Torsion constraints

Molecules without Strong Ring Torsional Constraints 698... 

The above discussion provides a background for a discussion of the conformations of various azocanes, oxocanes, thiocanes and related compounds which lack strong ring torsional constraints. 

The bridged biphenyl (472) (67ZN(B)1296> has a rigid twist-boat-chair conformation in the crystal (72CC985) and is an eight-membered heterocycle with two torsional constraints in a 1,3 relationship. It has been resolved by chromatography on cellulose acetate, and AG for racemization is 120 kJ mol-1 (67M1323). 

Ab initio, semi-empirical, and molecular mechanics calculations have been used extensively in the study of nine-membered heterocycles. Theoretical studies of heteronines have centered on the question of their aromaticity, which was surveyed as a part of general heterocycles aromaticity study <2004CRV2777>. Another important aspect is the conformation of the nonconjugated compounds (see Section 14.10.4.3). Computational aspects of conformational behavior of saturated nine-membered rings and nine-membered rings containing one torsional constraint were the part of the review <1999MI(5)89>. 

Nine-membered rings are strained in all of their conformations. Conformational studies of saturated heteronines and heteronines containing torsional constraint caused by double bonds, three-membered and benzo-annulated rings, lactams and lactones were the part of the survey <1999MI(5)89>. 

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