Ring inversion, spontaneous

Stacking of ring-shaped compounds can be another good alternative to create hollow tubular structures [26,27]. However, despite recent advances in synthetic tubular nanostructure using the self-assembly, the tubules have a limitation on the dynamic response characteristics. As a part of an effort to generate the stimuli-responsive tubular nanostructures, the self-assembled tubular nanostructure that can undergo a reversible expansion-contraction motion and chirality inversion was reported. Bent-shaped aromatic amphiphiles (enantiomers 1, 2) self-assembled into hexam-eric macrocyclic nanostructures in an aqueous solution (Fig. 2.1) [28]. When the concentration of the solution was increased, the chiral tubules were formed by spontaneous ID stacking of hexameric macrocycles with a mutual rotation in the one direction (Fig. 2.1c). 

Treatment of an oxirane bearing no silyl group with a silyl nucleophile leads to the same result. Both trimethylsilylpotassium and dimethylphenylsilyllithium effect smooth conversion of oxiranes into alkenes with inversion of configuration, nucleophilic ring opening being followed by spontaneous syn-y6-elimination (Scheme 2.121) [341, 342]. 

The effect of substitution on the aryl ring in this system was also studied by the same authors [140] (Scheme 67). The retention spiroacetals 268 were obtained under spontaneous cyclization conditions with good selectivity across most ring sizes, except for the electron-poor N02-substituted analogue. The selectivity could be completely reversed, in some cases, using the previously developed methanol-induced spirocyclization conditions to give the inversion spiroacetals 269 predominantly. 

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