giving Rise to Perpendicular Disymmetric Planes. Certain compounds that do not contain asymmetric atoms are nevertheless chiral because they contain a structure that can be schematically represented as in Figure 4.2. For these compounds, we can draw two perpendicular planes neither of which can be bisected by a plane of symmetry. If either plane could be so bisected, the molecule would be superimposable on its mirror image, since such a plane would be a plane of symmetry. These points will be illustrated by examples. 

Doucet, C. Vergely, I. Reboud-Ravaux, M. Guilhem, J. Kobaiter, R. Joyeau, R. Wakselman, M. Inhibition of human leucocyte elastase by functionalized V-aryl azeti-din-2-ones. Stereospecific synthesis and chiral recognition of disymmetrically C3-substi-tuted P-lactams. Tetrahedron Asymmetry 1997, 8, 739-751. 

In an interesting illustration of these reactions, the two disymmetric trienes (+)-33a and (—)-33b were found to preserve their chirality upon photolysis at 193 K and provide cyclohexadienes 34a and 34b, respectively (Scheme 8)18. Upon warming above 205 K, however, they lose their chiral integrity by competitive disrotatory cyclization to the achiral dienes 35a and 35b. The thermal disrotatory closure to the cis-fused ring isomer is generally found to be extremely facile in these systems. 

R)- and (S)-(6,6 -Dimethylbiphenyl-2,2 -diyl)bis(diphenylphosphine), BIPHEMP (1). This is the first example of an optically active bis(triarylphosphine) containing the axially disymmetric biphenyl group. The 6,6 -dimethyl groups are used to... 

Dinuclear asymmetric and /u-oxo-bridged complexes in which the two technetium atoms are in intimate electronic communication have been synthesized. The reaction of solutions of [TcOCU]" or [TcClg] with neat pyridines py-R (R = H, 2-Me, 3,5-Mc2) gave asymmetric complexes of the type [Cl2(py-R)3Tc(/r-0)TcCl3(py-R)2] (378), and disymmetric complexes [Cl(py-R)4Tc-... 

A convergent synthetic approach developed for 10,10-, 13,13-, and 20,20-difluoroarachidonic acids, based on the reactivity of disymmetric difluor-... 

Choiesteryi Ester Transfer Protein inhibitors Cholesterol ester transfer protein (CETP) is a glycoprotein that transfers choiesteryi ester from HDL (high density lipoprotein) to proatherogenic apolipoproteins (LDL—(low density lipoprotein). Its inhibition has beneficial effects at the level of HDL cholesterol. SC-71952 and torcetrapib are highly fluorinated CETP inhibitors. SC-71952 is a disymmetrical sulfide with 10 fluorine atoms. Torcetrapib contains three CF3 groups (Figure 8.61). ... 

On sonication, surfactants (4, 5) form vesicles which are polymerized by an initiator or by UV irradiation across either their bilayers or their head groups depending on the position of the double bond (Fig. 8). The polymeric vesicles are stable for extended periods even in 25 % C2H5OH. Efficient charge separation has been realized in such chemically disymmetrical polymerized vesicles. Photoexcitation of Ru(bpy)2 + placed on the outside of the vesicle resulted in the formation of long-lived reduced viologens on the inside. 

To explain these unexpected results, we assumed a disymmetrical structure of active species,... 

The results of this reaction changed when the conditions were slightly different. Other authors have reported that with MP in refluxing benzene for 2 days, the polymerization of 2-vinylthiophene 59 took place quite rapidly and no adducts could be isolated (89JHC397). However, when a neat 1 3 mixture of 59 and MP was heated in a sealed tube at 110° for 4 days, compound 69 was isolated in 70% yield. One of the striking contrasts of this reaction, in comparison with the addition of MP to l-methyl-2-vinylpyrrole 20a, is the different regioselectivity of the reaction between the disymmetric dienophile MP with the nonisolated intermediates. In this latter case the indole 27 was isolated. 

The reaction is quite sensitive to the chiral ligand used. Diphosphines with an axially disymmetric biaryl moiety in the backbone give the best results. The effectiveness of p-Tol-BINAP as ligand, for example, is similar to that of BINAP. The related atropisomeric ligand BIPHEMP can also be used [9]. Among chiral aliphatic diphosphines tested, CyDIOP, which only differs from DIOP in the type of P-substituents, also gives satisfactory results. 

Several optically active polymers of acrylates and methacrylates have been obtained by enantioselective polymerization of a racemic monomer initiated by a Grignard compound complexed with chiral reagent. Complexing agents for the polymerization of (K,S)-a-methylbenzyl methacrylate include chiral alcohols, such as quinine and cinchonine [63], (— )-sparteine and its derivatives [64-67], and other axially disymmetric biphenyl compounds [68,69]. Other racemic monomers used include (/ ,S)-a-methylbenzyl acrylate [70], (K,S)-l-phenylethyl acrylate, methacrylate and a-ethylacrylate [71], and 1,2-diphenylethylmethacrylate [72]. 

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