Chiral Induction in Quinoline-Derived Oligoamide Foldamers: Assignment of Helical Handedness and Role of Steric Effects

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• 作者：Christel Dolain ; Hua Jiang ; Jean-Michel Lé ; ger ; Philippe Guionneau ; and Ivan Huc
• 刊名：Journal of the American Chemical Society
• 出版年：2005
• 出版时间：September 21, 2005
• 年：2005
• 卷：127
• 期：37
• 页码：12943 - 12951
• 全文大小：522K
• 年卷期：v.127,no.37(September 21, 2005)
• ISSN：1520-5126

文摘

Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiralhelical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83%could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact thatdiastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sensefavored by the terminal chiral group could be determined unambiguously using X-ray crystallography.Assignment of chiral induction was performed in an original way using the strong tendency of racematesto cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that thestereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiralhelical induction was rationalized on the basis of sterics. Upon assigning an R^s or S^s chirality to thestereogenic center using a nomenclature where the four substituents are ranked according to decreasingsizes, it is observed that R^s chirality always favors left-handed helicity and S^s chirality favors right-handedhelicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction.The preferred conformation at the stereocenter is apparently one where the bulkiest group shouldpreferentially point away from the helix, the second largest group should be aligned with the helix backbone,and the smallest should point to the helix.