Synthesis of Enantiomerically Pure Oxa[9]helicene Derivatives by a Nucleophilic Cyclodehydration Reaction of Helical 1,1′-Bibenzo[c]phenanthrenylidene-2,2′-dione

Abstract

Enantiomerically pure 9-substituted 11-oxa[9]helicene derivatives have been synthesized through furan-ring formation by a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed 2,2′-diphenoquinone derivatives (1,1′-bibenzo[c]phenanthrenylidene-2,2′-diones). (P)-2,2′-diphenoquinone derivatives afforded (P)-oxa[9]helicenes, whereas (M)-2,2′-diphenoquinone derivatives afforded the corresponding (M)-oxa[9]helicenes. Therefore, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess, and it proceeded stereospecifically with retention of the configuration. The thermal stability of an oxa[9]helicene was studied by determining the decrease in its enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJ/mol.

Impact of helical elongation of symmetric oxa[n]helicenes on their structural, photophysical, and chiroptical characteristics

“…Notably, the remarkably high ( P / M ) enantiomerization barrier for OH9 contributed to the exceptional thermal stability of its enantiomers, observed when solutions of ( M )‐ OH9 were heated at 150 °C for 2 hours without any racemization. These results also compare well with the experimentally determined enantiomerization barrier of ( P )‐ OH9 (~40 kcal mol −1 ) as reported by Hossain and Karikomi using an Eyring plot 70 …”

“…CD spectra of isolated enantiomers ( P / M )‐ OH7 and ( P / M )‐ OH9 in chloroform solutions (1 × 10 −5 M) were measured (Figure 6A). To assign the configuration of each isomer, we compared the obtained CD spectra to those derived from TD‐DFT calculations (refer to Supporting Information) and the previously reported CD spectra of OH9 by Hossain and Karikomi 70 . The absolute configurations in the first and second fractions of the chiral HPLC analysis were designated as the ( P )‐ and ( M )‐enantiomers, respectively, for both OH7 and OH9 .…”

“…These results also compare well with the experimentally determined enantiomerization barrier of (P)-OH9 ($40 kcal mol À1 ) as reported by Hossain and Karikomi using an Eyring plot. 70…”

“…To assign the configuration of each isomer, we compared the obtained CD spectra to those derived from TD-DFT calculations (refer to Supporting Information) and the previously reported CD spectra of OH9 by Hossain and Karikomi. 70 The absolute configurations in the first and second fractions of the chiral HPLC analysis were designated as the (P)-and (M)-enantiomers, respectively, for both OH7 and OH9. Interestingly, due to the increase in helical length (n) from 7 to 9, OH9 exhibited a significantly higher Δε than OH7 in the long-wavelength region.…”

Helical Amide Derivatives: Synthesis by Insertion of Aliphatic ­Primary Amines into Benzo-Fused 2,2′-Diphenoquinones and Dia­stereomeric Resolution

“…[2] The introduction of heteroatoms into ortho-fused polycyclic arrangements significantly changes the electronic structure and helps improve a variety of optoelectrical properties. [3] In our previous studies, we reported the synthesis of functionalized or substituted oxa [9]helicene derivatives in racemic and enantiomerically pure forms by the reaction of various nucleophilic reagents [4] (such as Lawesson's reagent, phosphorus pentasulfide, [5] alcohols, [6] thiols, [7] and some halogenating reagents [8] ) with helical quinone derivatives 1 (Scheme 1). [9] However, when aliphatic primary amines were used as nucleophilic reagents, it was found that, unlike in the case of other nucleophilic reagents [4][5][6][7][8] , an amide derivative 2 composed of a sevenmembered ring was formed through the insertion of amines into helical cyclohexadienone rings.…”