Development of novel C2-symmetrical [5]oxahelicenoid diols

Abstract

Novel C2-symmetric [5]oxahelicenoid diols were synthesized via dehydroaromatization from the previous reported [5]oxahelicenoid dione 7 starting from 2-diazocyclohexane-1,3-dione and furan. The key dehydroaromatization reaction can be realized either by DDQ mediated or Pd/C catalyzed dehydrogenative aromatization. Optical resolution of (rac)-8 with N-benzylquininium chloride 9 and N-benzylquinidinium chloride 10 afforded the enantiomerically pure (P)-8 and (M)-8, respectively. In addition, six chiral phosphoric acids 13a ~ 13f were synthesized from (P)-8 and subjected to an enantioselective Friedel-Crafts reaction to afford the desired product 16 in up to great yield and moderate er value (94% yield, 76:24 er). The result indicates that these readily synthesized [5]oxahelicenoid diol based catalysts have comparable catalytic activity albeit lower enantioselectivity as BINOL-based and SPINOL-based ones. We believe that this work as the first try to introduce the helical diols-based chiral phosphoric acids as catalysts would draw the attention and interest of chemists.