ABCD system

From the fact that the ABCD system of dibenzotetraazapentalene 255 did not become an AA BB system on heating, even at 225°C, it was concluded339 that the valence isomerization tetraazapentalene tetraazacyclooctatetraene (Scheme 14, Section IV,B,2) did not occur. Hall, Stephanie, and Nordstrom341 have effected an analysis of the ABCD system of 255 and revealed the presence of two superimposed ABCD systems that give the molecule an element of symmetry. Chemical shifts conformed to electron densities calculated by Chia and Simmons (Section V,B,3).388 The same authors also studied the ABCD-... 

XYZ systems of 260 and 261 and found that the introduction of a methyl group perturbs the chemical shifts of the benzene ring protons some distance away. The transmission of this effect through many bonds led to the conclusion that these systems possessed aromatic character.341 The analysis of the ABCD system of 254b has also been performed by Hall.452... 

Table 15.2. The clinical features and scoring for the ABCD system of assessing risk of stroke in the seven days after a transient ischemic attack...

Table 15.3. The clinical features and scoring for the ABCD system...

The two alternate explanations for the low-temperature nmr spectra of VI are potentially distinguishable. Even the spectra are not quite the same the twist-boats should give two ABCD systems (with suitable coincidences in chemical shifts), while the 1 1 mixture of the boat-chair and twist-chair-chair should give two different AA BB systems. Unfortunately, vicinal fluorine-fluorine coupUng constants are extremely small, so that two apparent AB systems are expected from both explanations. 

Scheme 1-5 Virtual synthetic pathways toward the steroid skeleton with rings A, B, C, and D. Top row stepwise conversion of a ring A (B, C, or D)-building block into the ABCD system middle row expansion in a...

The steroidal alkaloids of the jervanine subgroup are composed of a hexacyclic framework with a tetrahydrofuran E-ring, spiro-cormected to the C-nor-D-homo ABCD-system and fused to a piperidine F-ring (see F pre 1). 

Four Coupled Nuclei (AX, AjXj, AA XX, A2B2, ABj, AA BB, ABCD Systems)... 

Figure 2.40. Conformations of a 1,2-disubstituted ethane drawn as Newman projections (a) trans conformation affords an AA BB (or AA XX ) system (b) gauche conformation affords an ABCD system.

Figure 2.44. Some typical ABCD systems found in (a) disubstituted benzenes and (b) epoxides.

The ABCDE system used in advanced trauma life support remains a useful structure for both primaiy and secondary surveys in cases of toxic trauma, but certain factors in the acronym may have determining effects on the others. 

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