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Synthesis and Stereochemical Analysis of Planar-Chiral (E)‑4-[7]Orthocyclophene

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posted on 16.11.2016, 13:19 authored by Kazunobu Igawa, Kouhei Machida, Kyouhei Noguchi, Kazuhiro Uehara, Katsuhiko Tomooka
An efficient synthesis of (E)-4-[7]­orthocyclophene (E)-1 via photochemical isomerization of (Z)-1 has been achieved. The key intermediate (Z)-1 was synthesized from commercially available 2-(hydroxymethyl)­benzenepropanol (3) in five steps: (i) group-selective Mitsunobu reaction with CH2CHCH2CH­(SO2Ph)2, (ii) oxidation of alcohol, (iii) olefination, (iv) RCM, and (v) removal of sulfones in an overall yield of 73%. The photochemical isomerization of (Z)-1 was efficiently performed in the presence of AgNO3-impregnated silica gel (AgNO3/SiO2). The resulting (E)-1 shows dynamic planar chirality at rt. Enantioenriched (E)-1 was prepared by the HPLC separation of enantiomers using a chiral stationary phase, and the absolute stereochemistry was determined by X-ray diffraction analysis of the Pt-coordinated crystalline derivative. The planar chirality of (E)-1 can be converted into the central chirality of carbon; e.g., the oxidation of (R)-(E)-1 using DMDO provided epoxide (8S,9S)-9 in a stereospecific manner. Furthermore, the Lewis acid-promoted reaction of (8S,9S)-9 afforded a unique tricyclic compound (8S,9S)-10 in an excellent yield and in a stereospecific manner.