Symmetric Derivatives

From a commercial viewpoint the most important compounds of this class are the 3,3-(bisindol-3-yl)phthalides. The first synthesis47 involved Route B as described in Scheme 7, in which a second indole derivative condenses with the indolylbenzoylbenzoic acid in acetic anhydride. However, for the preparation of symmetrical derivatives it has been shown61 that a one-pot process, avoiding isolation of the intermediate and use of aluminum chloride, is more convenient. 

Other N6-cages also prepared include symmetrical derivatives of type (161), the unsymmetrically capped species (162) and (163) (Boucher et al., 1983 Bond, Lawrance, Lay Sargeson, 1983 Hammershpi Sargeson, 1983) and a number of related cages derived from 1,2-diaminocyclohexane rather than 1,2-diaminoethane (Geue, McCarthy Sargeson, 1984). 

The oxidation of amidinothioureas by a range of oxidizing agents to give 3,5-diimino derivatives is still one of the most versatile methods for the synthesis of N-linked derivatives (see Section 5.08.9.4). An alternative method which gives symmetrical derivatives is the oxidation of A-arylthioureas to produce Hector s bases which readily isomerize to give 3,5-diamino-l,2,4-thiadiazoles (see Section 5.08.9.2). 

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 first 3,6-dialkoxy-2,5-dihydropyrazine used in asymmetric synthesis of amino acids 7 10 was the symmetrical derivative 2, derived from cyclo(L-Ala, L-Ala) (1). This dihydropyrazine can be prepared by direct condensation of the methyl ester of L-alanine and subsequent alkylation with trialkyloxonium tetrafluoroborate7. Although the condensation process results in partial racemization of the alanine moiety, recrystallization yields almost optically pure cyclo(L-Ala, L-Ala) (1). 

A special case, demonstrating the high versatility of the bis-lactimether method , utilizes the symmetrical derivative obtained from L-leucine, i.e, (2S,5S)-2,5-dihydro-3,6-dimethoxy-2,5-bis(2-methylpropyl)pyrazine (1)14. 

The first structural characterization of a tellurium diimide RNTe(p-N Bu)2TeNR (R = PPh2NSiMe3) was reported in 1994. Subsequently, the synthesis of the symmetric derivative, R = Bu, was achieved by the reaction of Li[HN Bu] with TeClq in a 4 1 molar ratio in toluene. Several LiCl-containing byproducts are also obtained in this solvent. In THF, however, the tellurium diimide, BuN-Te(p-N Bu)2TeN Bu, is obtained in 90% yield and is readily purified." Unlike sulfur or selenium diimides, the tellurium diimides are dimeric and form either cis or... 

The right-hand side of eqn. (9), which is the diffusion equation or Fick s second law, involves two spherically symmetric derivatives of p(r, t). In the general case of three-dimensional space, lacking any symmetry, it can be shown that the Laplacian operator... 

Cyclotrimerization of nitriles is the best-known route to 1,3,5-triazines (for a detailed discussion see CHEC 2.18). The reaction is of value for preparing the symmetrical derivatives only. Nevertheless, many important triazines, such as cyanuric chloride, are made in this way. Other cyclotrimerization reactions are useful thus, an easy route to 1,3,5-triazine involves heating ammonium acetate with ethyl orthoformate. 

Macrocycles of higher symmetry might possess better complexing properties than simple crowns. A model study on the synthesis of C2-symmetrical derivatives containing the sucrose molecule was reported. The synthesis was initiated from hexa-O-benzylsucrose, the crucial step being the selective silylation of the 6 -OH group. Further standard transformation provided two intermediate... 

Because F A V is antisymmetric, the symmetrical derivative ()/l dvFIJ V must vanish. This requires jv to satisfy the equation of continuity, 3vjv = 0, which implies charge conservation in an enclosed volume if net current flow vanishes across its spatial boundary. 

By contrast, however, 3,5-diamino-l,2,4-thiadiazoles are selectively converted into amides and sulfonamides in good yields. Thus, acetylation of 3,5-diamino-l,2,4-thiadiazole (53) with excess acetic anhydride yields the symmetrical derivative (133) whereas under similar conditions 3-amino-5-methylamino-l,2,4-thiadiazole and 3,5-diarylamino-l,2,4-thiadiazoles (65AHC(5)119) produce the monoacylated derivatives (134) and (135), respectively. Sulfonylation also results in the preferential formation of the 3-sulfonamides but reaction with excess reagent eventually produces the 3,5-disubstituted derivatives (136) and (137) in good yields (65AHC(5)119). 

The Comparison of Selected Molecular Parameters in the Symmetrical Bis(Silyl) Complexes 7163,164 WITH THE symmetrical Derivatives 89-92180,181... 

Based on residues Y that can pass the annulus99 but not each other, two chiral diastereomers are possible for 47 namely a / -symmetrical derivative (RRR/SSS) and a C2-symmetrical derivative (RRS/SSR). While the D3-symmetrical hexa-methylether was isolated (in low yield and impure form) by Yamato et al.,97 all our... 

In this section we will discuss not only asymmetric calixarenes (Cj-symmetry), but also those C2-symmetrical derivatives, which are prepared in a similar way. 

Naphthol may form cyclic tetrameric condensation products with formaldehyde among which 101 has a C4 axis.194 However, since the OH groups are in exo positions in this case (in contrast to the calix[4]arene 97g derived from 2-naphthol) it seems difficult to fix a nonplanar conformation in these cases.195 A possibility would be the connection of two opposite oxygens by a (crown) ether bridge which would lead to a C2-symmetrical derivative. 

The first convincing C4-symmetrical derivative of a resorcarene, tetra-lactone 108, was obtained by acid-catalyzed rearrangement of the cavitand-derived tetra-... 

Condensation of resorcarenes with primary amines and formaldehyde leads to tetrabenzoxazines 109, usually in excellent yield. First examples were described by Matsushita and Matsui45 without any discussion of the stereochemical aspects. In principle, four regioisomers can be formed in this reaction (see Figure 20) from which the C,- and the Cs-symmetrical isomers can be easily excluded on the basis of the H NMR spectra. The distinction between the C2V- and the C4-symmetrical derivatives relies only upon the difference of the CHR1 groups in the C2V isomer which could be isochronous by chance. 

Figure 22. Enantioselective preparation of C4-symmetrical derivatives of rccc-resor-carenes.

Let us now consider tetraurea derivatives possessing two different phenolic units A and B. This difference may be due to different residues R or Y or both. Tetraurea derivatives of the type ABAB are C2v -symmetrical. Their dimerization leads to chiral capsules II with an overall C2 symmetry. The situation is even more complicated for a Cs-symmetrical derivative AAAB. Two regioisomeric dimers are possible here, with a proximal (Ilia) or distal (Illb) arrangement of the phenolic unit B. Both are asymmetric and consequently chiral. (The same is true for a derivative AABB, not shown in Figure 32.)... 

Such a C4-symmetrical derivative is not inherently chiral in the strict sense of the definition given above, since after opening the macrocycle the -CHR-groups remain asymmetric. 

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