Second‐Generation Total Synthesis of the Pigment Aurantricholone

Abstract

AbstractOur first total synthesis of aurantricholone established its benzotropolone core by the ring‐enlargement of a tetralone. Here we describe another total synthesis of aurantricholone. It reaches the benzotropolone core from a known olefin metathesis product via an equally known dibromide, both of which contain a ketoketal moiety. The next transformation ‐ step 9 overall ‐ engaged this motif in a β‐elimination of ROH rather than in a hydrolysis under the forcing acidic conditions indispensable in all prior benzotropolone preparations from such an intermediate. In step 10, the Csp2−Br bonds of the elimination product underwent two doubly Z‐selective Suzuki couplings with a boronylated O‐methyl 4‐methylidenetetronate. This gave penta(O‐methyl)aurantricholone. Its NMR shifts matched essentially those of a derivative of natural aurantricholone by Steglich et al. Three O−Me bonds were cleaved with BBr3/CH2Cl2 (step 11) and two O−Me bonds with LiBr/DMF (step 12). A 1 : 3 co‐crystal of aurantricholone and DMSO allowed for an X‐ray structure analysis.