Ring combination

Rings combining with linear molecules to form longer linear species ... 

The third chapter Recent Developments in the Chemistry of N-Heterocyclic Phosphines presents a survey on five- and six-membered phosphorus-nitrogen heterocyclic compounds whose rings combine a phosphazene or phosphazane unit... 

Molecular models indicate that cavitand (259) has a tall, vase-shaped architecture in which the four benzene rings and four nine-membered rings combine to form a concave cavity to which is attached the four diazanaphthalenes (Moran, Karbach Cram, 1982). The latter groups resemble flaps which may occupy either equatorial or axial positions. In the former arrangement, the inner surface area of the cavity is quite reduced and hence a considerable decrease in the inclusion ability of this form is expected. 

For series-1, 10 novel ligand molecules with calculated ICVl values from 0.3 to 16 nM were tested. For series-2, the 48 tested molecules yielded calculated K, values from 21 nM to 2.5 pM. In both series, an isocoumarin ring combining H-bond-acceptor features with a delocalized, polarizable ring system yields best results (Fig. 5.8). 

Fig. 10.1. Structures of cholesterol and a commonly used phytosterol. Adding a branch to the side chain and removing the double bond in the sterol B ring combine to reduce absorption of sitostanol —20-fold as compared to cholesterol.

The substituents at C-3 and C-5 may also have stacking interactions in the crystal and thus packing forces can dictate the preferential conformation of the 1,2,4-trioxolane ring as the C-5 and peroxide envelope or even the half chair conformations are energetically close. For instance in the 6,7,8-trioxa-3-thiabicyclo[3.2.1]octane 21, centrosym-metrical pairs are formed by 71-71 stacking between two phenyl rings combined with two weak C-H- -n interactions <2000AXC1510>. 

The molecular formula C5H8O2 implies the presence of two elements of unsaturation. The carbonyl (C=0) group accounts for one, but there are no more carbonyl groups and no double-bonded alkene carbon atoms. The other element of unsaturation must be a ring. Combining the partial structures into a ring gives the complete structure. 

Besides the fragmentation of the 6/6 and 6/5 annelated rings, a number of other ring combinations in bicycles have been investigated. A 6/3 system3), Scheme VIII/6, has been used to explain the observation that the unexpected 6,7-dimethyl-3,4-benzotropolone (VIII/33) is formed when an acetic acid solution of VIII/30 is treated with zinc powder [25]4). 

Two substituents on the same side of a five-membered ring combine to dictate approach from the other side by any reagent, and the two epoxides can be formed each with essentially 100% selectivity. 

The five pp AOs of the C5H5 ring combine linearly to give an equal number of MOs having symmetry a... 

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