Three ring system

MO methods have been used to calculate dipole moments of each of the three ring systems (73MI50403, B-70MI50400). Calculated values for aziridine are somewhat higher (2.09-2.40 D) than the known experimental value (1.89 D). Dipole moment studies on a few simple aziridines have led to the determination of the preferred conformation of N-arylaziridines in solution and in the vapor state (71JCS(C)2104, 66DOK(169)839). For the 1-azirine system, no values have been determined experimentally, but values of 2.40-2.56 D for 1-azirine and 2.50-2.51 D for 2-azirine have been calculated (73MI50403). 

Similar number of the isomeric structures for thiazoloquinolines to that of the oxazoloquinolines can be drawn, but the reported examples for these isomers belong to only three ring systems thiazolo[3,2-u]quinoline, thiazolo[4,3-u]quinoline and thiazolo[5,4,3-i/]quinoline (Fig. 8). 

Fluorescein and its derivatives represent one of the most popular of all fluorescent labeling agents. Its fluorescent character is created by the presence of a multi-ring aromatic structure due to the planer nature of its upper, fused three-ring system (Figure 9.4). In its most elementary... 

Schmidpeter et al. studied the synthesis of various diazaphospholodiazines, and investigated the products by NMR spectroscopy <1995CB581>. In Table 5, 31P and 13C NMR chemical shifts as well as (P,C) coupling constants of three ring systems, [l,4,2]-diazaphospholo[4,5- ]pyrimidine e, [l,4,2]-diazaphospholo[4,5-< ]pyrimidine f, and [1,4,2]-diazaphospholo[4,5- ]pyrazine g, are compared. 

As indicated in the introductory part, three ring systems containing phosphoms atom have been synthesized. These syntheses are shown in Scheme 30. 

The most common organic derivatives of boron-sulfur rings are based on the three ring systems found in the binary boron sulfides, viz. the five-membered rings (RB)2S3 (9.27) and six-membered rings (RBS)3 (9.28) and, less frequently. 

The mass spectrometric analyses of the saturate fractions are reported in Table VI. These fractions appear to be composed mostly of alkanes and noncondensed cycloalkanes with smaller amounts of condensed cycloalkanes, mainly two- and three-ring systems. However, because of the presence of olefins in these fractions the analyses are only semiquantitative. In fact, an olefin should make a contribution to the cycloalkane group type which has the same molecular weight. That is, a monoolefin will contribute to the cycloalkanes, a diolefin or a cyclic olein will contribute to the bicycloalkanes, etc. However, to determine the extent of these contributions more analytical work is necessary. 

The flexibility of the pincer carbene ligand system can be seen from the crystal structures of their palladium complexes. The ligand without linker units features the central pyridine ring and the flanking NHC units strictly coplanar [431] whereas the linker unit introduces twisting of these three ring systems [490]. The flanking NHC units are now... 

The spectinomycins (O Fig. 20) have a fused three-ring system which consists of a 4,5-disub-stituted actinamine and an ulose. Spectinomycin is produced by Streptomyces spectabilis and Streptomyces flavopersius. It has been clinically used for treatment of the gonorrhea infection. A related structure has been discovered with spenolimycin which was isolated from the culture filtrate of Streptomyces gilvospiralis. Efforts have been directed toward semisynthetic aminoglycoside antibiotics to treat multi-drug resistant bacteria [94]. 

Intramolecular cyclohexadiene syntheses have been developed by Vollhardt [74], Enediynes with a terminal double bond react in isooctane at 100 °C in the presence of -Cp-cobalt dicarbonyl giving a three-ring system [75] according to eq. (26). 

There is one major restriction in this type of stabilisation, namely that the redistribution of the electrons must not result in a double bond at a bridgehead that is, at a junction point between three ring systems. This is Bredt s rule. 

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