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Second‐Generation Total Synthesis of the Pigment Aurantricholone

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Previously, 6,6-dimethoxy-6,7-dihydrobenzocyclohepten-5-ones (“ketoketals”) gained by ring-closing metatheses (“RCMs”) gave 6-hydroxybenzocyclohepten-5-ones (“benzotropolones”) by hydrolyses with 10 equiv. of hot TsOH. Now, an RCM-based ketoketal allowed to reach the benzotropolone aurantricholone by total synthesis for the second time and to avoid a forcing hydrolysis. Another key to success was establishing the pulvinone(−like) motifs by our recently developed Suzuki strategy.


Abstract

Our 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 C sp2−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.

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