This is demonstrated by analysis of the vicinal H,H-coupling constants along the backbones of compounds 19, 23, and 24. polymethylated alkane chains, which preferentially adopt a fully extended backbone conformation, have been identified.
We showed in the preceeding paper [1] that 2,4-disubsti-conformer preference in segment B amounts to 7:1 due to the presence and relative configuration of the C-5 substitu-tuted pentanes 1 adopt mainly two backbone conformations, and that the conformer equilibrium may be shifted ent. More precisely, it is the anti relative configuration of the (oxygen) substituent at C-5 and the methyl group at C-towards the conformation in which the carbon backbone is fully extended when the substitutents X are electronegative 4 which destabilizes one of the two low energy backbone conformations in segment B of 4. groups such as chlorine or phthalimido.
This led us to consider multisubstituted carbon chains of the type 6, which should have more pronounced conformational preferences. Structure 6 can also be derived by superposition of two building blocks 5 which have been calculated to possess a conformational preference of 80%. [3] MM3* calculations, however, indicate that the conformational preference for 6 is less than expected.
The problem is that multisubstitution creates a large number of gauche arrangements which destabilize all conformations, including the desired ground-state conformation. While such gauche interactions are an integral fea-A combination of such 2,4-disubstituted pentane units, ture of the substitution pattern in 5, higher conformational e.g. going from 1 to 2 or 3 should in principle result in preferences should result if substituents Ϫ other than larger flexible structures with a preference for the fully exmethyl groups Ϫ are chosen which, for steric or electronic tended conformation. But even if the local conformational reasons, prefer to occupy the positions lateral to the main preferences in each of the segments A remain the same in chain. [1] This led us to the structures 7 which are hybrids going from 1 to 2 or 3, the overall conformational preferof 2 and 6. MM3* calculations suggest that carbon backence for molecule 2 or 3 having four to six rotatable backbones 7 should indeed have high overall conformational bone bonds, will be significant only if the local conforpreferences when the substituents X are an oxygen, halogen mational preferences in each of the segments A exceed 95%. or an sp 2 -hybridized carbon or nitrogen atom. In the case The data reported previously [1] for compounds 1 with X ϭ of 7a (X ϭ OCH 3 ) the poor results are a consequence of Cl or phthalimido show that this is not the case.
the fact that these molecules become sterically overcrowded: Therefore, conformation control in larger open chain The O-methyl groups suffer syn-pentane interactions with structures would have to rely on effects other than or in addition to the polar and steric effects which control the conformation in the model compounds 1.
Higher conformation control in local backbone segments may be achieved [2] if any undesired conformation is minimized by destabilizing interactions such as syn-pentane interactions. An example is given by 4, in which the local [ ] Part XI: Ref.