Concise Synthesis of [1,1′‐Biisoquinoline]‐4,4′‐diol via a Protecting Group Strategy and Its Application for Potential Liquid‐Crystalline Compounds

The [1,1′‐biisoquinoline]‐4,4′‐diol ( 4a ), which was obtained as hydrochloride 4a ⋅2 HCl in two steps starting from the methoxymethyl (MOM)‐protected 1‐chloroisoquinoline 8 ( Scheme 3 ), opens access to further O‐functionalized biisoquinoline derivatives. Compound 4a ⋅2 HCl was esterified with 4‐(hexadecyloxy)benzoyl chloride ( 5b ) to give the corresponding diester 3b ( Scheme 4 ), which could not be obtained by Ni‐mediated homocoupling of 6b ( Scheme 2 ). The ether derivative 2b was accessible in good yield by reaction of 4a ⋅2 HCl with the respective alkyl bromide 9 under the conditions of Williamson etherification ( Scheme 4 ). Slightly modified conditions were applied to the esterification of 4a ⋅2 HCl with galloyl chlorides 10a – h as well as etherification of 4a ⋅2 HCl with 6‐bromohexyl tris(alkyloxy)benzoates 11b , d – h and [(6‐bromohexyl)oxy]‐substituted pentakis(alkyloxy)triphenylenes 14a – c ( Scheme 5 ). Despite the bulky substituents, the respective target 1,1′‐biisoquinolines 12, 13 , and 15 were isolated in 14–86% yield ( Table ).