Reaction of Bromonaphthofurans with Bis(pinacolato)diboron.

Abstract

The synthesis of a dimeric pyranonaphthoquinone (8) was investigated focusing on a late-stage biaryl coupling of suitably functionalized bromonaphthofurans by using Suzuki–Miyaura methodology. Bromonaphthofuran (16) underwent reaction with bis(pinacolato)diboron in the presence of PdCl₂(dppf) to afford boronate ester (21) and furonaphthofuran (22). ‘In situ’coupling of the boronate ester (21) with aryl bromide (16) to the desired dimer (11) was not realized. Bromonaphthofuran (17), prepared by Diels–Alder/retro-Claisen reaction of bromonaphthoquinone (24) with diene (25), underwent Suzuki–Miyaura coupling to naphthofuran (27) and boronate ester (28). Numerous attempts to alter the reaction conditions to effect homocoupling of bromide (17) to biaryl (19) were unsuccessful. Bromopyranonaphthoquinone (18) prepared by oxidative rearrangement of (17) failed to undergo Suzuki–Miyaura coupling.