title: Synthesis and ring size effect of macrocyclic ethynylhelicene oligomers.
abstract: [reaction: see text] 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 (1)H 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 Deltaepsilon 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.
authors.affiliation: Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Takahira
authors.forename: Hiroki
authors.initials: H
authors.lastname: Sugiura
authors.forename: Masahiko
authors.initials: M
authors.lastname: Yamaguchi