Synthesis of (−)-Conduramine A1, (−)-Conduramine A2 and (−)-Conduramine E2 in Six Steps from Cyclohexa-1,4-diene

A method to enable the synthesis of conduramines and their N-substituted derivatives (enantiopure or racemic form) in six steps (five steps for N-substituted derivatives) from cyclohexa-1,4-diene is reported. Key features of this reaction sequence include a preparation of benzene oxide that is amenable to multigram scale, and its efficient ring-opening upon treatment with a primary amine. Epoxidation of the resultant amino alcohols (40% aq HBF4 then m-CPBA) is accompanied by hydrolytic ring-opening in situ to give the corresponding N-substituted conduramine derivatives directly. These may undergo subsequent N-deprotection to give the parent conduramines, as demonstrated by the preparation of enantiopure (−)-conduramine A1, (−)-conduramine A2, and (−)-conduramine E2 (the latter two for the first time). The selectivity of the epoxidation reaction is proposed to be the result of competitive ammonium-directed and hydroxyl-directed epoxidation processes, followed by either direct (SN2-type) or conjugate (SN2′-type) ring-openings of the intermediate epoxides.