Highly Selective Three-Step Synthesis of Rhein in Chloroaluminate Molten Salt: Preclusion of the Hayashi Rearrangement

An expeditious, three-step synthesis of rhein (2) was optimized starting from bis(N,N-diethyl)-5-methoxybenzene-1,3-dicarboxamide. The key final step, involving deprotection/cyclization of ortho-benzoylbenzoic acid 9 in acidic chloroaluminate molten salts, yielded the desired natural product with high selectivity and good overall yield by precluding the competitive Hayashi rearrangement.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)