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Synthesis and Ring Size Effect of Macrocyclic Ethynylhelicene Oligomers

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journal contribution
posted on 20.01.2006, 00:00 by Yusuke Takahira, Hiroki Sugiura, Masahiko Yamaguchi
The cyclization of acyclic ethynylhelicene oligomers with decyl 3,5-diiodobenzoate under optimized conditions gave the corresponding optically active [n+n]cycloalkynes (n = 4−8) in high yields. Their structures were compared in terms of ring size by using 1H NMR, UV−vis, and CD spectroscopies and vapor pressure osmometry (VPO). The UV−vis spectra exhibited an increase in absorbance in proportion to n. In contrast, the CD spectra of the macrocycles exhibited a large ring size effect, comparable Δε values despite the increase in n and temperature-dependent properties of the [8+8]cycloalkyne. It was concluded that [4+4]cycloalkyne, [5+5]cycloalkyne, [6+6]cycloalkyne, and [7+7]cycloalkyne have rigid structures, while [8+8]cycloalkyne has a flexible structure.