No abstract

Bis(tetrahydropyran-2-yl)methane 4 along with its racemate have been synthesized. MM3 calculations suggest that the conformer 4a should be populated to cu. 85% in the conformer equilibrium. Analysis of the 'H NMR coupling constants show that one conformer predominates by about 9: 1. That this is the conformer 4a is shown by various NMR techniques, as well as by comparison of calculated with measured CD spectroscopic data. The study is extended to the methyl-substituted bis(tetrahydropyrany1)methanes 2 1 and 23 which show, as predicted from MM3 calculations, essentially monoconformational behaviour.Conformation design aims at the recognition and synthesis of segments of molecular backbones, which are conformationally flexible, but populate predominantly (ie. >90?/~) a single conformation. The starting point for the considerations is a multi-conformational carbon chain. A mono-conformational structure should result if destabilizing interactions could be introduced by structural modification into all but one of the low energy conformations. For instance, pentane has five low energy conformations (Ere, < 3 kcal mol-It) a~ailable.~ Introduction of two methyl groups into the 2 and 4 position reduces the conformational diversity to a bi-conformational situation,' as 2,4-dimethylpentane (1) populates just two enantiomorphic 1 : l c"3*H3

No abstract

An isobutyl group placed equatorially in the 2‐position of a 1‐equatorially substituted cyclohexane adopts a preferred conformation (cf. 5). This also holds when it is placed in the 2‐position on a 3‐equatorially substituted tetrahydropyran (cf. 6). The same conformational preference is found for 2‐methoxypropyl residues in the 2‐position of 3‐substituted tetrahydropyrans (cf. 8 and 10). The latter compounds chelate lithium cations as analogues of 1,2‐dimethoxyethane. Through this complexation, it is possible to effect a change in the side chain conformation.

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