Large rings transannular interactions

In eight-membered heterocycles containing atoms of silicon and donor (D) in positions 1 and 5, a coordinate bond Si<-D can be formed to yield two condensed five-membered rings. Such diptych structures (largely with transannular interaction... 

When substituents are attached to large rings, they tend to occupy only exterior positions to minimize the large transannular interactions. A fully substituted atom of a macrocycle will usually occupy a corner position since this is the only position where it does not cause severe transannular strain. For both substituted and unsubstituted macrocycles, the conformation of the ring will impose restraints on the torsional angles available to each unit of carbons (four carbons define a torsional angle). If these four atoms are shown... 

Since C-alkyl substitution enhances ring closure, the poly-merizability of a given lactam is expected to decrease with the inaease of substituent length. However, steric repulsions between large substituents and ring atoms (transannular interactions) may result in a relatively higher polymerizability. For example, on the basis of the equilibrium polymer content, the following order of polymerizability for e-alkyl-substituted CL has been reported ... 

Table 8 presents structures observed for monocyclic dienes and polyenes with rings large enough to accommodate trans C=C double bonds. In a cyclodecadiene molecule strain-free carbon skeletons can only be derived when two double bonds are diametrically placed and have the same configuration (as, cis or trans,trans). Cw,cis-Cyclodeca-1,6-diene (1,6-CDD) may exist in twelve different conformations, and it is therefore noteworthy that it almost exclusively prefers one of these, namely the one indicated in Table 8. This conformer does not have the repulsive transannular HH interactions that destabilize the corresponding saturated molecule in all conceivable conformers.

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