Improved asymmetric synthesis of dopamine D1 full agonist, dihydrexidine, employing chiral ligand-controlled asymmetric conjugate addition of aryllithium to a nitroalkene

Asymmetric conjugate addition of 2-trityloxymethylpheyllithium to a nitroalkene was mediated by a chiral ligand to give the key intermediate for dopamine D1 full agonist dihydrexidine 1. The shortcut of both Curtius rearrangement and Pictet-Spengler type cyclization, which were the drawback of the previously reported synthesis involving asymmetric conjugate addition of phenyllithium to an enoate, was realized by the newly developed asymmetric reaction. Short and efficient synthetic way gave optically pure dihydrexidine in 45% overall yield via eight steps. Improved synthesis of the best chiral ligand 13 was realized under the Buchwald conditions.